Conference Paper

Experimental Study of a Phase Modulator Using an Active Interferometric Device

Inst. de Telecomun., Campus Univ. de Santiago, Aveiro, Portugal
DOI: 10.1109/MELCON.2010.5476364 Conference: MELECON 2010 - 2010 15th IEEE Mediterranean Electrotechnical Conference
Source: IEEE Xplore
ABSTRACT
A novel architecture for an optical phase modulator is presented and experimentally demonstrated. This approach relies on a commercially available integrated Mach-Zehnder interferometer structure with Semiconductor Optical Amplifiers (MZI-SOA) and it is based in cross-phase modulation effect (XPM). The feasibility of the proposed optical phase modulator is experimentally investigated using different scenarios of input power and bit rates.

Full-text

Available from: Rogerio Dionisio
Experimental Study of a Phase Modulator Using an
Active Interferometric Device
Rogério Dionísio
#*1
, Cláudia Reis
#2
, Paulo André
#3
, Rogério Nogueira
#4
and António Teixeira
#+5
#
Instituto de Telecomunicações
Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
1,2,3,4
{rdionisio, creis, pandre, rnogueira}@av.it.pt
*
Escola Superior de Tecnologia do Instituto Politécnico de Castelo Branco
Avenida do Empresário, 6000-767 Castelo Branco, Portugal
+
Departamento de Electrónica, Telecomunicações e Informática, Universidade de Aveiro
Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
5
teixeira@ua.pt
Abstract A novel architecture for an optical phase
modulator is presented and experimentally demonstrated. This
approach relies on a commercially available integrated Mach-
Zehnder interferometer structure with Semiconductor Optical
Amplifiers (MZI-SOA) and it is based in cross-phase modulation
effect (XPM). The feasibility of the proposed optical phase
modulator is experimentally investigated using different
scenarios of input power and bit rates.
I. INTRODUCTION
In the past few years, the data volume of communication
networks increased dramatically, so there is a need for finding
fast optical transmission techniques, along with equipment
with low power consumption and integration facilities.
Among those techniques, optical phase modulation is an
option that allow greater transmission distances in both digital
and analog transmission systems [1].
Phase modulation generates signals of 1 and 0 by changing
the phase of light while allowing it to be in the on position. As
opposed to intensity modulation, phase modulation has
superior bandwidth efficiency and is not easily affected by
signal distortions caused by transmission fibers and relay
nodes.
Several optical techniques have been proposed to
implement optical phase modulators. In [2] the scheme
proposed is a phase modulation based on frequency shifters,
which consists of an acousto-optic modulator (AOM),
followed by a fiber. In [3] and [4] LiNbO3 waveguide-based
phase modulator and gain-transparent semiconductor optical
amplifier (GT-SOA) are used, respectively, as optical phase
modulators. In [5], a Highly Non-Linear Fiber (HNLF) is used
as the optical medium to phase modulate a continuous wave
(CW) laser.
In this paper, we propose an optical phase modulator based
on the XPM effect [6] using a MZI-SOA. To the authors’
knowledge, this is the first demonstration of optical phase
modulation using both interferometric arms of a MZI-SOA.
This technique can be used in multi-level modulation signals
generation [7] as well in OCDMA transmission systems [8].
II. SYSTEM DESCRIPTION
A. Principle of Operation
Fig. 1(a) is a schematic diagram of the MZI-SOA. We use a
commercial hybrid-integrated device consisting of a passive,
planar silica balanced Mach-Zehnder interferometer with
nonlinear Semiconductor Optical Amplifiers and phase
shifters assembled in each interferometer arm.
(a) Schematic diagram of the MZI-SOA
(b) Principle of operation
port A
port D
port I
(XOR)
0
0
0
0
1
1
1
0
1
1
1
0
(c) XOR truth table
Fig. 1. Schematic diagram of a MZI-SOA, principle of operation and XOR
truth table.
Two input data streams at the same wavelength
λ
s are
coupled into ports A and D of the MZI-SOA, while a CW
light at
λ
c is coupled into port B. Inside the interferometer,
data signals are launched into the two SOAs where they
modulate the carrier density and also the refractive index [9].
The intensity variations of the input optical signals cause a
phase modulation of the control CW signal propagating
through the SOAs. If both data signals are time synchronized,
λ
s
λ
s
λ
c
λ
c
XOR
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the CW light from the two SOAs interferes destructively.
According to the XOR operation in Fig. 1(c), no pulse is
observed at the output port I of the interferometer [10].
However, the phase
φ
of
λ
c
XOR
will vary in accordance to the
input pattern, as depicted Fig. 1(b) [11].
B. Experimental Setup
Fig. 2 illustrates the experimental setup of the proposed
optical phase modulator. It consists of an external cavity laser
peaking at 1549.32 nm, followed by a polarization controller
and a Mach-Zehnder external modulator. The NRZ data signal
generated by a serial BERT (Agilent N4901B) is then
amplified by an EDFA (IPG-EAD-500-C3-W) and split into
two equal parts using a 3 dB coupler. Both signals are
synchronized using optical delay lines. Polarization
controllers are included at ports A and D of a MZI-SOA (CIP
40G-2R2-ORP), in order to optimize the destructive output
signal at port I.
The control signal, a CW light beam lasing at 1546.12 nm,
is launched into port B of the MZI-SOA in a co-propagating
direction with the data signals.
