We now consider the diffraction of light by acoustic waves in an optically transparent medium in which the acoustic wave is excited. An optical beam is incident onto the cell and travels through the acoustic beam. Via the elasto-optical effect, the traveling acoustic wave sets up a spatial modulation of the refractive index which, under proper conditions, will diffract the incident beam into one or more directions.

The AO interaction can be viewed as a parametric process where the incident optical wave mixes with the acoustic wave to generate a number of polarization waves. The polarization waves in turn generate new optical waves at various diffraction orders.

But the most interesting particular case is Bragg diffraction.

In the Bragg limit, only the first-order diffracted light grows to a finite amplitude.

Wavevector diagram of Bragg diffraction in an optically anisotropic medium such as a birefringent crystal (TeO2) is shown below.

Significant diffraction of light occurs only when the exact momentum matching is met.

In the general case of AO interaction in an anisotropic medium, the magnitudes of the wave vectors are given by:

where Λ=V/f  is the acoustic wavelength, and V and f are the velocity and frequency of the acoustic wave.

In general, the refractive indices of the incident and diffracted light beams are different.

As an example, consider the Bragg diffraction in a positive uniaxial crystal.

From the wavevector diagram shown in the above figure and using the law of cosines one obtains:

One of the main parameters is diffraction efficiency that is given by

where P_a is the acoustic power, H is the acoustic beam height, ρ  is the mass density, V is the acoustic wave velocity, and is a material figure of merit.

The diffraction efficiency is thus linearly proportional to acoustic power.

TeO2 crystal has extremely high AO-efficiency (М₂ = 793 х 10^-18 s³/g and up with defined geometry interaction) that allows to use Paratellurite for low acoustic power with out decreasing diffractive efficiency in comparison with other materials.

As acoustic power increases, the diffraction efficiency saturates and approaches 100 percent.

Thus in the Bragg regime, complete depletion of the incident light is obtainable.

These features give possibility efficiently design all types of AO-devices with technical characteristics permitting, particularly, refuse from high-power cooling systems as well as construct a number of unique AO-devices, for instance, AOTF with high aperture to process images of multichannel devices.

 

Reference list:

[1] Xu J and Stroud R 1992 Acousto-Optic Devices (New York:Wiley)

[2] Handbook of optics. CHAPTER 12 ACOUSTO-OPTIC DEVICES AND APPLICATIONS I. C. Chang

[3] Goutzoulis A and Pape D 1994 Design and Fabrication of Acousto-Optic Devices (New York: Dekker)