Comparing ITU-T G.652,653 & 655 Fibres for RZ Modulation

Comparing ITU-T G.652,653 and 655 fibers for

RZ modulation

Shradha GUPTA1, N. K. SHUKLA

2 and Shikha JAISWAL

3

1,2Department of Electronic and Communication, University of Allahabad, Allahabad-211002, India

3DepartmentofPhysics,S.D.(PG)College(C.C.S.University,Meerut),Muzaffarnagar-251001, India

ABSTRACT : The study of advanced modulation formats for high speed internet services using potentially the

bandwidth of single mode embedded fiber is very important.An investigation is done on ITU-T G 6.652(single mode

fiber) ,6.653 and 6.655 transmission fibers for RZ pulse with different duty cycle. The variation of Q-value and

BER versus distance for different duty cycle for different fibers is compare on single channel signal transmission for

10 Gbit/sec system .

Keywords: Fiber optic; Modulation; Communications

1. Introduction

The choice of fiber infrastructure is an

important issue in the design of high

capacity optical networks. DSF have been

extensively studied at 10 Gbits per sec

transmission rate [1], but its application is

limited when high input power and low

channel spacing are involved.

The dependence of the 10-Gb/s/ch long

distance transmission performance on the

signal pulse duty factor is studied by

numerical calculation and experiments. The

duty factor of the signal pulse is defined as

the ratio of the signal pulse width to the bit

duration time (100 ps) [2]. It is clarified that

the optimum duty factor depends on GVD

compensation interval for single-channel

transmission. Reducing the duty factor was

effective in suppressing XPM induced

waveform distortion in WDM transmission.

Analysis of both single-channel and WDM

transmission showed that duty factors 0.5

were suitable for a dispersion managed

system with GVD compensation interval of

500 km and a fiber dispersion parameter of 1

ps/nm/km [2].

Going through the literature, a need was

felt to have a performance details of optical

RZ pulse in other fiber types especially

dispersion shifted fibers (DSF) of optical

communication systems [7-14]. Most optical

communication systems have adopted the

non return to zero (NRZ) signal pulse format

for data rates of up to 10 Gb/s/ch [6, 15-19]

however some experiments over 100 Gb/s

with dispersion management and WDM

technology have used return to zero signal

pulse formats at data rates of 5- 10

Gb/s/ch[3-5]. This is because the RZ signal

pulse format has several potential

advantages over the NRZ signal pulse

format including suppression of interchannel

interaction caused by the fiber nonlinearity,

such as cross-phase nonlinearity (XPM) and

Four Wave Mixing (FWM)

Shradha Gupta, N.K.Shukla and Shikha Jaiswal, Int. J. Comp. Tech. Appl., Vol 2 (1), 94-102 ISSN : 2229-6093

94

and increase in signal-to-noise Ratio’s

(SNR’s) for constant average optical

powers. The issue is which duty cycle is

better for RZ pulse for single channel 10

Gbps optical communication system

using the SSMF which is extended to the

dispersion shifted

regime [20]. The results for variable duty

cycle fraction for G.652, 653 and 655 are

not available in the literature, thus explored.

2. Performance measures

The right choice of the performance

evaluation criteria for the characterization of

optical transmission links represents one of

the key issues for an effective design of

future long-haul optical systems. Long-haul

transmission links experience performance

degradation due to ASE-noise from optical

amplifiers along the line. The most widely

used performance measures for performance

evaluation are Q-factor, BER and eye

diagram.

2.1. Q-factor

Q-factor measures the quality of an analog

transmission signal in terms of its signal-to-

noise ratio (SNR). As such, it takes into

account physical impairments to the signal

viz. noise, chromatic dispersion and any

polarization or non-linear effects which can

degrade the signal and ultimately cause bit

errors. In other words, higher the value of Q-

factor the better the SNR and therefore the

lower the probability of bit errors.

2.2. BER

The BER can be estimated from eq.(1), [21]

and requires Q>6 for the BER of 10-9

.This

BER gives the upper limit for the signal

because some degradation occurs at the

receiver end.

BER =)] ........(1) (1)

Optical communication systems use a BER

to specify the performance requirements for

a particular transmission link application.

