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IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 42, NO. 8, AUGUST 2006747

All-Optical Logic XOR Gate at 80 Gb/s

Using SOA-MZI-DI

Hongzhi Sun, Qiang Wang, Hao Dong, Zhe Chen, Niloy K. Dutta, Fellow, IEEE, James Jaques, and A. B. Piccirilli

Abstract—All-optical XOR operation has been demonstrated

using a semiconductor optical amplifier Mach–Zehnder interfer-

ometer (SOA-MZI) and delayed interferometer (DI) at 80 Gb/s.

The DI is based on a polarization maintaining loop (PML) mirror.

The results show using the PML-DI in series with the SOA-MZI

improves the pulse quality of the XOR result.

Index Terms—Delayed interferometer (DI), Mach–Zehnder in-

terferometer (MZI), photonic logic, polarization maintaining loop

(PML), semiconductor optical amplifier (SOA).

I. INTRODUCTION

F

switching, signal regeneration, addressing, header recognition,

data encoding, encryption, etc. All-optical signal processing

functionalities are important for some applications where

electrooptical conversion is not desired. Recently, optical logic

demonstrations using various schemes have been reported

[1]–[9]. For example, all optical XOR gate using semiconductor

optical amplifier (SOA)-based Mach–Zehnder interferometer

(MZI) [1], and NOR gate using cascaded SOAs [2].

In the case of XOR operation, SOA-based MZI is generally

used because it is compact and stable. However, conventional

techniquesusingSOA-MZIarelimitedbythegainandphasere-

sponsetimeofSOA(i.e.,50–200ps),whichlimitstheoperating

speed to

20 Gb s. Various differential schemes have been

proposed and realized [10]–[13], [18] to overcome the speed

limitation. A scheme which utilizes a delayed interferometer

(DI) after SOA-MZI has been proposed and analyzed theoret-

ically [13].

In this paper, XOR function is achieved experimentally using

thecrossphasemodulationintheSOA-MZI-DIatdatarateof80

Gb/s.Totheauthor’sknowledge,thishasbeenthehighestspeed

foralloptical XORoperationusingSOA-MZI-DI.Apolarization

maintainingloop(PML)mirroroperatesasaDIandfunctionsas

an “optical gate.” The quality of XOR output signals is improved

using a DI after the SOA-MZI. Theoretical verification of the

scheme has been carried out using a rate equation model.

ThepolarizationmaintainingloopmirrorthatweusedasaDI

in the experiment could be replaced by a InGaAsP/InP based or

OR high-speed optical communication networks, logic

operation is important for networking functions, such as

Manuscript received November 22, 2005; revised March 21, 2006.

H. Sun, H. Dong, Z. Chen, and N. K. Dutta are with the Department of

Physics, University Of Connecticut, Storrs, CT 06269 USA (e-mail: sun@phys.

uconn.edu; dong@phys.uconn.edu; chen@phys.uconn.edu; nkd@phys.uconn.

edu).

Q.WangiswiththeBeckmanLaserInstitute,UniversityofCalifornia,Irvine,

CA 92664 USA (e-mail: wq06269@yahoo.com).

J.JaquesandA.B.PiccirilliarewithLucentTechnologies,BellLaboratories,

Murray Hill, NJ 07974-0636 USA (e-mail: jjaques@lucent.com).

Digital Object Identifier 10.1109/JQE.2006.878184

a SiO -based waveguide structure. Both types of planar wave-

guide-based DI are likely to result in improved stability. The

InGaAsP/InP based DI could in principle be integrated with

the SOA-MZI devices. However, polarization maintaining loop

mirrors with different gate widths can be easily produced in the

laboratorysimplybychangingthelengthofthefiberintheloop.

II. OPERATING PRINCIPLES

The schematic diagram of the principle is shown in Fig. 1.

The signals 1 and 2 and a clockwise (CW) control signal (which

would carry the information of XOR output) are injected into the

SOA-MZI at port A, B, and C, respectively, the data signals 1

(wavelength

)and 2(wavelength

in the CW signal (wavelength

in the SOA. The wavelength

different from

and, and

At Port D, the modulated CW signal from the two arms of MZI

interfere obeying the following formula:

)willinduce a phaseshift

) via cross-phase modulation

must be chosen so that it is

, andneed not be different.

