40Gbits in WDM Submarine Transmission

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Seminar Report 2007 40Gbit/s in WDM Submarine Transmission

Chapter 1

INTRODUCTION

The most advanced submarine systems currently operating are

based or Dense Wavelength Division Multiplexing (DWDM) of channels at 10

Gbit/s. This means that the system capacity isNx 10 Gbit/s, whereNis the number of

optical carriers (channels), each at a separate wavelength and modulated at 10 Gbit/s.

The introduction of a larger bit rate (e.g. 40 Gbit/s) is not expected in deployed links

for a few years. However, the first WDM transmissions at a 40 Gbit/s channel rate

were reported in research laboratories over transatlantic distances . Subsequently, they

were extended to transpacific distances but did not exhibit adequate margins forindustrial implementation.

Despite using the most advanced Forward Error Correction (FEC) techniques,

the performance, measured in terms of Q-factor, was not sufficient in the laboratory

to ensure that the minimum Q factor required by the customer would be met after

factoring in all the expected repair and industrial penaltiesAlcatel Research and

Innovation has therefore achieved a world first, by demonstrating the transmission of

40 channels at 40 Gbit/s over a transpacific distance with industrial margins .One of

the key technological factors in achieving this result was updating the modulation

format to Alternate-Polarization Return-to-Zero Differential Phase Shift Keying

(APol RZ-DPSK). Furthermore, repeaters based on Raman amplification are used

instead of the conventional Erbium-Doped Fiber Amplifiers (EDFA) to limit the

accumulation of noise. These amplifiers require a specific fiber arrangement to

efficiently reduce propagation impairments at 40 Gbit/s.

Dept. of ECE Govt. College of Engg., Kannur1


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Chapter 2

MODULATION FORMAT

The modulation format designates the approach used to apply the

incoming digital information to each of the optical carriers. The most straightforward

way to do so at B bit/s (e.g. 40 Gbit/s) is by Amplitude Shift Keying (ASK), that is,

by switching the laser output light intensity ON or OFF, depending on whether the

symbol to be transmitted is a mark (1) or a space (0), at a rate equal to the

information frequency B GHz (e.g. 40 GHz).This operation is often achieved by

applying the electrical signal to an electro optical amplitude modulator fed with laser

light. It produces so-called Non- Return-to-Zero(NRZ) data. Figure depicts thecomputed waveform of an eight-bit binary sequence with corresponding intensity,

phase and eye-diagram, modulated with NRZ format, as well as with all the other

formats discussed later. The associated optical spectra are drawn, as well as

schematics of the most conventional ways of generating the formats.

The Return- Zero (RZ) format has often been viewed as a promising

ASK alternative to NRZ. Using RZ, any 1 symbol is represented by pulse, which

can be of variable duration. This into the NRZ electro-optical amplitude modulator.

Whereas ASK formats are used exclusively in products at 10 bit/s, Differential Phase

Shift Keying (DPSK) offers better performance at 40 Gbit/s. In this case, information

is carried by the phase itself. DPSK data is generated by passing laser light into an

electro-optical device which modulates the optical phase. Often this device is

concatenated with a pulse carver (identical to an RZ modulator but driven by a clock)

to make RZ-DPSK optical data. Whether DPSK or RZ-DPSK, a trick has to be used

at the receiver end, since the photodiodes used to convert the optical data into

electrical data are essentially intensity-sensitive.

The most common trick is differential detection, that is, comparison

of the phase of a given bit with that of the following bit, before the photodiode (hence

the D which completes PSK in DPSK or in RZ-DPSK). This operation is performed

in a passive fiber interferometer in which one arm is one bit longer than the other.

Note that differential detection scrambles the optical data, which can therefore only be



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recovered if passed into a pre-coder at the transmitter side. Besides, the interferometer

has the advantage of having two output arms carrying complementary outputs of the

intensity-converted signal. By departing from conventional receivers with ASK, and

having two photodiodes operating in parallel (their photocurrents being subtracted in

the end), it is possible to enhance the robustness to noise (and hence the system

margin) by a helpful ~3 dB. In other words, if the optical power level at each fiber

input were reduced by ~3 Db with respect to the power specifications for ASK

formats, the system performance would still fall within customer requirements.

