Download - POINT -TO - POINT OPTICAL FIBER LINK
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POINT -TO - POINT OPTICAL FIBER LINK
By Raja Phani Pappu
MSc Telecommunication Technology Email: [email protected] SUN: 059970419
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Contents
1. Introduction 3
2. Specifications Provided 3
2.1 Channel 3
2.2 Bit Rate 3
2.3 Distance between two nodes 4
3. Mathematical analysis 5
3.1 System Capacity 5
3.2 Bit Rate Distance Product 5
3.3 Channel Spacing 5
3.4 Spectral Window 6
4. Design specifications and selection criteria 7
4.1 Optical Fiber 7
4.2 Transmitter 13
4.3 Receiver 14
4.4 Multiplexer 15
4.5 Dispersion Compensator 17
4.6 Amplifier 18
4.7 Demultiplexer 20
4.8 Connector 20
5. Link Design 21
6. Conclusion 23
References 24
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1. Introduction
The assignment is aimed at designing an optical link between Coventry (United
Kingdom) and Paris (France) with a total of 16 channels each with a capacity of 5
Gbps. One end consists of 16 transmitters and a multiplexer to send and on the
other, a demultiplexer and 16 receivers to receive. The multiplexer and
demultiplexer is connected by optical fibre. Along this link, there are amplifiers to
amplify the signal and dispersion compensation modules to compensate the
dispersion in the fibre link.
The above diagram shows a schematic representation of the link. This report
tries to identify the various technologies that can be put to use for the fulfilment
and establishment of the optical link between the two cities. It will also discuss
the impairments encountered during the work and the ways to overcome it.
2. Specifications Provided
2.1 Channel
It is a communication path along which the signal is sent over. Through
multiplexing a number of channels voice and data channels can be sent over an
optical channel. The number of channels to be used is 16 in this project.
2.2 Bit Rate
It is number of bits that are transferred between several devices in a specified
amount of time. It is same as Data rate. The Data rate per channel is 5 Gbps
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2.3 Distance between nodes
It is important to determine the distance between the two cities, Coventry and
Paris, before commencing the design of the optical fiber link. Base on the result
from the RAC route planner the distance between Coventry and Paris is 479 km
(279 miles).
The link can be established as shown in MAP. This link will be a transoceanic link
as it passes through water. While designing the parameters like dispersion
compensation and losses compensation will have to be given top priority.
Table of given specifications
Parameters Specifications
1 Number of channels (M) 16
2 Bit Rate per channel (B) 5 Gb/s
3 Distance between nodes (L) 479 km
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3. Mathematical Analysis
3.1 System capacity
The total number of channels and the system bandwidth that a system can
handle is the system capacity or we can say the maximum number of channels
that a cable system can carry simultaneously. It determines the minimum
bandwidth requirement for the whole system and an important factor taken into
account while selecting an optical fiber.
This link has 16 channels at 5Gb/s.
System capacity = M*B
= 16 * 5 Gb/s
= 80Gb/s
The total system capacity is 80Gb/s.
3.2 Bit-Rate distance Product
The bit rate distance product of an optical fiber is a figure of merit equal to the
product of fiber’s length and it predicts the effective fiber bandwidth for other
lengths and for concatenated fibers.
System capacity *distance = B*L
= 5 Gb/s* 479km
= 2,395Gb/s*km
Since, the bit rate distance product is 2.395 Gb/s*Mm is low than ≤ 10(Gb/s)*Mm
it is a low specification link.
3.3 Channel spacing
It is the minimum frequency separation between two adjacent WDM signals. An
inverse proportion of frequency versus wavelength of operation calls for different
wavelengths to be introduced at each signal. The optical amplifiers bandwidth
and receivers ability to identify two close wavelength sets the channel spacing.
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The unique wavelengths passing through the amplifier are restricted by inter
channel cross talk. The significance of having adequate channel spacing is to
avoid any kind of cross-talk due to the interference between adjacent channels.
This is usually done by giving a guard band between adjacent channels which
acts as a buffer and prevents any kind of interaction between adjacent channels.
Smaller channel spacing leads to better system capacity.
δλ=1-2 nm for 8≤ m ≤ 32 Considering channel spacing δδδδλλλλ = 1nm as the link is
of 16 channels Or
Frequency bandwidth δf = 100GHz
Channel line width λ = 1550nm
Speed of light C = 3*108 m/s
Therefore the channel spacing equals
= [(1550nm) 2/3*108] * 100GHz
= 0.8nm ≈ 1nm
3.4 Spectral window
It is a band of wavelengths at which a fibre is sufficiently transparent for practical
use. It can be estimated from the calculation of spectral window the requirement
of source and the handling capacity of the optical fiber.
