Long-haul Coherent Optical OFDM Point-to-Point Transmission

using Optical Phase Conjugation

Thosaphon Jaemkarnjanaloha, Rachata Maneekut and Pasu Kaewplung

Department of Electrical Engineering, Faculty of Engineering, Chulalongkorn University, Pathumwan, Bangkok, Thailand

Keywords: Optical Fiber Transmission, Long-haul Transmission, Coherent Optical Orthogonal Frequency Division

Multiplexing (CO-OFDM), Optical Phase Conjugation (OPC), Superchannel, Polarization Division

Multiplexing (PDM), Coherent Detection, Nonlinearity, Kerr Effect.

Abstract: In this paper, we demonstrate the superchannel polarization division multiplexed coherent optical orthogonal

frequency-division Multiplexing (PDM-CO-OFDM) system employing midway optical phase conjugation

(OPC). The system is designed to show the optimum number of sub-carriers, amplifier spacing and the

maximum achievement reach at data rate 1Tb/s (10x100 Gb/s). The system is simulated with 10-WDM

superchannel at 50 GHz channel spacing. From the simulation results, PDM-CO-OFDM, with midway OPC

and the optimum system parameters, we can achieve the maximum reachable distance of 24,000 km at BER

4x10

-3

1 INTRODUCTION

Long-haul optical fiber transmissions with data rates

beyond Tb/s are next generation transmission systems

to support consumer applications such as video-on-

demand, cloud storage and social networking which

demand huge data load (Chandrasekhar and Liu,

2012). The performance of the optical transmission

system is limited by the waveform distortion induced

from signal attenuation, the fiber dispersion and the

nonlinear Kerr effect of optical fiber (Lowery et al.,

2006). The signal attenuation due to the ﬁber loss is

periodically compensated by using erbium-doped

ﬁber ampliﬁers (EDFA’s) as the optical ampliﬁer

gains. Moreover, we can mitigate the effect of the

dispersion and the nonlinear waveform by using

midway optical phase conjugation (OPC), since the

enhancement of phase noise through the interaction

between the fiber dispersion and nonlinear waveform

can be sufficiently compensated by the OPC.

Conventional digital signal processing (DSP) in

coherent optical fiber communication scheme can

compensate fiber dispersion. The OPC conjugates the

signal phase after transmission in the first section of

fiber link in order to achieve a net cancellation of the

nonlinear waveform by using the nonlinearity

generated in the second section of the fiber link (Le et

al., 2015). We can also use the advance modulation

formats to increase the bandwidth efficiency. One of

the modulation methods that can yield the highest

bandwidth efficiency in optical fiber transmission is

the orthogonal frequency division multiplexing

(OFDM). OFDM is a special form of multicarrier

modulation where a single data stream is transmitted

over a large number of lower rate orthogonal

subcarriers (Shieh et al., 2008). Comparing single-

carrier transmission with the coherent multiple-

carriers transmission, coherent optical orthogonal

frequency division multiplexing (CO-OFDM) has the

advantage of a well-defined power spectrum that

enables superchannel transmission achieving an

ultra-high spectral efficiency (Wei-Ren et al., 2011).

This modulation scheme provides the orthogonal

relation among subcarrier channels with the sufficient

tolerance to fiber dispersion comparing with the on-

off keying (OOK) modulation format. In addition,

CO-OFDM utilises available fiber bandwidth very

efficiency since each subcarrier can be placed with

channel spacing equivalent to the Nyquist frequency.

These closely packed carriers that travel from the

same origin to the same destination in a WDM system

collectively form a superchannel. Thus, Polarization

Division Multiplexed (PDM) CO-OFDM

superchannel is an efficiency system for long-haul

high-capacity optical transmission system.

In this paper, we propose the use of midway OPC

to improve the performance of long-distance CO-

Jaemkarnjanaloha, T., Maneekut, R. and Kaewplung, P.

Long-haul Coherent Optical OFDM Point-to-Point Transmission using Optical Phase Conjugation.

DOI: 10.5220/0005963700430047

In Proceedings of the 13th International Joint Conference on e-Business and Telecommunications (ICETE 2016) - Volume 3: OPTICS, pages 43-47

ISBN: 978-989-758-196-0

OFDM transmission system by employing midway

OPC at data rates of 10x100 Gb/s. Then, we analyse

the system parameters. For example, we use computer

simulation to analyse the number of subcarriers, the

amplifier spacing that affects the transmission

performance over c-band.