Additionally, the optical level could be increased with respect to the

power specifications for ASK formats because both the DPSK and RZ-DPSK formats

have excellent intrinsic resistance to power-dependent propagation impairments

(known as intra-channel nonlinear effects). In our experiment, we further increase this

resistance, using the fact that light is preferably modeled as a vector with two

transverse polarization components. By alternating the polarization of adjacent bits

into a dedicated modulator driven at half the information frequency B, to make

Alternate Polarization DPSK (APol RZ-DPSK) data, we show that the system

margins can be increased by another 3 dB. Note that this APol technique requires the

passive fiber interferometer in the receiver to have one arm longer than the other by

two bits instead of one. All these features make APol-RZDSPK one of the most

promising modulation formats for 40 Gbit/s submarine systems.



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Fig. 2.1 Typical Characteristics of Four Modulation systems



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Chapter 3

WAVELENGTH-DIVISION MULTIPLEXING

In fibre-optic communications, wavelength-division multiplexing

(WDM) is a technology which multiplexes multiple optical carriersignals on a single

optical fibreby using different wavelengths (colours) of laser light to carry different

signals. This allows for a multiplication in capacity, in addition to making it possible

to performbidirectional communications over one strand of fibre. "The true potential

of optical fibre is fully exploited when multiple beams of light at different frequencies

are transmitted on the same fibre. This is a form of frequency division multiplexing

(FDM) but is commonly called wavelength division multiplexing.

The term wavelength-division multiplexing is commonly applied to an

optical carrier (which is typically described by its wavelength), whereas frequency-

division multiplexing typically applies to a radio carrier (which is more often

described by frequency). However, since wavelength and frequency are inversely

proportional, and since radio and light are both forms ofelectromagnetic radiation, the

two terms are equal.

3.1 WDM SYSTEMS:

A WDM system uses a multiplexerat the transmitter to join the signals

together, and a demultiplexerat the receiver to split them apart. With the right type of

fibre it is possible to have a device that does both simultaneously. The first WDM

systems only combined two signals. Modern systems can handle up to 160 signals and

can thus expand a basic 10 Gbit/s fibre system to a theoretical total capacity of over

1.6 Tbit/s over a single fibre pair. WDM systems are popular with telecomm

unications companiesbecause they allow them to expand the capacity of the network

without laying more fibre.

By using WDM and optical amplifiers, they can accommodate several

generations of technology development in their optical infrastructure without having

to overhaul the backbone network. Capacity of a given link can be expanded by

simply upgrading the multiplexers and demultiplexers at each end.This is often done

by using optical-to-electrical-to-optical translation at the very edge of the transport

http://en.wikipedia.org/wiki/Fiber-optic_communicationhttp://en.wikipedia.org/wiki/Multiplexinghttp://en.wikipedia.org/wiki/Optical_Carrierhttp://en.wikipedia.org/wiki/Optical_fiberhttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Laserhttp://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Bidirectionalhttp://en.wikipedia.org/wiki/Frequency_division_multiplexinghttp://en.wikipedia.org/wiki/Frequency-division_multiplexinghttp://en.wikipedia.org/wiki/Frequency-division_multiplexinghttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Multiplexerhttp://en.wikipedia.org/wiki/Telephone_companyhttp://en.wikipedia.org/wiki/Telephone_companyhttp://en.wikipedia.org/wiki/Optical_amplifierhttp://en.wikipedia.org/wiki/Fiber-optic_communicationhttp://en.wikipedia.org/wiki/Multiplexinghttp://en.wikipedia.org/wiki/Optical_Carrierhttp://en.wikipedia.org/wiki/Optical_fiberhttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Laserhttp://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Bidirectionalhttp://en.wikipedia.org/wiki/Frequency_division_multiplexinghttp://en.wikipedia.org/wiki/Frequency-division_multiplexinghttp://en.wikipedia.org/wiki/Frequency-division_multiplexinghttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Multiplexerhttp://en.wikipedia.org/wiki/Telephone_companyhttp://en.wikipedia.org/wiki/Telephone_companyhttp://en.wikipedia.org/wiki/Optical_amplifier


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network, thus permitting interoperation with existing equipment with optical

interfaces.

Most WDM systems operate on single mode fibre optical cables,

which have a core diameter of 9 m. Certain forms of WDM can also be used in

multi-mode fibre cables (also known as premises cables) which have core diameters

of 50 or 62.5 m. Early WDM systems were expensive and complicated to run.

