Operational Conditions for Extinction Ratio Improvement in Ultralong ...

106 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 2, JANUARY 15, 2009
Operational Conditions for Extinction Ratio
Improvement in Ultralong SOAs
Patrick Runge, Robert Elschner, Student Member, IEEE, Christian-Alexander Bunge, Member, IEEE,
Klaus Petermann, Senior Member, IEEE, Michael Schlak, Walter Brinker, and Bernd Sartorius
Abstract—The extinction ratio (ER) improvement in ultralong
semiconductor optical amplifiers (UL-SOAs) is studied in dependence
on their driving conditions. A stable process potentially
useful for simple high-speed all-optical regeneration is observed.
Furthermore, results indicate that a Bogatov-like effect is the
reason for the ER improvement in the UL-SOA’s saturated section.
Index Terms—All-optical, Bogatov effect, extinction ratio
(ER) improvement, ultralong semiconductor optical amplifiers
(UL-SOAs).
I. INTRODUCTION
T
HE regeneration of data signals is an important issue
for long-haul optical communication systems. With increasing
data rates, all-optical solutions are needed to overcome
the electronic bottleneck. Opposite to regeneration concepts
with highly nonlinear fibers, most semiconductor optical amplifier
(SOA) schemes can be integrated. Typical SOA solutions
are based on inherently narrowband interferometer structures
[1], [2] and are due to the two interferometer paths sensitive to
data rates and wavelengths. In [3], a new single path technique
that uses the SOA’s nonlinear gain characteristic is presented.
Since this scheme is dependent on the slow interband effects,
the transmission speed is limited due to the carrier lifetime
(several hundred picoseconds). A novel single path extinction
ratio (ER) regeneration concept also using the nonlinear
transfer function of ultralong SOAs (UL-SOA), was presented
in [4]. This regenerator concept is based on the two-wave
competition (TWC) working with the fast intraband effects.
For this reason, the speed limitation should be in the range of
several hundred gigabits per second. An analytic description
of the ER improvement, where the TWC effect is explained
with the help of four-wave mixing, is given in [5]. In addition,
simulations as well as measurements for sine-modulated signals
were presented. In [6], a basic set of the UL-SOAs’ driving
conditions for the ER improvement have been investigated.
In this letter, further detailed experiments on the UL-SOAs
driving conditions are made. Due to a systematic variation of the
Manuscript received April 14, 2008; revised September 12, 2008. Current
version published January 14, 2009. This work was supported by the Deutsche
Forschungsgemeinschaft (DFG).
P. Runge, R. Elschner, C.-A. Bunge, and K. Petermann are with the Fachgebiet
Hochfrequenztechnik, Technische Universität Berlin, D-10587 Berlin, Germany
(e-mail: runge@hft.ee.tu-berlin.de).
M. Schlak, W. Brinker, and B. Sartorius are with the Fraunhofer-Institut
für Nachrichtentechnik, Heinrich-Hertz-Institut, D-10587 Berlin, Germany
(e-mail: sartorius@hhi.fhg.de).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2008.2008910
driving conditions, the UL-SOA’s optimal operating point for
the ER improvement was found. In addition, it could be proven
that the ER improvement in UL-SOAs is a stable process. Furthermore,
some of the experiments indicate that a Bogatov-like
effect (see Section II) in the UL-SOA’s saturated section seems
to be the reason for the ER improvement.
II. PROPERTIES OF UL-SOAs
Opposite to short SOAs, the major part of UL-SOAs is deeply
saturated due to the amplified input signals and the amplified
spontaneous emission (ASE). For this reason, the UL-SOA
can be divided into two sections: In the amplifying section,
the carrier density cannot follow high-speed modulated signals
because of the long carrier lifetime. As long as the signal
changes are fast enough, the carrier density experiences the
average signal power and no pattern effects occur. By contrast,
in the saturated section, the carrier density is clamped to the net
transparency level and only the fast intraband effects influence
the signals. In our simulations, we could observe a dynamical
gain and index grating created by carrier heating (CH) and
spectral hole burning (SHB) in this section. The pulsation is
caused by the beating of the two input signals.
In [9], Bogatov has demonstrated that due to the dynamical
gain and index grating in nonlinear semiconductor media,
the weaker signal’s amplification is dependent on the stronger
signal’s wavelength detuning. Due to the -factor causing the
index pulsation, the probe signal is stronger amplified on the
longer wavelength side of the pump signal. In [9], the pulsations
are due to the slow interband effects, while in [10], a similar effect
based on the fast intraband effects was presented. Due to
the fast intraband effects, this Bogatov-like effect is suitable for
high-speed operations. Furthermore, the effect can be observed
for a wavelength detuning up to several nanometers.
III. MEASUREMENT AND SIMULATION SETUP
As a simulation tool an improved time-domain SOA model
based on [7] is used. In order to apply the model on UL-SOAs,
a finite-impulse response filter used to model the wavelength dependence
of the gain in each SOA segment is adaptively fitted to
a cubic gain model [8]. Moreover, further nonlinear effects like
free carrier absorption and two-photon absorption were implemented,
since even these weak effects influence the signal over
such a long device length. For the simulation, typical parameters
for a 1550-nm InGaAsP bulk SOA had been taken from the
literature.
