optical networks assignment

Optical Networks Assignment φτυχ

Department of E & C, SJCE, Mysore

1. Distinguish between Isolators and circulators. Explain the operation of a polarization independent Isolator Optical Circulators:

An optical circulator is a multi-port (minimum three ports) nonreciprocal passive component.

The function of an optical circulator is similar to that of a microwave circulator—to transmit a light wave from one port to the next sequential port with a maximum intensity, but at the same time to block any light transmission from one port to the previous port.

The operation of optical circulators is based on two main principles.

1. Polarization splitting and recombining together with nonreciprocal polarization rotation.

2. Asymmetric field conversion with nonreciprocal phase shift.

In a three-port circulator, an input signal on port 1 is sent out on port 2, an input signal on port 2 is sent out on port 3, and an input signal on port 3 is sent out on port 1.

Circulators are useful to construct optical add/drop elements

Optical Isolators:

An isolator is a passive nonreciprocal device. Its main function is to allow transmission in one direction through it but block

all transmission in the other direction. Isolators are used in systems at the output of optical amplifiers and lasers

primarily to prevent reflections from entering these devices, which would otherwise degrade their performance.

The two key parameters of an isolator are its insertion loss, which is the loss in the forward direction and which should be as small as possible, and its isolation, which is the loss in the reverse direction and which should be as large as possible.

The typical insertion loss is around 1 dB, and the isolation is around 40–50 dB.

Polarization independent isolator In a polarization dependant isolator (which does not contain a half wave plate), if

light is entered from left to right at a particular state of polarization (SOP), light entering the device from the right to left due to a reflection, with the same SOP



orientation, is rotated by some more angle by the Faraday rotator, and thus blocked by the first polarizer.

Figure1: A polarization-independent isolator. (a) Propagation from left to right. (b) Propagation from right to left.

In a polarization independent isolator (Figure 1), the Faraday rotator is followed

by a half-wave plate. The half-wave plate (a reciprocal device) rotates the SOPs by 45◦ in the clockwise direction for signals propagating from left to right, and by 45◦ in the counter-clockwise direction for signals propagating from right to left.

Therefore, the combination of the Faraday rotator and the half-wave plate converts the horizontal polarization into a vertical polarization and vice versa, and the two signals are combined by another spatial walk-off polarizer (SWP) at the output.

For reflected signals in the reverse direction, the half-wave plate and Faraday rotator cancel each other’s effects, and the SOPs remain unchanged as they pass through these two devices and are thus not recombined by the SWP at the input.

2. What are the key characteristics of optical filtering technologies? Explain the principle of operation of Bragg Grating.

The characteristics of optical filtering are: Low insertion loss



The loss should be independent of the state of polarization of the input signals. The passband of a filter should be insensitive to variations in ambient

temperature. The individual filters should have very flat passbands, so as to accommodate

small changes in operating wavelengths of the lasers over time. The passband to stopband transition should be sharp to reduce the amount of

energy passed through from adjacent channels. Principle of operation of Fiber Bragg grating

Consider two waves propagating in opposite directions with propagation constants β0 and β1. Energy is coupled from one wave to the other if they satisfy the Bragg phase-matching condition where ∆ is the period of the grating.

In a Bragg grating, energy from the forward propagating mode of a wave at the right wavelength is coupled into a backward propagating mode. Consider a light wave with propagation constant β1 propagating from left to right. The energy from this wave is coupled onto a scattered wave traveling in the opposite direction at the same wavelength provided Letting β0 = 2πneff/λ0, λ0 being the wavelength of the incident wave and neff the effective refractive index of the waveguide or fiber, the wave is reflected provided λ0 = 2neff∆

Figure 2: Reflection spectra of Bragg gratings with uniform index profile



This wavelength λ0 is called the Bragg wavelength. In practice, the reflection efficiency decreases as the wavelength of the incident wave is detuned from the Bragg wavelength; this is plotted in Figure 2. Thus if several wavelengths are transmitted into a fiber Bragg grating, the Bragg wavelength is reflected while the other wavelengths are transmitted.

The operation of the Bragg grating can be understood looking at Figure 3, which shows a periodic variation in refractive index. The incident wave is reflected from each period of the grating. These reflections add in phase when the path length in wavelength λ0 each period is equal to half the incident wavelength λ0. This is equivalent to neff∆ = λ0/2, which is the Bragg condition.

Figure 3: Principle of operation of a Bragg grating.

In order to eliminate the undesirable side lobes, it is possible to obtain an

apodized grating, where the refractive index change is made smaller toward the edges of the grating as shown in Figure 4.

