optimization of four wave mixing effect in radio-over-fiber for a 32-channel 40-gbps dwdm system

8/22/2019 Optimization of Four Wave Mixing Effect in Radio-Over-Fiber for a 32-Channel 40-GBPS DWDM System

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Optimization of Four Wave Mixing Effect in

Radio-over-Fiber for a 32-Channel 40-GBPS DWDM

SystemBijayananda Patnaik and P.K.Sahu

Indian Institute of Technology Bhubaneswar

E-mail: [email protected], [email protected]

Abstract- In this paper, we have discussed in detail the four

wave mixing (FWM) effect of dense wavelength division

multiplexing (DWDM) in Radio-over-Fiber (RoF) system.

A 32-channel 40-Gbps system is considered. FWM effect

for various channel spacing, input power level, effective

fiber area and modulation formats are analyzed. Different

schemes for minimizing these effects are discussed for the

first time. Considering all these effects, an optimized

DWDM setup is designed using single parameter

optimization (SPO) technique.

Index Terms - Dense wave length division multiplexing,

four wave mixing, Optisystem 8.0, Q-value, radio over

fiber, single parameter optimization.

I. INTRODUCTION

Radio-over-fiber (RoF) technology entails the use of

optical fiber links to distribute radio frequency (RF) signals

from a central location to the remote antenna units (RAUs).

RoF systems with high data rates are used for wireless

broadband communications as they can utilize the low loss and

ultra wide bandwidth provided by optical fiber. The main

benefits of RoF is low attenuation loss, large bandwidth,

immunity to radio frequency interference, reduced power

consumption, multi-operator and multi-service operation,

dynamic resource allocation etc [1], [7]. Hence it is much more

preferable comparing to RF signal processing. The explosion

in demand for network bandwidth is largely due to the growth

in traffic such as video on demand, internet usages, and voice

over IP, streaming video and voice, etc. For utilizing the higher

band width of optical systems, multiplexed systems are

preferred for transmitting huge information on to a single fiber.

A multiplexed system with channel spacing of less than or

equal to 200GHz is called as Dense Wavelength Division

Multiplexing (DWDM). Similarly if the system uses two or

more signals multiplexed onto a single fiber, where one signal

is in the 1550 nm band, and the other in the 1310 nm band, it is

said to be Coarse Wavelength Division Multiplexing

system(CWDM). In CWDM systems, due to wide spacing

between channels, it doesnt supports erbium-doped fiber

amplifier (EDFA), which is the main disadvantage of CWDM

system. FWM is a phenomenon that occurs in DWDM/CWDM

systems, in which the channel spacing are very close to each

other. It is generated by the third order distortion that creates

third order harmonics [9], [2], [3]. These cross products

interfere with the original wavelength and cause the mixing.

These spurious signals may fall right on the original

wavelength which causes difficulty in filtering out them. This

effect is mainly due to narrow channel spacing, high input

power, small cross-sectional area of fiber and fiber dispersion

[5], [6], [8]. Hence, analysis of these effects and system

optimization is highly essential for DWDM/CWDM setup. Its

performance improvement is a challenging task in present

context to meet the demand.

II. THEORY

In DWDM system, for three continuous-wave channels of

input powers Pi, Pj, Pk at frequencies fi, fj, fk, the FWM power

Pijkat the outpXWRIDILEHUZLWKDWWHQXDWLRQDQGOHQJWK]LV>@]

ijkkjieff222

ijkijk ePPPLdP

(1)

Where dijkis the degeneracy factor, which takes value 1 and

2 for degenerate (i = j) and non-degenerate (i j) terms

respectively, Leffis the effective length, ijkis the efficiency is

the nonlinear coefficient, which is given by [4]

22

eff

22

1.

