50 GHz Spaced 4 × 40 Gbit/s WDM Transmission over 700 km Using 6 ps Bandlimited RZ Signals

doi:10.4236/opj.2011.13023

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Optics and Photonics Journal, 2011, 1, 137-141

doi:10.4236/opj.2011.13023 Published Online September 2011 (http://www.SciRP.org/journal/opj)

50 GHz Spaced 4 × 40 Gbit/s WDM Transmission over

700 km Using 6 ps Bandlimited RZ Signals

Mousaab M. Nahas

Electrical Engineering, Faculty of Engineering at Rabigh, King Abdulaziz University, Jeddah, KSA

E-mail: [email protected]

Received July 16, 2011; revised August 15, 2011; accepted August 25, 2011

Abstract

In this paper, we present 50 GHz spaced 4  40 Gbit/s WDM transmission over 700 km using SMF-based

Effective Area Enlarged Positive Dispersion Fiber in a recirculating loop. The paper uses bandlimited RZ

signals and shows that transmission distance of 700 km can be achieved with BER ≤ 10−9 using 6 ps

pulsewidth for each data signal. To attain this, optical filters with sharp transmission characteristics are used

in both transmitter and receiver. The results demonstrated in this paper are based on simulation, and the au-

thor believes the propagation distance reached in the paper is the longest distance achieved for such system.

Keywords: Telecommunications, Fiber Optics Communications, Wavelength Division Multiplexing (WDM),

Optical Time Division Multiplexing (OTDM), High Speed Optical Transmission

1. Introduction

There has been a big demand to increase the transmission

capacities of optical fiber communication systems since

these systems were first developed. In fact, increasing

the capacities is still under development as telecommu-

nications keep expanding in time. It is well known by

telecommunication people that increasing the capacity of

optical fiber systems can be either achieved through

wavelength division mutliplexing (WDM) or optical time

division multiplexing (OTDM) or by a combination of

both. OTDM has economic advantage for network op-

erators since the number of terminals is reduced and also

because it can accommodate the existing single-band and

narrow-band erbium-doped fiber amplifiers (EDFA’s),

thus no need to spend money on the replacement by

broadband amplifiers. Considering this, it would be more

practical in many cases to generate 40 Gbit/s signal

through OTDM rather than using 4 × 10 Gbit/s WDM

signal. Moreover, it is possible to multiply this band-

width by combining multiple 40 Gbit/s signals through

WDM so that the capacity increases significantly. This

approach is commonly used in high speed optical trans-

mission systems where a lot of work has already been

done like [1], in which nonzero dispersion shifted fiber is

used. However, since single mode fiber (SMF) is the

basis of most existing fiber optic networks, it has been

more realistic to develop and investigate systems using

similar fiber in their transmission links. The most world-

wide deployed SMF fibers are standard single mode fiber

(SSMF) and large effective area fiber (LEAF). Some

work has been done on multiple 40 Gbit/s signals using

SSMF like [2], which used NRZ modulation format and

reached 511 km propagation distance. Another work was

presented in [3] showing 4  40 Gbit/s WDM transmis-

sion over 300 km using RZ format over SSMF. By and

large, the LEAF has already shown better results in all

modulation formats due to reduced nonlinear effects in

the fiber during propagation [4]. Also, RZ signals are

more reliable than NRZ and most common in conven-

tional transmission systems using OTDM [5]. Based on

that, this paper shall concentrate on transmitting multiple

40 Gbit/s signals over LEAF using bandlimited RZ sig-

nals. Similar work was already presented in [6] showing

good transmission results over 480 km distance only.

Our paper demonstrates successful transmission of 4-

channels  40 Gbit/s WDM signals over 700 km using

SMF-based effective area enlarged fiber (or LEAF) with

positive dispersion. The four WDM signals are 50 GHz

spaced (i.e. 0.4 nm), thus the system is considered dense

wavelength division multiplexing (DWDM) system. We

believe that simulating four DWDM is somehow suffi-

cient to predict the behavior of systems carrying higher

number of channels while using less CPU time. In such

regime, the investigation would include finding the op-

timum input power and pulsewidth of the transmitted 4

M. M. NAHAS

138

WDM signals so that the longest possible transmission

distance is achieved with acceptable error rate. Indeed,

these two parameters play the major role as peak power

can cause nonlinearities while pulsewidth can lead to

distortion due to polarization mode dispersion (PMD)

within the fiber. Actually, the work presented in [6] used

10 ps pulsewidth to reach 480 km transmission distance.

However, that work did not show investigation of dif-

ferent pulsewidths thus it is not necessary that 10 ps is

the optimal value for the system described. Usually, the

pulsewidth used in experiments is limited by the band-

width of the transmitter’s filter available on the test bed.

