Theoretical study of all-optical RZ-OOK to NRZ-OOK
format conversion in uniform FBG for mixed
line-rate DWDM systems
Oskars Ozolins* and Vjaceslavs Bobrovs
Institute of Telecommunications, Riga Technical University, Azenes Street 14/16-500, Riga LV-1048, Latvia
*Corresponding author: oskars.ozolins@rtu.lv
Received January 30, 2015; accepted April 8, 2015; posted online May 11, 2015
In this work we study all-optical multi-channel return-to-zero (RZ)–on-off keying (OOK) to nonreturn-to-zero
(NRZ)–OOK format conversion in single uniform fiber Bragg grating (FBG) for mixed line-rate dense wave-
length-division multiplexing systems using mathematical simulations. Forty and 20 Gbit/s RZ–OOK signals
with 33% and 50% duty cycles are converted to NRZ–OOK signals in single uniform FBG with 21% reflectivity.
Impact of amplitude noise from FBG contrast profile on modulation format conversion efficiency is also
studied.
OCIS codes: 060.1155, 060.2340, 060.3735.
doi: 10.3788/COL201513.060603.
Fiber optic transmission systems (FOTSs) have been well-
established in last decades and have provided solutions for
every increasing demand for data amount in optical net-
works
[1]
. Therefore different modulation formats has been
used in FOTS for specific segments of optical networks
[2]
.
This leads to necessity to convert modulation formats
used in dense wavelength-division multiplexing (DWDM)
systems
[3]
. Different interfaces could be used to convert
modulation format for a specific channel. For example op-
tical to electrical-to-optical (OEO) conversion which
cause extra energy and time consumption
[3]
. In future, hy-
brid DWDM and optical time-division multiplexing trans-
mission systems will be a part of all-optical networks
[1,3]
.
Therefore all-optical modulation format conversion be-
tween return-to-zero (RZ)–on-off keying (OOK) and
nonreturn-to-zero (NRZ)–OOK becomes essential
[2,4,5]
.It
would increase a scalability and flexibility of the optical
network as well as reduced energy consumption
[6]
. Some
of the approaches for the all-optical modulation format
conversion have been demonstrated in past years. These
approaches could be classified as nonlinear and linear.
Modulation format conversion with nonlinear signal
processing include use of semiconductor optical amplifiers
(SOAs)/Mach–Zehnder interferometer (MZI)
[7]
, nano-
porous silicon waveguide
[8]
, nonlinear optical loop mirror
[9]
,
and stimulated Brillouin scattering
[10]
. Methods which in-
clude linear optical signal processing are mainly based on
the spectral shaping of the input optical signal. Some of
the earlier demonstrations included delay interferome-
ters/arrayed waveguide grating which now has been real-
ized also on integrated silicon-on-insulator technology
[11]
.
The large potential of the spectral shaping technique
could also be in capability to convert advanced modula-
tion formats from RZ to NRZ signal
[12]
. Another approach
is to use single microring resonator (MRR)
[13]
or even
cascaded MRRs
[14]
for simultaneous multi-channel RZ to
NRZ–OOK and differential phase-shift keying (DPSK)
modulation format conversion. With the increasing de-
mand for data amount, multi-channel operation seems to
be integral functionality for the modulation format
converters. RZ –OOK to NRZ–OOK modulation format
conversion with uniform fiber Bragg gratings (FBGs) so
far has been studied for a single-channel
[15–18]
and multi-
channel operations
[19]
for equal line-rate DWDM systems.
The various interfaces used in optical networks leads also
to mixed line-rate (MLR) FOTS. Therefore RZ–NRZ
modulation format conversion with single device for
MLR FOTS would be very attractive.
In this Letter, we study 33% and 50% duty cycle
RZ–OOK to NRZ–OOK modulation format conversion
with a single uniform FBG in seven-channel, 40 and
20 Gbit/s MLR DWDM system. Aspects of system and
device parameters, such as central wavelength deviation,
amplitude noise in uniform contrast profile, and deviation
are also taken in to account.
All-optical modulation format conversion with single
uniform FBG in MLR DWDM systems was studied using
commercial software Synopsys OptSim. The software
gives ability to design and simulate FOTS at the signal
propagation level. Synopsys OptSim employs method of
calculation that solves a complex set of differential equa-
tions, taking into account electrical and optical noise, and
linear and nonlinear effects
[15]
. The ability to use data from
external software was also employed. FBG design software
was used for synthesis of uniform FBG transfer function.
Transfer matrix method is employed in software to simu-
late different designs of FBG filters. It is applied to solve
the coupled mode equations and to obtain the spectral
response of the FBG filter. In this work three different
transfer functions were synthesized by given physical
parameters like length and index modulation with 21%
reflectivity value, and 0%, 10%, and 30% of amplitude
COL 13(6), 060603(2015) CHINESE OPTICS LETTERS June 10, 2015
1671-7694/2015/060603(5) 060603-1 © 2015 Chinese Optics Letters