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POINT -TO - POINT OPTICAL FIBER LINK

By Raja Phani Pappu

MSc Telecommunication Technology Email: pappurp@aston.ac.uk 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 ± 0.12@-0.5dB ¡¡ ITU ± 0.25@-0.5dB

¡¡ ¡¡ ITU ± 0.25@-3.0dB ¡¡ ITU ± 0.45@-3.0dB

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.