Biomedical Engineering Reference
In-Depth Information
3
Acousto-Optics,OpticalComputing,
and
Signal
Processing
We first address the acousto-optic (AO) effect called Bragg diffraction, uti-
lized in devices called Bragg cells. The basic operating principles of Bragg
cells, the components, the applications, and the performance of some sys-
tems developed using Bragg cells will be discussed. The implementations of
the AO effect are widespread. Much research continues due to the potential
for great speeds and small size of optical processing devices. AO Bragg cell
signal processing is an analog technology showing great promise, in an age
where digital electronics is most emphasized.
3.1 PrincipleofOperation
Acousto-optics deals with the interaction of sound and light. In Bragg cells,
the AO interaction takes place in media such as silicon, lithium niobate, tel-
lurium dioxide, or even glasses. Figure 3.1 shows a basic Bragg cell [1]. As
light enters from the left, it is interfered with by the upwardly propagating
longitudinal sound waves. The sound waves are introduced by a piezoelec-
tric transducer, which is vibrating at an RF frequency of 100 MHz to over
10 GHz. (This is the range of Bragg cell technology and not of any single
Bragg cell device. Bandwidth considerations will be covered later.) The vibra-
tions set up minute changes in the AO material's index of refraction via the
photoelastic effect which creates a diffraction grating [2]. When light travels
across the grating, assuming certain angular conditions, it is diffracted at an
angle proportional to the RF frequency of vibration.
The angular condition is called the Bragg angle and is given by [3]
λ
B
=
2
Λ
sin
α
(3.1)
where
λ is the wavelength of the light in the acoustic medium
Λ is the acoustic wavelength
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