Optics 101

Acousto-Optical Effects - Principles and Applications

Oct 8 2013

Acousto-optics is a phenomenon which deals with the interaction of light (optics) and sound (acoustics). The existence of an interaction between light and sound was discovered by Brillouin in the year 1922. The acousto-optic effect may be described as the diffraction of light brought about by sound waves.

The acousto-optic effect is caused due to the photo-elasticity of the interacting medium. This effect is useful in ultrasonic wave study. The presence of a sound wave causes changes in the refractive index of the medium, which renders the medium as a refractive index grating. This article will elaborate on the experimental set up, working and applications of the acousto-optic effect.

Experimental Set Up

An acousto-optic arrangement consists of an acousto-electric transducer which is bonded with a photo-elastic medium. The transducer is a piezoelectric crystal that is metalized at both ends for conduction of an electric current. The photo-elastic medium surrounds the transducer, such that the acoustic waves are launched into the optic medium.

Various acousto-optic devices are built based on the acousto-optic effect - for example acousto-optic modulators and acousto-optic filters.

Permittivity and Photo-Elasticity

The acousto-optic effect is brought about by the change in the permittivity of an optic medium due to the induced mechanical strain.

The variation in the permittivity is a function of photo-elasticity of the medium. The acoustic waves generate periodic modulation of the refractive index of the material, causing diffraction of the light wave.

Photoelasticity is a function of the stress distribution along the optic material. The two types of diffraction caused by the acoustic wave are Ram-Nath and Braggs diffraction.