However, recent standardization and better understanding of the dynamics of WDM

systems have made WDM much cheaper to deploy. Optical receivers, in contrast to

laser sources, tend to be wideband devices. Therefore the demultiplexer must provide

the wavelength selectivity of the receiver in the WDM system.

WDM systems are divided in different wavelength patterns,

conventional, dense WDM. Conventional WDM systems provide up to 16 channels

in the 3rd transmission window (C-band) of silica fibres around 1550 nm with a

channel spacing of 100 GHz. DWDM uses the same transmission window but with

less channel spacing enabling up to 31 channels with 50 GHz spacing, 62 channels

with 25 GHz spacing sometimes called ultra dense WDM. New amplification options

(Raman amplification) enable the extension of the usable wavelengths to the L-band,

more or less doubling these numbers.

WDM, DWDM are based on the same concept of using multiple

wavelengths of light on a single fibre, but differ in the spacing of the wavelengths,

number of channels, and the ability to amplify the multiplexed signals in the optical

space. EDFA provide an efficient wideband amplification for the C-band, Raman

amplification adds a mechanism for amplification in the L-band.

http://en.wikipedia.org/wiki/Raman_amplificationhttp://en.wikipedia.org/wiki/C-bandhttp://en.wikipedia.org/wiki/L-bandhttp://en.wikipedia.org/wiki/Raman_amplificationhttp://en.wikipedia.org/wiki/C-bandhttp://en.wikipedia.org/wiki/L-band


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3.2 BASIC OPERATION OF WDM

WDM enables the utilization of a significant portion of the

available fiber bandwidth by allowing many independent signals to be transmitted

simultaneously on one fiber, with each signal located at a different wavelength.

Routing and detection of these signals can be accomplished independently, with the

wavelength determining the communication path by acting as the signature address of

the origin, destination or routing. Components are therefore required that are

wavelength selective, allowing for the transmission, recovery, or routing of specific

wavelengths.

In a simple WDM system , each laser must emit light at a different

wavelength, with all the lasers light multiplexed together onto a single optical fiber.

After being transmitted through a high-bandwidth optical fiber, the combined optical

signals must be demultiplexed at the receiving end by distributing the total optical

power to each output port and then requiring that each receiver selectively recover

only one wavelength by using a tunable optical filter. Each laser is modulated at a

given speed, and the total aggregate capacity being transmitted along the high-

bandwidth fiber is the sum total of the bit rates of the individual lasers.

An example of the system capacity enhancement is the situation in

which ten 2.5-Gbps signals can be transmitted on one fiber, producing a system

capacity of 25 Gbps. This wavelength-parallelism circumvents the problem of typical

optoelectronic devices, which do not have bandwidths exceeding a few gigahertz

unless they are exotic and expensive. The speed requirements for the individual

optoelectronic components are, therefore, relaxed, even though a significant amount

of total fiber bandwidth is still being utilized.

Cha

pter

4


Fig.3.1 Diagram of a simple WDM system


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DWDM SYSTEM

Dense Wavelength Division Multiplexing refers originally to optical

signals multiplexed within the 1550-nm band so as to leverage the capabilities of

EDFAs, which are effective for wavelengths between approximately 1525 nm - 1565

nm (C band), or 1570 nm - 1610 nm (L band). EDFAs can amplify any optical signal

in their operating range, regardless of the modulated bit rate. In terms of multi-

wavelength signals, so long as the EDFA has enough pump energy available to it, it

can amplify as many optical signals as can be multiplexed into its amplification band

(though signal densities are limited by choice of modulation format). EDFAs

therefore allow a single-channel optical link to be upgraded in bit rate by replacing

only equipment at the ends of the link, while retaining the existing EDFA or series of

EDFAs along a long haul route. Furthermore, single-wavelength links using EDFAs

can similarly be upgraded to WDM links at reasonable cost.

The EDFAs cost is thus leveraged across as many channels as can be

Multiplexed into 1550 nm band..Optical networks use Dense Wavelength

Multiplexing as the underlying carrier. The most important components of any

DWDM system are transmitters, receivers, Erbium-doped fiber Amplifiers, DWDM

multiplexors and DWDM demultiplexor.

.



