All-Optical Frequency Shifting - Optical Signal Processing by Silicon Photonics - page 89

7.8 Stokes and Anti-Stokes Scattering
Basically, there are two types of Raman scattering, i.e., Stokes scattering and anti-
Stokes scattering. The different possibilities of visual light scattering are as follow:
• Rayleigh scattering (no Raman effect),
• Stokes scattering in which molecule absorbs energy, and
• Anti-Stokes scattering in which molecule loses energy.
7.8.1 Stimulated Scattering and Amplification
Raman amplification can be obtained by using SRS, which actually is a combina-
tion of a Raman process with stimulated emission. It is interesting for application
in telecommunication fibers to amplify inside the standard material with low noise
for the amplification process. However, the process requires significant power and
thus imposes more stringent limits on the material. The amplification band can be
up to 100 nm broad, depending on the availability of allowed photon states.
7.8.2 Spectrum Generation
For high intensity CW lasers, SRS can be used to produce broad bandwidth spec-
tra. This process can also be seen as a special case of FWM, where the frequencies
of the two incident photons are equal and the emitted spectra are found in two
bands separated from the incident light by the phonon energies. The initial Raman
spectrum is built-up with spontaneous emission and is amplified later on. At high
pumping levels in long fibers, higher order Raman spectra can be generated by
using the Raman spectrum as a new starting point, thereby building a chain of new
spectra with decreasing amplitude. The disadvantage of intrinsic noise due to ini-
tial spontaneous process can be overcome by seeding a spectrum at the beginning,
or even using a feedback loop like in a resonator to stabilize the process. Since this
technology easily fits into the fast evolving fiber laser field and there is demand for
transversal coherent high intensity light sources (i.e., broadband telecommunica-
tion, imaging applications), Raman amplification and spectrum generation might
be widely used in the near future.
7.9 Brillouin Scattering
Brillouin scattering, named after Léon Brillouin, which occurs when light in a
medium such as water or a crystal interacts with time-dependent density variations
and changes its energy (frequency) and path. The density variations may be due to