chapter i optical fiber in 1952, physicist narinder singh kapany conducted experiments that led to...

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CHAPTER I

OPTICAL FIBER

The usage of optical fibers in telecommunications has had a great importance in recent years, because such technology allows high rate and relatively low attenuation.

The first section focuses on the geometry of an optical fiber with particular attention to those physical phenomena that provide the optical transmission of a signal. Of great importance are the phenomena of dispersion, attenuation and nonlinear effects that often results in loss of performance. The second section describes the most important submicron electronic devices that allows an electro-optical (vice versa) communication and also describes active and passive optical components like lasers (exploiting stimulated emission of radiation) or photodiodes to understand the phenomenon of emission of photons.

In recent years, commercial solutions for optical transceivers have had a great success on the market. The third section focuses on a description of an optical transceiver module, that is, from physical point of view, an integrated circuit which allows both the transmission and the reception of optical signal.

As described in this third section the study of the performance of a generic optical signal transmitted or received is based on the definition of metrics related to signal transmission within the optical medium and on the receipt. Shortly, most of the metrics proposed are based on the generation of eye diagrams that provides an overall vision of the goodness of transmission, and the study of jitter, delays or errors in signal reception (BER and sensitivity).

The latest section of the chapter describes some of the most important optical fiber communication standard that allows an optical fiber transmission with a bit rates ranging from 4Gbps to 10Gbps.

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1. 1 Optical Fiber Communication

Optical fiber, though used extensively in the modern world, is a fairly simple and old technology. Guiding of light by refraction, the principle that makes optical fiber possible, was first demonstrated by Daniel Colladon and Jacques Babinet in Paris in the early 1840s. John Tyndall included a demonstration of it in his public lectures in London a dozen years later1.

Practical applications, such as close internal illumination during dentistry, appeared early in the twentieth century. Image transmission through tubes was demonstrated independently by the radio experimenter Clarence Hansell and the television pioneer John Logie Baird in the 1920s. The principle was first used for internal medical examinations by Heinrich Lamm in the following decade. In 1952, physicist Narinder Singh Kapany conducted experiments that led to the invention of optical fiber. Modern optical fibers, where the glass fiber is coated with a transparent cladding to offer a more suitable refractive index, appeared later in the decade2.

The use of optical fibers for communication purposes were first carried out in Western Europe in the late 19th and early 20th century, such as they were used to diagnose a patient's stomach by a doctor, and those communications within short ranges. Especially, the transfer of images by optical fibers was largely popularized at the beginning of 21st century, due to the growing medical and television demands3.

Optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. It is especially advantageous for long-distance communications, because light propagates through the fiber with little attenuation compared to electrical cables. This allows to cover long distances with few repeaters.

1 Bates, Regis J (2001). Optical Switching and Networking Handbook. New York: McGraw-Hill.

2 Bates, Regis J (2001). Optical Switching and Networking Handbook. New York: McGraw-Hill.

3 Mary Bellis , The Birth of Fiber Optics, http://inventors.about.com/library/weekly/aa980407.htm

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1.1.1 Optic Fiber Channel Specifications

The function of a transmission system is the transfer of a signal that carries information from a source to a user. The resource of greatest importance is the bandwidth offered by the medium. In a digital communication system, the bandwidth occupied by the transmitted signal is immediately connected to the speed of reporting. In the case of bit stream, a flow of information to Rb bits / s requires a bandwidth B of the order of Rb ÷ 2RB Hz depending on the type of modulation used.

Another key aspect is the power of the transmitted signal, due to the disturbances that accompany the transmission and reception of the signal (electronic noise, interference, etc..). Receivers may not detect and demodulate input signals with a power lower than a minimum level, called sensitivity of the receiver. Further specifications which affect the quality of service are given in terms of signal to noise rate, bit error rate. All this imposes a lower limit to the minimum power that can be received. Once known the characteristics of attenuation on physical media and the sensitivity of the receiver, the limitation on the received power, results in a limitation on the maximum distance L that the link is capable of supporting (assigned the maximum power that the transmitter can provide). One way of summarizing the two issues just mentioned spectral efficiency and energy of a given transmission system is to introduce a measure of overall efficiency given by the product δ between the maximum transmission rate B obtained over a half with a given technique modulation, and the maximum distance L can be covered by the same means, given the desired quality of service:

LB ⋅=δ [a]

Means of transmission is inherently characterized by fundamental physical limitations on bandwidth and attenuation. The evolution in the efficiency of telecommunication systems is in slow and continuous improvement of a certain kind of technology related to some physical medium of transmission. From the few bps of the telegraph, electromechanical technology data rate increased to the thousands of bps of the phone line, to the Mbps of copper coaxial cable and microwave radio links, satellite of radio relays. The technology that currently delivers the greatest transport capacity is that of fiber links4. The superiority of optical transmission is linked to the huge bandwidth that optical fiber makes available to the extremely low attenuation and immunity deals offered to the signal against electromagnetic interference. The optical fiber is characterized by near-infrared wavelengths, in particular λ = 0.8 ÷ 1.6 µm, corresponding to carrier frequencies between 190 and 380 THz.

The ability to reach large capacity transmission systems over fiber is linked to two factors: the development of semiconductor light sources of small size, high efficiency and high reliability, such as the laser (light amplification by stimulated-emission radiation) diodes, refinement of techniques for manufacturing optical fibers, which has made available high-quality fiber. The transmitted information is associated with an electrical signal that the transmitter can considered in two ways: it can indeed be seen as a modulator, which impresses the information given on an optical carrier frequency, or it can be seen as a transducer that transfers the information from an electrical signal to

4 G.P. Agrawal. Fiber- Optic Communication Systems. John Wiley, New York, 1997. Pag.7

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an optical signal. The same two also concerns the receiver, which performs the dual function transmitter, retrieving an electrical signal which is associated with the information received from the optical signal output of the fiber. The need for electro-optical conversion and opto-electronics is a limitation to the exploitation of the enormous bandwidth of optical fiber. In the current state of technology is in fact impossible to achieve single-channel digital transmissions at speeds greater than about 40 Gbps, because the opto-electronic component interfaced to the fiber does not allow them to handle signals with a higher speed. The total bandwidth available to the fiber can be exploited using various techniques, such as through the simultaneous transmission of multiple signals on different wavelengths, ie the technique of multiplexing5 in the frequency domain.

The fiber optic transmission systems are conventionally divided into successive generations, depending on the type of fiber and the wavelength of work. The possibility of using glass as a fiber optic waveguide to an optical signal has been known since the twenties, the first proposal to use the fibers as carriers for long-distance telecommunications dating back to 19666. The practical application of these concepts, however, was hampered by technological issues, in fact the glass fibers existing at that time, had a coefficient of attenuation of signal equal to about 1000 dB / km, which made them unusable for practical purposes. Around 1970 a procedure was developed for industrial production of fibers with attenuation less than 20 dB / km, which made these systems competitive with fiber-coaxial cable.

Figure 1: Historical Optical Fiber Evolution

The first generation of optical systems uses opto-electronic components in GaAs (gallium arsenide) operating at a wavelength of 850 nm (the window) over multimode fiber, which is able to propagate the signal according to a number of ways. The second


6 G.P. Agrawal. Fiber- Optic Communication Systems. John Wiley, New York, 1997. Pag.1-4

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generation is characterized by an operating wavelength of 1300 nm (second window) with single mode fiber propagation. All of these features bring a huge increase in capacity compared to the systems of generators. A further improvement is achieved in the third generation systems in operation around the wavelength of 1550 nm (area of minimum attenuation of the fiber, about 0.25 dB / km).

Figure 2: Light-wave system evolution

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1.1.2 Optic Fiber Propagation

An optical fiber is sketched according to a cylindrical structure7 with two concentric layers: the core and the cladding. Both regions are composed of silica glass obtained by melting of molecules of SiO2, and since the refractive index of the cladding is slightly lower than that of the nucleus mn nnn −=∆ , the propagation of guided light radiation can be obtained. Telecommunication fiber is aligned with the outer diameter of 125 µm, while the core diameter varies between a few µm to 50 µm.

Figure 3: Generic Optical Fiber Design

An important parameter that characterizes the operation of the fiber is the normalized frequency:

222mn nnaV −=

λπ

[b]

where a is the radius of the nucleus and λ the wavelength of the signal. This parameter determines the number of modes that can propagate in the fiber. For a fiber with refractive index step one has the propagation of only one way though 405.2<V . So, in relation to the size of the nucleus, there are:

• single-mode fiber (core diameter less than 9 µ m)

• Multimode fiber (diameter of the order of tens of µ m)


Micheal Bass, Eric Van Stryland. Fiber Optics HandBook.. MacGraw-Hill, 2002. Chapter 1.3 ‘Principles of Operation’

ITU-T G.652. Charactheristic of a single mode optical fiber and cable.06/2005

ITU-T G.651. Characteristics of a 50/125 µ m multimode graded index optical fiber cable. 02/1998

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1.1.2.1 The propagation wizard according to the geometrical optics

The principle of total reflection is the basis of operating all types of fiber. The index of refraction is defined as the ratio between the speed of propagation of the light in a vacuum and the speed of propagation in a medium different from vacuum (phase velocity):

v

cn =

[c]

where v depends, of course, the characteristics and physical properties of the medium itself, if the medium is isotropic and homogeneous then N is a number greater than one and constant inside the dielectric. A beam of light that affects an area of interface between two means with different refractive indices is partly reflected and partly refracted and transmitted, in the familiar Snell's law8:

mmnn sennsenn θθ ⋅=⋅ [d]

where θn is the angle of incidence of the line than the normal to the surface at the point of incidence and θm is the angle the refracted ray with the same normal form in the cladding. If ncore> ncladding the angle θm is greater than θn and it grows with increasing θn until you come to the condition:

[e]

or absence of the refracted ray. In the last situation you've witnessed the phenomenon of total reflection. The angle of incidence beyond which there is no refraction is:

n

mc n

narcsin=θ

[f]

called ‘critical angle’. It can be defined as cone of acceptance that contains all those rays that can spread within the nucleus for total reflection. The vertex of the cone is the

8 Fawwaz T. Ulaby. Fondamenti di Campi Elettromagnetici, Italian edition of Stefano Selleri. McGraw-Hill, Parma 2005

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π θ =m

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center of the face on the entry of the fiber and the angle at the vertex is called ‘acceptance angle’ θa to:

mn

mn

mna

nnn

nnn

nnnn

−=∆

+=

∆⋅≅−=

2

)2arcsin()arcsin( 22θ

Typically 48.1≅n and 005.0≅∆n . It also defines a similar parameter called ‘numerical aperture’ (NA) which is:

nnnn mna ∆⋅≅−= 2sin 22θ [g]

Figure 4: Path of a ray propagation at the geometric angle for total internal reflection

A high value of N.A. is preferable for easier coupling of the light source with the fiber, obviously this operation is easier in multimode fiber for the largest radius of the nucleus.

1.1.2.2 Internal Structure

Optical fibers can be classified according to the profile of their index of refraction. This profile is simply the curve that describes the variation of the refractive index of the fiber axis, moving toward the surface, from the core to the cladding. The simplest type of fiber consists of a core and a mantle that have a step discontinuity between the two indices of refraction and therefore it is called step-index. The current standard single-mode fibers are of step-index9.


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Another class of optical fibers is one in which the variation of the 'refractive index is continuous between the core and cladding, these fibers are called graded-index10.

10

G.P. Agrawal. Fiber- Optic Communication Systems. John Wiley, New York, 1997. Pag.27

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1.1.3 Dispersion, Attenuation and Non Linear Effects

Principles characteristic of Optical Fiber are:

a) Dispersion

b) Attenuation

c) Non Linear Effects

1.1.3.1 Dispersion

The dispersion11 is a characteristic that determines the upper limit of the product δ regardless of the optical fiber attenuation. The term refers to any dispersion effect for which the spectral components of the transmitted signal travel at different speeds along the fiber, reaching the section of the receiver at different times. This phenomenon means that the transmitted waveform undergoes a progressive deformation, thus causing inter-symbol interference in the reception and then power loss in terms of performance of the system.

For each mode that propagates in the fiber can be defined:

• Fase Speed: βk

cn

cv

efff ==

neffm nnn ≤≤ [h]

• Group Speed:

−

==

λλ

βd

dnn

c

dk

dc

veff

eff

g

[i]

Both employees β from using λ and k, it is clear that components at different wavelengths travel at different speeds, causing dispersion.

There are two types of dispersion: 1. In a multimode fiber, each mode carries a part of the energy with his speed, causing the so-called inter-modal dispersion12. This effect can be partially mitigated employing graded-index fibers to minimize the differences between the “group velocity” and totally eliminated employing single-mode fiber. 2. In single-mode fiber dispersion is called the residual intra-modal dispersion13 (also called the group velocity dispersion GVD), refers to the uneven distribution of


12 Micheal Bass, Eric Van Stryland. Fiber Optics HandBook.. MacGraw-Hill, 2002. Chapter 1.4 ‘Fiber Dipsersion and Attenuation’

13 G.P. Agrawal. Fiber- Optic Communication Systems. John Wiley, New York, 1997. Pag.39-40

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velocities of propagation for different spectral components of the electromagnetic field propagating. This dispersion is caused by two different phenomena:

a. The dispersion of the material (also called chromatic dispersion14). This is linked to the index of refraction with frequency and is due to the presence of a series of resonant frequencies along the characteristic absorption spectrum of silica corresponding to fluctuations of their electrons to bond. In the spectral region of interest for optical communications, the wavelength at which this dispersion is zero, is 1276 nm.

b. The waveguide dispersion15. The propagation constant varies non-linearly with the frequency; this translates into a variation of the confinement of the field in the nucleus. With increasing frequency the modal distribution tends to be increasingly confined within the nucleus, taking as the index of refraction ŋeff that is the effective value of the mantle than the core. The contribution of this effect is negligible in fibers "standard" and is expressed only in a slight shift of the zero chromatic dispersion toward higher wavelengths.