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Fig. 4.1 The Structure of DWDM System

4.1 DWDM SYSTEMS

At this stage, a basic DWDM system contains several main components:

1. A DWDM terminal multiplexer. The terminal multiplexer actually contains one

wavelength converting transponder for each wavelength signal it will carry. The

wavelength converting transponders receive the input optical signal , convert that

signal into the electrical domain, and retransmit the signal using a 1550-nm band

laser. (Early DWDM systems contained 4 or 8 wavelength converting

transponders in the mid 1990s. By 2000 or so, commercial systems capable of

carrying 128 signals were available.) The terminal mux also contains an optical

multiplexer, which takes the various 1550-nm band signals and places them onto a

single SMF-28 fibre. The terminal multiplexer may or may not also support a

local EDFA for power amplification of the multi-wavelength optical signal.

2. An intermediate optical terminal, or Optical Add-drop multiplexer

This is a remote amplification site that amplifies the multi-

wavelength signal that may have traversed up to 140 km or more before reaching

the remote site. In more sophisticated systems (which are no longer point-to-

point), several signals out of the multiwavelength signal may be removed and

dropped locally.

3. A DWDM terminal demultiplexer.

The terminal demultiplexer breaks the multi-wavelength signal back into

individual signals and outputs them on separate fibres to detect. Originally, this

demultiplexing was performed entirely passively.

WDM wavelengths are positioned in a grid having exactly 100 GHz

(about 0.8nm) spacing in optical frequency, with a reference frequency fixed at

193.10 THz (1552.52nm). The main grid is placed inside the optical fibre amplifier



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bandwidth, but can be extended to wider bandwidths. Today's DWDM systems use 50

GHz or even 25 GHz channel spacing for up to 160 channel operation.

DWDM systems have to maintain more stable wavelength or frequency than those

needed for CWDM because of the closer spacing of the wavelengths. Precision

temperature control of laser transmitter is required in DWDM systems to prevent

"drift" off a very narrow frequency window of the order of a few GHz.



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Chapter 5

REPEATER DESIGN

To compensate for fiber loss, the optical signal must be returned to its

initial value as often as possible. To achieve this, repeaters are inserted periodically

along the link (typically 150 to 200 over a transpacific submarine distance). In all

deployed optical systems, the repeaters rely on EDFA technology. By contrast, in our

experiment they are based on Raman amplification technology. EDFA technology

provides optical amplification in a short section of specially-designed fiber, and can

therefore be viewed as lumped when compared with the typical repeater span length

of over 50 km. In contrast, Raman amplification technology uses the transmission

fiber as the amplification medium, and should therefore be considered as distributed.

At each Raman repeater, a strong continuous wave is launched (generally backwards)

into the transmission fiber. Through the effect of stimulated Raman scattering, this

wave serves as a pump to amplify the WDM channels propagating in the opposite

direction, provided that its wavelength is approximately 100 nm smaller than the

wavelength region where gain is needed.This technique can improve the Optical

Signal-to-Noise (OSNR) because of its distributed nature.

A better insight into this phenomenon can be obtained by computing

the relative change in signal power along a link with a 66 km repeater span length, as

shown in Figure 2. When using all-Raman amplification, the power decay resulting

from fiber loss is stopped at about 20 km before the next repeater. At this point, the

power level is P higher than at the input of a regular EDFA. This minimum power

level in the repeater span Pmin mainly sets the amount of noise generated by the

overall amplification process. Therefore, deploying Raman amplifiers effectively

reduces the repeater span loss by several dB (smaller than but in the range of P), or

effectively increases the OSNR by the same amount. However, at the same time, the

average power along the line is higher than with EDFA. This means that the benefits

in terms of OSNR cannot be fully exploited for extra margins, since the power at each

fiber input must be reduced to avoid breaching the upper power limit set by nonlinear

impairments.



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A better insight into Raman versus erbium performance is

provided in the next section. Other detrimental effects, such as double Rayleigh

backscattering, should also be taken into account when assessing the true system

margins provided by Raman amplification. Stimulated Raman scattering provides

gain only over a limited wavelength region, and gain uniformity is generally obtained

by inserting several pumps at different wavelengths simultaneously into the

transmission fiber.