In our work, since we have simulator, it would be possi-

ble and easy to examine different pulsewidths in addition

to the power to reach the optimum case.

2. Experimental Setup

The experimental setup of our work is shown in Figure 1.

At transmitter, four laser diodes are used with wave-

lengths ranging from 1554.4 nm to 1555.6 nm using 50

GHz spacing. The four wavelengths are WDM multi-

plexed and then modulated by 10 Gbit/s bandlimited RZ

data signal to give 4  10 Gbit/s signals. The data pattern

used is random 128 bits with 50% ones (note: 128 is the

data length limit of the simulator). A 4  40 Gbit/s bit

stream is then generated through two stages co-polarized

OTDM, as depicted in Figure 1. A fiber link of 1600 km

is composed using 40  40 km recirculating loop con-

sisting of 2  10 km SMF-based effective area enlarged

positive dispersion fiber, one 20 km dispersion slope

compensating fiber (SCF) and one erbium-doped fiber

amplifier (EDFA) repeater. This SMF-SCF configuration

allows dispersion flattening over the fiber span within

the loop with reduced intra-span dispersion excursion [6].

The dispersion, dispersion slope and effective area of the

SMF are 20 ps/nm/km, 0.06 ps/km/nm2 and 110 m2,

respectively. The dispersion and dispersion slope of the

SCF are the same as for the SMF but in the opposite sign

and the effective area is 30 m2. Each SMF has an aver-

age loss of 0.2 dB/km at around 1550 nm while the

SCF’s loss is 0.24 dB/km, thus the total loss in the loop

span is 8.8 dB. The EDFA is set to 8.8 dB gain to com-

pensate for the entire loop loss. The EDFA noise is 1.5

mW. Optical bandpass filters with ideal Gaussian curve

of 0.048 THz bandwidth are used at transmitter and re-

ceiver for fine filtration of the unwanted components and

to allow little guard-bands between the neighboring

channels. The received 40 Gbit/s signals are optically

time division demultiplexed back into 4  10 Gbit/s sig-

nals via two DEMUX stages using two clock recovery

circuits as seen in the setup diagram.

Figure 1. Experimental setup.

M. M. NAHAS

139

For evaluation, the simulator is set such that it pro-

duces performance results every 1 km transmission.

3. Results and Analysis

To study the performance of the system described, dif-

ferent peak power and pulsewidth (through full wave

half maximum, FWHM) values of the propagating 4  40

Gbit/s signals were examined against transmission dis-

tance. Commonly, the performance is evaluated via Q-

value or BER where good transmission should exhibit Q

≥ 6 or BER ≤ 10−9. Therefore, the optimum distance is

effectively the maximum distance that satisfies the above

condition. The simulation used peak power values in the

range between 1 - 10 mW and FWHM between 5 - 8 ps,

where outside these intervals the performance degrades

dramatically. This is explained as if the peak power was

too small, the system would be impaired by noise where

optical signal to noise ratio (OSNR) decreases over short

distance. In contrast, if the peak power was too high, the

system would be impaired by nonlinearities thus the sig-

nal distorts shortly, resulting in high BER. On the other

hand, if the pulsewidth was too small, the data signal

would loose some of its information and thus errors will

be counted upon transmission. If the pulsewidth was too

broad, polarization mode dispersion (PMD) would result

in inter-symbol interference (ISI) between the neighbor-

ing bits thus data will face considerable distortion over

short propagation distance. This argument leads us to

explore the best values among those in the intervals

mentioned above, at which signals are allowed to reach

their maximum possible propagation distance. For accu-

racy, the simulation was run four times for each test and

the results were based on average values. This was done

as amplifier noise is random thus the results slightly de-

viate every time the same test is run. Figure 2 shows the

major results obtained from this experiment. It presents

the maximum transmission distance obtained with Q ≥ 6

versus peak power for each 40 Gbit/s signal using dif-

ferent FWHM values.

It is obvious from Figure 2 that the maximum propa-

gation distance differs from one channel to another, and

also differs for different parameters. Apparently, the side

signals (Ch. 1 and Ch. 4) often performs better than the

FWHM = 5 ps

100

200

300

400

500

600

700

800

900

1000

01234567891011

Peak Power (mW)

Transmission Distance (km

)

Ch. 1

Ch. 2

Ch. 3

Ch. 4

FWHM = 6 ps

100

200

300

400

500

600

700

800

900

1000

0123456789101

Peak Power (mW)

Transmission Distance (km

)

Ch. 1

Ch. 2

Ch. 3

Ch. 4

(a) (b)

FWHM = 7 ps

100

200

300

400

500

600

700

800

900

1000

01234567891011

P eak P ower (mW)

Transmi ss ion Di st ance (k m)

Ch. 1

Ch. 2

Ch. 3

Ch. 4

FWHM = 8 ps

100

200

300

400

500

600

700

800

900

1000

01234567891011

Peak Power (mW)

Transmi ss i on Di stanc e (k m )

Ch. 1

Ch. 2

Ch. 3

Ch. 4

Figure 2. Transmission distance versus peak power for different values of FWHM: (a) 5 ps; (b) 6 ps; (c) 7 ps; and (d) 8 ps.