Said τ the time taken by an impulse to travel along a stretch of fiber length L, the GVD is characterized by parameter D (ps / nm / km) mean:

dk

d

c

L

v

Ld

d

LD

g

βτ

λτ

==

= 1

[l]

The picosecond measures deformation of the pulse, nanometer the width of the spectral distance and kilometers the miles traveled. The following illustrations show the dispersion curves for different types of fiber:

Figure 5: Dispersion of different types of fiber



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Figure 6: Dispersion of different types of fiber

Finally there is a further contribution to the total dispersion, polarization dispersion16 (PMD, Polarization Mode Dispersion) which produces a broadening of the pulse increases with the distance traveled. The cause of this dispersion is related to the fact that fiber is propagated, even under single-mode, two independent modes orthogonal to each other and linearly polarized. If the fiber has a perfect circular cross-section symmetry, these modes have the same group velocity, in reality any cause of anisotropy (for stress or manufacturing) makes the fiber birefringence. For this reason, the two polarization states are propagated with different speeds and you can give them a differential group delay (DGD Differential Group Delay), this effect is called Polarization Dispersion.

Unlike the GVD, DGD does not accumulate linearly with the propagation distance and is considered the random variable with Maxwell probability density function. In a first order approximate description, PMD is characterized by assigning the coefficient of PMD, which is the ratio between the average value of the DGD and the root length is considered.

Typical values are 0.1-1 ps/ km1/2or even 0.05 ps/km1/2.

Figure 7: Polarization Mode Dispersion


Micheal Bass, Eric Van Stryland. Fiber Optics HandBook.. MacGraw-Hill, 2002. Chapter 1.5 ‘Polarization characteristic of Fibers’

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1.1.3.2 Attenuation

Analysis conducted has always overlooked the possibility that this could be a phenomenon of loss during propagation inside the core. Not taken into account that the light signal, while propagating guided, undergoes an attenuation that can be obtained from the following differential equation:

zePzPzPdz

zdP ⋅−=⇒−= αα )0()()()(

[m]

where P (z) indicates the signal strength on a cross section of the nucleus, α is the coefficient of attenuation17. The power decays exponentially along the fiber (z-axis positive). Given a section of fiber length L is the attenuation coefficient expressed in dB / km, can be written as:

)0(

)(log10

1

P

LP

L−=α

[n]

The coefficient α is dependent on the wavelength. The following Figure8 shows Attenuation characteristic18:

Figure 8: Attenuation Characteristic of typical fiber

1.1.3.3 Non-Linear Effects

In order to obtain a good signal to noise ratio in reception would better increase the average power launched into fiber. However this average power can’t exceed a certain threshold otherwise meets the action of nonlinear effects19. The nonlinearity in optical


18 Micheal Bass, Eric Van Stryland. Fiber Optics HandBook.. MacGraw-Hill, 2002. Chapter 1.4 ‘Fiber Dipsersion and Attenuation’

19 Micheal Bass, Eric Van Stryland. Fiber Optics HandBook.. MacGraw-Hill, 2002. Chapter 3.1 ‘Key Issues in Nonlinear Optics in Fibers’

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fibers falls into two categories. One belonging to a scattering effects due to the stimulus (for Raman20 and Brillouin21), while the other belong to optical Kerr22 effect related to the dependence of the refractive index of the intensity of the optical signal. Stimulated scattering in the optical field transfers part of its energy to the medium nonlinear, causing amplification or attenuation of the optical signal. The effects of nonlinear Kerr are linked to the modulation of the phase of the signal.

20

Micheal Bass, Eric Van Stryland. Fiber Optics HandBook.. MacGraw-Hill, 2002. Chapter 3.3 ‘Stimulated Raman Scattering’. ITU-T G.665. Generic characteristics of Raman amplifiers and Raman amplified subsystems. 01/2005 21

Micheal Bass, Eric Van Stryland. Fiber Optics HandBook.. MacGraw-Hill, 2002. Chapter 3.4 ‘Stimulated Brillouin Scattering’

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G.P. Agrawal. Fiber- Optic Communication Systems. John Wiley, New York, 1997. Pag.62-63 : The refraction index of the optical fiber is

weakly dependent on the intensity of the optical signal propagating. eff

tot

A

tPnntInnn

)()( 2020 +=+=

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1.2 Active and Passive Optical Components

In a system of fiber-optic communication there are different types of devices: transmitters (LEDs and lasers, modulators), multiplexers, de-multiplexers, optical amplifiers, optical filters, interleavers, receivers (photo-detectors), as well as the whole electronic control and management system.

The sources used in optical transmission systems are light-emitting diodes (LEDs) or laser diodes. The operation of the LED is based on the phenomenon of spontaneous emission of photons, while that of lasers on the phenomenon of stimulated emission of photons. In photo-detectors the basic process is the absorption. Let us examine in detail the various types of radiative transition in a material.

1.2.1 Radiative Transition

There are three basic processes of interaction between a photon and an electron in a solid

• Absorption • spontaneous emission • stimulated emission

Referring to a simple physical system, considering two energy levels that an atom can take, E1 and E2, the first corresponding to the ground state, the second to excited state.

Each transition between these states involves the emission or absorption of a photon

with frequency ν12 and energy 1212 EEh −=ν . At room temperature the majority of

atoms in a solid are in the ground state. If a photon of energy 12νh affects the system,

this equilibrium is disturbed. An atom 1E absorbs the photon, acquires its energy

through the state 2E .

This type of transition is called absorption23.

The excited state is unstable then, after a short time, the atom returns to ground state,

emitting a photon of energy 12νh . This process is called spontaneous emission24.

When a photon interacts with an atom while it is in the excited state, the atom can be

stimulated to make a transition to the ground state by emitting a photon of energy 12νh

23

G.P. Agrawal. Fiber- Optic Communication Systems III edition. John Wiley, New York, 15 june 2002. Pag.77-78 24

G.P. Agrawal. Fiber- Optic Communication Systems III edition. John Wiley, New York, 15 june 2002. Pag.77-78

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in phase with the incident radiation. This process is called stimulated emission25 and the

resulting radiation is monochromatic because each photon has energy 12νh and is also consistent because all the photons emitted are in phase.

Figure 9: Energy-Matter Inteeraction

Let us calling as 1n and 2n the instantaneous population distributions in the state 1E

and

2E . At thermodynamic equilibrium if ( ) kTEE 312 >− , the populations are described by the Boltzmann distribution:

kT

h

kT

EE

een

n 1212 )(

1

2ν−−−

== [o]

So 12 nn < , as most of the electrons of the material is on the energy level of the ground state. In stationary condition, the rate of stimulated emission and the rate of spontaneous

emission must be balanced by the absorption rate to maintain constant populations 1n

and 2n .

To make stimulated emission dominates the spontaneous emission, must obtain a high density of photons, to achieve this condition is used a resonant optical cavity, while making dominant stimulated emission than absorption is required that:

12 nn > [p]

This condition is called ‘population inversion’26.

25

G.P. Agrawal. Fiber- Optic Communication Systems III edition. John Wiley, New York, 15 june 2002. Pag.77-78

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G.P. Agrawal. Fiber- Optic Communication Systems III edition. John Wiley, New York, 15 june 2002. Pag.79-81 Micheal Bass, Eric Van Stryland,William Wolfe, Joey M. Enoch, HandBook of Optics II edition Volume IV.. MacGraw-Hill, 2001. Chapter 4.2 ‘Double Heterostructure Laser Diode’

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1.2.2 Laser (Light Amplification by Stimulated Emission of Radiation)

Sources commonly used in modern optical transmission systems are the laser diode. The operation of lasers is based on the phenomenon of stimulated emission of photons in terms of population inversion. These devices, based on a p-n junction, are extremely compact because produced with technology of integrated circuits and provide output power levels in the range of tens of mW.

The ‘stimulated emission’ process happens in a p-i-n diode after an electrical pumping27 through current. Following this pumping causes ‘population inversion’, and so the conduction band is more populated of the valence band.

Figure 10: Laser Stimulated Emission

Figure 11: Light Sources in comparison

27

Internet: www.corecom.it/education/tecnologie/IX_TX1 .pdf

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1.2.3 Modulator

The transmitters consists of a laser source and a device which performs the function of modulation necessary for moving the optical carrier information previously encoded in the form of electrical signal.

This device is called the modulator28. There are two ways to modulate the intensity of the wave emitted by the laser:

• Direct modulation

• External modulation

The first allows to check the electrical signal that must be transmitted through the drive current of the laser. This technique generates pulses with a high chirping factor29 (ie, a high oscillation frequency of the carrier lens) and does not allow the processing of bit rate higher than 2.5 Gbit / s.

External modulation provides a mechanism to be positioned at the exit of the laser, which in this case operates under a continuous wave30 (CW, continuous wave) and then in a state that has a very small factor chirping. The external device can be of two types:

• Mach-Zehnder modulator (MZM, Mach-Zehnder modulator). This device is based on the phenomenon of interference. Interferometric structure separates the incoming electromagnetic radiation into two equal parts making them interfere constructively (ON) or destructively (OFF) depending on the phase shift introduced by the applied voltage. This device is made with the technology of planar waveguides in lithium niobate (LiNbO3) and requires a voltage of 4-8 V peak to peak modulation. The transmission characteristics of optical power (transmissivity) is:

+=

π

πV

VT sin1

2

1

[q]

• Electro-Absorption modulator (EAM, electro-absorption modulator). This device exploits the variation with the applied voltage, the absorption of light radiation from a structure similar to that of the semiconductor laser, but inversely polarized. The device

28

G.P. Agrawal. Fiber- Optic Communication Systems III edition. John Wiley, New York, 15 june 2002. Pag.122

29

Internet: www.corecom.it/education/tecnologie/IX_TX1 .pdf

The chirped pulse has a bit instantaneous frequency increasing or decreasing according to the sign of the chirp C. 30


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requires a voltage modulation Vm of no more than 3 volts, peak to peak. If Vm = 0 the device is transparent.

1.2.4 Photodiode

The photodiode is based on a reverse-biased p-n junction. When an optical signal at wavelength λ (which is energy νh ) affects it, it generates electron-hole pairs that are separated by the electric field in the depletion region, resulting in a conduction current in the external circuit :

inRPI = [r]

Where R is the ‘responsivity’31 and Pin is the power of the optical signal which affects the photodiode. Essential for assessing the performance of the receiver, η

is the

parameter called ‘quantum efficiency32:

[s]

where q is the electron charge, h is Planck's constant, v is the frequency of the optical signal.

In order to have greater ‘quantum efficiency’ is necessary that the absorption coefficient of the material should be higher and this factor has a large value for only a small range of frequencies, so a given material has good characteristics of photo-detectors only for these frequencies. In addition to work at high frequencies the photodiode must have an adequate speed response. This is highly dependent on the transit time of carriers in the depletion region, which then should be very close but without compromising the ability of absorption (which is greater if the depletion zone is wide). A good compromise is to impose a transit time equal to half the period of modulation33.

The photodiodes used in most fiber-optic telecommunications systems are:

• p-i-n photodiode34

• avalanche photodiode (APD, avalanche photodiode)35

The pin photodiode is the most common, since the thickness of the intrinsic layer, which is the depletion region can be designed to optimize the quantum efficiency and speed of response.

31


32


33


34


35


20

The avalanche photodiode APD36, operates in the reverse bias region that triggers the phenomenon of avalanche carrier multiplication. This phenomenon results in a current internal amplification, that enables the detection of signals that have a power level less than the pin photodiode. Infact, the ‘responsivity’ of an APD is greater than a factor M:

MRRAPD = [t]

Figure 12: Receiver Sensitivity of Photodiode

1.2.5 Optical Amplifiers

There are different types of optical amplifiers: semiconductor or fiber based on Brillouin scattering or that of Raman. One type of amplifier that has revolutionized the field of optical communications and is widely used in WDM systems is the Erbium doped fiber amplifier (EDFA37, Erbium doped fiber amplifier).

This type of device whose center is made of a section of optical fiber doped with erbium, amplifies the incident light through stimulated emission mechanism, as for semiconductor lasers. The gain is achieved when the amplifier is "pumped" to reach the condition of ‘population inversion’

The EDFA can be used within the transmission system, in three different configurations:

• as a booster to increase the signal level before entering the fiber optic in this configuration, the EDFA is designed to provide maximum power output.

• as a line amplifier in order to increase the distance between two regenerations enclosable signal (optical-electrical conversion and viceversa).

36

Ivan Kaminov, Tingye Li. Optical Fiber Telecommunications. 2002.Chapter 16, subchapter 1.4.2 Receiver Sensitivity APD Receiver

“For avalnche photodiode receivers, sensitivity is given by: ” 37

G.P. Agrawal. Fiber- Optic Communication Systems III edition. John Wiley, New York, 15 june 2002. Pag.250 Micheal Bass, Eric Van Stryland,William Wolfe, Joey M. Enoch, HandBook of Optics II edition Volume IV.. MacGraw-Hill, 2001. Chapter 13.4 ‘EDFA in WDM networks’

21

• as a pre-amplifier, just before the receiver in order to improve the overall sensitivity of the latter, for these purposes, the noise figure of EDFA should be as low as possible.

1.2.6 Passive Optical Components

Passive components can be divided into three categories according to the technique of construction:

• integrated optic

• micro-optics

• all-fiber

The first and the second are capable of handling a large number of optical channels on the same chip, and are currently the most widely used. The last category also has an important role, especially with regard to filters with Bragg grating38 and couplers FBT (Fused biconical tapered). These have allowed the introduction of new devices for multiplexing DWDM39, especially for those applications which require low insertion loss (IL, insertion loss) and high directivity. Furthermore, these components enable lower costs.

The development in the technique of construction of this component allows to project new solutions such as the ‘slicing-interleaving’. Slicing is the separation of a group of wavelengths regularly spaced along the spectrum, in two groups of double-spaced wavelengths than the original (even and odd channels). The reverse technique is called ‘interleaving’. This technique provides increased design flexibility since it is completely symmetrical and can be applied anywhere in the transmission system.