Fig. 1 shows a conceptual setup for the investigations. To keep
the influence of the bandpass filter after the UL-SOA on the ER
improvement as small as possible, a sine-modulated continuouswave
(CW) signal was used as a data signal. The investigated
device is an 8-mm-long bulk SOA. For all measurements and
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RUNGE et al.: OPERATIONAL CONDITIONS FOR ER IMPROVEMENT IN UL-SOAs 107
Fig. 1.
Setup used for simulations and measurements.
Fig. 3. Output ER in dependence of the wavelength detuning, optimized detuning4nm(
=const:; solid lines—measurement; dashed lines—simulation).
Fig. 2. Output ER in dependence of the data signal input power (solid
lines—measurement; dashed lines—simulation).
simulations, the driving current was set to 310 mA/mm. The default
input parameters for the experimental series in Section IV
are dBm, dB, nm, and
GHz (corresponds to a 40 Gb/s “10101 ”-sequence).
The measured device was fabricated at the Heinrich-Hertz
Institute and has a buried InGaAsP–InP waveguide structure.
The bandgap wavelength of the active material is 1537 nm. The
confinement factor of this waveguide is approximately 0.35. To
avoid lasing modes, the facets are tilted by 10 and are antireflective
coated.
IV. ER IMPROVEMENT DEPENDENCE ON THE UL-SOA’s
OPERATING CONDITIONS
In this section, the results of the systematic study of the
UL-SOA’s operating conditions are reported and the experiments
are compared to simulations.
Fig. 2 shows the data signal’s output ER for various power
ratios of the CW signal and the data signal. Depending on the
power ratio an increased output ER compared to the input ER
can be observed. This behavior is unknown from conventional
assumptions and theories. In the following investigations, the
average ER
is used for plotting these
characteristics.
The first test series investigates the ER’s improvement in dependence
of the data signal input power (Fig. 2). As a result
there is only a small influence of the data signal’s input power
on the ER improvement efficiency as long as the power ratio is
readjusted. Furthermore, a qualitative match between the measurements
and the simulations can be seen, as we will observe
it for all following experimental series.
Another driving condition of interest is the wavelength detuning.
Fig. 3 shows, that there is an optimized wavelength detuning
around 4 nm.
Fig. 4. ER after passing the UL-SOA against the input ER (ER taken from the
optimized power ratio).
Next the ER improvement in dependence of the input ER is
investigated. With increasing ER at the UL-SOA’s input, the
ER improvement efficiency increases (in Fig. 4 slope ). The
discrepancy of the measured ER between Fig. 4 and the other
figures can be ascribed to different setup conditions during the
measurements.
So far the studied driving conditions have only influenced the
ER improvement efficiency. Driving conditions that enable and
disable the ER improvement are investigated in the following
part of this section and will be interpreted with the help of the
Bogatov-like effect.
Fig. 5 demonstrates the dependence of the ER improvement
on the polarization. The input signals need to be parallel polarized
in order to obtain an ER improvement. The polarization dependence
can be explained with the Bogatov-like effect. If the
signals are orthogonally polarised, there is no dynamical gain
and index grating, resulting in a decreasing ER.
The impact of the signal’s relative wavelength alignment
on the ER improvement is analyzed in Fig. 6. Having the CW
signal located on the shorter wavelength side, the ER behind
the UL-SOA is decreased more than it is conventionally expected
(compare to the ER in Fig. 5 for orthogonal polarized
input signals). The CW signal has to be located on the longer
wavelength side in order to obtain an ER improvement. This
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108 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 2, JANUARY 15, 2009
Fig. 5. Measured ER improvement in dependence of polarization; both signals
need to be parallel polarized in order to create a intensity beating.
Fig. 7. Simulated ER improvement in dependence of the -factor; inverting the
-factor leads to an inverse behavior of the TWC (solid line— < ;
dashed line— > ).
rameters (signals’ input power, signals’ wavelength detuning
and signals’ input ER) only influence the ER improvement efficiency.
But there are also driving conditions (signals’ polarization,
SOA’s -factor and signals’ wavelength alignment) which
can disable the effect. Furthermore, latter driving conditions indicate
that a Bogatov-like effect in the UL-SOA’s saturated section
seems to be the reason for the ER improvement. Since this
effect is caused by the fast intraband effects, the regenerative
mechanism should be usable for data rates fairly above the carrier
lifetime’s resonance frequency up to several hundred gigabits
per second.
Fig. 6. Relative wavelength alignment of the data signal to the CW signal; the
data signal has to be located on the shorter wavelength side in order to achieve
an ER improvement (solid line—measurement; dashed line—simulation).
asymmetry regarding the wavelength detuning is a consequence
of the asymmetric probe amplification due to the Bogatov-like
effect as presented in [10]. Similar to the ER improvement, the
probe gets amplified for an alignment on the pump’s longer
wavelength side while for the shorter wavelength side it will be
attenuated.
At the end the dependence of the -factor on the ER improvement
is investigated (Fig. 7). Inverting the -factor results in
an inverse behavior of the ER development. For this reason,
the alignment of the data signal to the CW signal also has to
be changed in order to obtain an increasing ER. In the same
manner, the Bogatov-like asymmetry flips over with a negative
-factor. For devices with a decreased -factor like multiple
quantum-well and quantum-dash SOAs the ER improvement
should also be observable. On the one hand, the Bogatov-like
effect should be reduced due to the decreased -factor but on
the other hand CH and SHB are more pronounced in such devices.
V. CONCLUSION
We could prove that the ER improvement is a stable process
if the UL-SOA is driven in a correct operating range. Some pa-
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