In our experiment, we use four pumps at 1439.5 nm, 1450 nm, 1461

nm and 1493 nm, providing gain to the 32 nm-wide multiplex, all sent backwards in

each repeater span. More generally, the compatibility of Raman amplification with

large bandwidths (larger than EDFAs) is probably where its greatest potential lies.

However, the need for large bandwidths is not expected to drive the market for the

next few years. Moreover, this compatibility should be balanced against one of the

major drawbacks of Raman repeaters, namely their electrical power consumption.

Electrical power is a scarce resource in submarine systems because it is supplied over

the cable itself.

Fig. 5.1 Schematic of distributed Raman amplification, as compared with lumped

erbium amplification



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Chapter 6

FIBER ARRANGEMENT

Since Raman repeaters use the transmission fiber as the

amplification medium, careful consideration needs to be given to adapting the fiber

parametersfor optimal system performance. Fiber types differ mainly in terms of their

loss and effective area (which describes the energy confinement within the fiber and

hence its sensitivity to nonlinear effects), as well as their Polarization Mode

Dispersion (PMD) and chromatic dispersion properties. PMD originates from minute

imperfections in fiber manufacturing which alter the material composition or the local

fiber geometry, causing anisotropy.Because of this anisotropy, the two polarization

components of light travel at different velocities, causing unwanted waveform

distortions. The effect of PMD is especially detrimental at high bit rates (e.g. 40

Gbit/s). We assume that the transmission fiber and all the system components exhibit

low PMD, so that transpacific distances are achievable. This is a difficult target to

meet at an industrial level with existing technologies.

Chromatic dispersion is a phenomenon caused by different spectralcomponents not traveling at the same speed. It distorts the waveforms and therefore

requires compensation. Almost all deployed submarine links are based on Non-Zero-

Dispersion Shifted Fiber (NZDSF). In this type of fiber, the strong wavelength

dependence of the chromatic dispersion characteristics makes it difficult to

compensate the dispersion uniformly across all WDM channels, which is required to

achieve uniform performance versus wavelength. This is especially true at 40 Gbit/s

where the sensitivity to dispersion is greater than at 10 Gbit/s.

Here we consider a different fiber scheme involving two types of

fiber with positive (+D) and negative (-D) dispersion characteristics. These have been

considered for the longest and most stringent 10 Gbit/s submarine systems, and

appear to be better suited to a 40 Gbit/s channel rate than NZDSF. The typical loss

and effective area for +D fiber are 0.185 dB/km and 110 m2, respectively, compared

with 0.23 dB/km and 30 m2, respectively, for the D fiber. The total lengths of +D

and D fiber are dictated by the need to compensate for the overall dispersion.



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This still leaves various possibilities for concatenating the +D/D

fibers, which we study next as a function of the amplifier configuration. We compare

four fiber/amplifier configurations,as shown inFigure 3. Configuration (a) is the most

common configuration involving EDFA repeaters. Configuration (b) is the replica of

configuration (a), but combines a Raman preamplifier and an EDFA in the repeaters,

whereas configuration (c) uses only Raman repeaters. Finally, configuration (d)

differs from configuration (c) in that the transmission fiber consists of three fiber

sections (+D/-D/+D) rather than two. We compare the relative performance of all four

configurations by evaluating the maximum reachable distance Dmax versus the

repeater spanlength. Dmax is obtained when the lower optical power limit given by

the tolerance to noise coincides with the higher optical power limit given by the

accumulation of non-linear impairments .

For an accurate evaluation, we have taken into account a realistic

insertion loss for all the critical components of the repeaters. In particular, a gain

equalizing filter is needed at every amplifier to contain the power excursion versus

wavelength along the link. In our simulations, we assume a maximum insertion loss

of the gainequalizing filter, which is 1.2 dB higher with EDFA-based repeaters than

with those based on all-Raman. This accounts for the fact that EDFA gain is generally

less uniform than Raman amplifier gain. We have also taken into account the

relatively high 0.3 dB per splice loss of the D-fiber.

Figure 4 is a plot ofDmax, normalized to the maximum

distance that can be achieved with the optimum 40 km span length in the EDFA

configuration (a).In most cases, configurations using some form of Raman

amplification perform better than those based solely on EDFAs, but the advantage

does not translate into more than a 20% increase in the maximum reach Dmax.

