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Abstract-The objective of this study is toanalyse the performance of dispersion managed

RZ. The dispersion managed system is a

promising way to transmit data in optical

communication networks. The performance of

10 Gbps optical communication system with the

dispersion managed return-to-zero (RZ) pulse

has been reported. The return-to-zero (RZ)

pulse is efficient for long-distance, high-bit-rate,

wavelength division multiplexed (WDM)transmission dispersion-managed systems. In

RZ pulse, the power is transmitted only for a

fraction of bit period. In this thesis, predictions

are made by varying the dispersion parameter

of single mode fiber in optical communication

system. It has been reported that the

performance of the system is improved with

increase in the value of dispersion parameter.

Using the different types of modulation formats,

it is predicted that the novel modulation formats

enhance the overall performance of the optical

communication systems at high bit rate.

1. INTRODUCTION

Wireless optical communications is one of the most

promising candidates for future broadband

communications, offering transmission rates far

beyond possible by RF technology.

A lot ofresearch has been going on for wireless

optical communications systems that can provide

reliable communications under such conditions. In

order to maximize channel throughput, powerful

modulation schemes need to be employed and

provide reliable communications.

The simplest and the most widely used

modulation scheme is non -return-to-zero (NRZ)

format,where the pulse is on for the entire bitperiod.Alternatively, a return-to-zero (RZ) format

can be used where the pulse is on only for a portion

of the bit period. Optical return-to-zero (RZ)

signals are becoming increasingly important in

optical communication systems. They have

proven to be superior to the non return- to-zero

(NRZ) format both in terms of receiver sensitivity

and fiber transmission performance .

The RZ format has the better receiver sensitivity

and nonlinearity tolerance due to which thismodulation format is of great interest for research

scholars these days. The work is going on

achieving high bit rates which is above 40 Gb/s.

Due to its relatively broad optical spectrum which

results is results in reduced dispersion toleranceand a reduced spectral efficiency. RZ pulse isless susceptible to inter symbol interference and

better nonlinear robustness. RZ modulation

scheme has became a popular solution for 10 Gb/s

systems because it has higher peak power, higher

signal-to-noise ratio, and lower bit error rate than

NRZ encoding. Dispersion and attenuation are the

two factors which degrade the performance of the

system. Several techniques are developed for

compensating the dispersion.

Problems with nrz:

In the case of the traditional non-return-to-zero (NRZ)keying technique, the faster the light is switched, the

less energy there is per bit, as the line is on for shorter

period and pulses are closer to each other. Hence, the

signal is more susceptible to attenuation, chromaticdispersion, polarization mode dispersion, and other

impairments, so the reach is decreased substantially.

A typical 40/43 Gbit/s signal can reach a fewkilometers; while a similar 10/10.7 Gbit/s NRZ could

reach 40 km. One huge impact to the network would

be the number of extra repeaters required between

nodes. Installing

more repeaters would not be an easy decision to make,not just because of the cost, but because these DWDM

networks carry multiple live channels, all full of

customer traffic.As transmission rate increases from 10 Gbit/s to 40 or

even 100 Gbit/s, the spectral characteristics of the

signal become wider, and this has another list of

consequences. In practical terms, the DWDMnetworks built in the past few years have filters

(add/drop) designed for the spectral characteristics

and channel spacing of 10/10.7G signals. This would

imply replacing key network components in thenodes, at a great expense and downtime.

Design and Simulation of a Optical communication

system with dispersion managed RZ pulse

Neha Nalini (09BEC426),Sharmila Jayakkumar (09BEC415),V. Sarvani(09BEC451)


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RETURN-TO-ZERO (RZ) modulation

formats are becoming increasingly popular forlong-haul optical fiber transmission systems at bit

rates of 10 Gb/s and above . Previously, the

benefits of RZ formats were often overlooked,

because they require larger bandwidth than non-

return-to-zero (NRZ) formats, and their generation

typically requires two cascaded MachZehnder

(MZ) modulators. In recent years, it has beenshown that RZ can have superior performance over

NRZ in certain regimes where chromatic

dispersion and fiber nonlinearities are present , as

the RZ pulse may exhibit soliton-like properties.

In addition, RZ has greater tolerance to

polarization-mode dispersion than NRZ . Recentresearch has compared the performance of RZ with

different modulation techniques, including binary

ONOFF keying (OOK) and binary differential

phase-shift keying (2-DPSK).

2. FIBER TRANSMISSION

1. Dispersion

The refractive index of glass is a function of

wavelength, which results in the spectral

components of a pulse traveling at different

group velocities along the fiber. Hence

chromatic (material) dispersion broadens

optical pulses beyond their time slot, leading

to intersymbol interference (ISI). A second

component of dispersion in optical fibers is

known as waveguide dispersion. This

component arises because the proportion of

light traveling in the fiber core versus

cladding is a function of wavelength. The

dispersion coefficient of a fiber is defined as

D = d(1/vg)/d. The typical dispersion

coefficient of single mode fiber (SMF) is 16

ps/nm.km. Non-zero dispersion-shifted

fibers (NZDSF) such as LEAF and

TrueWave-RS

have lower dispersion

coefficients than SMF. The dispersion

coefficient of a fiber is also a function of

wavelength, otherwise known as dispersion

slope.

