All-Optical Wavelength Conversion With Extinction Ratio ...

938 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 46, NO. 6, JUNE 2010
Fig. 2. Conceptual setup of the wavelength converter including the ER improvement;
is either , +1 or 0 1 depending which
kind of wavelength conversion mechanism shall be used.
III. SIMULATION SETUP
Fig. 1. Probe signal’s amplification asymmetry due to the dynamical part of the
gain suppressions in the UL-SOA’s saturated section (1 = 0 ;
jE j P ).
Unlike in SOAs, the main part of UL-SOAs is saturated by
the amplified signals and the amplified spontaneous emission
(ASE) noise after a certain length. For this reason, UL-SOAs
can be regarded as being divided into an amplifying section
and a saturated section. For bulk SOAs with typical dimensions,
the transition between these sections takes place after approximately
1 mm of propagation. The amplifying section has the
same properties as a short SOA and the saturated section can be
regarded as another device with different properties. Because of
the high optical power after the amplifying section, the carrier
density in the saturated section is fixed to the net transparency
level. If the signal levels change much faster than the carrier lifetime,
the carrier density in the amplifying section is only dependent
on the average optical power. If the signal remains at one
of the states longer than the carrier lifetime, the length and the
gain of the amplifying section changes. As a result bit pattern effects
occur. For high data rates with fast changing signal states,
the performance is not limited by the slow interband effects.
Only the fast nonlinear intraband effects influence the signals in
the saturated section. Their response time is about 1000 times
shorter compared to the slow interband effects, but their impact
is also weaker. For this reason, the desired operation point for
signal processing is when the carrier density inside the UL-SOA
is shifted as far as possible towards the front of the device in
order to efficiently use the length of the device and to avoid the
influence of back-propagating ASE.
Moreover, the UL-SOA shows a Bogatov-like effect over several
nm due to the increased efficiency of the fast intraband effects.
In [15] Bogatov has demonstrated that the beating of two
input signals creates dynamic gain and index gratings in nonlinear
semiconductor media. Because of the interaction of these
dynamic gratings, 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
(Fig. 1). In [15] the pulsations are due to the slow interband
effects, while in [14], [16] 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.
The conceptual setup of the AOWC with ER improvement
is shown in Fig. 2. Due to the two input signals, a parametric
amplification of the data signal in dependence of the CW signal
can be obtained, resulting in a nonlinear transfer characteristic
over
. With the help of the nonlinear
gain characteristic, the ER can be improved. To achieve
a maximum ER improvement the UL-SOA’s operating point is
set at a steep slope of the nonlinear transfer characteristic.
The investigated device is a 4 mm-long bulk SOA. The geometrical
structure has been optimised in order to increase the
efficiency of the nonlinear effects (decreased coupling losses
and increased mode confinement [20]). The values are given in
Table I. The driving current is 300 mA/mm.
In later applications the presented scheme might be used
at a switching node in long-haul transmission systems. After
being transmitted over several spans, a degenerated channel
approaches the node. First the whole channel will be optically
amplified with an EDFA before the WDM channel of interest
will be converted to the new wavelength. For this reason, we
assume the following parameters for the input data signal after
an EDFA: , . The power for
the additionally injected CW is:
. The signals’
wavelength are 1560 nm and 1564 nm, respectively. The
wavelength alignment depends on which kind of wavelength
conversion mechanism shall be used. The data signals being
investigated are 100 Gb/s OOK-RZ-50% modulated PRBSs
of orders up to 25. The bandpass filter after the UL-SOA
is a second-order Gaussian filter with a 3 dB-bandwidth of
275 GHz.
Due to limited computation capacity it was not possible to
simulate complete PRBSs longer than . For this reason,
critical bit sequences were taken to estimate the impact of pattern
effects on the regeneration scheme. As already mentioned
in Section II, pattern effects occur if the data signal remains long
at one of the states. Therefore, the critical bit sequences contain
various combinations of the following bit sequences: constant
signal states, one inverted bit within a constant signal state and
alternating bit sequences. These combinations of bit sequences
are expected to create worst-case bit pattern effects. A detailed
description of the critical bit sequence is given in the Appendix .
The simulations have been done with the same model as in
[21], [22] where a very good match between measurements and
simulation results has been demonstrated.
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