38


39

ITU-T G.696.1 Longitudinally compatible intra-domain DWDM applications 07/2005

22

1.3 Optical Transceivers

An optical transceiver chip is an integrated circuit (IC) that transmits and receives data using optical fiber rather than electrical wire. Optical fiber, also called fiber optic, refers to the technology associated with the transfer of information in light beams or pulses along solid transparent fibers or cables.

An Optical transceiver chip facilitates the use of fiber to the premises40 (FTTP) services, in which optical fiber runs all the way from central hubs to the final users. This can provide extremely high-speed Internet access. Optical fiber systems can also be used to transmit and receive telephone communications and to receive digital television broadcasts.

1.3.1 Wavelength Division multiplexing (WDM)

The potentially achievable transmission bandwidth of optical fiber exceeds the Tbps. However, nowadays it isn’t possible to reach such a performance because of the limitation of available electrical components. Maximum transmission speed single channel currently in use is of the order of 40 Gbit / s (with multilevel modulation).

To exploit the enormous capacity of fiber it is possible to use multichannel systems, where each transmission channel is transmitted independently of the others, modulating a carrier at a certain wavelength. This results in a Wavelength Division Multiplexing41 (WDM) system as shown in the following figure:

Figure 13: Wavelength Division Multiplexing scheme

The components comprising a typical WDM system are:

• Transmitter: DFB laser, Lithium niobate external modulators;

40

Don McCullough, Flexibility is key to successful fiber to the premises deployments, July 31, 2005

41

ITU-T G.692. ‘Optical interfaces for multichannel systems with optical amplifiers’, 10/1998

23

• Optical multiplexers and de-multiplexers: AWG filters;

• Optical In-line amplifier: EDFA;

• The receiver: Photodiodes PINFET broadband networks;

In optical transmission is usually indicated by an WDM spacing between the carriers with > 100 GHz (or 1 nm), while speaking of DWDM (Dense WDM)42 for dense WDM-space, where the spacing between words bearing is of the order of magnitude as the bandwidth of individual channels B. Coarse Wavelength Division Multiplexing (CWDM)43 is the technique of combining several optical channels onto a single fiber. CWDM technology uses an ITU standard 20nm spacing between the wavelengths, from 1310nm to 1610nm. With CWDM technology, since the wavelengths are relatively far apart (compared to DWDM), the transponders, lasers and filters are generally not very expensive.

All the optical transmission system that use WDM technique suffer of problem of Attenuation, Dispersion and Non Linearity as previously44 described.

1.3.2 Transceiver Commercial Modules

A typical system that gives the possibility both to transmit and receive data is known as Transceiver. An optical transceiver module for the wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) applications is successfully realized with the so called small form factor pluggable (SFP)45 module.

Figure 14: Transceiver Module

Typically an SFP module is constituted by a TOSA (transmitter optical sub assembly) and a ROSA (receiver optical sub assembly). A TOSA is constituted by an LD (laser diode) and a ROSA composed by a PD (photodiode) and a TIA (trans-impedance amplifier) to give a high minimum sensitivity to the receiver. Together with TOSA and

42

ITU-T G.694.1 ‘Spectral grids for WDM applications: DWDM frequency grid’. 06/2002 43

ITU-T G.694.2 ‘Spectral grids for WDM applications: CWDM frequency grid’. 12/2003 44

See SubChapter 1.3 45

SFF Committee. INF-8074i. ‘Specifications for SFP (Small Form factor Pluggable) Transceiver’. Rev 1.0 May 12, 2001

24

ROSA an SFP module is constituted by two other IC (Integrated Circuit) that are LDD (Laser Diode Driver) and an LA (Limiting Amplifier).

Depending on the transmission rate, it have been developed, over the years, various transceiver modules. Particularly an SFP (small form factor pluggable) module allows a transmission rate under 5Gbps, an SFP+46 (Enhanced small form factor pluggable) module allows a data rate range from 1Gps to 10Gbps and finally an XFP47 (10 gigabit small form factor pluggable) allows a data rate at about 12Gbps.

1.3.3 Transceiver Module Characterization: Eye Diagram

To evaluate an optical TRx it is necessary to collect a set of metrics capable of quantifying the performance of its Tx and Rx parts48.

When evaluating the performance of an optical Tx, it is very important to study the characteristics of its optical output signal. For this purpose it is necessary to focus on power levels and general waveform characteristics. This information can be extracted from the Tx optical eye diagram.

To evaluate the performance of an optical Rx can be analized the Bit Error Rate (BER) curve in order to extract the Rx sensitivity (previously discussed).

The eye diagram49 is created by taking the time domain signal and overlapping the traces for a certain number of symbols.

Figure 15: Eye Diagram and what it means

46

SFF Committee , SFF-8431, ‘ Specifications for Enhanced Small Form Factor Pluggable Module SFP+’ .Revision 4, July 16, 2009 47

SFF Committee, INF-8077i, ‘10 Gigabit Small Form Factor Pluggable Module’.Revision 4, April 13, 2004 48

Luis Amaral, Jan Troska, Alberto Jimenez Pacheco, Stefanos Dris,,Daniel Ricci, Christophe Sigaud and Francois Vasey ‘Evaluation of Multi-Gbps Optical Transceivers for Use in Future HEP Experiments’, Geneva 23 49

Internet: http://www.complextoreal.com/chapters/eye.pdf

25

The following information can be obtained from an eye diagram:

Jitter: Jitter is a measure of signal quality and is defined as the measure of variance in signal characteristics. A zero jitter measurement indicates that the signal transition occurs at exactly the same point in time for each transition. The wide superimposed transitions in the eye pattern diagram are the result of high jitter associated with the signals, implying that the signals are not consistently transitioning at the required time. Small eye width implies a large variance in signal transition time.

Voltage Swing: The voltage swing has to be above the required LVDS voltage specification. The signals in the eye diagram have to "swing" between levels for the transitions to be valid.

Transition Time: Transition time is the rise and fall time that is associated with the signals. Transition times can be measured using the slope of the transitioning signals in the eye diagram. A 90 degree slope implies a rise and fall time of 0 ns. In reality, however, there is a certain transition time associated with rising and falling signals. A smaller transition time indicates that the signal is valid for a longer time at the next time period.

1.3.3.1 Definition of the Jitter and relations with BER

Jitter can be defined as “the deviation of the significant instances of a signal from their ideal location in time.”50 Briefly jitter represents how early or late a signal transition is with ideal reference transition. In a digital signal the significant instances are the transition (crossover) points.

Figure 16: Receiver misinterprets transmitted signal

Shown in Figure 17 is an idealized eye diagram, with very smooth and symmetrical transitions at the left and right crossing points. A large, wide-open “eye” in the center shows the ideal location (marked by an “x”) for sampling each bit. At this sample point the waveform should be set to its high or low value and sampling in this point, is probably that the result will be a bit error.

50

Jonnhie Hancock. ‘Jitter-Understanding it,Measuring it, Eliminating it’. Agilent technologies, April 2004

26

Figure 17: An idealized eye diagram

Phenomena that cause jitter are listed below51:

1. System phenomena

These are effects on a signal that result from the characteristics of its being a digital system in an analog environment. Examples of these system-related sources include:

• Crosstalk from radiated or conducted signals

• Dispersion effects

• Impedance mismatch

2. Data-dependent phenomena

These are patterns or other characteristics of the data being transferred that affects the net jitter arriving in the receiver. Data-dependent jitter sources include:

• Intersymbol interference

• Duty-cycle distortion

• Pseudorandom, bit-sequence periodicity

3. Random noise phenomena

These are phenomena that randomly introduce noise in a system. These sources include:

• Thermal noise—kTB noise, which is associated with electron flow in conductors and increases with bandwidth, temperature, and noise resistance

• Shot noise—electron and hole noise in semiconductors in which the magnitude is governed by bias current and measurement bandwidth

• “Pink” noise—noise that is spectrally related to 1/f

The sources of jitter are often categorized as “bounded” and “unbounded”: Bounded jitter sources reach maximum and minimum phase deviation values within an identifiable time interval. This type of jitter is also called deterministic52, and results from systematic and data-dependent jitter- producing phenomena (the first and second groups identified above).

51

Jonnhie Hancock. ‘Jitter-Understanding it,Measuring it, Eliminating it: Sources of Jitter’. Agilent technologies, April 2004

52

INCITS - T11-2 Technical Committee, “Methodologies for Jitter and Signal Quality Specification”, Rev. 13.1, May 2004.

27

Unbounded jitter sources do not achieve a maximum or minimum phase deviation within any time interval, and jitter amplitude from these sources approaches infinity, at least theoretically. This type of jitter is also referred to as random53 and results from random noise sources identified in the third group above. The total jitter on a signal, specified by the phase error function ϕj(t), is the sum of the deterministic and random jitter components affecting the signal:

ϕj(t) = ϕj(t)D + ϕj(t)R [u]

where ϕj(t)D, the deterministic jitter component, quantified as a peak-topeak value, JppD, is determined by adding the maximum phase (or time) advance and phase (or time) delay produced by the deterministic (bounded) jitter sources. ϕj(t)R, the random jitter component, quantified as a standard deviation value, JrmsR, is the aggregate of all the random noise sources affecting the signal. Random jitter is assumed to follow a Gaussian distribution and is defined by the mean and sigma of that Gaussian distribution.

Shown in Figure 18 is an eye diagram of a waveform that is even less ideal. The bottom appears to have a smaller amplitude variation than the top, so the signal seems to carry more 0s than 1s. There are four different trajectories in the bottom, so at least four 0s in a row are possible. Whereas on the top only two trajectories appear, indicating that the waveform contains at most two 1s in a row. The waveform has two different rising and falling edges, denoting the presence of deterministic jitter. The rising edges have a greater spread than the falling edges, and some of the crossover points intersect below the threshold level, denoting duty-cycle distortion, with 0s having a longer cycle or on-time than 1s.

Figure 18: Eye Diagram: Deterministic and Random Jitter characterization

An important viewpoint of jitter is provided by the “bathtub plot”54, depicted in Figure 19. It is so named because its characteristic curve looks like the cross-section of a bathtub. A bathtub curve is a graph of BER versus sampling point throughout the Unit Interval.

53

INCITS - T11-2 Technical Committee, “Methodologies for Jitter and Signal Quality Specification”, Rev. 13.1, May 2004. 54

Jonnhie Hancock. ‘Jitter-Understanding it,Measuring it, Eliminating it: ‘The Bathtub Plot’. Agilent technologies, April 2004

28

Figure 19: Bathtub Curve

A bathtub plot is typically shown with a log scale that illustrates the functional relationship between sampling time and BER. When the sampling point is at or near the transition points, the BER is 0.5 equal to probability for success or failure of a bit transmission. The curve is fairly flat in these regions, which are dominated by deterministic jitter phenomena. As the sampling point moves inward from both ends of the unit interval, the BER drops off precipitously. These regions are dominated by random-jitter phenomena and the BER is determined by the sigma of the Gaussian processes producing the random jitter. As one would expect, the center of the unit interval provides the optimum sampling point.

Typical measurable metrics that influence and characterize Jitter behavior are:

• Eye Width;

• Deterministic Jitter (Dj);

• Random Jitter (Rj);

• Total Jitter (Tj);

1.3.3.2 Voltage Swing: Eye Opening

The swing between transition levels of the signal that generates the eye diagram is an important factor that affects the eye opening.

Defining level 1 as the power level 1 and level 0 as the power level 0, the eye opening has an effect on the optical power with which the signal is transmitted55.

55

Luis Amaral, Jan Troska, Alberto Jimenez Pacheco, Stefanos Dris,,Daniel Ricci, Christophe Sigaud and Francois Vasey ‘Evaluation of Multi-Gbps Optical Transceivers for Use in Future HEP Experiments’, Geneva 23

29

Figure 20: Eye Diagram: Optical Power Levels

If the signal power which arrives to the receiver is too low, the BER (Bit Error Rate) of the transmission will be high. In this way you can characterize the ‘sensitivity’ of the receiver as a factor related to the eye opening.

Typical measurable metrics that influence and characterize Eye Opening are:

• Eye Height;

• Level1:

• Level0;

• OMA (Optical Modulation Amplitude) = level1 – level0;

• ER (Extinction Ratio) = level1/level0;

1.3.3.3 Transition Time: Eye Degradation

The rise56 and fall times represents the transition between a high logical level and a low logical level and generally are calculated between the 20% and 80% amplitude points. They are a measure of the ability of a circuit to respond to fast input signals. Incorrect rise time could cause signal distortions, such as ringing and overshoot, or, if too slow, it could reduce the time available for sampling within the eye.

Figure 21: Eye Diagram, Rise and Fall Time

Overshoot could be the result of incorrect rise time but will more likely be caused by impedance discontinuities or poor return loss at the receiving or sending terminations.

56

Guy Foster,Joan Gibson. ‘10 Gb/s Optical Transmitter Testing’. SyntheSys Research, December 2006

30

Typical measurable metrics that influence and characterize Transition Time are:

• Rise Time (20%-80%);

• Fall Time (20%-80%);

• Positive Overshoot;

• Negative Overshoot;

1.3.3.4 Eye Mask: BER and Sensitivity Estimator

The digital transmission systems and, in particular, the eye diagram have a particular shape to achieve a good BER. Often it is convenient to construct areas, or masks, inside and around the eye diagram. The eye diagram waveform should not enter into these masked areas. The polygons57 in the center of the eye diagram shown in Figure 22 and the lines at the top and bottom correspond to the mask used to evaluate optical transmitters intended for use in SONET58/SDH59 systems.

Figure 22: Eye Mask in SDH/SONET systems

Depending on the transmission bit rate, the size and shape of the mask changes. The x and y coordinates are specified for each bit rate and their relative positions are based on the mean logical 1 level and the mean logical 0 level. The receiver bandwidth for the measurement of the transmitted eye diagram is specified to be a fourth-order Bessel-Thomson60 response with a reference frequency at three fourths of the bit rate. This ensures a common reference bandwidth for transmitter evaluation. Hardware low-pass filters are commonly employed to achieve this response.