Chromatic dispersion in a fiber can be

compensated by specially designed fiberwith a refractive index profile (core

composition) that leads to negative

waveguide dispersion characteristics.

Another approach is zero dispersion-shifted

fibers, designed such that the dispersion

coefficient at the loss minimum (1550 nm) is

zero. Dispersion compensation schemes

must compensate not only for dispersion but

also for dispersion slope. In dense WDM

systems, it is a challenge to compensate forthe dispersion and its slope for each channel

over the entire optical spectrum.

Since an RZ pulse has a wider optical

bandwidth than an NRZ pulse, it is more

affected by dispersion, as can be seen from

the eye diagrams in Figure 4. For the same

reason, slope compensation for an RZ signal

is also more difficult. RZ transmission

through dispersion-shifted fiber would still

require the appropriate slope

compensation.

2. Bit rate

Higher bit-rate systems are limited by

dispersion. The RZ format would be

beneficial for systems with few channels but

would require NRZ as the number of

channels increase.5

Dispersion compensation

based on chirped Fiber-Bragg gratings

(FBG) to compensate for the residual

dispersion of dispersion compensation fibers

(DCFs) is under development. The

effectiveness of FBG modules in mitigating

residual dispersion effects at 40 Gb/s over

the multiple channels of the transmission

spectrum is being explored.


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3. BLOCK DIAGRAMA typical digital fiber optic link is depicted in

Figure 1. Electrical data signals are

converted to optical signals via a modulator.

A 1 is transmitted as a pulse of light while

a 0 has no light output. The number of

1s and 0s transmitted per second

determines the speed of the link (bit rate).

Glass optical fibers have a wide

transmission window over which a number

of optical signal channels may be

transmitted simultaneously by wavelength

division multiplexing (WDM). The power of

all the channels combined is boosted by anoptical amplifier before being launched into

an optical fiber. The launched power

generally compensates for the fiber

transmission loss of a given fiber stage

(span). After each span, the signals are

amplified by an optical line amplifier (e.g.,

Erbium doped fiber amplifier), or repeater.

Since transmission fiber is a dispersive

medium, implying that pulses spread asthey travel through the fiber, some form of

dispersion compensation is applied at each

repeater stage. At the receiving end of the

link, the WDM optical signal is de-

multiplexed. Each channel is optically pre-

amplified and then detected by an optical-

to-electrical (O/E) converter (e.g., a

photodiode). A decision circuit identifies the

1s and 0s in the signal. An optical filtercan be inserted before the O/E converter to

filter out amplifier noise.

The choice of duty cycle will impact other

system design parameters such as

transmission at a higher bit-rate, closerchannel spacing, dispersion management,and polarization mode dispersion (PMD).

These issues will be discussed in the

following sections.

4. SIMULATION SETUP


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5. RESULTS AND DISCUSSIONSThe duty cycle of a pulse is = T

on/(T

on+

Toff

). The eye diagram and frequency

spectrum of a 10 Gb/s NRZ pulse and an RZ

pulse with a 50% duty cycle are shown in

Figure. Observe that the RZ spectrum has a

wider bandwidth than the NRZ spectrum.

The spectrum of an NRZ signal at 20 Gb/s is

the same as that of an RZ signal except for

the tones at 10 and 30 GHz.


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6. SUMMARY AND CONCLUSIONIn this paper we have discussed that the RZ

format has better baseline receiver

sensitivity when the average power into the

fiber is kept constant. RZ is more affected bydispersion and dispersion slope. For 10-20

Gb/s systems, where dispersion and its slope

are well compensated, RZ will perform

better than NRZ in most cases. The

exception is the zero dispersion regime in

zero dispersion shifted fiber,where non-

linearities will dominate. Because 40 Gb/s

systems are limited by dispersion and

dispersion slope, NRZ may be a better

choice for a system with a large number of

channels, Implementing the RZ modulationscheme requires a higher bandwidth driver

on the transmit end. This scheme can also be

implemented using two optical

modulators.This solution may be expensive,

however, since a higher bandwidth driver

may be more cost effective than two optical

modulators. At the receiver end, we have

shown that the optimal filter bandwidth for

an RZ system may be the same as that of an

NRZ system.

7. REFERENCES[1] Belal Hamzeh and Mohsen Kavehrad

(FIEEE) Multirate RZ Communications for

Dispersive Wireless Optical Channels 2006

[2] Ezra Ip and Joseph M. Kahn,Fellow,

IEEEPower Spectra of Return-to-ZeroOptical Signals JOURNAL OF

LIGHTWAVE TECHNOLOGY, VOL.

24, NO. 3, MARCH 2006

[3] Anjali Singh, Ph.D. Modulation Formats

for High-Speed, Long-Haul Fiber Optic

Communication Systems

Ildefonso M. Polo I October, 2009

Optical Modulation for High

Bit Rate Transport Technologies

[4] Gerardo Castanont anid Takeshi

Hoshida Impact of Filter DispersionSlope in NRZ, CS-RZ,IMDPSK and RZ

formats on Ultra High

-Bit-rate Systems

[5] Rahul Chhilar1, Jitender Khurana2,

Shubham Gandhi3

MODULATION FORMATS IN

OPTICAL COMMUNICATION

SYSTEM IJCEM International Journal of

Computational Engineering &Management, Vol. 13, July 2011

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