Figure 4: Reflection spectra of Bragg gratings with apodized index profile The bandwidth of the grating, which can be measured by the width of the

main lobe, is inversely proportional to the length of the grating. Typically, the grating is a few millimeters long in order to achieve a bandwidth of 1 nm.



3. What are long period fiber gratings? Discuss the implementation of OADM using FBG

Long-period fiber gratings are gratings having periods that are much greater than the wavelength, ranging from a few hundred micrometers to a few millimeters.

Fiber Bragg gratings find variety of uses in WDM systems, ranging from filters and optical add/drop elements to dispersion compensators.

Long-period fiber Bragg gratings are used as a gain equalizer for erbium-doped fiber amplifiers.

A simple optical add drop element (figure 5) based on fiber Bragg gratings consists of a three-port circulator with a fiber Bragg grating and a coupler.

The circulator transmits light coming in on port 1 out on port 2 and transmits light coming in on port 2 out on port 3.

Figure 5: OADM based on fiber bragg grating The grating reflects the desired wavelength λ2, which is dropped at port 3. The remaining three wavelengths are passed through. The coupler can add the same wavelength that was earlier dropped. Many variations of this simple add/drop element can be realized by using gratings

in combination with couplers and circulators. A major concern in these designs is that the reflection of these gratings is not

perfect, and as a result, some power at the selected wavelength leaks through the grating. This can cause undesirable crosstalk.

4. Distinguish between 1G and 2G optical networks. How does the transition from

transmission links to networks take place?

1G 2G Optics was used only for transmission

and to provide capacity Also have routing, switching and

intelligence in optical layer All switching and other intelligent network Switching performed optically



functions were handled by electronics Ex: Synchronous Optical NETwork Ex: MPLS

Low speed Speed is high, so no electronic

component

Packet switched lightpath Since packet switched lightpath is costly,

electronics is used for switching Relatively cheap Relatively costly

Most of the early experimental efforts were focused on optical networks for local-

area network applications. Next, optical packet-switched networks and local-area optical networks were

introduced. Meanwhile, wavelength-routing networks became a major focus area for several

researchers in the early 1990s as people realized the benefits of having an optical layer.

Optical add/drop multiplexers and crossconnects are now available as commercial products and are beginning to be introduced into telecommunications networks, stimulated by the fact that switching and routing high-capacity connections is much more economical at the optical layer than in the electrical layer.

At the same time, the optical layer is evolving to provide additional functionality, including the ability to set up and take down lightpaths across the network in a dynamic fashion, and the ability to reroute lightpaths rapidly in case of a failure in the network.

A combination of these factors is resulting in the introduction of intelligent optical ring and mesh networks, which provide lightpaths on demand and incorporate built-in restoration capabilities to deal with network failures.

5. What are the various techniques to control dispersion in fibers? What do you

mean by effective area and effective length? Differentiate between positive dispersion and negative dispersion fibers Chromatic dispersion,

D=DM+DW Where Dm – material dispersion, Dw – waveguide dispersion For standard single-mode fiber, the chromatic dispersion effects are small in the

1.3 μm band, and systems operating in this wavelength range are loss limited.



On the other hand, most optical communication systems operate in the 1.55 μm band today because of the low loss in this region and the well-developed erbium-doped fiber amplifier technology.

Optical communication systems in this band are chromatic dispersion limited. This limitation can be reduced if somehow the zero-dispersion wavelength were shifted to the 1.55 μm band.

We do not have much control over the material dispersion DM, though it can be varied slightly by doping the core and cladding regions of the fiber.

However, waveguide dispersion DW can be varied considerably so as to shift the zero-dispersion wavelength into the 1.55 μm band. Fibers with this property are called dispersion-shifted fibers (DSF) (Figure 6).

The waveguide dispersion can be varied by varying the refractive index profile of the fiber (figure 6 b, c), that is, the variation of refractive index in the fiber core and cladding.

Figure 6: Refractive index profiles of a) Step index fiber b) Dispersion shifted fiber c) Dispersion compensating fiber

Effective length The nonlinear interaction depends on the transmission length and the cross-

sectional area of the fiber. The longer the link length, the more the interaction and the worse the effect of the

nonlinearity. However, as the signal propagates along the link, its power decreases because

of fiber attenuation. Thus, most of the nonlinear effects occur early in the fiber span and diminish as the signal propagates.

Effective length (Le) is the length over which power is assumed to be constant in order to understand the effects of non-linearities.