57

ITU-T G.691 ‘Optical interfaces for single channel STM-64 and other SDH systems with optical amplifiers’ 03/2006 58

SONET, Synchronous Optical NETwork. ANSI standard (American National Standards Institute). 59

SDH, Synchronous Digital Hierarchy ITU Standard (International Telecommunication Union).

60

ITU-T G.957 ‘Annex B, Optical interfaces for equipments and systems relating to the synchronous digital hierarchy’ 03/2006

31

1.3.3.5 Evaluating TRx Performance

The previous defined metrics can generate a very large data set and so it is necessary to focus on the TRx performance at established bit rate. In particular it is possible to develop a Figure of Merit (FoM) to combine all the performance data into three numbers: TxFoM for the Tx, RxFoM for the Rx and PwrFoM for the TRx power dissipation. For example, following an analysis carried out on commercial devices in Luis Amaral’s study61, a FoM has been proposed, evaluated according to the following expressions:

[v]

61

Luis Amaral, Jan Troska, Alberto Jimenez Pacheco, Stefanos Dris,,Daniel Ricci, Christophe Sigaud and Francois Vasey ‘Evaluation of Multi-Gbps Optical Transceivers for Use in Future HEP Experiments’, Geneva 23

32

1.4 Optical Fiber Communication Standard

This chapter describes some optical fiber communication standard with bit rates range from 4Gbps to 10Gbps. In particular the electrical characteristics of transmission of the laser driver are described, tha allow to program the optical transmission of the laser diode as previously presented in Figure 14. Such characteristics of transmission have been used for a comparison with the similar study on GBLD (Giga Bit Laser Driver) carried out in the INFN laboratory of Perugia, for the planning of a laser driver to be used in future SLHC (Super Large Hadron Collider) experiment under study at CERN in Geneva.

1.4.1 IEEE 802.3 10 Gigabit Ethernet

The IEEE 802.3 standard defines protocol and MAC specification of devices that operate at a speed of 10 Gb/s62.

TX Electrical Characteristic:

Table 1: Electrical Characteristic

Parameter

Value

Max Min

Untis

Baud Rate 10.3125 Gbps

UI 320 ps

Differential Amplitude max 1600 mVp-p

Output voltage limit 2.3 -0.4 V

Output jitter Near End max Total Jitter Deterministic Jitter Far end max Total Jitter Deterministic Jitter

± 0.175 peak form the mean ± 0. 085 peak from the mean

± 0.275 peak from the mean ± 0.185 peak from the mean

UI UI

UI UI

Rise time/Fall time 20% - 80% 130 60 ps

62

‘Ethernet standard IEEE 802.3’ published in 30 august 2002.

33

Eye Mask:

Figure 23: Eye Mask in 802.3ae standard

Symbol Near end value Far end value Untis

X1 0.175 0.275 UI X2 0.390 0.400 UI A1 400 100 mV A2 800 800 mV

1.4.2 8GFC standard at 8.5Gbps

8.5 Gbps Optical Fiber Channel63 communication standard.

Tx Electrical Characteristics:


Parameter

Value

Max Min

Untis

Baud Rate 8.5 Gbps

UI UI ps



Output jitter Total jtter Deterministic jitter Uncorrelated jitter

0.31 0.17 0.020

UI UI UI

Rise time/Fall time 20% - 80% NA 40 ps

63

INCITS. FC-PI-4, rev.7 sep. 20, 2007

34

Eye Mask:

Figure 24: Eye Mask in 8GFC standard

Symbol Value Untis

X1 0.155 UI X2 0.345 UI A 155 mV B 800 mV

1.4.3 4GFC standard at 4.25Gbps

4.25 Gbps Optical Fiber Channel64 communication standard.



Parameter Value Max Min

Untis

Baud Rate 4250 MBd ± 100 ppm MBd ppm

UI UI ps



Output jitter Total jtter Deterministic jitter

0.52 0.33

UI UI

Rise time/Fall time 20% - 80% NA 60 ps

64

INCITS.FC-PI2, rev.8 march 29, 2005

35

Eye Mask:


Symbol Value Untis

X1 0.26 UI X2 0.45 UI A 155 mV B 800 mV

1.4.4 10GFC standard at 10Gbps

10 Gbps Optical Fiber Channel65 communication standard.



Parameter Value Max Min

Untis

Baud Rate 10.519 Gbps

UI UI ps



Output jitter Total jtter Deterministic jitter

0.30 0.16

UI UI

Rise time/Fall time 20% - 80% 128 60 ps

65

INCITS ‘10GFC’ rev.1.99 December 1, 2001

36

Eye Mask:


Symbol Value Untis

X1 0.15 UI X2 0.34 UI A 550 mV B 1000 mV

37

CHAPTER II

THE DACEL2 PROJECT

Future collider experiments (SLHC, ILC, CLIC) will be designed with increasingly innovative technologies both for the detectors that Trigger and readout electronics of the equipment. Consequently, it is necessary to have:

• Large number of readout channels • Tolerant technology to radiation • Low power consumption

The recent Project Dacel2, inherent in the development of an upgrade for the current LHC (Large Hadron Collider) in Geneva, provides the Design and Characterization of deep sub-micron Electronics devices for future particle accelerators.

In this context the work done in INFN laboratories of Perugia consisted in testing and characterization of the new version of laser driver operating at 5Gbps (developed in CERN and Turin INFN section).

In order to test Laser Driver it has been necessary to focus to some test targets that shortly are given below:

• I2C Test; • Laser Driver Power Consumption; • TDR of impedance of board traces; • Laser Driver Output Eye Diagram; • Laser Driver Output Jitter; • Laser Driver Rise and Fall time; • Laser Driver Output noise; • Laser Driver Overshoot; • DAC Linearity;

This chapter is organized into two sections. The first are dealt with topics related to the first version of the GBT (developed in collaboration with CERN in Geneva and INFN in Turin), called version A. This first section examines all the tests to measure the performance of the laser driver and also explains the development environment that made it necessary to begin testing. The second section of the chapter provides a brief discussion of some tests on the second version B of the GBT, developed to improve the performance achieved in the previous version A. Only few tests of significant importance are reported to better compare the different performances of the two version of the GBT.

38

The information concerning the study phase of the programming of Laser Driver and target points relating to the tests defined above are detailed in the document of specifications given in the Appendix B in this text.

39

2.1 Laser Driver CHIP 1 Mode “A” Test

The optoelectronic system for the data transfer between the front-end and the system DAQ (developed in collaboration with Turin INFN section and CERN) allows to use GBT (Giga Bit Transceiver) chipset (elaborated in collaboration with CERN and Turin INFN), that includes a laser driver targeted iat driving VCSELs (Device Under Test). In order to test the DUT in Perugia and in order to perform data post processing, a LeCroy (WE-100H) sampling oscilloscope has been used an some ancillary electronics has been developed.

2.1.1 GBT CHIP 1 Mode “A” (DUT) The GBT chip includes a laser driver targeted at driving VCSELs. The block diagram of the laser driver is represented in figure 27.

Figure 27 : GBLD Block diagram

The ASIC is basically composed by a series of stages based on NMOS differential pairs with resistive load. The laser modulation, the bias current and the pre-emphasis settings are programmable through an I2C serial port and only modulation and bias current are also programmable through hard wired pins. Serial programmed settings are stored in the configuration registers.

40

The GBLD contains an I2C slave with changeable slave address: in this case it has been chosen ‘00010000’ (bin) as I2C slave address. It contains seven 8-bit configuration registers, each with an individual internal address (0-6 hex). To access a register, the I2C master must issue the correct slave-address, write the register address and then write/read the register data.

Chip specifications66:

• Source mode: Current sink;

• Modulation type: Single ended and differential;

• Max bit Rate: 5Gbps;

• Power supply 1(Vdd1): 1.5V;

• Power supply2 (Vdd2): 2.5V;

66

Appendix B: GBLD Specifications, 3 April 2009

41

2.1.2 Environment Tools

With a PPG (pulse pattern generator), lodged in the WE 100H, it is possible to produce a pattern of variable length and bit-rate. This signal is the input of the board and will drive the laser driver. The electric exit of the board (the signal produced by the laser driver) will be acquired through the St-20 (LeCroy module, described later in detail) and analyzed with special software.

The modulation current and laser diode bias current (of GBT chip) have been programmed through an I2C serial port. The GBLD contains an I2C slave with changeable slave address (in our case $10). So it has been possible to work to produce a fixed I2C-Master system able to change the configuration registers of the GBLD. The whole setup is represented in figure 28 and 29.

Figure 28: Environment SETUP scheme

I2C

slave

I2C Master

RS-232

42

Figure 29: Environment SETUP photo

2.1.2.1 LeCroy WE100H

Wave Expert 100H is an "equivalent time sampling oscilloscope" consistent of a mainframe and of four plug-in acquisition modules. Different modules allow the acquisition of electrical and optic signals up to frequencies of 20 Ghz but also the possibility to produce long pattern for the characterization of cards and devices.

The core of the Wave Expert is the timebase lodged inside the mainframe. The timebase checks the acquisition of the electric and optic signals giving an opportune signal of sampling and converts the position acquired by the forms in opportune tension signals.

The WE works only in equivalent time, or it samples with a very low but synchronous frequency with the signal under test. This type of sampling requires that the signal is necessarily periodic.

The Wave Expert 100H exploits a new acquisition technique called CIS (Coherent Interleaved Sampling) to allow for an acquisition at the rate of 10 MS / s. This is possible thanks to the use of a PLL that produces a signal of sampling hooked in phase to the clock of the signal under test. The signal of sampling will be cleared by the present noise on the clock, making the reliable and repeatable measures.

WE100H specifics:

• 10 MHZ of sampling frequency

• Intrinsic jitter very low-230 fs RMS

• automatic pattern locking till PRBS23

• max 510 M samples that can be memorize

43

Two modules are used:

PPG-E135 PRBS PULSE PATTERN GENERATOR WITH CLOCK Output

The PPG is a Pseudo Random Bit Sequence (PRBS) pulse pattern generator. It is able to produce PRBS long pattern up to 231 -1 bits to a maximum bit-rate of 11.5 Gbps. Its intrinsic jitter and its rise and fall time very low, make the PPG an ideal source of digital signals to characterize high bit rate serial transmissions or optic components for telecommunications.

St-20

The St-20 it is the electrical sampling module used to acquire the electric signals given by the card. Its band is of 20 GHz and is characterized by a RMS noise of around 700 µV. Using the St-20, is possible to do some TDR analyses and is possible to calculate the S parameters of devices up to 20 GHz.

The software used for data analysis is the LeCroy SDA (Serial Data Acquisition), a packet that allows the complete analysis of serial signals characterized by high data-rate.

Following some of the practicable analyses:

• Representation of eye diagrams and possibility to visualize possible violations;

• Accurate and repeatable analyses of the jitter;

• Bit error analysis;

2.1.2.2 I2C Master

To perform I2C link it has been used EasyPIC5 development board for Microchip 8/14/18/20/28/40-pin PIC microcontrollers. EasyPIC5 includes: touch-screen controller, USB2.0 programmer, USB2.0 In-Circuit Debugger, display interfaces, and many others useful I/O devices (see figure 30).

44

Figure 30: EasyPic5 board

The board features a 40-pin microcontroller (Pic16F877A, figure 31) whose main features are listed below:

Microcontroller Core Features:

• High Performance RISC CPU

• 35 single-word instructions

• All single-cycle instructions except for program branches , which are two cycles

• Operating speed : DC – 20 MHZ clock input, DC – 200 ns instruction cycle

• Up 256 * 8 bytes of EEprom data memory

Peripheral Features:

45

• Synchronous serial port (SSP) with SPI (master mode) and I2C (Master/Slave)

• USART with 9 bit address detection

• Timer0: 8 bit timer/counter with 8 bit timer prescaler

• Timer1: 16 bit timer/counter with timer prescaler, can be incremented during sleep mode via external crystal/clock

• Timer2: 8 bit timer/counter with 8 bit period register, prescaler and postscaler

Analog Features:

• 10 bit, up to 8-channel A/D

• Brown out reset (BOR)

• Analog comparator module

CMOS technology:

• Low-power, high-speed Flash/EEprom technology

• Full static design

• Wide Operating voltage range (2.0 V to 5.5 V)

• Low power consumption

Figure 31: Pic16F877A pin diagrams

Programming the 16F877A PIC (Programmable Intelligence Controller)67 is based on three main points:

1- On the communication via Serial Port (RS232) situated on EasyPic5 board with PC. It is possible to send from PC, via a USB to serial Converter linker, the address

67

Appendix A: Details of software programming

46

register and data register to Pic Microcontroller housed on EasyPic5. Since the voltage levels of the microcontroller and PC serial port are not directly compatible with each other, a level converter, Max232, must be used (component included on EasyPic5 board);

2- Pic receives address and data registers from PC and sends them via I2C to GBT chip connected to PORTC of Easypic5 board;

3- Pic writes address and data register on I2C bus. After it reads these data from GBT chip and shows read data on 7- segment displays (to check the goodness of written data);

For Serial Communication between PIC and PC two pins are available:

RC6 (Pic pin) for TX transfer and RC7 (Pic pin) for RX transfer (see figure 32);

Figure 32: Communication between PIC and PC via Serial Port

So, if the channel is free, respecting RS232 protocol, Pc sends a RTS (request to send) to Easypic5 board that answers with CTS (clear to send), if there aren’t simultaneous communications. Subsequently it is possible to send address and data register to Pic via serial port.

For I2C Communication two pins have been used:

Serial clock (SCL) – RC3/SCL;

Serial data (SDA) – RC4/SDA;

It has been possible to send from PC address and data register to PIC, which, subsequently, sends address and data byte to GBT via I2C bus. Consecutively PIC reads via I2C data written to GBT.

47

2.1.2.3 Level Translator

Since the voltage levels of the microcontroller I2C interface and GBT I2C interface are not directly compatible with each other, a level converter, must be used.

