COL 11(3), 030604(2013) CHINESE OPTICS LETTERS March 10, 2013
All-optical format conversion of NRZ-OOK to QPSK and
16QAM signals via XPM in a SOA-MZI
Yueying Zhan (
===
)
∗
, Min Zhang (
ÜÜÜ
¬¬¬
), Mintao Liu (
444
²²²
777
),
Lei Liu (
444
[[[
), and Xue Chen (
ÈÈÈ
)
State Key Laboratory of Information Photonics and Optical Communications,
Beijing University of Posts and Telecommunications, Beijing 100876, China
∗
Corresponding author: zhanyueying@gmail.com
Received September 4, 2012; accepted November 16, 2012; posted online February 28, 2013
This letter proposes a scheme for the format conversion of on-off keying (OOK) signal to quadrature
phase-shift keying (QPSK) and 16-ary quadrature amplitude modulation (16QAM) signals via cross-phase
modulation (XPM) in a semiconductor optical amplifier (SOA). Theoretical and experimental analyses
of the format conversion scheme are conducted to validate its feasibility. The phase changing is obtained
because of the XPM in the SOA. The QPSK and 16QAM signals are converted from the OOK signal. The
performance of the 10 Gb/s format conversion system is evaluated and discussed. The receiver sensitivities
of the converted QPSK and 16QAM signals after detection are –27.25 and –23.5 dBm, respectively, at a
bit error rate (BER) of 10
−9
.
OCIS codes: 060.2330, 230.4480, 070.4340.
doi: 10.3788/COL201311.030604.
All-optical signal processing is a promising technology for
future optical transparent networks because it helps re-
duce implementation costs a nd removes the bottleneck
in ultrafast signal processing without requiring costly
optical-electrical- optical equipment
[1,2]
. Optical format
convers ion, one of the key optical s ignal processing tech-
nologies, has been extensively studied. Researchers have
mainly focused on the conversion b e tween conventional
on-off keying (OOK) and binary phase shift ke ying
(BPSK)
[3−6]
. By co ntrast, the multiple-OOK to M -ary
phase-shift keying and/or quadrature amplitude modu-
lation (QAM) conversion is rarely demonstrated. The
OOK to BPSK conversion can be conducted in commer-
cially available s e miconductor optical amplifier (SOA)
because of the simplicity of the BPSK format. However,
the conve rsion of multiple OO K signals to higher order
constellation signa ls is more difficult to achieve and re-
quires a more flexible device approach than that of a
single SOA. Based on previous reports, s chemes are usu-
ally complex and cos tly, such as cascaded Mach–Zehnder
modulators (MZMs) followed by a phase modulator
[7]
,
dual-parallel MZM
[8]
, dual-drive MZM
[9]
, and parallel
SOA Mach-Zehnder interferometer (MZI)
[10]
.
In this letter, a novel scheme for the fo rmat conve rsion
of nonreturn to zero (NRZ)-OOK signal to QPSK or 16-
ary QAM (16QAM) signal via cross-phase modulation
(XPM) in a SOA based MZI (SOA-MZI) is proposed and
verified. The return to zero (RZ) pulse and dual NRZ-
OOK signals are injected into a SOA, and the quadra-
ture phase-shift keying (QPSK) signal is converted from
an OOK signal via the XPM in the SOA. The dual NRZ-
OOK signals are then injected into the SOAs located at
the upper and lower MZI arm. Therefore, the dual QPSK
signal is conver ted at the lower and upper arm via the
XPM in the SOA-MZI. The 16QAM signal can then be
converted by coupling the double QPSK signa ls. The
scheme is simpler, more compact, and cos t-efficient than
traditional schemes. Our format conversion scheme is
entirely in the optical domain a nd reduces the unstable
factors in the electrical domain. The feasibility of this
technique is evaluated through theore tical and experi-
mental a nalyses.
Figure 1 shows the theoretical analysis of the format
convers ion based on the XPM effect in a SOA
[11]
. Two
NRZ-OOK signa ls with wavelengths λ
1
and λ
2
and a
RZ clock pulse signal with wavelength λ
3
were injected
into the SOA. The OOK signals modulate the SOA gain
by depleting the carriers, thereby changing the refra ctive
index and phase of the RZ clock pulse signal. The SOA
was divided into a number of smaller sections to analyze
the carrier change in the SOA. The rate equation of the
carrier density is given by
[11]
dN
i
dt
=
J
qd
−
N
i
τ
i
−
X
x=1,2,3
g
(x)
m
i
I
(x)
i
E
(x)
−g
m
p
S
t
i
, (1)
where N is the carrier dens ity, J is the injection current
density, d is the active layer thickness, τ is the c arrier
life time, g
m
is the material ga in, i is the ith section of
the SOA, index x refers to the different optical input
(NRZ-OOK1 sig nal, NRZ-OOK2 signal, and RZ clock
pulse signal), g
mp
is the value at peak ga in wavelength,
S
t
i
is the average amplified spontaneous emission in sec-
tion i, E is the photon energy, and I is the injection light
intensity.
The nonlinear phase change, arising from the carrier
density-induced changes in the refractive index, is given
by
[10,11]
φ
i
=
2πL
λ
ω
N
r
+ Γn
p
dN
dn
+
2πL
i
(n
i
− n
0
)
λ
ω
dN
dn
=
π∆Lg
m
i
2a
1
λ
ω
∆n
N
, (2)
where Γ is the optical confinement factor, L
i
is the cavity
1671-7694/2013/030604(4) 030604-1
c
2013 Chinese Optics Letters