Spectral window ∆λ
∆λ = m* δλ
= 16 * 1*10-9
∆∆∆∆λλλλ = 16nm
typically, which is greater than 2B i.e. 10Gbps
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4. Design Specifications and Selection criteria Based on the calculations above for system capacity, channel spacing and with
the knowledge of attenuation we can select the components required for our link.
The standard specifications that should be in all the components are:
1. Operational at 1550nm wavelength
2. Functional in 5 Gbps data rate range
1. Optical Fiber
Corning SMF-28e® Photonic Fiber
The key optical performance parameters for single-mode fibers are attenuation,
dispersion, and mode-field diameter. Optical fiber performance parameters can
vary significantly among fibers from different manufacturers in ways that can
affect your system’s performance. It is important to understand how to specify the
fiber that best meets system requirements.
Impairments in performance:
Attenuation
Attenuation is decrease in the signal strength in a fiber optic cable because of
absorption and scattering. It is the loss of optical power as light travels down a
fiber and measured in decibels (dB/km). Over a set distance, a fiber with a lower
attenuation should be opted will allow more power to reach its receiver than a
fiber with higher attenuation.
While low-loss optical systems are always desirable, it is possible to lose a large
portion of the initial signal power without significant problems. A loss of 50% of
initial power is equal to a 3.0 dB loss.
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Any time fibers are joined together there will be some loss. Losses for fusion
splicing and for mechanical splicing are typically 0.2 dB or less.
Dispersion
Dispersion is the time distortion of an optical signal that results from the time o
flight differences of different components of that signal, typically resulting in pulse
broadening.
Impact of Dispersion
In digital transmission, dispersion limits the maximum data rate, the maximum
distance, or the information-carrying capacity of a single-mode fiber link. In
analog transmission, dispersion can cause a waveform to become significantly
distorted and can result in unacceptable levels of composite second-order
distortion (CSO). single-mode fiber that eliminated severe multimode fiber related
dispersion and left only chromatic dispersion and polarization mode dispersion to
be dealt with.
Chromatic dispersion
It represents the fact that different colors or wavelengths travel at different
speeds, even within the same mode. Chromatic dispersion is the result of
material dispersion, waveguide dispersion, or profile dispersion. Figure below
shows chromatic dispersion along with key component waveguide dispersion and
material dispersion.
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The example shows chromatic dispersion going to zero at the wavelength near
1550 nm. This is characteristic of bandwidth dispersion-shifted fiber. Standard
fiber, single-mode, and multimode have zero dispersion at a wavelength of 1310
nm. Every laser has a range of optical wavelengths, and the speed of light in
fused silica (fiber) varies with the wavelength of the light. Since a pulse of light
from the laser usually contains several wavelengths, these wavelengths tend to
get spread out in time after travelling some distance in the fiber. The refractive
index of fiber decreases as wavelength increases, so longer wavelengths travel
faster. The net result is that the received pulse is wider than the transmitted one,
or more precisely, is a superposition of the variously delayed pulses at the
different wavelengths. Further complication is that lasers, when they are being
turned on, have a tendency to shift slightly in wavelength, effectively adding
some Frequency Modulation (FM) to the signal. This effect, called “chirp,” causes
the laser to have an even wider optical line width. The effect on transmission is
most significant at 1550 nm using non-dispersion-shifted fiber because that fiber
has the highest dispersion usually encountered in any real-world installation.
Polarization mode dispersion
It is another complex optical effect that can occur in single-mode optical fibers.
Single-mode fibers support two perpendicular polarizations of the original
transmitted signal. If it is perfectly round and free from all stresses, both
polarization modes would propagate at exactly the same speed, resulting in zero
PMD.
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However, practical fibers are not perfect; thus, the two perpendicular
polarizations may travel at different speeds and, consequently, arrive at the end
of the fiber at different times. Figure below illustrates this condition.
The fiber is said to have a fast axis, and a slow axis. The difference in arrival
times, normalized with length, is known as PMD (ps/km0.5). Excessive levels of
PMD, combined with laser chirp and chromatic dispersion, can produce time-
varying composite second order distortion. Like chromatic dispersion, PMD
causes digital transmitted pulses to spread out as the polarization modes arrive
at their destination at different times. For digital high bit rate transmission, this
can lead to bit errors at the receiver or limit receiver sensitivity.