I2C’s typical voltages used are +5 V or +3.3 V although systems with other, higher or lower, voltages are permitted. PIC 16F877A have Wide Operating voltage range (2.0 V to 5.5 V). GBT uses I2C bus at voltage of +1.5V with pull-up resistor of 10KΩ as shown in figure 33.

Figure 33: GBT’s I2C interface

To connect I2C system operating at +1.5 V it has been necessary to use a level translator. To have a fixed system, also pull-up resistors, on I2C level Translator board, have been implemented.

SDA and SCL bus are connected in GBLD board by mean of a jumper (JP10) with tension voltage of +1.5V (as shown in figure 33). Considerate is worthy of notice that pull-up resistors are just implemented on level translator board.

48

Figure 34: Level Translator schematic

49

2.1.3 Test Targets

In order to verify the expected laser driver functionalities a list of tests has been defined. In the following the considered tests are listed and shortly described, the related results being shown in the paragraph: "Electric tests results."

Test I2C:

It is necessary to be sure that our I2C master (PIC16F877A), is able to write/read correctly the internal registers of laser driver.

Laser Driver power consumption:

It is necessary to measure the values of power consumption.

Test TDR for the impedance of board traces:

It is necessary to define the impedance of the GBLD board traces (just the ones dedicated to Gbit throughput).

Laser Driver Output Eye Diagram:

The eye diagram is obtained by overlapping all the symbols of the waveform and plotting them in a single UI (interval of symbol). This graph is particularly useful because it allows to describe in a compact way the waveform and to estimate important parameter. According to the quality of the transmission system the eye will be more or less open.

Laser Driver Output Jitter:

The jitter is one of the problems that influences the quality of a transmission system at high bit-rate. Ideally that laser driver pulses would present stable transients with high slope. To characterize the jitter it is necessary to decompose it in its random and deterministic components, to do so SDA software of LeCroy oscilloscope has been used.

Laser Driver Rise and Fall time:

Rise time represents the temporal interval that the signal employs to pass from a low logical level to high logical level (tipically calculated by estimating the time needed to rise the signal from 20% to the 80% of its total amplitude). Fall time is dually defined.

50

Laser Driver Overshoot:

The overshoot represents (in %) how much the signal in the transition from a high level to a low level and viceversa exceed its typical stationary value.

Laser Driver Output noise:

Various parameters depend on noise power (BER, eye opening, etc.) which should be estimated and reported. In this case some analysis of the BER and SNR has been carried out.

DAC linearity:

Is it necessary to define the accuracy of the GBT DAC in compare to laser driver specifications.

2.1.4 Power Supply and Pulses

As described in GBT specification document68, GBLD board requires in input a differential signal with a Vp-p voltage ranging from 100 mV up to 1200 mV. This signal is provided by the PPG. It is possible to produce a LVDS signal with a Vp-p of 500 mV and bit-rate of 5 Gbit/s or 2.5 Gbit/s. The produced pattern is characterized by a length of 127 bits and by a mark space of 0.5.

The output signal from the card, read from LeCroy, is a train of rectangular pulses with a peak-peak voltage equal to Imod * 50 Ω (it represents the signal that drives laser diode). The board requires a power supply with a voltage of 5 V and a current of about 120 mA.

68


51

2.1.5 Electric Tests Results

The electric tests have been performed using as input for the card the signal PRBS previously described and setting the Imod <5÷0> in Modulation current register69 (01h), and setting preDrvBias <2÷0> in Pre-Driver Register70 (06h) via I2C interface.

The tests have been repeated at 2.5Gbps and 5Gbps.

2.1.5.1 Test @ 2.5 Gbps

I2C Test:

To set the laser driver internal registers, it has been possible to use a PIC16F877A as I2C master defined previously.

Figure 35: I2C environment setup

In order to be sure that I2C master correctly write and read GBT chip (I2C slave), different tests have been performed:

• A simple algorithm has been written in Basic (developed in Mikrobasic environment release 7.271), which performs a write cycle followed by a read cycle just to verify that the register is written properly.

• Signals analyses: thanks to an oscilloscope it has been studied the signal that goes through SDA and SCL bus lines (see figure 36). In this way it is possible to estimate Acknowledge character (ACK) that represents a transmission control character

69

Appendix B: GBLD Specifications, 3 April 2009 70


Appendix A: Details of software programming

52

transmitted by the receiving station (I2C slave) as an affirmative response to the sending station (I2C master).

Figure 36: SDA (yellow), SCL (purple) tension signals

Laser Driver power consumption:

Thanks to these measures it is possible to estimate the power consumption of the laser driver in default state (when it is sent on laser driver, via I2C interface, the Control Register (00h) with the data value ‘1001 0111’ (bin)).

Condition(Test Point-Jumper

N°)

current Power supply

Power

dissipate

Test point3 - measure on

JP14

15.4mA 2.491V 38.36 mW

Test point2- measure on

JP15

99 mA 1.462V 144.74 mW

TDR test:

These measurements allows to estimate the impedance of PCB traces (especially the ones with high frequency signal).

The mean value of the impedance is 54.2 Ω. The mean value of reflected coefficient |ρ| is about 4% (0.04).

This measurement is done with soldered SMA connectors, which do not exhibit any value of impedance tolerance.

As a matter of fact measurements done directly to the PCB trace have shown an impedance Zo of 49.69 Ω.

53

Figure 37: TDR measure of impedance trace of PCB with SMA connectors

Laser Driver Output Eye Diagrams:

In the following, different eye diagrams are shown, obtained for different values of Imod (figure 38 and 39). It is possible to view that the eye has the tendency to close itself for low values of the Imod (according to the relationship between Imod and the output signal peak to peak voltage Vpp).

Figure 38: Eye Diagram Imod= 3.3 mA

54

Figure 39: Eye Diagram Imod= 11 mA

These two eye diagrams show that with high Imod it is possible to have a more open eye, with a contribution of jitter that decreases (see jitter test reported later in this text).

Figure 40: Eye Heigth preDrvBias<2÷0> =100(bin)

55

Figure 41: Eye Width preDrvBias<2÷0> =100(bin)

In figure 42 and 43 there are two eye diagrams at different preDrvBias<2÷0> values:

Figure 42: Eye Diagram preDrvBias<2÷0> =100(bin) with Imod = 7.1 mA

56


In the following figures the Eye diagram is reported, obtained by increasing Imod and preDrvBias:

Figure 44: Eye Heigth

57

Figure 45: Eye Width

Figure 46: Eye Mean

Figure 47: Eye Amplitude

58

Figure 48: Eye Level 0



In the following, we can see, in figure 50 and 51, different eye jitter diagrams when Imod changes.

59

Figure 50: Jitter Diagram Imod= 3.3 mA

Figure 51: Jitter Diagram Imod= 11 mA

In the following graphs (figure 52, 53 and 54) Deterministic, Random and Total Jitter are shown when in default condition of pre-Driver register (06h), preDrvBias<2-0> is set to 100(bin).

Figure 52: Deterministic Jitter preDrvBias<2÷0> =100(bin)

60

Figure 53: Random Jitter preDrvBias<2÷0> =100(bin)

Figure 54: Total Jitter preDrvBias<2÷0> =100(bin)

The following graphs (figure 55, 56 and 57) show the characteristics of Jitter measures obtained when increase Imod and preDrvBias:

61

Figure 55: Deterministic Jitter

Figure 56: Random Jitter

62

Figure 57: Total Jitter

It is possible to note that Random jitter decreases when preDrvBias increases instead of Deterministic jitter.


Following figures (58 and 59) show rise time and the fall time variations versus Imod changes.

Figure 58: Rise time preDrvBias<2÷0> =100 (bin)

63

Figure 59: Fall time preDrvBias<2÷0> =100 (bin)

With higher Imod we have higher values of Rise and Fall time.

Moreover, the following figures (60 and 61), show the characteristics of Rise and Fall time obtained when Imod and preDrvBias increase:

Figure 60: Rise time

64

Figure 61: Fall time

In particular, it can be observed that Rise and Fall time decrease when preDrvBias increases.

Laser Driver overshoot:

No significant overshoot has been observed. Such a result may be due to the same phenomena that affect both rise and fall time, resulting in slow transitions.

Laser Driver Output Noise:

To characterize the performances of the laser driver it is necessary to see how the noise changes when currents is varied, and how it influences the data transmission system performances. Laser Driver Output Noise refers to SNR and BER measures (figure 62 and 63).

65

Figure 62: SNR preDrvBias<2÷0>= 100 (bin)

Figure 63: BER preDrvBias<2÷0>= 100 (bin)

In the following three graphs (figure 64) it has been summarized the characteristics of Eye Heigth, SNR and BER obtained when increase Imod and preDrvBias:

66

Figure 64: Laser Driver output noise diagrams

DAC linearity:

According to GBLD specifics72 Imod is given by:

• 2+(0.16* modulation (Imod M)<0÷63>) mA.

The aim of this test is to be confident that the setting modulation current (imposed by means of I2C) is the real value which it is possible to measure at the GBLD output.

Similarly, it is possible to examine the modulation current graphs obtained by changing preDrvBias<2÷0> 73configuration bits.

If preDrvBias<2÷0> = 100 (bin) Imod is given by:

• 1.8+(0.14* modulation (Imod M)<0÷63>) mA (see figure 65).

72

Appendix B: GBLD Specifications, 3 April 2009; 73


67

Figure 65: Laser Driver Imod equation preDrvBias<2÷0>= 100 (bin)

If preDrvBias<2÷0> = 000 (bin) is given by:

• 2+(0.13* modulation (Imod M)<0÷63>) mA (see figure 66).


If preDrvBias<2÷0> = 111 (bin) Imod is given by:

68

• 1.9+(0.14* modulation (Imod M)<0÷63>) mA (see figure 67).


In conclusion figure 68 graphically compares all the characteristics.

Figure 68: Laser driver Imod for different value of preDrvBias

The same test has been done for the laser driver bias current. It has been estimated the value of laser bias current measuring it, thanks to an ampere-meter, on JP3 (jumper 3),as described in GBLD schematic (figure 69).

The aim of this test is to be confident that the setting bias current (imposed by means of I2C) is the real value set into the laser driver.

69

Figure 69: Laser Bias on Laser Driver Schematic

Via I2C it is possible to change Bias Current register74 (02h):

According to GBLD specifics Ibias is given by :

• 2+(0.16* bias (Ibias N)<0÷255>) mA

74


Condition(address,data register)

(decimal)

Laser Bias Current measured

(2,0) default state 2 mA

(2,32) 7.1 mA

(2,64) 12.25 mA

(2,96) 17.52 mA

(2,128) 22.87 mA

(2,160) 28.22 mA

(2,192) 33.33 mA

(2,205) 35.1 mA

(2,236) 40.4 mA

(2,255) 42.9 mA

70

Figure 70: Laser Bias current linearity

2.1.5.2 Test @ 5 Gbps

Concerning DAC linearity, Laser Driver power consumption, I2C and overshoot the results are the same to the ones relative to 2.5Gbps.

Laser Driver Output Eye Diagrams:

Following, two different eye diagrams, when Imod changes (figure 71 and 72) are shown.

71

Figure 71: Eye Diagram Imod= 3.3 mA

Figure 72: Eye Diagram Imod= 11 mA

Figure 73: Eye Heigth preDrvBias<2÷0> =100(bin)

72

Figure 74: Eye Width preDrvBias<2÷0> =100(bin)

In figure 75 and 76 there are two eye diagrams at different preDrvBias<2÷0> values:


73


In the following figures (from figure 77 to 82) we show the characteristics of Eye measurements, obtained when Imod and preDrvBias are increased:

Figure 77: Eye Heigth

Figure 78: Eye Width

74

Figure 79: Eye Mean

Figure 80: Eye Amplitude

75



Also at 5 Gbps the measures obtained thanks to laser driver Output Eye Diagram are similar to the results obtained at 2.5 Gbps. Eye is opening when Imod and preDrvBias is increasing.


Following, it is possible to see, in figure 83 and 84, different eye jitter diagrams when Imod changes.

76

Figure 83: Jitter Diagram Imod= 3.3 mA

Figure 84: Jitter Diagram Imod= 11 mA

In the following figures we show Determinist, Random and Total Jitter in default condition of pre-Driver register (06h), when preDrvBias<2-0>=100(bin).

77

Figure 85: Deterministic Jitter preDrvBias<2÷0> =100(bin)

Figure 86: Random Jitter preDrvBias<2÷0> =100(bin)

78

Figure 87: Total Jitter preDrvBias<2÷0> =100(bin)

The following graphss (figure 88, 89 and 90) describe the characteristics of Jitter measurements, obtained when Imod and preDrvBias are increased:

Figure 88: Deterministic Jitter

79

Figure 89: Random Jitter

Figure 90: Total Jitter


The following graphs report the rise time and the fall time characteristics as a funct9ion of Imod.

80

Figure 91: Rise time preDrvBias<2÷0> =100 (bin)

Figure 92: Fall time preDrvBias<2÷0> =100 (bin)

In the following figures (93 and 94) it is possible to see the characteristics of Rise and Fall time obtained when increase Imod and preDrvBias:

81

Figure 93: Rise time

Figure 94: Fall time

Rise and Fall time go to decrease when preDrvBias increases.

Laser Driver Output Noise:

To study Laser Driver Output Noise we can refer to SNR and BER measures (see 95 and 96).

82

Figure 95: SNR preDrvBias<2÷0>= 100 (bin)

Figure 96: BER preDrvBias<2÷0>= 100 (bin)

In the following figures the characteristics of Eye Heigth, SNR and BER, obtained when increase Imod and preDrvBias, are shown:

83

Figure 97: Laser Driver output noise diagrams

When we increase preDrvBias, SNR has very high value, instead of BER that decreases.

84

2.1.6 Conclusion Chip ‘A’

the tests did highlight a very bad behavior of the laser driver with increasing current supplied (IMOD). According to the standards mentioned above, the system does not guarantee the minimum required to be validated as electro-optical transmission system 5Gbps. In particular specifications concerning the values of rise time and fall time and deterministic jitter values are too high.