Channels’ definitions: Ch. 1 = 1554.4 nm; Ch. 2 = 1554.8 nm; Ch. 3 = 1555.2 nm; Ch. 4 = 1555.6 nm).

(

M. M. NAHAS

140

mid channels due to one side interaction thus less in-

ter-channel crosstalk caused by cross phase modulation

(XPM) and four wave mixing (FWM). Furthermore, Ch.

4 is still better than Ch. 1 due to better noise characteris-

tics within its band at the transmitter. This can be noticed

in Figure 3 that shows the spectra of the four channels in

the transmitter’s filter (note that Ch. 4 is the first channel

on the right).

To decide on the optimum parameters, the perform-

ance of the four signals was compared for different

FWHM, and the optimal transmission distance was de-

termined based on a comparison between the worst sig-

nals’ behaviors. In details, for FWHM = 5 ps shown in

(a), good overall performance was achieved for peak

power around 4 mW where the maximum distance of the

worst signal was 540 km (Ch. 3) although Ch. 4 reached

820 km. Comparing this with other graphs, for FWHM =

6, 7 and 8 ps as in (b), (c) and (d), respectively, good

performance was achieved at around 3 mW peak power.

This difference in optimum peak power is understood as

for 5 ps the signal lost little part of its power thus it

needed more power to hit the nonlinear window. How-

ever, for 6, 7 and 8 ps pulsewidths, the worst case was:

700 km (Ch. 2), 623 km (Ch. 3) and 580 km (Ch. 2),

respectively. As a result of comparing the worst cases,

the maximum transmission distance with Q ≥ 6 achieved

for this system can be 700 km and the optimum peak

power and pulsewidth are 3 mW and 6 ps, respectively.

At these particular values, a good compromise between

noise and nonlinear impairments has been attained, and

the pulses are broad enough to contain full information

and power of the data bits while do not overlap due to

PMD effect. Since the transmission results shown in Fig-

ure 2 based on Q-value assessment, it would have been

necessary to shows Q-value versus transmission distance

for the given parameters. However, as we are most inter-

ested in the optimum parameters, Figure 4 shows Q-

value evolution with distance for all channels using 3

mW peak power and 6 ps pulsewidth.

4. Conclusions

In this paper, we demonstrated simulation results for 50

GHz spaced 4  40 Gbit/s WDM signals transmission

using bandlimited RZ modulation format over SMF-

Based Effective Area Enlarged Positive Dispersion Fiber.

Transmission performance with BER ≤ 10−9 was suc-

cessfully achieved over 700 km using 3 mW peak power

and 6 ps pulsewidth for each data signal. The experiment

used optical filters with sharp transmission characteris-

tics in both transmitter and receiver.

5. Acknowledgements

The author would like to thank Marc Eberhard, who is a

lecturer in Electrical Engineering at Aston University

Figure 3. Signals’ spectra using Gaussian filter with 48 GHz bandwidth.

M. M. NAHAS

141

0100 200 300 400 500600 700 800 90010001100

Trans m i ssi on Distanc e (km )

Q-Val ue

Ch. 1

Ch. 2

Ch. 3

Ch. 4

Figure 4. Q-value versus propagation distance for all chan-

nels using peak power = 3 mW and FWHM = 6 ps. (Chan-

nels’ definitions: Ch. 1 = 1554.4 nm; Ch. 2 = 1554.8 nm; Ch.

3 = 1555.2 nm; Ch. 4 = 1555.6 nm).

(UK), for providing the author with his self-developed

XML-based simulation code.

6. References

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Anderson, Y. Cai, L. Li, A. N. Pilipetskii, D. G. Foursa,

W. W. Patterson, P. C. Corbett, A. J. Lucero and N. S.

Bergano, “Transmission of 40-Gb/s WDM Signals over

Transoceanic Distance Using Conventional NZ-DSF with

Receiver Dispersion Slope Compensation,” Journal of

Lightwave Technology, Vol. 24, No. 1, 2006, pp. 191-

200. doi:10.1109/JLT.2005.861138

[2] S. S. Lee, H.-W. Cho, S.-K. Lim, K.-M. Yoon, Y.-G. Lee,

and J. Ko, “Field Trial of 1.6 Tb/s (40 channels × 40 Gb/s)

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[5] R. Ludwig, U. Fieste, E. Dietrich, H. G. Weber, D. Breuer,

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