High-Nonlinearity in Glass Fibers and Cross-Phase Modulation - Optical Signal Processing by Silicon Photonics - page 56

the polarization controllers continuously whenever required, and moreover, huge
built-in stress birefringence in conventional low-index cladding PMFs depends
highly on temperature variation. In particular, nonlinear fiber devices based on
the optical Kerr effect always use PMFs to ensure that the control pulse which is
linearly polarized in either one of the two principal axis maintains its SOP along
the fiber, because the optical Kerr effect operates based on the variation of SOP.
In such devices, since the direction of the linearly polarized signal pulse makes
a 45° angle with the two principal axes, the signal pulse will suffer PMD by the
built-in birefringence. Moreover, this intentional nonvarying built-in birefringence
in PMFs is much higher than the varying birefringence in standard fibers. As a
result, the use of PMFs also brings serious problems which cause the limitation
of response time and bit-rate of signals. The problems include the signal pulse
broadening caused by PMD and the pulse walk-off between the control and signal
pulses caused by the birefringence-induced group velocity mismatch. For the non-
linear fiber devices based on mode interference, SOPs of the two modes should be
maintained and remain the same during operation for better performance, but it is
hard to achieve due to temperature dependence of the SO. Short lengths of devices
are able to solve these problems of the signal pulse broadening caused by PMD
and the pulse walk-off by the birefringence-induced group velocity mismatch.
Devices having short lengths are hardly affected by environmental disturbances.
Therefore, the use of HNL glasses for short devices would be the better way to
effectively reduce polarization sensitivity.
5.5 Cross-Phase Modulation
Many nonlinear dispersive systems like optical fiber communications exhibit insta-
bility, known as the modulation instability, which has already been discovered in
Physics [ 2 - 10 ]. In case, weak perturbations grow exponentially causing interplay
between nonlinearity and GVD then this process is known as modulation insta-
bility. But as far as optical fiber is concerned, modulation instability occurs due
to anomalous dispersion, so, manifests itself as breakup of the continuous waves/
quasi continuous waves radiation in a stream of extremely short pulses [ 6 - 10 ].
Anomalous dispersion is also necessary for solitons [ 11 , 12 ] which is the result
of a balance between the nonlinearity induced SPM and GVD. However, recently
it has been observed that the observation of modulation instability with cw prop-
agating beams is hampered by competing nonlinear effects like SBS [ 9 , 10 ],
under quasi cw conditions. These experiments were performed in infrared region
beyond 1.3 μ m (micrometer), in order to operate in anomalous dispersion regime
of silica fiber.
When two or more optical fields propagate inside the fiber then a new type of
modulation instability occurs, even in the normal dispersion regime. But the actual
mechanism behind this phenomenon is XPM which occurs due to nonlinear phase
change of an optical field induced by other co-propagating fields. The modulation