COL 10(9), 091902(2012) CHINESE OPTICS LETTERS September 10, 2012
Design of ultrafast all-optical PolSK DMUX based on
semiconductor optical amplifiers
Hassan Kaatuzian
∗∗
and Hamed Ahmadi
∗
Photonics Research Lab, Electrical Engineering Department, Amirkabir University of Technology, Tehran-15914, Iran
∗
Corresponding author: ha.ahmadi@aut.ac.ir;
∗∗
corresponding author: hsnkato@aut.ac.ir
Received February 28, 2012; accepted Ap ril 17, 2012; posted online August 3, 2012
A novel ultrahigh-speed all-optical demultiplexer (DMUX) with polarization-shift-key ing (PolSK) modula-
tion input signals is proposed. This design is based on four-wave mixing (FWM) in a semiconductor optical
amplifier (SOA). For analyzing each amplifier, we use finite-difference method (FDM) based on solution
of the traveling wave coupled equations. Using numerical simulation, th e all-optical DMUX is theoretically
realized at 40 Gb/s. We also study the relation between optical confinement factor and thickness of active
layer of the SOA section successfully, and investigate the increasing effect of confinement factor on the
DMUX optical output power. With this work, t he confinement factor is increased from 0.3 to 0.48, and
as a result, the output power approximately twice of its initial value is achieved. Moreover, the effects
of polarization dependence of SOA on the output performance of all-optical DMUX for PolSK signal are
theoretically investigated in detail.
OCIS codes: 190.4380, 230.3750, 230.4480, 230.5440.
doi: 10.3788/COL201210.091902.
An all-optical demultiplexer (DMUX) is a necessary
element fo r optical signal processing in future ultrahigh-
sp e ed optical networks. In optical communication sys-
tems, one of the important devices is the all-optical log ic
gate or all-optical DMUX
[1,2]
. Various designs of all-
optical logic gates have been created and tested, and one
of the best results is the setup using a semiconductor
optical amplifier (SOA). SOAs ar e k nown for their non-
linear o ptical effects, such as fo ur-wave mixing (FWM).
In SOAs, FWM has been used as a technique for perform-
ing wavelength conversion and ultrahigh-speed response
for wavelength division multiplexing (WDM) networks.
FWM can also be used for all-optica l logic gates and
optical DMUX in high-speed transmission systems
[3]
.
In recent years, polarizatio n-shift-keying (PolSK) mod-
ulation, which works according to the polarization state
of the light wave, has been greatly investigated due to
its unique advantages
[4]
. PolSK modulation can become
one of the new modulation methods for future optical
communication systems. In past few years, SOA has
been used to implement all-optical logic gates
[1,4,5]
, both
exp erimentally and theoretically. In the present study,
we investigate the FWM effect for all-optical DMUX
in SOA with PolSK modulation signals. For analyzing
this model, we use the FDM method to solve traveling
wave-coupled equations numerically
[6]
. In the following
sections, we review the operational principle, theoretical
model, simulation and results, and conclusion regarding
this s cheme.
The suggested quantum well SOA is a 1.55-µm InP-
In
1−x
Ga
x
As
y
P
1−y
device (see Fig. 1). Here, x and
y are the molar fractions of gallium and arsenide,
respectively, in the ac tive layer. We use the InP-
In
0.58
Ga
0.42
As
0.89
P
0.11
device for our simulation because
of good lattice matching. This device is specifically
appropriate for amplifiers in the 1 550 nm wavelength,
which has very low optical loss for silica optical fibers .
Pertinent geometrical and material parameters for the
device under consideration were reported in Ref. [7].
In fact, the gain of a SOA depends on the polarization
state of the input signals. This dependency is due to
a number of factors, including the waveguide structure,
the pola rization-dependent nature of the antireflection
coatings, and the gain materia l. The amplifier waveguide
is characterized by two mutually orthogonal po larization
modes: the TE and TM modes. When two optical pulses
with different central frequencies, f
1
and f
2
, are injected
into the SOA simultaneously, the FWM signal is gener-
ated in the SOA at a frequency of 2f
1
−f
[8]
2
. Nevertheless,
producing these signals depend on the polarization state.
In other words, if two input signals have one polarization
state, the phase conjugate signal will be produced in the
output; however, if the polarization states of two input
signals are perpendicula r, no signal will be produced in
the output.
Figure 2 shows the schematic model of the ultrahigh-
sp e ed all-optical DMUX. In this diagram, the FWM
effects in the fo ur SOAs are used for PolSK modulatio n
signals. In this modulation, logic “1” is a given linear
polarization state, and the “0” is the orthogonal state.
If polariz ation states of two input sig nals are similar, the
phase conjugate signal will be produced in the output
[8]
.
Figure 2 shows that the two input signals are s plit by
the red optical couplers (OC). Thus, the sig nals past
Fig. 1. Schematic crossection of homogenous buried ridge
stripe S OA.
1671-7694/2012/091902(5) 091902-1
c
2012 Chinese Optics Letters