85

2.2 Laser Driver Chip 2 Mode “B” Test

The necessity to improve performance of version A of GBLD has led to the development of a second version of GBLD. This new version (Chip 2 mode B) presents a different option for the packaged respect to old GBLD version (Chip 1 mode A).

In the first version (Chip 1 mode A) only driver A is connected to the output pins 25 and 26 (see Figure 98). The output impedance in this case is 50 Ω and the output current range is 2÷12 mA.

In this second version (Chip 2 mode B) both output drivers (A and B) are connected in parallel to pins 25 (positive output) and 26 (negative output). The output impedance is therefore 25 Ω and the output current range is 4÷24 mA.

Chip 2 mode B allows the test of the full chip functionality (including Pre-emphasis75 current) while Chip 1 mode A allows the test of the single modulator without pre-emphasis.

Figure 98: GBLD version B pin out

75


86

As better explained in Appendix B of this text, the modulation current, the laser diode bias current and the Pre-Emphasis settings are programmable through an I2C serial port. Serial programmed settings are stored in the configuration registers. The GBLD contains seven 8-bit configuration registers, each with an individual internal address (0 – 6).

Here is a simple discussion on the first of the seven configuration register of the GBLD:

The control register76 is the first of these seven configuration registers. It enables/disables the laser diode bias and modulation currents of driver A or B, determines if the current and the Pre-Emphasis settings are taken from the internal registers or from the external pins, and gives the possibility to set Emphasis sign of rising and falling edge.

Figure 99: Control Register (00h)

76

Appendix B: GBLD Specifications 2.0, 3 April 2009

87

2.2.1 Comparison between Driver ‘Enable’ State

It is necessary to compare the GBLD behavior in default condition. Two tests are curried out:

1) When simultaneously driver ‘A’ and ‘B’ have enabled (fig.74)

2) When only driver ‘B’ has been enabled (fig.75).

The waveform shown in Figure 100 has been measured:

- Working @2.5 Gbit/s;

- Using Default Conditions according to GBLD Release 2.0 (April 3, 2009)77 that are: Modulation current of 7.12 mA, Pre-Emaphasis current of 6.4 mA;

- Enabling both laser driver A and B and setting Control Register (00h) <7:0> to ‘11011111’ (bin).

Figure 100: Driver A and B Waveform

Below the results of the most significant measures:


77


88

- Working @ 2.5 Gbit/s;

- Using Default Conditions according to GBLD Release 2.0 (April 3, 2009)78 that are: Modulation current of 7.12 mA, Pre-Emaphasis current of 6.4 mA;

- Enabling only laser driver B and setting Control Register (00h) <7:0> to ‘01011111’ (bin): as shown in figure 2 only laser driver B has been enabled, setting Control Register (00h) bit <6> to ‘1’ and disabling laser driver A, setting Control Register (00h) bit <7> to ‘0’.

Figure 101: Driver B Waveform


When only driver B has been enabled it is possible to note an improvement in performance respect to the case when driver A and B have enabled together. In particular Rise and Fall time is about 40 ps better (respect to the case of A e B simultaneously enabled). The amplitudes value measured with only driver B enabled or by enabling driver A and B together are lower than expected values; this is due to mismatch between the GBLD output impedance (25 Ω) and board trace impedance (50 Ω).

78


89

2.2.2 Chip “B” Pre-Emphasis and De-Emphasis

The laser driver provides independently programmable Pre-Emphasis or De-Emphasis on the rising and falling edges of the modulation current. Figure 102 shows a schematic eye diagram to illustrate the Pre-Emphasis and the De-Emphasis definition. In that figure, Imod is the laser modulation current, Ipeak is the additional current that adds to/subtracts from Imod during the period Tpeak over which the Pre-Emphasis/De-Emphasis takes place.

Figure 102: Pre and De Emphasis definitions

Positive values of Ipeak correspond to Pre-Emphasis while negative values correspond to De-Emphasis. Pre-Emphasis or De-Emphasis current varies from 0 mA to ±12 mA.

It is necessary to verify the correct behavior of Pre-Emphasis current (fig. 103 and fig. 105) and De-Emphasis current on Chip 2 mode B (fig. 104 and fig. 106). In order to do this it has been necessary to worke on Control Register (00h), changing emphasis current sign (positive is Pre-Emphasis, negative is De-Emphasis, see Figure 102) and studying the output waveform. In the next tests it has been set Pre-emphasis current to MAX (12 mA) in all cases except in figure 104, where is in default condition (6.4 mA). Modulation current is always in default conditions, 7.12 mA.



- Setting Control Register (00h) <7:0> to ‘01011111’ (bin):

Enabling only driver version B (bit <6> to ‘1’) and setting positive the Emphasis

current sign (bits <1:0> to ‘1’);

- Setting pre-Emphasis register (03h) <7:0> to ‘11111111’ (bin):

90

Pre-Emphasis current is set to MAX that is 12 mA;

- Using default conditions in other registers according to GBLD specs release 2.0 (April 3, 2009)79.

Figure 103: Chip 2 mode B, Pre Emphasis activated Waveform




- Setting Control Register (00h) <7-0> to ‘01011100’ (bin):

Enabling only driver version B (bit <6> to ‘1’) and seting negative the Emphasis


- Using default conditions in pre-Emphasis register (03h) <7:0> to ‘10001000’ (bin): that is De-Emphasis current is 6.4 mA;


79



91

Figure 104: Chip 2 mode B, De Emphasis activated Waveform


Following two eye diagrams are shown, obtained when Pre Emphasis Current (fig. 105) or De Emphasis Current (fig. 106) have been set.

The eye diagram shown in Figure 105 has been measured:



Enabling only driver version B (bit <6> to ‘1’) and setting positive the Emphasis



Pre-Emphasis current has been set to MAX that is 12 mA;


81


92

Figure 105: Chip 2 mode B, Pre-Emphasis activated Eye Diagram


The eye diagram shown in Figure 106 has been measured:



Enabling only driver version B (bit <6> to ‘1’) and setting negative the Emphasis



De-Emphasis current has been set to MAX that is 12 mA;


82


93

Figure 106: Chip 2 mode B, De-Emphasis activated Eye Diagram


Enabling driver B, De-Emphasis and Pre-Emphasis current work well. Besides it is possible to say that working on Pre-Emphasis current there is an improvement in performances. Rise and Fall Time decrease of about 50 ps compared with Rise and Fall time of Chip 1 mode A, that was about 130 ps. Rising the Emphasis current it is possible to note a more open eye diagram with an elevated overshoot (see figure 105-106).

94

2.2.3 Chip “B” Pre-Emphasis Test @2.5 Gbit

Working @2.5 Gbit/s, it is possible to study (enabling only driver ‘B’) the Laser Driver behavior at the raising of Modulation Current (from 2 mA to 12.08 mA) for some Pre-Emphasis current values:

- Pre Emphasis min=2.4 mA;

- Pre Emphasis default = 6.4 mA;

- Pre Emphasis max = 12 mA;

• Laser Driver Vertical Measures:

In the following Rise and Fall time graphs are reported, measured on Chip 2 mode B.

Figure 107: Rise Time

Figure 108: Fall Time

95

results in Rise and Fall time reduction.

The following figures show Positive and Negative overshoot graphs.

Figure 109: Positive Overshoot

Figure 110: Negative Overshoot

It is possible to note that an increase of Pre-Emphasis current results in an increase in of the vershoot, which can exceed 100%. As shown in figure 102 Ipeak is an additional current, that adds (Pre-Emphasis) or subtract (De-Emphasis) current to Imod: large positive Ipeak values can create strong overshoots.

• Laser Driver Eye Measures:

The two following plots are Eye Diagrams, obtained with Pre-Emphasis min value (2.4 mA, figure 111) and Pre-Emphasis max value (12 mA, figure 112).

96

Figure 111: Pre-Emphasis MIN, Eye Diagram

Figure 112: Pre-Emphasis MAX, Eye Diagram

In the following figure it is possible to graphically superimpose these two eye diagrams obtained with Pre-Emphasis MIN and MAX values.

97

Figure 113: Pre-Emphasis MIN and MAX eye diagrams

Figure 113 shows clearly how high Pre-Emphasis values can improve the eye diagram, opening the eye and speeding its rising and falling transients.

When Pre-Emphasis current increases it is possible to note a more open eye diagram, with better rise and fall time (see figure 107-108). As expected, setting a high Pre-Emphasis current results in a strong overshoot (see figure 109 and 110).

98

2.2.4 Chip “B” Pre Width of Pre Emphasis Current

Working @ 2.5 Gbit/s, it has been possible to study (enabling only driver ‘B’) the Laser Driver behavior with two different Pre-Width83 of Emphasis current values:

- Pre-Width of Pre-Emphasis min value = 50 ps (figure 114);

- Pre-Width of Pre-Emphasis max value = 110 ps (figure 115).

Pre-Width represent the time-space in which it is possible to use Ipeak current (see figure 102).

This test was obtained using the Modulation and Pre-Enphasis current in default condition, respectively 7.12 mA and 6.4 mA.

In the following two Eye Diagrams are shown, obtained with Pre-Width min value (50 ps, figure 114) and Pre-Width max value (110 ps, figure 115).

Figure 114: Pre Width MIN, Eye Diagram

83

Appendix B; GBLD Specifics 2.0, 3 April 2009

99

Figure 115: Pre Width MAX, Eye Diagram

In the following figure, on the other hand, it is possible to graphically compare the two eye diagrams obtained with Pre-Width MIN and MAX values.

Figure 116: Pre Width MIN and MAX eye diagram

How it is shown in figure 116, the eye diagrams are very similar. It is possible to note a slight growth of overshoot with lower Pre-Width value.

Conversely, changing Pre-Width of Pre-Emphasis current does not appear to introduce significant changes.

100

2.2.5 Conclusion Chip ‘B’

The tests on the B version of the chip have confirmed a better functioning of the laser driver compared with specific standards designed. In particular, the possibility of exploiting the current pre-emphasis to have fast rise and fall in particular has improved the performance above the band of 2.5Gbps.

It is necessary to consider that the system tested is still out of specification due to the high deterministic jitter still persistent.

101

CHAPTER III

ANALYSIS OF MEASUREMENT RESULTS

After defining the most important metrics that characterize electro-optic relationship between laser driver and laser diode and after showing electrical results obtained working on GBLD (Giga Bit Laser Driver) for DACEL2 project, it is necessary to study the behavior of a laser diode driven by a laser driver modeling an electro-optic connection between these two devices.

In particular, by using in Matlab environment it has been possible to model the connection between laser driver and laser diode. To this aim, the eye diagram has been studied, obtained by simulating electrical signals characterized by AWGN (Additive White Gaussian Noise), Random Jitter and ISI (Inter Symbol Interference) jitter at 2.5Gbps.

Considering that the tests carried out on GBLD (chip 'B') specifie that the DUT is substantially out of specification due to the high values of deterministic jitter, the third chapter describes a possible model of electro-optic connection in Matlab environment and focuses on generation of eye diagrams in presence of high Deterministic Jitter, in order to understand the effects of such phenomenon on the received signal characteristics.

It was studied a model to obtain a possible electro-optic connection between laser driver and laser diode (under AWR enviroment) based on the study of scattering parameters at the frequency of 2.5Gbps and 5Gbps in order to use this model to extrapolate an electric form wavelengths that can be used in Matlab enviroment to get an eye diagram for a possible study of hardware design changes in order to obtain values of deterministic jitter small or in specific.

102

3.1 Modeling Laser Driver- Laser Diode Connection

Using the MATLAB development environment, it is possible to generate a signal similar to the GBLD output, by properly setting Bit Rate, Rise / Fall time, SNR, and signal output levels.In particular, a specific proprietary matlab tool has been used, developing a specific software, part of which is reported in the following.

In particular, the most significant code lines are reported in the following84:

% Initialize system parameters

.....

Rs = 2.5e+9; % Symbol rate (Sps)

nSamps = Fs/Rs; % Number of samples per symbol

SNR = 40; % Signal to noise ratio (dB)

Trise = 50e-12; % Rise time of the NRZ signal

Tfall = 70e-12; % Fall time of the NRZ signal

.......

%Generate a Data Pattern

hSrc = commsrc.pattern('SamplingFrequency', 250e+9, ...

'SamplesPerSymbol',100, ...

'RiseTime', Trise, ...

'FallTime', Tfall, ...

'DataPattern', 'PRBS7')

% Generate NRZ signal and add AWGN

msgSymbols = generate(hSrc, frameLen);

msgRx = awgn(msgSymbols, SNR, 'measured');

From this obtained signal it is possible to elaborate an eye-diagram using a Matlab function:

% Create an eye diagram and display properties

84

Appendix C: “Modeling Laser Driver- Laser Diode connection”; Programming Code

103

eyeObj = commscope.eyediagram(.................);

% Update the eye diagram object with the noisy NRZ signal

update(eyeObj, msgRx);

In the following, figure 117 and figure 118 show Data Pattern generated at 2.5Gbps, and the Eye Diagram obtained from this signal.

Figure 117: Pattern Generated at 2.5Gbps

104

Figure 118: Eye Diagram generated from data pattern

The signal obtained is still not affected by white noise and jitter.

Using a few more programming code lines is possible to introduce jitter effects (Random jitter, Periodic jitter and ISI jitter)

% Obtain PDF of random jitter generated by the combined jitter object

hJitter = commsrc.combinedjitter('RandomJitter', 'on', 'RandomStd', 0.8e-12);

.......

% Obtain PDF of periodic jitter

set(hJitter, 'RandomJitter', 'off', 'PeriodicJitter', 'on', ... 'PeriodicAmplitude', 10e-12, 'PeriodicFrequency', 1/33);

..............

The following figure is a simple graphic representation of the most important type of jitter.

105

Figure 119: Jitter graphic representation

In figure 120 the data pattern of a signal affected by AWGN and Jitter is represented.

