Introduction to Acousto-Optics Technology

Acousto-Optic devices are used in laser equipment for electronic control of the intensity and position of the laser beam. In this note, we will explain the theory and application of acousto-optic modulators.
Acousto-optic interaction occurs in all optical mediums when an acoustic wave and a laser beam are present and interact with each other. When an acoustic wave is launched into the optical medium, it generates a refractive index wave that behaves like a sinusoidal grating. An incident laser beam passing through this granting will be diffracted into several orders. With appropriate design, the first order beam has the highest efficiency. Its angular position is linearly proportional to the acoustic frequency, so that the higher the frequency, the larger the diffracted angle.

Where, λ, is the optical wavelength in air, fa, is the acoustic frequently, and , Va is the acoustic velocity, and Q, is the angle between the incident laser beam and the diffracted laser beam.
A diagram of the angular relationship between the acoustic wave and the laser beams is shown in Figure 1.
The intensity of light diffracted (deflected) is proportional to the acoustic power (Pac), the material figure of merit (M2 ), geometric factors (L/H) and inversely proportional to the square of the wavelength.

With acousto-optics, both the deflection as well as the modulation of the amplitude of the optical beam are possible. Also, in the acousto-optic interaction, the laser beam frequency is shifted by an amount equal to the acoustic frequency. This frequency shift can be used for heterodyne detection applications, where precise phase information is measured.

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