Besides, with a conventional two-section +D/-D fiber configuration, a hybrid

EDFA/Raman configuration (b) slightly out performs the all-Raman configuration

(c), especially when the repeater span length exceeds 40 km.



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However, the winning configuration (d) uses only Raman amplifiers,

but with three +D/- D/+D sections of fiber. Whereas a three section scheme would be

of limited interest with EDFAs, it enhances the benefits of Raman amplification by

ensuring that the maximum optical power level is found at the beginning and at the

end of each repeater span in the fiber type (+D), which has a larger effective area and

is therefore the least sensitive to nonlinear impairments. Note that the greatest benefits

are obtained with an optimal repeater span length of ~60-80 km, significantly larger

than the optimal 40 km span length with configuration (a) based on EDFAs.

Fig. 6.1 Impact of amplification scheme and span length on system performance



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Chapter 7

WDM LABORATORY EXPERIMENT

Figure 5 shows the setup for the laboratory experiment conducted atAlcatel Research and Innovation to emulate an Nx 40 Gbit/s submarine system. The

WDM transmitter consists of 40 laser diodes with wavelengths ranging from 1569.54

nm to 1602.32 nm. Because of the cost issues, not all the lasers are modulated with

independent information; instead, in each band they are combined into two sets of 200

GHz spaced channels, corresponding to odd and even channels, through 1 x 20 array-

waveguide multiplexers. These sets are modulated independently using the APol RZ-

DPSK format by 223-1 bit-long pseudo-random bit sequences at 40 Gbit/s with 7%

overhead data (i.e. at 43 Gbit/s). The overhead emulates the presence of Forward

Error Correction (FEC), a digital technique used to correct errors in the received data

stream, and which is now part of all WDM transmission systems. Odd and even

channels are then interleaved, boosted into an EDFA and fed to our recirculating loop

through a switch SW1 and a 3 dB coupler.

A loop is a convenient way to emulate transmission over long distances

without resorting to the required quantity of fiber and repeaters. It incorporates a

switch SW2, which, like SW1, is controlled by delay generators, triggering the

measuring equipment synchronously with the loop. Initially, the loop is filled with a

flow of data traveling clockwise after closing SW1 while keeping SW2 open. Then

SW2 is closed and SW1 opened at time t0, which initiates circulation of the loaded

data. A fraction of the light from this data is continuously extracted via the loop

coupler and detected by the receiver. The transmission quality is evaluated by

comparing the received data sequence with the initial one. The number of bit errors

over the number of transmitted bits yields the Bit Error Ratio (BER).

The BER is measured only within a small gating window starting at time t>

t0 to select only data that has traveled a given number n of round trips (i.e. a given

distance), nbeing derived from the loop trip time and tt0. The recirculating loop

consists of seven 65 km fiber spans (near the optimum ofFigure 4), plus one 55 km

span, giving a total distance of 510 km. Each span incorporates three sections of +D/

-D/+D fiber using the (d) fiber configuration. Spans are separated by repeaters based

on Raman amplification. They all incorporate a gain-equalizing filter, but further



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power equalization is performed in the laboratory by a dedicated dynamic device. To

compensate for its loss and for all the other loop-specific losses, a Raman amplifier is

inserted at the end of the loop, based on a dedicated fiber section. At the receiver end,

the signal is sent into a preamplifier EDFA.

The chromatic dispersion is finely tuned before the photodiode so as to meet

the receiver requirements. Channel selection is performed by a tunable optical filter

emulating a wavelength demultiplexer. The two complementary outputs of the

demodulator are input to a 43 Gbit/s balanced receiver. The BERs of all 40 channels

were recorded after 18 loops and 22 loops, corresponding to distances of 9180 km and

11 220 km, respectively. The BERs were then numerically converted into Q-factors,

which are plotted for both distances in Figure 6. at 9180 km, the Q-factors of all the

channels are better than 11.5 dB.

A third-generation FEC would achieve better than 10-13 BER after correction

when the Q-factor is higher than the 8.5 dB limit (4 x 10-3 BER). Thus, 11.5 dB is 3

dB above the FEC limit, and should be compatible with industrial margin

requirements. When the transmission distance is increased to 11 220 km (a record

distance for 40 Gbit/s systems), the performance of the worst channel (9.9 dB) is still

1.4 dB above the FEC limit.