Figure 120: AWGN and Jitter

The following programming code is a simple simulation of a signal affected by AWGN, Random jitter and ISI jitter. ISI is introduced by passing the signal through a raised cosine filter85. The Jitter Hysteresis property is set to a value such that level crossings due to noise are suppressed. In Figure 121 the obtained eye diagram.

% Set the jitter parameters

hSrc.Jitter.RandomStd = 0.6e-12;

85

Appendix C: “Modeling Laser Driver- Laser Diode Connection”; Raised Cosine Filter.

106

% Set jitter hysteresis value, measurement delay, and color scale

eyeObj.MeasurementSetup.JitterHysteresis = 0.1;

eyeObj.MeasurementDelay = 1/Rs; eyeObj.ColorScale = 'log';

% Set up the square root raised cosine filter

hdFilt = fdesign.pulseshaping(nSamps, 'Raised cosine', ...

'Nsym,Beta', 8, 0.5);

hTxFlt = design(hdFilt); hTxFlt.PersistentMemory = true;

% Run simulation

........

for p=1:numFrames

% Generate jittered signal


% Add ISI and noise

msgISI = hTxFlt.filter(msgSymbols);

% Update the eye diagram object with the signal

eyeObj.update(msgRx);

end

Figure 121: Eye Diagram (Random jitter, ISI and AWGN)

107

Using analyze(eyeObj) function and eye scope Tool, is possible to scan and view some possible eye diagram measurements.

Figure 122: Eye Diagram: eye scope Matlab Tool

108

3.1.1 Jitter classification and sources

Knowing that Random Jitter (RJ) is Gaussian in distribution and Deterministic Jitter is non-Gaussian is a good beginning, but there is more. Total Jitter86 is composed of a Random Jitter component and a Deterministic Jitter component, as denoted in Figure 123.

Figure 123: Jitter Definition and causes

The sub-components of Deterministic Jitter consist of Duty Cycle Distortion (DCD), Inter-Symbol Interference (ISI), and Periodic Jitter.

Random Jitter is predictable in terms of distribution. Its probability distribution function is always Gaussian in shape. On the other hand, Deterministic jitter is bounded and always measured in terms of a peak-to-peak value. Although the distribution of deterministic jitter can be unpredictable, the most likely causes and characteristics of the individual sub-components of measured deterministic jitter can usually be easily identified.

86

Jhonnie Hancock. Jitter-Understanding it, measuring it, eliminating it: Causes of jitter; Agilent technologies, 2004

109

3.1.2 GBLD (Giga Bit Laser Driver): Deterministic Jitter

According to GBLD specification document87,

Random Jitter: 0.6ps (RMS);

Deterministic Jitter: 6ps (peak to peak);

Considering electrical results obtained in chapter II, inherent to Laser Driver Output Jitter (see figure 124 and figure 125),

Figure 124: GBLD (chip 1 version A): Deterministic Jitter

Figure 125: GBLD (chip 1 version A): Random Jitter

It is possible to say that the values of deterministic jitter are objectively out of specification.

87

Appendix B: GBLD Specifications; 3 April 2009

110

The sub-components of Deterministic Jitter88 consist of Duty Cycle Distortion (DCD), Inter-Symbol Interference (ISI), and Periodic Jitter. Following the different causes of the components of deterministic jitter:

• DCD (Duty Cycle Distortion): There are two primary causes of DCD jitter. If the data input to a transmitter is theoretically perfect, but the transmitter threshold is offset from its ideal level, then the DCD at the output of the transmitter will be a function of the slew rate of the data signal’s edge transitions. Another cause of DCD is asymmetry in rising and falling edge speeds.

• ISI (Inter Symbol Interference): ISI, sometimes called data dependent jitter, is usually the result of a bandwidth limitation problem in either the transmitter or physical media. With a reduction in transmitter or media bandwidth, limited rise and fall times of the signal will result in varying amplitudes of data bits dependent on not only repeating-bit lengths, but also dependent on preceding bit states. In addition, improper impedance termination and physical media discontinuities will also result in ISI due to reflections that cause signal distortions.

• Periodic Jitter (PJ): Usually is the result of a cross-coupling or EMI (Electromagnetic Interference) problem in the system. PJ can be either correlated or uncorrelated to the data signal.

88

Jhonnie Hancock. Jitter-Understanding it, measuring it, eliminating it: Causes of jitter; Agilent technologies, 2004

111

3.2 Modeling of package and laser diode electrical equivalent circuits

In order to estimate the achievbale signal integrity, an accurate simulation of microstrip lines has been carried out, as a function of the microstrip geometry and size.

Figure 126: Microstrip Analysis

Working in Matlab enviroment it is possible to create a miscrostrip object89 in order to study, at different frequencies levels, scattering parameters S22 and S21 of the microstrip lines. This activity aim at ensuring to have the required 50Ω impedance, so avoiding mismatching and reflections.

tx1=rfckt.microstrip('Thickness',1e-6, ....

……..

'Termination', 'Short', ...

'Width', 9.6703E-05, ....

'Height', 0.0001, ....

'EpsilonR', 9.8000, ....

'SigmaCond', 5.88E+07 , ....

'LossTangent', 0.0035)

…..

analyze(tx1,[2.5e9:0.5e9:5e9]);

tx1.AnalyzedResult;

[data,params,freq] = calculate(tx1,'S22','S21','dB')

89

Appendix C: “Modeling Laser Driver- Laser Diode Connection”; Modeling Microstrip : matlab enviroment

112

[outmatrix, freq] = extract(tx1,'S_parameters',z0);

Figure 127: Microstrip object: Scattering parameter ‘S21’

Figure 128: Microstrip object: Smith chart

To this aim, RF tool in Matlab has been used, as it provides a visual interface for creating and analyzing RF component and networks. By adding microstrip lines object

113

previously created is possible to have a complete analysis of scattering parameters. ( figures 129 and 130 ).

Figure 129: RF tool: microstrip analysis

Figure 130: RF tool: Scattering parameters analysis

3.2.1 Intrinsic equivalent circuit of Package and Laser Diode

The complete equivalent circuit of a laser diode can be separated into two parts. The first part represents the intrinsic electrical equivalent circuit of the laser chip itself. The second part is the electrical equivalent circuit of the package including the major parasitic elements. The elements of the intrinsic laser equivalent circuit are derived from the coupled rate equations which describe the interplay between the injected carrier and photon densities in the active region of the laser diode.

114

The impedance function of Laser Diode is same as a parallel RLC90 circuit with a resistor Rse in series with the inductor. The obtained intrinsic electrical equivalent circuit of the laser diode is shown in Figure 131.

Figure 131: Intrinsic equivalent circuit of Laser diode

The resistance Ri including the differential resistance of the laser diode models damping due to the spontaneous and stimulated recombination terms in the rate equations. The resistance Rse models damping due to spontaneous emission mode. So, damping of the electro-optical resonance is due to the resistances Ri and Rse. The capacitance Ci represents the active layer diffusion capacitance of the laser diode. The inductance Li arises from the small signal analysis of the rate equations and represents the resonance phenomenon of the laser diode with the capacitance Ci.

The resonant frequency of the circuit shown in Figure 131 is given by:

[z]

The electrical equivalent circuit of the package and parasitics can easily be found by considering the geometry of the package and the main parasitics elements associated with the laser chip. Since the small-signal ac analysis results in the linearization of the rate equations, the elements of the package and parasitics circuit can directly be added to the intrinsic equivalent circuit of the laser diode. The electrical equivalent circuit of the package and parasitics for laser diode91 is shown in Figure 132. In Figure 132, Lb is the bond wire inductance, Cm is the shunt package capacitance (ceramic capacitance), Rb is the bond wire resistance, Cp is the parasitic capacitance associated with the laser chip (it is the capacitance between the top and bottom contacts in the area outside the lasing region) and Rc is the contact resistance including the semiconductor bulk resistance. The main contribution to Rc comes from the contacts because the bulk resistance of semiconductor is very low.

90

M. S. Ozyazici; ‘THE COMPLETE ELECTRICAL EQUIVALENT CIRCUIT OF A DOUBLE HETEROJUNCTION LASER DIODE USING SCATTERRING PARAMETERS’; December 2004. 91

M. S. Ozyazici; ‘THE COMPLETE ELECTRICAL EQUIVALENT CIRCUIT OF A DOUBLE HETEROJUNCTION LASER DIODE USING SCATTERRING PARAMETERS’; December 2004.

115

Figure 132: Package, parasitic and laser diode equivalent electrical circuit

3.2.2 Simulation and design of the electrical paramenter for PCB and Laser Diode connection

Sizing of the electrical parameters is made possible by developing the PCB (Print Circuit Board) and laser diode conection in AWR (AWR Design Enviroment 2006).

Figure 133: Package and Laser diode Connection in AWR enviroment

The connection has been considered as a 2 ports network made in Microstrip technology, previously sized working in Matlab and using the TXLine AWR tools (Figure 134). Txline tool allows to size Microstrip physical characteristic according to frequency and impedance.

116

Figure 134: TXLine Tool

The simulation of the circuit allows to see on a chart | S11 | and | S21 |, expressed in dB. The value of | S11 | and | S21 | were selected in order to avoid mismatch and reflections.

The optimizer, by imposing Option Goal, allows to obtain value of S11 and S21 according to those obtained in Matlab RFTool, previously described (S11 lower possible and S21 greater than 20dB ).

Figure 135: AWR Optimization Goal

Figure 136 is the graphically result of the simulation:

117

Figure 136: S11 and S21 simulation graphically results

118

CHAPTER IV

Conclusions

The work carried out at INFN Perugia, divided into a part of programming of a GBLD via I2C interface and the acquisition of electrical measurements of the electro-optical system under test, has allowed to study phenomena that are the basis of any system of TRx of an electrical signal, and to characterize two transceivers under test. To this aim, an experimental test setup has been developed, managing both hardware and software related issues. The experimental verification has allowed to identify potential improvements for the developed transceivers. The behavior of the board and its effects on the signal eye diagram has been also modeled, by means of simulation tools.

It is also necessary to consider two important aspects:

• The acquisition system, obtained by programming GBLD via I2C has proven extremely reliable, with an error rate below 0.02%. • The study conducted in the third chapter could afford, in a future development, to better understand the main issues when designing an electro-optical transceiver, and achieve, from the point of view of the selected performance metrics, values very similar to those proposed by the most common optical fiber communication standards. This would be able to obtain a figure of merit (FoM) in order to study the quality of a system for transmission and reception of a electro-optical system as a function of the most common parameters of an electro-optic connection.

119

Appendix A:

DETAILS OF SOFTWARE PROGRAMMING

I2C “BASIC” CODE

To pursue the task to communicate via I2C with GBT chip we have worked under Mikrobasic enviroment, release 7.2. It is designed to provide the customer with the easiest possible solution for developing applications in embedding systems.

Figure 137: Mikrobasic enviroment

I2C communication

According to I2C protocol specified in GBT document we have programmed our PIC microcontroller to write data and address byte to the slave and read data byte from the slave:

I2C_Init(100000) ' Initialize full master mode

I2C_Start ' Master transmits START command

I2C_Wr($10) ' Master transmits7-bit slave address 0001000 (bin) followed by 8th bit(R/W_) set to zero

I2C_Wr(ee_adr) ' Master transmits 8-bit register address

I2C_Wr(ee_data) ' Master transmits 8-bit register data

I2C_Stop ' Master transmits STOP command

120

delay_us(500)

I2C_Start ' Master transmits START command

I2C_Wr($10) ' Master transmits7-bit slave address 0001000 (bin) followed by 8th bit(R/W_) set to zero

I2C_Wr(ee_adr) ' Master transmits 8-bit register address

I2C_Repeated_Start ' Master transmits repeated start

I2C_Wr($11) ' Master transmits7-bit slave address 0001000 (bin) followed by 8th bit(R/W_) set to one

ee_datax = I2C_Rd(0) ' Slave transmits 8-bit register data and save in ee_datax

I2C_Stop ' Master transmits STOP command

Usart Communication

In order to have the possibility to send a byte sequence which enjoys our test, we have implemented USART communication between PC (Serial port) and PIC (Easypic5 serial port):

Soft_Uart_Init(PORTC, 7, 6, 2400, 0) ' Initialize soft UART using RC6(TX) and RC7(RX) pins

‘with baud rate of 2400

ee_adr = Soft_Uart_Read(er) ' Read received address byte

ee_data = Soft_Uart_Read(er) ' Read received data byte

7-segment Display

The necessity to view at once the behavior of I2C link, has given to us the possibility to implement the show of data written to GBT I2C slave in a 7-segment displays.

Sub Function Mask is only a conversion function of a data from ASCII to Decimal format:

sub function Mask(dim num as byte) as byte ' this function returns masks

select case num ' for common cathode 7-seg. display

case 0 result = $3F

case 1 result = $06

121

case 2 result = $5B

case 3 result = $4F

case 4 result = $66

case 5 result = $6D

case 6 result = $7D

case 7 result = $07

case 8 result = $7F

case 9 result = $6F

end select 'case end

end sub

Sub Procedure interrupts manages the 7- segment displays multiplexing in order to use only PORTA and PORTD of our PIC microcontroller:

sub procedure interrupt

PORTA = shifter ' turn on appropriate 7seg. Display

PORTD = por[portd_index] ‘ turn appropriate 7seg. Display in counter

Inc(portd_index) ' increment value of variable portd_index

shifter = shifter << 1

if shifter > 8 then

shifter = 1 ' prepare mask for digit

end if

if portd_index > 3 then

portd_index = 0 ' turn on 1st, turn off 2nd 7seg.

end if

TMR0 = 0

INTCON = $20

end sub

At last we show read data from I2C slave:

digit = ee_datax div 1000 ' prepare digits for diplays

por[3] = Mask(digit)

122

digit = ee_datax div 100 mod 10


digit = ee_datax div 10 mod 10


digit = ee_datax mod 10


Delay_ms(1500)

Figure 138 : I2C master write data 151 (decimal) in address register (00h): Default State

I2C “MATLAB” CODE

To create a fixed system that permits the communication among PC serial port, PIC and I2C slave as well as to elaborate data in real time, thanks to LeCroy WE100H, we have improved Serial Communication in Matlab (release 2008b) environment.

function writeI2C_n2 (given,data)

Bit=[];

% Create a serial port object.

obj1 = instrfind('Type', 'serial', 'Port', 'COM4', 'Tag', '');

% Create the serial port object if it does not exist.