For example SONET/SDH networks specify

that the BER must be 10-10

or lower,

whereas gigabit Ethernet &fiber channel

requires a BER of no more than 10-12

.

2.3. EYE DIAGRAM

The eye diagram is powerful measurement

tool for assessing the data-handling ability

of a digital transmission system. The eye-

pattern measurements are made in the time

domain and allow the effects of waveform

distortion to be shown immediately on the

display screen. The basic upper and lower

bounds are determined by the logic one and

zero levels.

The width of the eye opening defines the

time interval over which the received signal

can be sampled without error due to

interference from adjacent pulses (known as

intersysmbol interference).The vertical

distance between the top of the eye opening

and the maximum signal level gives the

degree of distortion. The more eye closes,

the more difficult is to distinguish between

ones and zeroes in the signal.

3. System modeling for transmission

performance evaluation


95

Fig.1 Optical communication link

The simulated diagram of optical

communication system considered is given

in Fig.1 to examine the performance of RZ

modulated signals on different fibers for

different duty cycle. The simulation has

been carried out using a commercial

package OptSimTM

. The data is pseudo

random having bit rate 10 Gbits/s with 16

samples per bit using polynomial of degree

7. The adjustable duty cycle RZ driver

converts input to electrical outputs -2.5 low

level to 2.5 high level. The numbers of poles

in low pass Bessel filter has been kept to 1

and uses the -3dB cutoff frequency 9.953

GHz. The Modulator is a single arm mach-

zehnder amplitude modulator with sin2

electrical shaped input-output (P-V)

characteristics. Typical transfer function is

taken for a mach-zehnder external

modulator based on the eletro-optic effects

in the LiNbO3 devices. The level of

extinction ratio (corresponding to the ratio

between the maximum and minimum values

of the input-output transmission

characteristics) is kept ideal.Fixed output

power optical amplifier is used which sets

the amplifier gain so as to have a total

output power equal to a 5 dBm, flat gain

shape and noise figure value 4.5.

The fiber length is varied up to 180 km in

presence of fiber nonlinearity and

polarization mode dispersion but without

Raman cross talk to analyze the variation of

various factors with respect to distance.

At the receiver, optical signal is passed

through band pass raised cosine optical filter

for centre wavelength 1552 nm. The

detection is done with the use of pin

Photodiode at 1552 nm wavelength with

quantum efficiency of 0.75 and 0.939 A/W

responsitivity. Electrical filter of low pass

Bessel types with 5 poles & -3dB BW gives

the electrical signal which is subsequently

measured for Q values, BER and eye

diagram.

Table 1. Characteristics of fibers used in

set up.

S.

No ITU-T

Standard G.6

52 G.6

53 G.655

(NZDSF) Parameter

s SM

F DSF Cor

ning

LEA

F

Luc

ent

True

Wav

e 1 Core

effective

area (m2)

80×

10-12

55×

10-12

72×

10-12

55×

10-12

2 Loss at

1550 nm

(in dB)

0.2 0.2 0.2 0.2

3 Zero

dispersion

wavelengt

h (nm)

139

1.5 158

0.24 151

5.43 146

6.22

4 Fiber non-

linearity(1

/W/km)

1.27 1.84 1.40

8 1.84

5 First order

dispersion

D (ps/nm-

km)

16 -2 4 4.5

6 Second

order

dispersion

0.07 0.07 0.07 0.10

8


96

D’

(ps/nm2-

km) 7 β2(ps

2/km) -

20.4

07

2.55

1 -

5.10

2

-

5.74

8 Β3(ps3/km

) 0.14

745 0.10

96 0.18

4 0.08

3 9 Non-

linearity

refractive

index

2.5

×10-

20

2.5

×10-

20

2.5

×10-

20

2.5

×10-

20

10 Non-

linearity

refer.

wavelengt

h (nm)

155

0 155

0 155

0 155

0

4. Results and Discussion

The investigation has been carried out at a

bit rate of 10Gb/s for variable length link to

yield a performance analysis of RZ data

format for different duty cycle under

different ITU-T recommended fibers.

