All-Optical Frequency Shifting - Optical Signal Processing by Silicon Photonics - page 90

acoustic modes such as phonons, magnetic modes like magnons, or temperature
gradients. As described in classical physics, when the medium is compressed its
index of refraction changes and the light's path necessarily bends.
7.9.1 Relationship to Raman Scattering
Brillouin scattering is related to Raman scattering in a way that both represent
light's inelastic scattering processes with quasi-particles. The distinction exists
in the type of information extracted from the sample and the detected range of
frequency shift. Brillouin scattering denominates the photons scattering from
quasi-particles while in case of Raman scattering, interaction with vibrational and
rotational transitions in molecules makes photon scatter. Therefore, the informa-
tion provided by these two techniques about the sample is different. Raman spec-
troscopy is used to determine the molecular structure and chemical composition,
while Brillouin scattering is capable of measuring the properties on a larger scale
such as the elastic behavior. From an experimental point of view, in Brillouin scat-
tering the frequency shifts are detected with an interferometer while Raman setup
can be based on either dispersive (grating) spectrometer or interferometer.
7.9.2 Stimulated Brillouin Scattering
For intense beams (e.g. laser light) traveling in a medium such as an optical fiber,
acoustic vibrations in the medium is created due to variations in the electric field
of the beam itself through electrostriction. As a result of these vibrations, the beam
might endure Brillouin scattering, which usually occurs in the direction opposite
to the incoming beam, a phenomenon known as stimulated Brillouin scattering.
For gases and liquids, the frequency shifts typically are of the order of 1-10 GHz.
The effect by which optical phase conjugation can take place is Stimulated
Brillouin scattering.
7.10 XPM or Cross-Phase Modulation
Generally, the cross-phase modulation or XPM is an optical upshot nonlinear in
nature, in which the phase of wavelengths can get effected through the optical
Kerr effect. This implies the modulation technique that can be used for the addi-
tion of a light stream to the information by modification of the coherent optical
beam phase with another beam through interactions in a proper nonlinear medium,
which is cross-phase modulation. This method is functional to optical fiber com-
munications for various applications. In DWDM applications with direct detection