High-Nonlinearity in Glass Fibers and Cross-Phase Modulation - Optical Signal Processing by Silicon Photonics - page 53

5.4 Major Nonlinear Effects
As already discussed, nonlinearity plays an important role in the output response
of the systems. Following are the major nonlinear effects encountered in optical
fiber communication systems.
(a) Stimulated Brillouin Scattering (SBS)
(b) Stimulated Raman Scattering (SRS)
(c) Cross-Phase Modulation (XPM)
(d) Self-Phase Modulation (SPM)
(e) Four-Wave Mixing (FWM).
The role of nonlinear fiber devices has been highly appreciated in the recent years.
This appreciation has been given to nonlinear fiber devices due to their built-in
property of ultrafast response time along with the potential application in optical
communication systems. In order to generate sufficient nonlinear phase shifts,
long fibers are required because nonlinearities of conventional silica-core silica-
clad fibers are too low. Nonetheless, these long length devices cause under-men-
tioned serious problems which have to be traded-off one way or other.
(a) Pulse walk-off
(b) Pulse broadening
(c) Polarization fluctuation.
The above-mentioned problems act as limiting factors for response time, band-
width, and maximum attainable bit-rate. Thus, in view of the prevailing sce-
nario, the shorter length is preferably important in order to achieve ultrafast
switching and higher bit-rate data transmission. Same time, we simultaneously
need shorter length and higher nonlinearities to achieve our goals so in order to
increase the fiber nonlinearities, selection of high-nonlinearity (HNL) materials
is of prime importance too. These materials include Litharge, Bismite, Tellurite,
and Chalcogenide glasses which serve the purpose of nonlinearity as such. Along
with high nonlinearity, these materials also exhibit high-group velocity disper-
sion and high losses. These factors adversely affect the performance of nonlinear
fiber devices. Therefore, it has to be taken into account too that how HNL glasses
affect the performance of nonlinear fiber devices, so both the advantages and
the disadvantages has to be considered in parallel fashion. For this purpose, we
can evaluate different types of fibers which are constructed from different types
of HNL glasses. By using HNL glasses for fibers, we can effectively reduce the
device length of nonlinear fiber devices. In addition to the length, other problems
of group velocity dispersion (GVD) and losses could also be controlled because
of the extremely short device length. Once the above-mentioned effects are under
control, walk-off, pulse broadening, and polarization fluctuation in nonlinear fiber
devices can also be suppressed. Thus, this would lead to ultrafast switching and
higher bit-rate. In this respect, a scheme of wavelength division demultiplex-
ing is in place that is based upon the optical Kerr effects. The most important