Chapter 3 Acousto-Optics of the textbook Crystal Optics – Properties and Applications by Ashim Kumar Bain says the following:
In general, acousto-optic effects are based on the change in the refractive index of a medium caused by the mechanical strain produced by an acoustic wave. Since the mechanical strain varies periodically in the acoustic wave, the refractive index of the medium also varies periodically leading to a refractive index grating. When a light beam is incident on such a refractive index grating, diffraction takes place and this produces either multiple-order diffraction or only single-order diffraction. The former is referred to as Raman-Nath diffraction and is usually observed at low acoustic frequencies. The latter is analogous to Bragg diffraction of X-rays in crystals and is referred to as Bragg diffraction. It is usually observed at high acoustic frequencies.
It is said that the mechanical strain causes a change in the refractive index. Since the refractive index is changing, that means that the position/direction of the beam exiting the material will change (by the angle of refraction). Furthermore, I know that acousto-optic modulators are used for shifting the frequency of a laser beam. But doesn't this mean that the use of an acousto-optic modulator for the purpose of shifting the frequency of a laser beam will also mean that your laser beam will be changing position/direction, since both effects are due to the changing refractive index? And so, if one is using an acousto-optic modulator for the sole purpose of changing the frequency of a laser beam, doesn't that mean they would also have to deal with the (potentially undesirable) effect of having the beam change position/direction? In other words, doesn't this create other problems? And yet people do indeed use acousto-optic modulators when they only want the frequency-shifting effect. Is this because the effect of the changing direction/position of the beam is negligible/insignificant?