To optimize the amplifier spacing it is

important to investigate the performance of

different fibers listed in Table1 for variable

length. Fig.(2)-(3) represents the variation of

Q-value and BER with duty-cycle

respectively for different fibers. Fig.4.

represents the comparison of eye diagram

for different fibers for fixed length (i.e.

100km) & duty cycle 0.8.

Fig 2.1. Q-value vs distance for SSMF

Fig 2.2. Q-value vs distance for DSF fiber

Fig 2.3

Q-value vs distance for Corning LEAF


97

Fig 2.4. Q-value vs distance for Lucent TW

Fig 2. Q-value vs distance for different

fibers

SONET/SDH networks specify that BER

must be 10-10

or less. The maximum length

range attainable with duty cycle variation

from 0.2 to 0.8 is listed in Table 2 (Fig 3).

Thus, amplifier distance can be increased up

to 30 km for SSMF with duty cycle selection

from 0.2 – 0.8. The amplifier distance can

be increased only 10 km for DSF whereas

this value is very low for LEAF and TW

fibers.

From fig 2.3 and 2.4 it is clear that LEAF

and TW fiber are showing similar trend, the

Q-value variation with duty cycle is

observed only for 40 – 120 km length of

fiber. On comparing these 4 fibers for fixed

length (100 km) of fiber & fixed duty cycle

(0.8) then the eye diagram is shown in fig.4.

Table 2. Fiber length range for duty cycle

variation from 0.2 - 0.8, for BER=10-10

ITU-

T Fiber used Fiber span length

(kms)

G65

2 SSMF 60-90

G65

3 DSF 150-160

G65

5 Lucent TW 150-160

G65

5

Corning

LEAF

155-157

Fig

3.1 BER vs single mode fiber length

Fig

3.2 BER vs DSF length


98

Fig 3.3 BER vs Corning LEAF fiber length

Fig 3.4 BER vs Lucent TW fiber length

Fig 3. BER vs fiber length for different

ITU-T transmission fibers for variable

duty cycle 0.2, 0.4, 0.6, 0.8 of RZ pulse.

Fig 3 shows the variation of BER with fiber

length for different ITU-T standard fibers

for different duty cycle. Fig 4 shows the eye

diagram for different fibers at span length of

100 kms and fixed duty cycle value of

0.8.On comparing Fig 4.1, 4.2, 4.3 and 4.4 it

is clear that DSF gives the best results in the

optical communication link of 100 kms at 10

Gb/s/ch with duty cycle 0.8 of RZ pulse.

Fig 4.1 Eye diagram for SSMF

Fig 4.2 Eye Diagram for DS normal

Fig 4.3 Eye diagram for LEAF


99

Fig 4.4 Eye diagram for True Wave

Fig 4 Eye diagram for different ITU-T

transmission fiber.

The Q-value for 100 km fiber length & 0.8

duty cycle is listed in Table.3.The higher

duty cycles gives higher Q values in SSMF,

while it is least effected in DSF. The Lucent

TW and Corning LEAF shows exactly

reverse trend i.e. the Q-value decreases at

higher duty cycles.

Table 3 .Q-value for 100 km fixed fiber

length

ITU

-T Fiber

used Q –Value for different

duty cycle

Duty cycle 0.2 0.4 0.6 0.8

G65

2 SSMF 6.02 6.85 8.55 13.6

2

G65

3 DSF 34.0

2 34.7

4 33.4

1 34.1

4

G65

5 Lucent

TW 36.2

8 35.9

1 32.6

8 26.2

1

G65

5 Corning

LEAF 35.8

8 33.6

2 30.7

6 27.6

6

5. Conclusion

The ITU-T recommended fibers are

compared for RZ modulation by varying the

span length to 180 km without post

compensation for different duty cycle at 10

Gb/s/ch optical communication system. For

SSMF, the variation of duty cycle help to

increase the amplifier distance from 60 to 90

km. Whereas DSF fiber shows negligible

change with duty cycle but the maximum

span length travelled is about 150 km which

is better than SSMF. Corning and Lucent

fiber are showing similar trends i.e span

length travelled increases upto 120 kms on

increasing duty cycle value and the Q-value

decreases with the increase in duty cycle

value

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Documents

Comparing ITU-T G.652,653 & 655 Fibres for RZ Modulation