Acousto-optic modulator

They are used in lasers for Q-switching, telecommunications for signal modulation, and in spectroscopy for frequency control.

An oscillating electric signal drives the transducer to vibrate, which creates sound waves in the material.

These can be thought of as moving periodic planes of expansion and compression that change the index of refraction.

[1][2] When the incident light beam is at Bragg angle, a diffraction pattern emerges where an order of diffracted beam occurs at each angle θ that satisfies:[3]

The angular deflection can range from 1 to 5000 beam widths (the number of resolvable spots).

Note that this formula also tells us that, when we start at a high RF power P, it might be higher than the first peak in the sine squared function, in which case as we increase P, we would settle at the second peak with a very high RF power, leading to overdriving the AOM and potential damage to the crystal or other components.

To avoid this problem, one should always start with a very low RF power, and slowly increase it to settle at the first peak.

Note that there are two configurations that satisfies Bragg Condition: If the incident beam's wavevector's component on the sound wave's propagation direction goes against the sound wave, the Bragg diffraction/scattering process will result in the maximum efficiency into m = +1 order, which has a positive frequency shift; However, if the incident beam goes along the sound wave, the maximum diffraction efficiency into m = –1 order is achieved, which has a negative frequency shift.

One difference from Bragg diffraction is that the light is scattering from moving planes.

This frequency shift can be also understood by the fact that energy and momentum (of the photons and phonons) are conserved in the scattering process.

A typical frequency shift varies from 27 MHz, for a less-expensive AOM, to 1 GHz, for a state-of-the-art commercial device.

The acoustic waves induce a birefringent phase-shift, much like in a Pockels cell[dubious – discuss].

The acousto-optic tunable filter, especially the dazzler, which can generate variable pulse shapes, is based on this principle.

[6] Acousto-optic modulators are much faster than typical mechanical devices such as tiltable mirrors.

However, these require very high voltages (e.g. 1...10 kV), whereas AOMs offer more deflection range, simple design, and low power consumption (less than 3 W).

An acousto-optic modulator consists of a piezoelectric transducer which creates sound waves in a material like glass or quartz . A light beam is diffracted into several orders. By vibrating the material with a pure sinusoid and tilting the AOM so the light is reflected from the flat sound waves into the first diffraction order, up to 90% deflection efficiency can be achieved.
A sketch to explain the Bragg condition for an AOD. Λ is the wavelength of the sound wave, λ is that of the light wave, and n is the refractive index of the crystal in the AOD (which should be omitted. This is a mistake). The +1 order has a positive frequency shift compared to the incident light; The 0th order has the same frequency as the incident light. The minor transverse displacement of 0th order from the incident light represents the refraction inside the crystal.