2 COHERENT OPTICAL OFDM

TRANSMISSION SYSTEM

EMPLOYING OPC

WDM systems classification based on the channel

spacing or bandwidth allocation. Figure 1 shows the

proposed high spectral efficiency systems using

advanced modulation formats with coherent detection

as CO-OFDM.

Figure 1: Configuration of Long-distance CO-OFDM

transmission system employing midway OPC.

We simulate at 1-Tb (10x100-Gb/s PDM-CO-

OFDM). The CO-OFDM condition for forming a

superchannel can be shown as follow, each subcarrier

must be placed with channel spacing equivalent to the

Nyquist frequency to form a superchannel. For WDM

superchannel systems detection, wavelength channels

are first de-multiplexed before being received. The

modulated carriers inside each superchannel are

closely packed to be separated by WDM filters as

coherent detection enables banded-detection of a

superchannel.

Figure 2 shows the spectrum of wavelength-

division-multiplexed (WDM) N-channels with CO-

OFDM modulation in optical fiber transmission and

Figure 3 shows the zoomed-in optical spectrum for

each wavelength channel. We use the bandwidth of

the first one WDM channel to set channel spacing of

each wavelength channel (Shieh, 2011).

Figure 2: The optical spectrum for N wavelength-division-

multiplexed CO-OFDM channels.

Figure 3: The zoomed-in optical spectrum for one WDM

channel.

Midway optical phase conjugation was used to

mitigate the nonlinear phase noise (Lorattanasane and

Kikuchi, 1997). As shown in Figure 4, a long-distance

transmission system employing midway optical

phase conjugation (OPC) where the two repeated

section fiber spans are sandwiched with OPC. OPC

uses to compensate for fiber nonlinearity. By

conjugating the signal in the first half of the system

near the midway of the link, the fiber nonlinearity that

generated in the second half of the system mitigate

the fiber nonlinearity generated in the first half. It

shows that by using midway optical phase

conjugation, the self-phase modulation (SPM) is

totally removed and phase noise can be significantly

reduced (Watanabe and Shirasaki, 1996); (Jansen et

al., 2005).

Figure 4: Long-distance transmission system employing

midway optical phase conjugation (OPC).

OPTICS 2016 - International Conference on Optical Communication Systems

(a)

(b)

Figure 5: Conceptual diagram for complete CO-OFDM

system (a) T

, and (b) R

S/P: serial-to-parallel; (I)DFT: (inverse) discrete Fourier

transform; CP: cyclic prefix; D/A: digital-to-analog; LPF:

lowpass filter; MZM: Mach-Zehnder modulator; A/D:

analog-to-digital; PD: photodiode; LD: laser diode.

A conceptual diagram of an OFDM transmitter is

shown in Figure 5. The function of the OFDM

transmitter is mapping the data bits into each OFDM

symbol by subcarriers symbol mapper. Next, generate

the time series by inverse discrete Fourier transform

(IDFT) that used in OFDM modulation. It can be

represented like a digital filter that naturally produces

a well-confined square-like signal spectrum.

Afterwards including cyclic prefix then converted

back to serial signal at digital to analog conversion

(D/A). Next, upconvert to an appropriate RF

frequency to be fed into an optical upconverter. The

function of the optical upconverter is to linearly shift

the OFDM spectrum from the RF domain to the

optical domain. After that used I/Q modulator to form

CO-OFDM signal shows an approach using two

optical Mach-Zehnder modulator and subsequently

signals transmit through the single-mode fiber (SMF)

ITU-T G.655.D.

Figure 6: Conceptual diagram for PDM-CO-OFDM

system.

To generate a PDM-CO-OFDM signal, we use

CWlaser connect to polarization Splitter to split laser

into both axis of CO-OFDM signal transmission then

transmit to CO-OFDM Tx (X-axis) and CO-OFDM

Tx (Y-axis) afterward use polarization combiner to

combine the signal from both axis together.

According to Figure 6, we use computer

simulation for the PDM-CO-OFDM transmission

system by Optical simulation software. The PDM-

CO-OFDM signal is generated by continuous-wave

(CW) laser then transmit into both axis of CO-OFDM

Tx then split by polarization splitter. After that the

signals after CO-OFDM Tx are combined again by

polarization combiner .The data rate is 100 Gb/s per

channel x 10 channels that mean data rate is 50 Gb/s

for one polarization axis, sequence length is 8192

bits. The channel spacing is 50 GHz. The optical

spectrum for 10 WDM CO-OFDM channels is

showed in Figure 7 which is the optical spectrum of

signal system after WDM superchannel multiplexing.

Figure 7: The optical spectrum for 10 WDM CO-OFDM

channels.