Where, α is the attenuation constant in dB/km



L is the actual link length in km Effective area The effect of a nonlinearity also grows with the intensity in the fiber. For a given

power, the intensity is inversely proportional to the area of the core. Effective area is the cross-sectional area over which intensity is assumed to be

constant. It is given by

Where, F(r, θ)is the fundamental mode, r and θ denote the polar coordinates

The effective area has the significance that the dependence of most nonlinear effects can be expressed in terms of the effective area for the fundamental mode propagating in the given type of fiber. For example, the effective intensity of the pulse can be taken to be Ie = P/Ae, where P is the pulse power. The effective area of SMF is around 85 μm2 and that of DSF around 50 μm2. Positive and negative dispersion shifted fibers

Fibers can be designed to have either positive chromatic dispersion or

negative chromatic dispersion in the 1.55 μm band based on the algebraic sign of their dispersion. Typical chromatic dispersion profiles of fibers, having positive and negative chromatic dispersion in the 1.55 μm band, are shown in Figure 7.

Figure 7: Typical chromatic dispersion profiles of fibers with positive and negative chromatic dispersion in the 1.55 μm band



6. Show how wavelength conversion is done by Cross phase Modulation using

Semiconductor amplifier embedded inside a MZI

As the carrier density in the amplifier varies with the input signal, it changes the refractive index as well, which in turn affects the phase of the probe and creates a large amount of pulse distortion.

This phase-change effect can be used to effect wavelength conversion. The term cross-phase modulation is used to define this phenomenon. This phase modulation can be converted into intensity modulation by using an interferometer such as a Mach-Zehnder interferometer (MZI).

Figure 8: Wavelength conversion by cross-gain modulation in a semiconductor optical amplifier.

Figure 8 shows one possible configuration of a wavelength converter using

cross-phase modulation. Both arms of the MZI have exactly the same length, with each arm incorporating an SOA.

The signal is sent in at one end (A) and the probe at the other end (B). If no signal is present, then the probe signal comes out unmodulated.

The couplers in the MZI are designed with an asymmetric coupling ratio γ = 0.5. When the signal is present, it induces a phase change in each amplifier.

The phase change induced by each amplifier on the probe is different because different amounts of signal power are present in the two amplifiers.



The MZI translates this relative phase difference between its two arms on the probe into an intensity-modulated signal at the output.

7. Show how a 3R regeneration is done using a combination of CGM and CPM

Figure 9: All optical regeneration using reshaping and retiming (3R) using a combination of CGM and CPM

We assume that a local clock is available to sample the incoming data. This

clock needs to be recovered from the data. The regenerator consists of three stages as shown in figure 9. The first stage

samples the signal. It makes use of CGM in an SOA. The incoming signal is probed using two separate signals at different

wavelengths. The two probe signals are synchronized and modulated at twice the data rate of the incoming signal.



Figure 10: Waveforms at various stager during regeneration

Since the clock is available, the phase of the probe signals is adjusted to sample the input signal in the middle of the bit interval.

At the output of the first stage, the two probe signals have reduced power levels when the input signal is present and higher power levels when the input signal is absent.

In the second stage, one of the probe signals is delayed by half a bit period with respect to the other.

At the output of this stage, the combined signal has a bit rate that matches the bit rate of the input signal and has been regenerated and retimed.

This signal is then sent through a CPM-based interferometric converter stage, which then regenerates and reshapes the signal to create an output signal that has been regenerated, retimed, and reshaped.

8. Write the schematic diagram of a MZI. Discuss its principle of operation

Figure 11: Schematic diagram of MZI Principle of operation Consider the operation of the MZI as a demultiplexer; so only one input, say,

input 1, has a signal (Figure 8). After the first directional coupler, the input signal power is divided equally between the two arms of the MZI, but the signal in one arm has a phase shift of π/2 with respect to the other. Specifically, the signal in the lower arm lags the one in the upper arm in phase by π/2,

Since there is a length difference of ΔL between the two arms, there is a further phase lag of βΔL introduced in the signal in the lower arm. In the second directional coupler, the signal from the lower arm undergoes another phase delay of π/2 in going to the first output relative to the signal from the upper arm.



Thus the total relative phase difference at the first or upper output between the two signals is π/2 + βΔL + π/2. At the output directional coupler, in going to the second output, the signal from the upper arm lags the signal from the lower arm in phase by π/2.

Thus the total relative phase difference at the second or lower output between the two signals is π/2 + β Δ L − π/2 = β Δ L.

If β Δ L = kπ and k is odd, the signals at the first output add in phase, whereas the signals at the second output add with opposite phases and thus cancel each other. Thus the wavelengths passed from the first input to the first output are those wavelengths for which β Δ L = kπ and k is odd. The wavelengths passed from the first input to the second output are those wavelengths for which β Δ L = kπ and k is even.

Thus the path difference between the two arms, Δ L, is the key parameter characterizing the transfer function of the MZI.

optical networks assignment

Documents

port circulator

sequential port

previous port

horizontal polarization

vertical polarization

light wave

halfwave plate

multiport minimum