Single mode fiber is the most suitable choice for this link. A laser is used to
launch light into this fiber, which have a small core and diameter. Corning SMF-
28® Photonic fiber is a single mode fiber designed for optical customisation and
component applications, has low manufacturing cost, standardised processes
and improved performance. The key technical features and optical performances
of this fiber are listed below:
1. Good optical and geometric specifications
2. Exceptional performance and splice- ability
3. Low loss and high effective area
4. Attenuation <= 0.2 dB/km
5. Dispersion <=18[ps/(nm*km)]
6. Functional at low temperature like –600C to upto +850C.
7. Polarisation mode Dispersion <=0.2(ps/\/km)
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Also it can be used as a cost-effective fiber for the periodic in-line dispersion
compensation that is usually required.
Specification sheet of Manufacturer for Corning SMF-28e® Fiber
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Specification sheet of Manufacturer for Corning SMF-28e® Fiber
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2. Transmitter
MAP 1550 nm Optical Transmitter
An optical transmitter is used to convert the electrical signal into optical form and
to launch the resulting optical signal into the optical fibre. Semiconductor lasers
do the encoding to allow an optical output of 850nm, 1330nm or 1550nm. There
are 16 channels in the link from Coventry to Paris and thus 16 transmitters; one
for each link is required. The Multiple Application platform (MAP) 1550nm Optical
Transmitter is used in this link. It is an externally modulated 1550nm transmitter.
Manufacturer’s Specification Sheet of MAP 1550 nm Optical Transmitter
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The keys features associated with this Transmitter that makes it the most
suitable choice is:
1. High output power
2. Wide frequency range
3. Operational from 155 Mb/s to 12.5 Gb/s data rates
4. Functional Optical wavelength 1550nm
5. Extinction ratio 11dB
6. Various options for Optical connector FC/PC, SC/PC
3. Receiver
OPTICAL RECEIVER MO10GB1550
A receiver is a fibre-optic device that is responsible for converting the weakened
signal back to an electrical signal. It accepts optical signals from the optical fiber
and converts it into electrical signal. A typical one consists of optical detector, a
low noise amplifier and other circuitry used to produce the output electrical
signal.
Optical receiver MO10GB1550 is the receiver used in this link. The key features
of this receiver are:
1. Receiver sensitivity >-19dBm
2. Maximum Optical Input Power 2dBm
3. Low power consumption
4. Low cut off frequency 50 KHz
5. Supports upto 10Gbps Data rate
6. Maximum output power >+6.5dBm
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Manufacturer’s Receiver Specification sheet
Property Unit Worst Case Typ Comments
Receiver Sensitivity dBm 17.5 >-19 10Gb/s BER at 1X10-10
λ=1.5um Receiver Transimpedence Gain Ω 2K
Max Optical Input Power dBm +1 +2
PIN Responsivity A/W 0.75 >0.85
Receiver 3dB Bandwidth GHz 8 >8.5 Small signal
Low Frequency Cutoff KHz 50
Phase Linearity Deviation Degree 20 <10
Amplitude Peaking dB 2.5 <1.5
Input Optical Reflection dB -25 -30
Output Return Loss dB -10 -15
Total Power Consumption mW 550 <400
PIN Diode Bias V +5
Amplifier Bias V 3.5/5.5
Total DC Current mA 100
Out Power dBm +5 >+6.5
4. Multiplexer
AOC 100/200 GHz Configurable MUX Module
The multiplexing technique used for this system is DWDM (Dense Wavelength
division multiplex). Since the link has 16 channels and DWDM increases the
capacity signal of embedded fiber i.e. the incoming optical signals are assigned
to specific wavelengths within a designated frequency band then multiplexed on
to a single fiber.
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This process allows multiple video, audio and data channels to be transmitted
over one fiber while maintaining system performance and enhancing transport
systems.