For optical fiber link, we use ITU-T G.655.D

single mode optical fiber ITU-T standard. The fiber is

connected to EDFA to compensate signal attenuation

from fiber loss. We use midway OPC between the

fiber repeated sections. At the receiver, the optical

coherent detection converts CO-OFDM signal to

electrical signal followed by four balance

photodetector with thermal noise at 18.14 pA/ and

dark current at 5 nA. Then down-conversion

modulates RF at 50 GHz.

3 SIMULATION RESULT

We can mitigate the effect of the dispersion and the

nonlinear waveform by using midway OPC, since the

enhancement of phase noise through the interaction

between the fiber dispersion and nonlinear waveform

distortion can be sufficiently compensated by the

OPC as shown in Figure 8. The constellation of PDM-

CO-OFDM transmission system employing OPC (b)

after 350 km transmission is recovered with four

Long-haul Coherent Optical OFDM Point-to-Point Transmission using Optical Phase Conjugation

distinct clusters compare to the residual noises

spreading constellation of PDM-CO-OFDM

transmission system (a) point are from nonlinearity of

optical fiber. In conclusion, the distorted waveform

due to dispersion in the first fiber section is

compensated by the OPC and the following

propagation through the second fiber section has the

same dispersive characteristics.

(a) (b)

Figure 8: Received constellation diagram after 350 km

transmission. The PDM-CO-OFDM transmission system

(a) without OPC and (b) with OPC.

Figure 9: Relation between log(BER) and receiver power

[dBm] for various number of sub-carriers at 128, 256, 512,

1024 and 2048 respectively.

Figure 9 shows the relation between log(BER)

and receiver power for all 10 wavelength-division-

multiplexed PDM-CO-OFDM channels. The

simulation results indicate that the BER of the signals

from 512 sub-carriers has the lowest BER. The

number of sub-carriers affects the system

performance. This is due to the phase noise effects.

The other is loss of orthogonality or inter-symbol

interference (ISI) that are error terms which affect the

demultiplexed signal and a common phase rotation

while a greater number of sub-carriers can cause a

better protection in preparation for multipath delay

spread. In contrast phase noise effects are worsened

in this situation.

Figure 10: Maximum reach of 10-WDM channels PDM-

CO-OFDM 4-QAM systems at 100 Gb/s per channel for

each amplifier spacing.

We demonstrate maximum reach of the system for

various values of the amplifier spacing. At BER

4x10

-3

, the transmission system at amplifier spacing

80 km was chosen to match the PDM-CO-OFDM 4-

QAM system. To characterize the linear and

nonlinear transmission performance of 10-WDM

channels PDM-CO-OFDM 4-QAM system, the

maximum reach is displayed in Figure 10. The results

show that the maximum reachable distance is 24,000

km at BER 4x10

-3

4 CONCLUSION

We demonstrated the superchannel PDM-CO-OFDM

system employing midway OPC by using computer

simulations in long-distance transmission systems

using EDFA’s at data rate 1Tb/s (10x100 Gb/s). From

the simulation results, the following points can be

concluded. For the PDM-CO-OFDM system

operating with 4-QAM at 1 Tb/s, the bandwidth of the

optical signal is 50GHz. The PDM-CO-OFDM 4-

QAM signal also has a high tolerance to nonlinearity.

The advantage is increased spectral efficiency. The

results have also confirmed that midway OPC can

compensate for the nonlinearity and dispersive

waveform distortion. The simulation results indicate

that the BER of the PDM-CO-OFDM signals from

512 sub-carriers have the lowest BER. The optimum

input power corresponds to the maximum reach of

24,000 km for PDM-CO-OFDM.

OPTICS 2016 - International Conference on Optical Communication Systems

ACKNOWLEDGEMENTS

Obtaining a Master’s is always a challenging task as

the unexpected can frustrate our initial enthusiasm

and dampen our quest to know more about the

unknown world. As I take a walk down memory lane,

there are many who have made this journey

meaningful and purposeful. Firstly, I would like to

thank my dissertation advisor, Asst. Prof. Pasu

Kaewplung, Ph.D., for his guidance and support over

the past 2 years. His encouragement during my downs

has made the fulfilment of this dissertation more

bearable and achievable. I would like to thank

Mr.Rachata Maneekut, for his generous help and

numerous advices. I have learned from them not only

the knowledge but also the invaluable methodologies

to solve problems. I would also like to thank my

friends from the Master’s group for sharing our woes

and spurring one another to complete our search to

know more about the unknown.

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Long-haul Coherent Optical OFDM Point-to-Point Transmission using Optical Phase Conjugation