All-Optical Frequency Shifting - Optical Signal Processing by Silicon Photonics - page 88

7.6.1 Optical Parametric Generation
This light emission is based on the nonlinear optical principle. By a nonlinear
optical crystal, the photon of an incident laser pulse (pump) is divided into two
photons. The sum energy of these two photons is equivalent to the energy of the
photon of the pump. Ordinary and extraordinary polarizations generate light; the
ordinary light is called the signal and the extraordinary light is called the idler. By
phase matching condition, the wavelengths of the signal and the idler are deter-
mined. The wavelength is changed by the angle between the incident pump laser
ray and the axes of the crystal. By changing the phase matching condition, the
wavelengths of the signal and the idler lights can be tuned. This process is termed
as optical parametric generation or OPG.
7.6.2 Optical Parametric Amplification
After separation of the signal light from the OPG outputs, the remaining idler
passes through a nonlinear optical crystal collinearly with light of the same wave-
length as the pump, and a stronger output of the same wavelength as the signal
and idler is acquired as the output of the OPA. These wavelength-variable outputs
are efficiently used in many spectroscopic methods. As an example of OPA, the
incident pump pulse is the 800 nm (12,500 cm − 1 ) output of a Ti:sapphire laser,
and the two outputs, signal and idler, are in the near-infrared region, the sum of the
wave number of which is equal to 12,500 cm − 1 .
7.7 Raman Amplification
Usually, Raman amplification is based on the SRS phenomenon. In the non-
linear regime when a lower frequency 'signal' photon that induces the inelas-
tic scattering of a higher frequency 'pump' photon in an optical medium, as a
result, another 'signal' photon is produced with the surplus energy resonantly
that passed to the vibrational states of the medium. This procedure allows all-
optical amplification as with other stimulated emission processes. Nowadays,
mostly optical fiber is used as the nonlinear medium for SRS for telecommunica-
tion systems. In this case, it is characterized by a resonance frequency downshift
of ~11 THz (corresponding to a wavelength shift at ~1,550 nm of ~90 nm). The
SRS amplification process can be readily cascaded, thus accessing essentially any
wavelength in the fiber low-loss guiding windows (both 1,300 and 1,550). Raman
amplification is used in optical telecommunications allowing all-band wavelength
coverage and in-line distributed signal amplification in addition to applications in
nonlinear and ultrafast optics.