Acousto-Optic Devices: Optical Elements for Spectroscopy

Abstract

Seventy years ago, Brillouin theorized that light could be diffracted by an acoustic wave (1). This prediction was based on the fact that when an acoustic wave is propagated in a transparent material, it produces a periodic modulation of the index of refraction. The perturbation in the refractive index arises from the change in the number density of the acoustic medium induced by the compression and rarefaction of the traveling sound wave. As a consequence, the propagating acoustic wave produces a moving grating that will diffract portions of an incident light beam.This sound-light interaction phenomenon, which is called the acousto-optic interaction, was initially observed by Debye and Sears in 1932 (2). Early studies were limited to interactions of low-frequency acoustic waves with incoherent light sources in liquids and gases. As a result, the phenomenon had an impact primarily on the academic community; experimental results provided a basic understanding of the thermal and acoustic properties of liquids and gases.The first applications of the acousto-optic interaction did not appear until the 1960s. Since then, significant scientific and technological advances have occurred in optics, elec-INSTRUMENTATION

Electro-optic and Acousto-optic Laser Beam Scanners

“…The changing refractive index is the result of rarefaction and compression of the material, inducing a changing density of the material. This periodically changing refractive index n acts like an optical grating, moving at the speed of sound in the crystal, that will diffract a laser beam traveling through the material (Tran, 1992). Fig.…”

“…This settling time is referred to as the response time τ a (or transit time or random-access time) of the AOD, and depends on the acoustic velocity v a and on D, which denotes the beam diameter or the optical aperture of the AOD, whichever is smaller, as (Tran, 1992;Chang, 1995) …”

FIDO science payload simulating the Athena Payload

“…The acousto‐optical filter is an electro‐optical device that can electronically tune its transmission wavelength by changing a RF signal applied to the AOTF crystal. The applied acoustic wave sets up a periodic grating in the crystal by modulating the crystal lattice spacing [ Harris and Wallace , 1969; Tran , 1992]. Incoming light is diffracted by the induced grating with a narrow spectral band pass.…”

Near-infrared thermal lens spectrometer based on an erbium-doped fiber amplifier and an acousto-optic tunable filter, and its application in the determination of nucleotides

“…Its spectral bandwidth is wider and its output intensity higher than the other near-IR ͑continuous wave͒ sources that are currently available. 1 The recently developed near-IR spectrophotometer, 1 based on the use of the EDFA as a light source and an acousto-optic tunable filter [2][3][4][5][6] ͑AOTF͒ as a dispersive element, can be used for measurements that are not possible with spectrophotometers based on halogen tungsten lamps. For instance, the absorption spectrum of a 1.0M Pr 3ϩ aqueous solution can be successfully measured through four sheets of paper with this spectrophotometer.…”

“…[2][3][4][5][6] Conventionally, it is this diffracted light that is used in all experiments. [2][3][4][5][6] The intensity of this diffracted light is only a fraction of the total intensity of the incident light. The transmitted light whose intensity is much higher than the diffracted light has not been used in all cases.…”