Finally, the control signal is recovered at port I, using a filter
with a 40 GHz bandwidth (X-tract Net Test). We use two
measurement instruments to analyze the output signal: an
oscilloscope (Agilent 86100A), connected through a PIN
photodiode (HP-11982A), and an optical complex spectrum
analyzer (APEX AP 2441A) to gather phase and power
information of the output signal, for time domain
characterization.
III. EXPERIMENTAL RESULTS AND DISCUSSION
In order to validate the feasibility of MZI-SOA based phase
modulators, experiments at 2.5 and 10 Gb/s were carried out
with the same experimental setup.
An average extinction ratio (ER) of 11.3 dB and a mean
power of 2.5 dBm were measured for data signals launched
into ports A and D of the MZI-SOA.
The bias current (Isoa) of both SOAs were varied
simultaneously, from 150 to 300 mA for 2.5 Gb/s and from
150 to 400 mA for 10 Gb/s. For each bias current, the mean
power of the control signal (P
CW
) was increased from -6 to
2 dBm.
The voltage applied to the phase shifters was adjusted in
order to maximize the destructive interference at output port I.
A. Phase modulator experiments at 2.5 Gb/s
Due to limitations imposed by the OCSA, the length’s
sequence at 2.5 Gb/s was restricted to 4 bits [12].
Fig. 3(c) shows the bit pattern launched at the
interferometric ports (A and D) of the MZI-SOA. Fig. 3(d)
illustrate the output signal at port I for bias current at 250 mA
and control signal at -4 dBm. The phase shift related to
different logic levels is well defined and the output power
signal is inverted when compared with the input data signals.
In Fig. 3(a), phase span is plotted as a function of P
CW
for
several bias currents. The results show that they increase as
the bias current is raised. Mean values vary between 35º and
50º.
It can be observed in Fig. 3(b) that the mean output power
is also proportional to the increase of Isoa and P
CW
since the
SOAs gain is not saturated.
B. Phase modulator experiments at 10 Gb/s
The proposed optical phase modulator was also evaluated
at 10 Gb/s. The tests were performed using data sequences
with 16 bits [12].
Fig. 2. Experimental setup of the optical phase modulator. DFB: distributed-feedback laser; PC: polarization controller; MZM: Mach-Zehnder modulator;
EDFA: Erbium Doped Fiber Amplifier; ODL: optical delay line; CW: continuous wave laser; VOA: variable optical attenuator; SOA: semiconductor optical
amplifier; PS1, 2: phase shifters; OCSA: optical complex spectrum analyzer; PG: pattern generator; O/E: PIN photodiode. Solid lines represent fiber-optic paths
and dashed lines indicate electronic connections.
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(a) Output phase span
(b) Output mean power
(c) Input signal with sequence “1000”
(d) Phase and power output for Isoa = 250 mA and P
CW
= -4 dBm
Fig. 3. Experimental results at 2.5 Gb/s
Fig. 4(c) shows the bit pattern coupled at the input ports
(A and D) of the MZI-SOA. The resulting output signal
with bias current at 150 mA and control signal at 0 dBm is
depicted in Fig. 4(d). Output power fluctuations are mainly
due to noise. As with 2.5 Gb/s experiments, phase shifts
are inverted when compared with data signal logic levels.
However, due to the dynamics of the SOA and the carrier
recovery time, output phase levels are less pronounced at
10 Gb/s when fast variations occurs at the MZI-SOA data
inputs. Phase constellations diagrams in Fig. 4(a) shows
that phase logic levels are evenly defined when the bias
current is increased.
As it can be seen in Fig. 4(a) and Fig 4(b), we obtain
higher values of phase span and output mean power,
respectively, by increasing the bias current.
For P
CW
ranging from -6 to 0 dBm, SOAs are in linear
amplification regime. In this case, the mean values of the
phase span vary between 70º and 170º. For P
CW
above
0 dBm, the SOAs saturates, which reduces phase span
values and output mean powers.
IV. CONCLUSION
In this paper, a new way of performing optical phase
modulation has been presented. We assess the impact of
SOAs bias current and input CW power on the phase of the
destructive output of a MZI-SOA.
We observed that an increase of the bias current
produces higher values of phase spans and output mean
powers. However, SOAs gain saturation has an opposite
effect on the output signal.
The experimental results demonstrate the feasibility of
an MZI-SOA device as an optical phase modulator. Other
options for phase modulation exist, using a single
waveguide embedded in an electro-optical substrate
(LiNbO
3
) or using the principle of interference with two
waveguides to cause also an amplitude modulation of the
optical signal (as in a MZM). However, those methods
introduce insertion losses. Using an MZI-SOA, not only
the losses are compensated, but also the optical power can
be increased by the SOAs.
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(a) Output phase span
(b) Output mean power
(c) Input sequence ”1110010010101100
(d) Phase and power output for Isoa = 150 mA and P
CW
= 0 dBm
Fig. 4. Experimental results at 10 Gb/s
Future research and application include high-level modulation
formats (m-QSK, m-QAM), using several MZI-SOAs in serial
or parallel configuration, within access to metro or long-haul
connection nodes, making at the same time amplification and
conversion of amplitude modulated signals to high level
advanced modulation formats.
ACKNOWLEDGMENT
The authors greatly acknowledge Networks of Excellence
EURO-FOS, BONE and also PANORAMA (ADI
2009/003144) and THRONE (PTDC/EEA-TEL/66840/2006)
projects.
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