Brillouin Scattering

Scattering of light from acoustic modes is called Brillouin scattering. From a strictly classical point of view, the compression of the medium will change the index of refraction and therefore lead to some reflection or scattering at any point where the index changes. From a quantum point of view, the process can be considered one of interaction of light photons with acoustic or vibrational quanta (phonons). Two examples will provide some context for this phenomenon.

Acoustic Mode Scattering

When acoustic standing waves are produced in a solid, they create a periodic condition which can scatter light waves according to the Bragg law. In addition to its application to x-ray diffraction, the Bragg law applies to some cases of light scattering from acoustic standing wave modes in a solid. This is an example of Brillouin scattering. It is used in acousto-optic modulators.

Laser-Water Scattering

Benedek, et al., reported the production of sidebands from the interaction of a helium-neon laser with water. This can be considered to be Brillouin scattering.

Acousto-Optic Modulator

Light traveling through a quartz crystal can be diverted from its path by an acoustic wave. Light can be scattered off the areas of altered density caused by the acoustic wave in a process called Brillouin scattering. The analogy is made to Bragg scattering in that the sound waves produce effective planes for scattering the incident waves. The light reaching the point directly along the incident light path will be modulated by the presence of the acoustic wave. If that path is part of the amplification path to produce laser action, then the acoustic wave will "Q-switch" the laser on and off.

An acoustooptic coupler accomplishes Q-switching by scattering the incoming light from acoustic waves in a crystal. The periodic scattering of the light reduces the "quality" of the cavity and prevents laser action. Only during brief intervals when there is no scattering is laser action possible, so pulses are produced at these times when there is no density change along the light path through the crystal.

Moller reports the frequencies applied to the crystal for the production of the acoustic standing waves to be a few Hertz to 50 kHz for the acoustooptical modulator and that the resultant laser pulses are on the order of 100-500 picoseconds.

Electrooptical Q-switching

An electrooptical Q-switcher uses the Pockels effect. A Pockels cell between crossed polarizers normally transmits no light. A voltage pulse which rotates the plane of polarization by 90¡ will allow the light to pass and this shutter effect transmits a brief pulse of light enabling laser action during that brief interval. KDP is a common material for the Pockels cell - it can be used for modulation up into the GHz range.

Dye Cell Shutter

A dye such as Eastman Kodak 9860 polymethine dye dissolved in dichloroethane serves as a saturable absorber. Subjected to a 30 ps pulse, the dye may saturate and become transparent and then become opaque again, acting as a shutter of approximately 10 ps duration. This saturable absorber is used to both Q-switch and mode lock the laser. The result may be a train of 10 ps pulses some 1 microsecond in duration. Such a saturable absorber is an important component of a laser oscillator.