% otherwise use the object that was found.

if isempty(obj1)

obj1 = serial('COM4');

123

else

fclose(obj1);

obj1 = obj1(1);

end

% Set flags

set(obj1, 'Timeout', 2);

set(obj1, 'BaudRate', 2400);

% Disconnect from instrument object, obj1.

fclose(obj1);

% Connect to instrument object, obj1.

fopen(obj1);

% Send address and data in char format.

pause(0.5)

fwrite(obj1,char(given,data), 'uchar','sync');

pause(2)

fclose(obj1);

Appendix B:

124

GBLD SPECIFICATIONS, Release 2.0, 3 April 2009

DESCRIPTION

The GBT chip set includes a laser driver targeted at driving VCSELs. However the range of selectable modulation and laser bias currents allow to drive some types of edge-emitting laser diodes. The block diagram of the laser driver is represented in figure 139. The ASIC is essentially composed of an NMOS differential pair for laser modulation and of a current sink for laser bias. Both the laser modulation and bias currents are programmable. To optimize the system response the driver circuit has programmable pre-emphasis. The ASIC is programmable either via hardwired pins (at power on) or via an I2C interface.

Figure 139: GBLD simplified block diagram

LASER DRIVER SPECIFICATIONS

126

Figure 140 illustrates some general relations between the laser input-output characteristics to help clarifying the laser driver specifications.

Figure 140: Laser Driver Input-Output Characteristic and related definitions

127

PRE-EMPHASIS and DE-EMPHASIS CURRENT

The laser driver will provide independently programmable emphasis or de-emphasis of the raising and falling edges of the modulation current. Alternatively, eye-crossing control has to be provided. Figure 141 shows a schematic eye diagram to illustrate the emphasis and the de-emphasis definition. In that figure, Imod is the laser modulation current, Ipeak is the additional current that adds to/subtracts from Imod during the period Tpeak over which the pre emphasis/de-emphasis takes place.

Figure 141: Pre and De – Emphasis Definitions

Positive values of Ipeak correspond to emphasis while negative values correspond to de-emphasis. Note that the emphasis/de-emphasis definition is made in terms of the modulation current across the load, that is, the definition of emphasis and de-emphasis currents correspond to what would be measured across a 0 Ω resistor connected as the laser driver load.

128

PACKAGE AND PIN OUT

The current version of the GBLD is packaged in a small footprint 5 mm * 5 mm 28 pin QFN package with a lead pitch of 0.5 mm. The pin out is shown in figure 142.

Figure 142: GBLD Pin Out

For the current GBLD version there are two different options available for the packaged GBLD. In the first option both output drivers are connected in parallel to pins 25 ( positive output ) and 26 ( negative output ). The output impedance is therefore 25 Ω and the output current range is 4÷24 mA. In the second option only driver A is connected to the output pins 25 and 26. The output impedance in this case is 50 Ω and the output current range is 2÷12 mA. Option 1 allows the test of the full chip functionality while option 2 is for the test of the single modulator without pre-emphasis.

129

PROGRAMMING THE GBLD

The modulation current, the laser diode bias current and the pre-emphasis settings are programmable either through hard wired signals or through an I2C serial port. Serial programmed settings are stored in the configuration register. The configuration register contains a "configuration select" bit which establishes if the configuration is to be taken from the configuration pins or from the configuration register. At power-on, the "external" bit is set and the configuration pins are used to set the laser driver configuration. To use the configuration register is thus necessary to load a configuration word that sets the "external" bit to "0".

• Register File

The GBLD contains an I2C slave with fixed slave-address 0001000 (bin). It contains seven 8-bit configuration registers, each with an individual internal address (0 - 6) as defined in table I to VII. At power-on, these registers will assume their default values as indicated in those tables.

To access a register, the I2C master must issue the correct slave-address, write the register address and then write/read the register data. The steps in the protocol are as follows:

1. WRITE to Register

- Master transmits START Command.

- Master transmits 7-bit slave address (0001000) followed by the 8th bit (R/W_) set to zero.

- Master transmits 8-bit register address.

- Master transmits 8-bit register data word (can be repeated).

- Master transmits STOP command.

2. READ from Register

- Master transmits START Command.

- Master transmits 7-bit slave address (0001000) followed by the 8th bit (R/W_) set to zero.

- Master transmits 8-bit register address.

- Master transmits repeated START command. - Master transmits 7-bit slave address (0001000) followed by the 8th bit (R/W_) set to

one. - Slave transmits 8-bit register data word (can be repeated). Master transmits STOP

command.

130

• Control Register The control register enables/disables the laser diode bias and modulation currents and determines if the current and pre-emphasis settings are taken from the internal registers or from the external pins.

• Modulation Current Register

The laser modulation current is given by 2 + (0.16* modulation<5:0>) mA. The time width of the emphasis is given by 50 + (20* preWidth<1:0>) ps. Note that all 8 bits are masked by the modulator mask register (address 04h)

131

• Bias Current Register

The laser diode bias current is given by 2 + (0.16* bias<7:0>) mA. Note that all bits are masked by the bias-mask register (address = 5).

• Pre-Emphasis Current Register

The laser diode pre-emphasis current is given by 0.95 ∗ pre_height<3:0> mA. Other bits used for emphasis settings are in the control register (address 00h) and in the modulation current register (address 01h). • Pre-Driver Register

The pre-driver register is used to control the bias current of the pre-driver stage and the eye crossing control circuit.

133

Appendix C:

Modeling Laser Driver-Laser diode Connection

Matlab Code:

% Initialize system parameters

Fs = 250e+9; % Sampling frequency (Hz)

Rs = 2.5e+9; % Symbol rate (Sps)

nSamps = Fs/Rs; % Number of samples per symbol

SNR = 25; % Signal to noise ratio (dB)

Trise = 90e-12; % Rise time of the NRZ signal

Tfall = 90e-12; % Fall time of the NRZ signal

frameLen = 4000; % Number of symbols in a frame

%Generate a Data Pattern

hSrc = commsrc.pattern('SamplingFrequency', 250e+9, ...

'SamplesPerSymbol',100, ...

'PulseType', 'NRZ', ...

'OutputLevels', [-0.5 0.5], ...

'RiseTime', Trise, ...

'FallTime', Tfall, ...

'DataPattern', 'PRBS7',...

'Jitter' , commsrc.combinedjitter)

% Generate NRZ signal and add AWGN



% Create an eye diagram and display properties

eyeObj = commscope.eyediagram('SamplingFrequency',250e+9, ...

'SamplesPerSymbol', 100,...

'SymbolsPerTrace', 2,...

134

'MinimumAmplitude', -1, ...

'MaximumAmplitude', 1, ...

'AmplitudeResolution', 0.00100, ...

'MeasurementDelay', 0, ...

'OperationMode', 'Real Signal',...

'PlotType', '2D Color', ...

'PlotTimeOffset', 2e-10, ...

'PlotPDFRange', [0 1], ...

'ColorScale', 'log', ...

'RefreshPlot', 'on');

% Update the eye diagram object with the noisy NRZ signal

update(eyeObj, msgRx);

% Plot the time domain signal

t = 0:1/Fs:50/Rs-1/Fs; idx = round(t*Fs+1);

hFig = figure('Position', [0 0 460 360]); plot(t, msgRx(idx));

title('Noisy NRZ signal');xlabel('Time (sec)');ylabel('Amplitude');grid on;

managescattereyefig(hFig, eyeObj, 'left')

histEdges = -0.1/Rs:1/(10*Fs):0.1/Rs; hFigPdf = figure;

% Obtain PDF of random jitter generated by the combined jitter object

hJitter = commsrc.combinedjitter('RandomJitter', 'on', 'RandomStd', 0.8e-12);

jitter = generate(hJitter, 3e5);

rjPdf = histc(jitter, histEdges); rjPdf = rjPdf / sum(rjPdf);

subplot(221);plot(histEdges*1e3,rjPdf);grid on;

title('Random Jitter');xlabel('Time (ms)');ylabel('PDF')

% Obtain PDF of periodic jitter

set(hJitter, 'RandomJitter', 'off', 'PeriodicJitter', 'on', ...

'PeriodicAmplitude', 10e-12, 'PeriodicFrequency', 1/33);




title('Periodic Jitter');xlabel('Time (ms)');ylabel('PDF')

135

% Obtain PDF of random and periodic jitter

hJitter.RandomJitter = 'on';




title('Periodic and Random Jitter');xlabel('Time (ms)');ylabel('PDF')

% Obtain PDF of ISI and Random Jitter

hJitter.PeriodicJitter = 'off';

set(hJitter, 'DiracJitter', 'on', 'DiracDelta', 0.05/Rs*[-0.5 0.5]);




title('ISI and Random Jitter');xlabel('Time (ms)');ylabel('PDF')

close(hFigPdf);close(eyeObj);

% Attach the jitter object to the pattern generator

hSrc.Jitter = hJitter;

% Generate only random jitter with standard deviation 0.6 ps.

hSrc.Jitter.DiracJitter = 'off'; hSrc.Jitter.RandomJitter = 'on';


% Generate NRZ signal with random jitter and add AWGN

reset(hSrc); msgSymbols = generate(hSrc, frameLen);


% Plot the jittered noisy NRZ signal with the noisy signal

t = 0:1/Fs:50/Rs-1/Fs; idx = round(t*Fs+1);

figure(hFig);hold on;plot(t, msgRx(idx), 'r');

title('Noisy and Jittered NRZ signal');

136

xlabel('Time (sec)'); ylabel('Amplitude'); grid on;

close(hFig)

% Make a copy of the eye diagram object and reset

eyeObjJitter = copy(eyeObj); reset(eyeObjJitter);

% Update the eye diagram object with the noisy, jittered signal

update(eyeObjJitter, msgRx);

% Bring up the previous eye diagram for comparison

plot(eyeObj); plot(eyeObjJitter);

managescattereyefig([], [eyeObjJitter eyeObj])

close(eyeObj)

% Echo measurements setup

eyeObjJitter.MeasurementSetup

% Export the histogram data

[verHist eyeLine horHist] = exportdata(eyeObjJitter);

% Plot the horizontal histogram

t = 0:1/Fs:(eyeObjJitter.SymbolsPerTrace/eyeObjJitter.SymbolRate)-1/Fs;

hFig=figure('Position', [0 0 460 360]); refAmpIdx=(size(horHist,1)+1)/2;

plot(1e4*t,horHist(refAmpIdx,:)/sum(horHist(refAmpIdx,:)));

grid on;xlabel('Time (ms)');ylabel('PDF');

title(sprintf('Horizontal histogram at ReferenceAmplitude = %d AU', ...

eyeObjJitter.MeasurementSetup.ReferenceAmplitude(1)))

managescattereyefig(hFig, eyeObjJitter)

close(eyeObjJitter); close(hFig)

% Set the jitter parameters


137

% Set jitter hysteresis value, measurement delay, and color scale

eyeObj.MeasurementSetup.JitterHysteresis = 0.1;

eyeObj.MeasurementDelay = 1/Rs; eyeObj.ColorScale = 'log';

% Set up the square root raised cosine filter

hdFilt = fdesign.pulseshaping(nSamps, 'Raised cosine', ...

'Nsym,Beta', 8, 0.5);

hTxFlt = design(hdFilt); hTxFlt.PersistentMemory = true;

% Run simulation

frameLen = 1000;numFrames = 20;lastSymbol = 0;lastJitter = 0;

for p=1:numFrames

% Generate jittered signal


% Add ISI and noise

msgISI = hTxFlt.filter(msgSymbols);

msgRx = awgn(msgISI, SNR, 'measured');

% Update the eye diagram object with the signal

eyeObj.update(msgRx);

end

% % Center the eye

timeOffsetSamps = eyeObj.SamplesPerSymbol - Fs*eyeObj.Measurements.EyeDelay;

eyeObj.PlotTimeOffset = round(timeOffsetSamps)/Fs;

% Perform eye diagram measurements

analyze(eyeObj);

% Display the measurement results

eyeObj.Measurements

close(eyeObj)

138

% % Load the simulation results

%load('commeye_EyeMeasureDemoData')

% Start the EyeScope with the first eye diagram object

hEyeScope2 = eyescope(eyeObj);

% % Add the rest of the eye diagram objects to the EyeScope

commeye_addEyeDiagramObjects(hEyeScope2, load('commeye_EyeMeasureDemoData'))

Raised Cosine Filter

The raised-cosine filter is a filter frequently used for pulse-shaping in digital modulation, due to its ability to minimize intersymbol interference (ISI). Its name stems from the fact that the non-zero portion of the frequency spectrum of its simplest form (β = 1) is a cosine function, 'raised' up to sit above the f (horizontal) axis.

The raised-cosine filter is an implementation of a low-pass Nyquist filter, i.e., one that has the property of vestigial symmetry. This means that its spectrum exhibits odd symmetry about 1/2T, where T is the symbol-period of the communications system.

Its frequency-domain description is a piecewise function, given by:

and characterised by two values; β, the roll-off factor, and T, the reciprocal of the symbol-rate.

The impulse response of such a filter is given by:

The roll-off factor, β, is a measure of the excess bandwidth of the filter, i.e. the bandwidth occupied beyond the Nyquist bandwidth of 1/2T. If we denote the excess bandwidth as ∆f, then:

Where Rs = 1/T is symbol rate.

139

Figure 143: Frequency response of raised cosine filter at different roll-off factor

Figure 144: Impulse response of raised cosine filter at different roll-off factor

140

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SITE LINKS

http://www.corecom.it/education/tecnologie/IX_TX1.pdf

http://www.complextoreal.com/chapters/eye.pdf

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