Fig. 7.1 Experimental setup: 18 loops of 510 km emulate a 9180 km transpacificdistance



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Chapter 8

CONCLUSION

The growing demand for broadband services is generating additional traffic

in optical networks, which will be reflected in submarine WDM systems. In the not

too distant future, such systems will likely move to a 40 Gbit/s channel rate. In this

article, the authors have highlighted some of the technologies that could enable a 40

Gbit/s channel rate to be introduced. Raman technology may bring some benefits to

system performance, but whether these benefits are worth the development of the

associated disruptive repeater design remains a matter of debate. It has been shown

that the result of this debate should influence the fiber arrangement between two

repeaters, and that the effect of polarization mode dispersion should be contained.

However, adapting the modulation format to the requirements of 40 Gbit/s operation

is the direction that will bring us closest to the actual implementation of 40 Gbit/s

systems. In this respect, the APol-RZ-DPSK format must be short-listed as one of the

most promising formats. Based on these elements, Alcatel Research and Innovation

has demonstrated, in a world first, that 40 Gbit/scould be transmitted over both

transatlantic and transpacific distances with industrial margins. This experiment has

confirmed Alcatels leading position in optical transmission research.



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REFERENCES

[1] H. Sugahara et al: 6,050 km transmission of 32 x 42.7 Gbit/s DWDM signals

using Raman-amplified quadruplehybrid span configuration, Proceedings of the

Optical Fiber Communications Conference 2002, paper FC6, Anaheim, 18-22 March

2002.

[2] J.-X. Cai et al: Transmission of thirtyeight 40 Gbit/s channels (>1.5 Tbit/s) over

transoceanic distance, Proceedings of the Optical FiberCommunications Conference

2002, paper FC4, Anaheim, 18-22 March 2002.

[3] C. Rasmussen et al: DWDM 40G transmission over trans-pacific distance

(10,000 km) using CSRZ-DPSK, enhanced FEC and all-Raman amplified

100 km UltraWaveTM fiber spans, Proceedings of the Optical Fiber

Communications Conference 2003, paper PD18, Atlanta.

[4] T. Tsuritani et al: 70 GHz spaced 40 x 42.7 Gbit/s transmission over 8700 km

using CS-RZ DPSK signal



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ACKNOWLEDGEMENT

I express my sincere gratitude to the entire faculty of Electronics &

Communication Dept. of Govt. College of Engg., Kannur for their support and

guidance provided for this seminar.

I am highly indebted to our respected guide Mr.B.S. Shajeemohan, Asst.

Prof. Dept. of Electronics & Communication for his excellent guidance and

cooperation.

I am also grateful to Mr. Rishidas, Asst. Prof. Dept. of Electronics &

Communication for his support.

I also extend my thanks to Mr. Dinesh Babu, Asst. Prof. Dept. of Electronics

& Communication for his useful suggestions.

I would also like to thank all my friends and my family, who were the sourceof constant encouragement.

Above all I thank the Almighty for His grace.

DHANYA. P.T

Dept. of ECE Govt. College of Engg., Kannur


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ABSTRACT

The most advanced submarine systems currently operating are based or Dense

Wavelength Division Multiplexing (DWDM) of channels at 10 Gbit/s. This means

that the system capacity is Nx 10 Gbit/s, where N is the number of optical carriers

(channels), each at a separate wavelength and modulated at 10 Gbit/s. The

introduction of a larger bit rate (e.g. 40 Gbit/s) is not expected in deployed links for a

few years. However, the first WDM transmissions at a 40 Gbit/s channel rate were

reported in research laboratories over transatlantic distances . Subsequently, they were

extended to transpacific distances but did not exhibit adequate margins for industrial

implementation.

Dept. of ECE Govt. College of Engg., Kannur


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CONTENTS

1. INTRODUCTION 1

MODULATION FORMAT 2

2. WAVELENGTH-DIVISION MULTIPLEXING 5

3.1 WDM SYSTEMS 5

3.2 BASIC OPERATION OF WDM 7

3. DWDM SYSTEM 8

4.1 DWDM SYSTEMS 9

4. REPEATER DESIGN 11

5. FIBER ARRANGEMENT 13

6. WDM LABORATORY EXPERIMENT 16

CONCLUSION 18

7. REFERENCES 19

Documents

40Gbits in WDM Submarine Transmission