(1)

where

are the gain in two arms of SOA-MZI, and

difference of the CW signal at wavelength of

A XOR gate using SOA-MZI has been built [1], [18]. However,

for nondifferential operation, the XOR performance is limited to

20 Gb/s at this stage due to the long carrier life time in con-

ventional SOA (i.e.,

ps). We added a PML mirror as a

delayed intererometer after MZI [7] to improve the high-speed

performance. The output of the MZI is injected into the PML

where at the input port it is split into a clockwise component

and a counter-clockwise (CCW) component. The input signal is

polarized along either the fast or the slow axis of the polariza-

tion maintaining fiber in the loop. Since the optical path is bire-

fringent, the phase difference between the clockwise and CCW

pulses with polarization along the fast and slow axis will ac-

cumulate and will have a differential phase delay of

(where

is the wave vector in the vacuum,

ence in index seen by the light propagating along the fast and

slow axis, and,

is the fiber length in the PML) when the sig-

nals arrive back at the input splitter.

The CW and CCW components traverses the in-loop polar-

ization controller (PC in Fig. 1) once and arrives at the coupler

with the same polarization where they interfere. Fig. 2 shows

the output result of an erbium-doped fiber amplifier (EDFA)

ASEinputthatgoesthroughthePMLinfrequencydomain.The

is the input optical power of CW signal,and

is the phase

in two arms.

,

is the differ-

0018-9197/$20.00 © 2006 IEEE

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748IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 42, NO. 8, AUGUST 2006

Fig. 1. Schematic of the differential phase modulation scheme using MZI-DI at 80 GHz. PC-polarization controller, 50/50 3-dB coupler.

Fig. 2. PML transmission spectrum.

transmissionisminimumatwavelengthswherethephasediffer-

ence between clockwise and counterclockwise component is

while it is maximum at phase differences of 0 or

XOR scheme, the PML transmission peak spacing is designed to

be 0.64 nm.

The CCW and clockwise CW signals have a relative delay

when they arrive at the input coupler. The maximum achievable

relativedelayis3ps(forthefiberlengthchosenhere).Themax-

imumrelativedelayisexpectedforthecasewhenCCWandCW

signal goes through the PMF along its fast and slow axis. The

delayed optical signals will interfere at the PML coupler with

the following resultant intensity:

. In this

(2)

The output of the MZI at high data rate will be reshaped by this

PML (DI) and gives better result of XOR. The PML behaves as

an optical gate (

3 ps wide) for the output of the MZI and it

reshapes the pulsed output.

III. EXPERIMENT

The experimentalalset up for differential XOR operation at80

Gb/s is shown in Fig. 3.

A20-GHzpulsetrainisproducedbyrationalharmonicmode-

lock laser [11] which is driven using a synthesized signal gen-

erator at 10 GHz. The wavelength is 1557 nm. At 20 GHz, the

pulse width is about 5 ps. The pulses are further amplified and

compressed to

ps using 4-km dispersion shifted fiber and

10-m standard single mode fiber [10]. The pulses are combined

with a passive multiplexer (delay 12.5 ps) to make “1100” at

80 Gbit/s as shown in Fig. 4(a). Then, the pulse train is split

into two parts. The first signal (signal A) is sent to port 1 of

SOA-MZI. The other is delayed by 12.5 ps to make “0110”

[Fig. 4(b)] and injected into port 2 (signal B) of SOA-MZI. An-

other CW signal (signal C) at a different wavelength is injected

intoport3.Atallinputports,polarizationcontrollersareusedto

adjustthepolarizationofthesignal.ThesignalCcarriesthe XOR

result.Itisthenextractedbythebandpassfilter.Fig.5showsthe

result for 80-GHz operation with and without PML-DI after the

MZI.

The ? factor of XOR output are readily calculated by

(3)

where

and

mission systems, a

values are calculated in the next section.

andare average power of pulses 1 and 0, and

arenoisepowerofpulses1and0,respectively.Fortrans-

provides a bit-error rate of

,

.

IV. SIMULATION

XORgateoperationhasbeenanalyzedbyanumericalsolution

of the SOA rate equations. The time dependent gain of the SOA

satisfies the temporal gain rate equations [12], [13]

(4)

(5)

(6)

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SUN et al.: ALL-OPTICAL LOGIC XOR GATE AT 80 GB/S USING SOA-MZI-DI749

Fig. 3. Experimental setup for SOA-MZI-DI XOR.

Fig. 4. Input data signal trace in two arms of MZI at 80 Gb/s. (a) Patterned as

“11001100.” (b) Patterned as “01100110.”

where

and

lifetime,

is the saturation energy of the SOA.

input optical intensity inside the SOA and

the

-factor values for carrier recombination, carrier heating,

and spectral hole burning, respectively. Equations (4) and (5)

account for the intraband carrier dynamics (i.e., spectral hole

burning and carrier heating effects). Here, we assume the data

stream pulses to be Gaussian pulses. So

is a integral of optical gain over the length of SOA

equals the sum of, , and

is the unsaturated power gain and

. is the carrier

is the instantaneous

,, are

(7)

where

data in data stream A and B,

is the energy of a single pulse,represents

or 0. To simulate the

XOR gate performance, we assume both input signals are of RZ

pseudorandom bit sequence.