Manufacturers Data Sheet for multiplexer
Optical Specifications ¡¡ ¡¡ ¡¡ ¡¡
Add/Drop Number ¡¡ 4-Channel 8-Channel 16-Channel
Operating Wavelength (nm) ITU-T Grid, C-band: 1528 - 1568 ,
L-band: 1568 – 1610
Center Wavelength Difference (nm) 0.1
Channel Spacing (GHz) 100 ( ~ 0.8 nm) ¡¡ 200 ( ~ 1.6 nm)
Channel Passband (nm) ITU ± [email protected] ¡¡ ITU ± [email protected]
¡¡ ¡¡ ITU ± [email protected] ¡¡ ITU ± [email protected]
Insertion Loss (Input to Drop, Add to Out) (dB) 4.0 5.5 6.5
Insertion Loss (Input to Out, Without Add/Drop) (dB) 5.8 7.0 8.5
Add/Drop Uniformity (dB) 1.0 1.2 1.5
Add/Drop Passband Ripple (dB) ¡¡ 0.5 ¡¡
Add/Drop Adjacent Channel Crosstalk (dB) ¡¡ 30 ¡¡
Add/Drop Non-Adjacent Channel Isolation (dB) ¡¡ 45 ¡¡
Add/Drop Channel Switching Speed (ms) ¡¡ 10 ¡¡
Directivity (All ports) (dB) ¡¡ 60 ¡¡
Return Loss (All ports) (dB) ¡¡ 55 ¡¡
Polarization Dependence Loss (dB) ¡¡ 0.15 ¡¡
Polarization Mode Dispersion (ps) ¡¡ 0.15 ¡¡
Max. Operating Power (mW) ¡¡ 300 ¡¡
Operating Temperature Range (°C) ¡¡ -5 ~ +65 ¡¡
Storage Temperature Range (°C) ¡¡ -40 ~ +85 ¡¡
Package Dimension (mm)3 ¡¡ Custom ¡¡
Electronic Specifications ¡¡ ¡¡ Custom ¡¡
Control Interface ¡¡ ¡¡ Custom ¡¡
Power Consumption ¡¡ 3W (9V) 5W (9V) 10W (9V)
The AOC 100/200 GHz Configurable MUX Module is the multiplexer used and its
attributes are as follows:
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1. Supports 16 channels
2. Operates on different wavelengths including 1550nm which is required
3. Has a channel spacing of 200GHz (~1.6nm)
4. Low insertion loss <=6.5dB
5. Low Power consumption 10W(9V)
6. High isolation
5. Dispersion Compensator
ClearSpectrumTM DC Fixed Dispersion Compensator
Dispersion is the dominating factor limiting transmission performance in the
optical systems and in trans oceanic links it is the most important factor to come
over. Dispersion is the time distortion of an optical signal, i.e. each spectral
component of the mode takes a different time to travel through the fibre, typically
resulting in pulse broadening. Dispersion can limit the maximum data rate, the
maximum distance, or the information carrying capacity of a SM fibre. The
compensating devices are designed to have dispersion of the opposite sign to
that of the fibre in the link so as to eliminate the delay difference between
spectral components.
Manufacturers Data Sheet Dispersion Compensator
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ClearSpectrumTM DC Fixed Dispersion Compensator is a suitable dispersion
compensator for the link and has following features:
1. Operates on 1550 nm wavelength
2. Channel Spacing of 100 GHz
3. Supports many channels
4. Insertion loss < 1dB
5. Maximum input power 27dBm
6. Dispersion level upto +/- 2000ps/nm
7. Operates on customised range of bandwidths
6. Amplifiers
NP2000-MSA EDFA Block Gain
The optical amplifiers are used to boost transmitter power, eliminate the need for
electronic regenerators and improve receiver sensitivity, to increase the capacity
of fibre-optic networks, opening up new wavelength windows for WDM such as
1300nm, 1550nm etc. Some of the technical advantages are improved noise
figure and reduced non-linear penalty of fibre system, allowing longer amplifier
spans, higher bit rates, closer channel spacing and operation near the zero-
dispersion wavelength. Optical amplifiers can be placed at intervals along a fiber
link to provide linear amplification of the transmitted optical signal. It provides
much simpler solution, which can be used for any kind of modulation at any
transmission rate. Moreover, if it is sufficiently linear it may allow multiplex
operation at different wavelength. Since the link is nearly 500km long it definitely
needs intermediate amplifiers, which will boost up the travelling signal.
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Manufacturer’s Data Sheet Amplifiers
NP2000-MSA EDFA Block gain is the chosen amplifier for this link. The main
features of this amplifier are listed below:
1. High Power up to 23 dBm
2. Operates at various wavelengths
3. Low Noise
4. Automatic Gain Control
5. Wide Signal Bandwidth
6. Transient Control
7. Excellent Gain Flatness
8. Dynamic Gain & Power Control
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7. Demultiplexer
MRV SFP Media Connect
At the receiver end, the demultiplexer will then separate the signals according to
its wavelength. The demultiplexer used is MRV SFP Media Connect and has the
following specifications:
8. Connector
All the design components are standing individually. Hence, to set-up the optical
fiber link, connectors are needed to connect them together. FIS SC/APC
Connector (part number: F1-3069APC) from Fiber Instrument Sales Incorporated
is used in this design. This connector is designed for top optical performance and
greatly reduces termination time. The connector features are pre-radiused
zirconia ferrue, pre-assembled body and precision moulded plastic body. This SC
connector achieves low optical loss with high performance physical contact and
maximum repeatability.
