Introduction - Optical Signal Processing by Silicon Photonics - page 6

The all-optical wavelength conversions (AOWC) is significant enough to be
an indispensable technology for the switching architectures of future. There are
numerous schemes, such as a cross-phase modulation (XPM), a self-phase modu-
lation also known as SPM, a cross polarization modulation (XPolM), a cross-gain
modulation (XGM), can be used to realize the all optical wavelength conversion.
Nevertheless, the AOWC is based on a four-wave mixing, one of the most promis-
ing schemes, because of the reason that it is fully see-through to the signal modu-
lation format and bit rate of signal.
In optical fibers the processes of scattering mostly depends on molecular vibra-
tions or variations in density of silica. In another category of nonlinear phenom-
ena, optical fibers participate by playing a passive role excluding the mediating
interaction among a number of optical waves. The processes that are nonlinear are
referred to as parametric processes, it is because of the involvement of medium
parameters modulation, such as the refractive index, and phase-matching is
required before they can build up along with the fiber. Among these, four-wave
mixing (FWM) is a technique that plays a vital role. Although FWM can be unfa-
vorable for Wavelength Division Multiplexing systems that should be designed in
such a way that the impact is reduced, it is also functional for a range of applica-
tions like generation of a spectrally inverted signal through the optical phase con-
jugation process, designing light wave systems and wavelength conversion. The
FWM can also be made applicable for holographic imaging, phase conjugation,
and optical image processing.
FWM can be performed in optical amplifiers like semiconductor optical ampli-
fier (SOA) or fiber, in general, the AOWC is based in fiber on the FWM, it can be
operated at a higher speed, while the AOWC speed in the semiconductor optical
amplifier is inadequate due to the reason that the response time of the carriers is
long. The AOWC is basically based on the FWM with a single pump, has been
demonstrated in a high-nonlinear dispersion shifted fiber, but the conversion is
sensitive in polarizing aspect, and the spectral in converted form is inverted com-
parative to that of the original signal. Switching network is a potentially promising
scheme of switching in the next routing generation and forwarding an ultrahigh
bit-rate data in the optical layer without detecting the payload.
There are numerous schemes in order to reduce the sensitivity of polarization
of the FWM-based wavelength conversion by using the dual pumps. A polariza-
tion scheme with multiplicity architecture requires an additional optical compo-
nents as well as an accurate polarization control in the loop. In the case of twisted
optical fiber, the dual-pump FWM with circular-polarization pumps and signal can
also minimize the sensitivity of polarization of the signal in converted form, the
setup although, is complex. The converted signal is a circular polarization, and it
has limited conversion bandwidth because of the circular polarization-mode dis-
persion of the twisted fiber. For the fiber which has standard of tens-kilometer,
with a birefringence that is low, the birefringence reorientation changes its axes
randomly along the fiber, this eliminates the reliance of the signal polarization.
Referring the Manakov equation, Inoue and McKinstrie theoretically revealed that
the scheme with orthogonal pumps can minimize the sensitivity of polarization.