The carrier density induced phase change is described by

(8)

where

),

SOA parameters are as follows:

ps,

is the linear linewidth enhancement factor (i.e.,

is the carrier heating alpha factor(i.e.,), other

mW,

fs,

dB,

fs, ps,

ps.

Fig. 6 shows the calculated XOR results using the above

equations, signal A has (01100110) pattern, and signal B has

(11001100) pattern. Signals A and B are on the left figure and

the XOR output is on the right.

Fig.7illustratesthe XORoperationwithoutPML-DIusingthe

aboveequations.SignalsAandBareof

patterns. In the simulation, the input powers of the two signals

are equal, and the saturated gain for the two wavelengths are

alsoequal.Fig.7(b)showsthesimulatedresultof XORoperation

with PML-DI. An undesired side-pulse emerges in this scheme.

This phenomenon is a result of carrier recovery processes in

SOA [13]. At the receiving end of XOR, the side pulse can be

filtered out using a low-pass filter. The calculated

filter is larger with DI than without DI. We believe the operating

speedlimitis

GbsforthebulkactiveSOAgainandphase

response parameters used here. For MZI utilizing fast SOA, i.e.,

SOA with fast gain and phase recovery times, (such as quantum

dot SOA), the operating speed is

According to the experiment results in Fig. 5, the scheme

with MZI followed by DI produces signals with higher extinc-

tion ratio than using MZI alone. The simulated XOR output

using MZI followed by DI and MZI alone are shown in Figs. 6

and 7. The simulation also shows a performance improvement

(higher

) if the MZI is followed by a DI. The simulation

results for pseudorandom pulses suggest

pseudorandombit

value after

250 Gbs [19].

with DI.

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750IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 42, NO. 8, AUGUST 2006

Fig. 5.

XOR output trace. (a) Scheme without PML-DI. (b) After PML DI.

Fig. 6. (a)Input 80-Gb/s patterns. (b) Comparisonof XOR outputbefore and after PML-DI. Signals A andB are on the left figure andthe XOR outputis on the right.

Fig. 7. (a) Simulated XOR eye-diagram with MZI alone (without PML-DI) with input 80 Gb/s pseudorandom pattern. (b) Eye-diagram with MZI followed by

PML-DI after lowpass filter (the inset shows side pulse before filter).

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SUN et al.: ALL-OPTICAL LOGIC XOR GATE AT 80 GB/S USING SOA-MZI-DI751

V. SUMMARY

An all-optical XOR gate at 80 Gb/s has been demonstrated

based on a semiconductor optical amplifier Mach–Zehnder in-

terferometer and DI device. A polarization maintaining loop

mirror is used as a delayed interferometer. We investigated the

performanceof XORoperationusingnumericalsimulations.The

quality of the XOR result is improved using a DI after the MZI.

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Nor-

Hongzhi Sun received the M.S. degree in physics from the University of Con-

necticut, Storrs, where he is currently pursuing the Ph.D. degree.

He has published several papers and given a couple of presentations in inter-

national conventions.

Qiang Wang received the Ph.D. degree in physics from the University of Con-

necticut, Storrs.

He is currently with the Beckman Laser Institute, University of California,

Irvine. He has published several papers and given several presentations in inter-

national conventions.

Hao Dong received the M.S. degree in physics from the University of Con-

necticut, Storrs, where he is currently pursuing the Ph.D. degree.

He has published several papers and given a couple of presentations in inter-

national conventions.

Zhe Chen is currently pursuing the Ph.D. degree at the University of Con-

necticut, Storrs.

NiloyK.Dutta(M’82–SM’84–F’90)receivedthe Ph.D.degreein physicsfrom

Cornell University, Ithaca, NY.

He is a Professor of Physics with the University of Connecticut, Storrs. He

has published many papers in the area of semiconductor lasers, semiconductor

amplifiers, high-power lasers, high-speed transmission, and photonic logic.

Dr. Dutta is a Fellow of the Optical Society of America and SPIE and is a

member of the Connecticut Academy of Science and Engineering.

JamesJaquesreceivedthePh.D.degreeinphysicsfromtheUniversityofNotre

Dame, South Bend, IN.

He is a Member of Technical Staff with Lucent Technologies, Bell Laborato-

ries, Murray Hill, NJ. He has published many papers in the area of high-power

lasers, fiber transmission, and photonic logic.

A. B. Piccirilli received the M.S. degree in materials science from the Stevens

Institute of Technology, Hoboken, NJ.

He is a Technical Manager with Lucent Technologies, Bell Laboratories,

Murray Hill, NJ. He has published many papers in the area of semiconductor

amplifiers, high-power lasers, high-speed transmission, and photonic logic.