Acousto-optics
Acousto-optics is a branch of physics that studies the interactions between sound waves and light waves, especially the diffraction of laser light by ultrasound (or sound in general) through an ultrasonic grating.
However, the growing principal area of interest is in acousto-optical devices for the deflection, modulation, signal processing and frequency shifting of light beams.
This is due to the increasing availability and performance of lasers, which have made the acousto-optic effect easier to observe and measure.
Technical progress in both crystal growth and high frequency piezoelectric transducers has brought valuable benefits to acousto-optic components' improvements.
Optics has had a very long and full history, from ancient Greece, through the renaissance and modern times.
[3] As with optics, acoustics has a history of similar duration, again starting with the ancient Greeks.
[4] In contrast, the acousto-optic effect has had a relatively short history, beginning with Brillouin predicting the diffraction of light by an acoustic wave, being propagated in a medium of interaction, in 1922.
[7] The particular case of diffraction on the first order, under a certain angle of incidence, (also predicted by Brillouin), has been observed by Rytow in 1935.
Raman and Nath (1937) have designed a general ideal model of interaction taking into account several orders.
The acousto-optic effect is a specific case of photoelasticity, where there is a change of a material's permittivity,
For a plane acoustic wave propagating along the z axis, the change in the refractive index can be expressed as [8] where
is the amplitude of variation in the refractive index generated by the acoustic wave, and is given as,[8] The generated refractive index, (2), gives a diffraction grating moving with the velocity given by the speed of the sound wave in the medium.
In contrast, Bragg diffraction occurs at higher acoustic frequencies, usually exceeding 100 MHz.
It is simply a fact that as the acoustic frequency increases, the number of observed maxima is gradually reduced due to the angular selectivity of the acousto-optic interaction.
A simple method of modulating the optical beam travelling through the acousto-optic device is done by switching the acoustic field on and off.
So the acousto-optic device is modulating the output along the Bragg diffraction angle, switching it on and off.
The acousto-optic medium must be designed carefully to provide maximum light intensity in a single diffracted beam.
So to increase the bandwidth the light must be focused to a small diameter at the location of the acousto-optic interaction.
This minimum focused size of the beam represents the limit for the bandwidth.
The principle behind the operation of acousto-optic tunable filters is based on the wavelength of the diffracted light being dependent on the acoustic frequency.
: the wedge angle between the input and output faces of the filter cell (the wedge angle is necessary for eliminating the angular shift of the diffracted beam caused by frequency changing);
: the angle between the incident light wave vector and [110] axis of the crystal;
: the angle between the input face of the cell and acoustic wave vector;
of the filter are defined by the following set of equations,[10] Refractive indices of the ordinary (
) polarized beams are determined by taking into account their dispersive dependence.
between the diffracted and non-diffracted beams defines the view field of the filter; it can be calculated from the formula,[10] Input light need not be polarized for a non-collinear design.
AOD technology has made practical the Bose–Einstein condensation for which the 2001 Nobel Prize in Physics was awarded to Eric A. Cornell, Wolfgang Ketterle and Carl E.
In an AOM, only the amplitude of the sound wave is modulated (to modulate the intensity of the diffracted laser beam), whereas in an AOD, both the amplitude and frequency are adjusted, making the engineering requirements tighter for an AOD than an AOM.
Fused silica is used as a standard to compare when measuring photoelastic coefficients.
Softer materials, such as arsenic trisulfide, tellurium dioxide and tellurite glasses, lead silicate, Ge55As12S33, mercury(I) chloride, lead(II) bromide, with slow acoustic waves make high efficiency devices at lower frequencies, and give high resolution.