A

)n(2 (2)

Where n2 is the fiber nonlinearity coefficient, Aeff is the core

effective cross-sectional area, is the central wavelength. In

general, the number of cross mixing products M, for N input

channels are given by [9]

1)(N2

NM

2

(3)

Thus as the number of input channels increases FWM effect

is more prominent and it is to be taken care for increasing the

efficiency of DWDM system.

III. SIMULATION

The proposed system consists of a transmitter, fiber and an

optical receiver as shown in Fig. 1. Transmitter consists of 32

number of DWDM channels, optical multiplexer. It supports

different modulation formats e.g., no modulation, non-return to

zero (NRZ) and return to zero (RZ) modulation etc. Standard

single mode fiber (SSMF) is used to achieve long distance

communication in this setup. The simulation parameters used

is given in Table I.

The transmission link is designed suitably, so that the first-

order dispersion is compensated exactly (D =0), that is [10],

DCFDCFSMFSMF LDLD (4)

2010 International Symposium on Electronic System Design

978-0-7695-4294-2/10 $26.00 2010 IEEE

DOI 10.1109/ISED.2010.31

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D stands for first-order dispersion and L stands for length of

the respective fiber. Fiber parameters used in the system model

is as given in Table II, which is practically available and

suitable. The EDFA is used for compensating the linear loss

and the noise figure of which is set to 6dB. The model is

simulated using symmetrical dispersion compensation

technique. Thereafter, the SPO approach is used to design the

optimal system.

In the receiver the signal is passed through an optical

demultiplexer, detected by PIN detector, filtered, applied to

power meter and BER analyzer. The responsivity and dark

current of PIN diode taken is 1A/W & 10nA respectively and a

4th

order filter is used. 3R regenerator is used to generate an

electrical signal connected directly to the BER analyzer which

is used to visualize the graphs and results such as eye diagram,

eye opening, BER, Q value of the setup. For analyzing the

FWM effect, we have used the WDM analyzer, spectrum

analyzer at the input and output of the standard single mode

fiber(SSMF).

Fig. 1 Schematic of the DWDM setup using symmetric compensation

technique

Table I: Simulation parameters

Bit rate 40Gbps

Sequence length 64

Samples/bit 256

DWDM channel spacing 200GHz

Capacity 32-channel 40-Gbps

Distance 120 Km

Input Power 5dBm

Table II: Fiber Parameters

Fiber Atten.

(dB/Km)

Disp.

(ps/km-

nm)

Disp. Slope

(ps/km-nm2)

Effective

core

area(m2)

SMF 0.2 17 0.075 70

DCF 0.5 -85 -0.3 22

IV. RESULTS AND DISCUSSION

A. FWM effect for varying channel spacing

For analyzing this effect in the proposed setup (Fig. 1), the

following parameters are considered: input power level of

0dBm; no modulation format; laser line-width of 0Hz and

P2 as the effective cross-sectional area of fiber. The input

and output spectrum of SSMF is observed by varying the

channel spacing. Fig. 2 and 3 shows the input and output

spectrum of the setup respectively, for channel spacing of

25GHz. From these Figures, we observed the effect of FWM

that is the spectrum is broadened and many side bands arenoticed. Fig. 4 shows the output spectrum of SSMF for

200GHz channel spacing. In a similar fashion the output

spectrum can be observed for 100GHz spacing. Table III

summarizes the effect of FWM for various channel spacing.

Comparing the Fig. 3, 4 (maximum side lobe power) and from

table 3, it can be concluded that the FWM effect increases, as

the channel spacing decreases. However it is not possible to

increase the channel spacing too much, because in the fiber

link faithful amplification can't be achieved for wideband

signal. Again for DWDM system maximum channel spacing

should be 200GHz.

Fig. 2 Input spectrum of SSMF of 50Km length (25GHz spacing)

31

32

31

1

WDM &

SPECTRUM

ANALYZER

2

WDM &

SPECTRUM

ANALYZER

SMF

50K

DCF

20K

SMF

50Km

EDFA

10dB

EDFA

10dB

EDFA

10dB

.