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5. Link Design
From the Data sheets of manufacturers
Transmitter power Pt =0.2dBm
Receiver sensitivity Pr = -19dBm
Attenuation = 0.2 db/km
Line Losses
Attenuation is the reduction in optical power as light travels through the fibre. The
main causes of optical attenuation in fibres are: coupling loss, splice loss, optical
fibre loss and connector loss, and also scattering, absorption of the light,
irregularities in the glass structure. Apart from actual losses suffered, while
designing the system it is also important to incorporate a margin of 6 -8 dB to
account for losses from splices or other components that may have to be added
at a future date and also to allow for any deterioration of components due to
aging. For the given link, which has attenuation loss of 0.2dB, the fibre loss is
calculated as follows.
A [dB] = α * L
= 0.2dB/km * 479km
= 95.8dB (for the whole link)
This shows the requirement of deploying amplifiers in the link to make up for the
lost power. This lost power must be recovered so that the output power should
be high. Since, ∆λ< 50 nm therefore, multiple combination of amplifiers are not
required.
Amplifier spacing
This is the space between two adjacent amplifiers in the link.
Amplifier spacing = LA[km]
LA[km] = Pt [dBm] –Pr [dBm]
α [dB/km]
= [0.2 + 19] / 0.2
= 96km
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Number of Amplifiers
The number of amplifier required (N) = L/LA
Where L = distance for the link (479 km)
N = 479 /96
= 4.99 ~5 amplifiers
The link requires 5 amplifiers with 96 km spacing.
Dispersion calculation
SMF-28e, made by Corning, is among the most popular NDSF (non –dispersion-
shifted- fiber). It exhibits zero chromatic dispersion at 1313nm.
The dispersion can be calculated as:
Dispersion = D (λ) ≈ (s0/4) [λ -(λ04/ λ3)] ps/(nm*km)
For 1200nm 1600nm where λ = 1550 nm
s0 = 0.086 ps/(nm2*km)
λ0 = 1313 nm
By applying the values to Dispersion formulae we get D (λ)=16.17 ps/(nm*km)
Now since the amplifiers are deployed at every 96 km of fiber calculating the
dispersion at that distance we get the following table:
Distance (km) Total Dispersion (ps/nm)
96 1552.32
192 3104.64
288 4656.96
384 6209.28
480 7761.6
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Based on the above readings the following graph is plotted
The above graph shows as the distance is increased the dispersion also
increases and thus dispersion compensators come into the picture. To reduce
the overall dispersion the dispersion compensators are deployed in equal
numbers and intervals with the amplifiers.
6. Conclusion
As a technology Optical communication has proven to become one of the fastest
growing segments of the telecommunications industry worldwide. Designing a
fiber optic system needs a whole length of specifications and considerations
related to power, dispersion, capacity etc. The components were selected on the
basis of data rate 5Gbps and wavelength 1550nm of the laser source. Source
was selected as laser as it has the potential to carry the signal in long distance
fibers.
Other components were selected carefully after calculations of the required
parameters keeping in mind the cost and efficiency of that component. 5Gbps as
data rate is not standard one. The present day standards are 2.5Gbps, 10Gbps,
20 Gbps and above. Still there are equipments that are available and which
operate on a variable data range.
Dispersion Map
0100020003000400050006000700080009000
0 100 200 300 400 500 600
Distance (km)
Dis
pers
ion
(ps/
nm)
Total Dispersion (ps/nm)
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While designing a system the major parameters to be taken into account are
BER (Bit error rate) and SNR (Signal to Noise ratio). Designs should be flexible
so as to ensure system upgrade. The transmitter laser used in this experiment is
capable of 10Gb/s data rate where as link only requires 5Gb/s for 16 channels.
The extra 5Gb/s bandwidth could be used cost effectively as the system is
upgraded. For multi-channel transmission, WDM is used to combine and
separate all the wavelengths. Since there will be loss and dispersion in the fiber
optic link, amplifier such as EDFA is used to amplify the signal and DCM for
dispersion compensation. The design for the optical link built, satisfies the
requirement for this project.
Web References
http://www.fiber-optics.info
http://www.rad.com
http://www.corning.com
http://www.globalspec.com
http://www.jdsu.com
http://www.nuphoton.com
http://www.teraxion.com
http://www.mrv.com/technology/
Technical References
1) G.P.Agarwal,”Optical Fiber Communications” 2) G. Keiser, “Optical Fiber Communications”, McGraw-Hill Inc., 2000.