Acousto Optic

Acousto Optic


The efficiency of an acousto-optic modulator or deflector is increased by re-using the undiffracted beam, causing the same light beam to travel multiple times through the modulator. In order to maintain the bandwidth of the modulator, the multiple passes are made co-axial. A polarizing beam combiner and a waveplate are used to transmit the incoming beam but reflect the beam on the second pass. The gain in the efficiency of the modulator is particularly large for low efficiency modulators.



This tutorial provides an overview of the technical approaches most commonly used to change the amplitude (modulation) of laser light in the nanosecond or sub-nanosecond time domain in fiber-coupled laser systems. More specifically, this tutorial gives a summary of the pros and cons of the four primary technical approaches to laser modulation. Three of these are based on external modulation : AOM (Acousto-Optic Modulators), EOM (Electro-Optic Modulators), SOA (Semiconductor Optical Amplifiers) and the fourth is by directly driving the laser diode.


acousto optic Q & A


I wonder if the 1st order (or any higher order) light carries both the laser frequency and the 80MHz (the driving signal) or just the laser frequency? The example seems that it is only used for changing the direction of the laser light based on the driving frequency, but will it change the laser frequency also?

The acoustic wave in the material causes a variation in refractive index and the amount of light scattered at a particular angle (its similar to Bragg scattering) depends on the intensity of the modulation. The scattered light does carry both frequencies in the form of blue or red shifting. It shifts by mf where f is the driving frequency and m is the order.

The answer to your question is generally yes, but it depends on how the AOM is driven. Typically AOMs have a piezoelectric transducer on one side of the crystal. In this case the frequency of the diffracted beams is shifted; if the laser frequency is ω0 and the modulation frequency is ωm, then the light in the first order beams will have frequency ω0±ωm, and the light in the second order beams will have frequency ω0±2ωm.

As is mentioned in this Wikipedia article, this is due classically to the fact that the light is diffracted off of compression waves which are moving across the crystal. This causes the diffracted beams to pick up a Doppler shift. In the condensed matter world they would say that the photons absorb or emit a phonon (lattice vibration), and that conservation of energy and momentum implies that the shifted beams must be shifted in frequency.

In some AOMs there are piezoelectric transducer elements on both sides, and driving both of them at the same time produces a standing wave. In this case the diffracted beams do not experience a frequency shift.

The first order is shifted by +80 Mhz. The -1st order is shifted by -80 Mhz The second order is shifted by +160 Mhz.