.

.

.

.

.

.

.

.

.

.

.

1

2

32

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Fig. 3 Output spectrum of SSMF of 50Km length (25GHz spacing)

Fig. 4 Output spectrum of SSMF of 50Km length (200GHz spacing)

Table III: Maximum FWM power with respect to Channel spacing

Channel Spacing

(GHz)

25 100 200

Approximate Max.

FWM side band

power (dBm)

-36 -38 -39

B. FWM effect for varying Input power

For this analysis of the setup of Fig. 1 the input and output

spectrum of SSMF is observed by varying the input power

levels, keeping other parameters as constant: the channel

spacing is 200GHz; no modulation format is used; the laser

line-width taken is 0Hz and effective cross- sectional area of

ILEHU LV FRQVLGHUHG DV P2. Fig. 5 and Fig. 6 show the

output spectrum for input power level of 20 dBm and -10

dBm respectively. Fig. 4 shows the output spectrum for input

power level of 0 dBm and spacing of 200GHz. Table IV

summarizes the maximum sideband power of FWM, forvarying input power. Comparing Fig.4, 5, and 6 and from

Table IV, it can be concluded that the FWM effect decreases

as the input signal power decreases. However, the input

power level can't be reduced below a certain limit, as it may

not be sufficient to drive the components.

Fig. 5 Output spectrum of SSMF of 50Km length (20 dBm input Power)

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Fig. 6 Output spectrum of SSMF of 50Km length (-10 dBm Input Power)

Table IV: Maximum FWM side band power value w.r.t Input power

Input power

(dBm)

-10 0 10 20

Approximate Max.

FWM side band

power (dBm)

-59 -41 -20 -12

C. FWM effect for various Modulation formats.

For this analysis modulation formats considered are RZ

and NRZ. All other parameters are considered as constant: the

input power level is 0dBm; channel spacing is 200GHz; fiber

effective cross-sectional area is P2 and laser line-width is

0Hz. From WDM analyzer shown in Fig 7 and 9, we found that

OSNR is 29.83 for NRZ but 33.77 for RZ of channel no 1.

Again from Fig. 8 and 10 we have observed that, RZ

modulation has less FWM effect comparing to NRZ

modulation. Hence RZ is preferable in case of high data rate

and long-haul communication system for minimizing the

FWM effect.

Fig. 7 WDM analyzer output for NRZ modulation (After transmission through

SSMF of 50Km length)

Fig. 8 Output spectrum for NRZ modulation (After transmission through


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Fig. 9 WDM analyzer output for RZ modulation (After transmission through


Fig. 10 Output spectrum for RZ modulation (After transmission through SSMF

of 50Km length)

D. FWM effect for varying fiber effective cross-sectional

area

For this analysis, the fiber effective cross-sectional area is

varied, by considering all other parameters as constant: input

power is 0dBm; channel spacing is 200GHz; laser line-width is

0Hz; no modulation format is used. Effective cross-sectional

area of fiber is considered as 64P2

and then 80 P2. Table V

summarizes the approximate maximum FWM side band

power. It can be concluded that FWM effect decreases as the

effective area of the fiber increases.

Table V: Maximum FWM side band value with respect to effective cross-sectional area of fiber

Effective cross-sectional

area (mt2)

64 80

Approximate maximum

FWM side band power

(dBm)

-39 -41

At the end a complete DWDM system is designed and

simulated as shown in Fig. 1 by considering, a) without

minimizing and b) with minimizing the FWM effect. The

worse case performance is always observed at channel no 1

(end Channel) for the DWDM system. Hence channel no 1

output is considered for analysis. Fig. 11 shows the eye

diagram of channel no 1, when the FWM minimization

technique is not considered. The eye is completely a distorted

one. Then considering all these minimizing effects of FWM

such as modulation technique of RZ; channel spacing of

200GHz; input power of 5dBm; fiber effective cross-sectionalarea of 70 mt2 the set up is again designed. Symmetrical

dispersion compensation scheme and single parameter

optimization technique is utilized for simulating the link length

of 120Km. Fig. 12 and 13 shows the eye diagram and Q-value

of the system for Channel no1 before and after optimization. It

is observed that the maximum Q-value is improved from 19.33

to 36.24 after optimization. Fig. 13 also shows the 3-

dimentional BER plot of the optimized link. Hence the system

performance is found to be improved by taking care of FWM

effect.

