Acousto-optic modulators are devices that use sound waves to modify the amplitude, frequency, or phase of light passing through an acousto-optic material. For materials with a fixed acoustic velocity, the acoustic wavelength is a function of the frequency of the drive signal. The acoustic wavelength controls the angle of deflection. In turn, the acoustic wave’s amplitude controls the fraction of the electromagnetic radiation that is deflected. Typically, acousto-optic modulators consist of an acousto-electric transducer bonded onto a photo-elastic medium. Often, the transducer is a piezoelectric crystal that is metallized on both faces so that an electric field can be applied and a strain induced. The crystal cut and orientation determine how the time-dependent strain within the transducer is coupled with the frequency-dependent efficiency into the photo-elastic medium. The acoustic impedance matching between the transducer and bonding layers determines this frequency-dependent efficiency for acousto-optic modulators.
Important specifications for acousto-optic modulators include aperture size, wavelength range, center frequency, rise time, insertion loss, and modulation bandwidth. Aperture size is typically expressed in millimeters and represents the largest diameter. Rise time, which approximates a step function and represents the overall speed of the device, is the amount of time required for a signal to change from 10% to 90% of full power. Insertion loss is the attenuation caused by the insertion of an optical component. Modulation bandwidth, which is sometimes called tuning bandwidth or video bandwidth, is the input (sinewave) modulation frequency at which frequency deviation decreases to – 3 dB of its direct current (DC) value. Some acousto-optic modulators are rack-mounted and have a fiber pigtail attached. Other devices are freestanding and preserve the polarization of the incoming signal.
Acousto-optic modulators are used in a variety of laser printers, video recorders, and video projection systems. Principally, they are used to vary and control laser beam intensity. Bragg configurations provide a single first order output beam with an intensity that is linked directly to the power of a radio frequency (RF) control signal. Polychromatic modulation systems enable the use of up to 12 separate lines that can be mixed or modulated separately. Special acousto-optic modulators with tunable filters use anisotropic interactions inside tellurium dioxide crystals to provide independent and simultaneous control of different lines of incoming laser light. Crystal cuts produce good diffraction efficiency (> 90%), narrow resolution (1 – 2 nm), low cross talk between lines, and a high extinction ratio.
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Electro-optic modulators are analog or digital devices that use an electric field to alter the characteristics (e.g., band gap and index of refraction) of a material through which light is traveling, changing the characteristics of the light itself.