Fig. 11 Eye diagram and Q-value of the link without optimization

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Fig. 12 Eye diagram and Q-value of the link after optimization

Fig. 13 Three-dimensional BER plot of the link after optimization

V. CONCLUSION

In our work, an optimized 32-channel 40-GBPS DWDM

system is designed, by considering all the effects of four wave

mixing. The Q-value obtained for channel no. 1 of the

proposed system at a distance of 120Km is 36.24. From our

observations we found that the detrimental FWM effect can be

minimized for higher data rate and long-haul communicationsystem by maintaining the following conditions: a) the channel

spacing should be as high as possible; b) input power should be

as low as possible; c) the fiber cross-sectional area should be as

high as possible; d) preferably RZ modulation is to be used

comparing to NRZ modulation. The above system may be

further analyzed and optimized for long distance

communication, for more number of input channels and also

for advanced modulation formats.

VI. R EFERENCES

[1] Muhammad Ifran Memon, Gabor Mezosi, Bei Li, Dan Lu, ZhuoranWang, Marc Sorel, and Siyuan Yu Generation and Modulation ofTunable mm-Wave Optical Signals Using Semiconductor Ring Laser,

IEEE Photonics Technology Letters, Vol. 21, No. 11, June 1, 2009.[2] M. Forzati, J. Martensson, A. Bernutson, A. Djupsjobaska and P.

Johannisson, Reduction of intrachannel four-wave mixing using thealternate-phase RZ modulation format, IEEE Photonics Technol. Lett.14 (9, 14) ,pp. 1285, 2002.

[3] K.S. Cheng and J. Conradi, Reduction of pulse-to-pulse interactionusing alternative RZ Formats in 40-Gb/s systems, IEEE PhotonicsTechnol. Lett. 14 (1), pp. 98, 2002.

[4] Antonella Bogoni and Luca Poti, Effective channel allocation to reduce

inband FWM crosstalk in DWDM transmission systems, IEEE Journalof selected topics in Quantum Electronics, Vol. 10, No. 2,April 2004.

[5] Yannis, L. G. New optical Microwave Up-Conversion Solution in Radioover Fiber Network, Journal of lightwave technology, 24 (3) pp. 1277-1282, 2006.

[6] Caiqin Wu and Xiupu Zhang, Impact of Nonlinear Distortion in RadioOver Fiber Systems With Single-Sideband and Tandem Single-SidebandSubcarrier Modulations, Journal of light wave technology, 24 (5), pp.2076 2090, 2006.

[7] S. Ohmori, The Future Generations of Mobile Communications Based onBroadband Access Technologies,IEEE Communications Magazine, 134

142, December 2000.

[8] Hiroshi F., Koji Y., Tetsufumi S, and Sei-ichi Itabashi Four-wave mixingin silicon wire Waveguides, Optics Express 13 (12) pp.4629-4637,2005.

[9] Hafiz Abd El Latif Ahmed Habib, Four wave mixing non-linearityeffect in wave length division multiplexing Radio over Fiber system,

Master thesis, University Teknology Malaysia, 2007.[10] I. Hayee and A.E. Willner, NRZ Versus RZ in 1040-Gb/s dispersion

managed WDM transmission systems, IEEE Photonics Technol. Lett.11, pp. 991993,1999.

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optimization of four wave mixing effect in radio-over-fiber for a 32-channel 40-gbps dwdm system

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