I am trying to understand the salient differences between photoelastic modulators (PEMs) and acousto-optic modulators (AOMs).

Both of these devices are based on the photoelastic effect - that is, under mechanical stress, a medium's optical properties (e.g., refractive index) change. PEMs are generally used as tune-able phase retarders (refractive index along one axis changes compared to the orthogonal direction), whereas AOMs induce a periodic index variation, yielding effects similar to Bragg diffraction. AOMs are often said to utilize the acousto-optic effect, but that is a subset of the photoelastic effect (not clear on what the distinction is).

Am I correct to assume that the same underlying principles apply to both the PEM and AOM? The only difference is that in the PEM, we have a standing (sound) half-wave, whereas in the AOM we have a travelling wave with many cycles present in the medium, yielding spatially-periodic modulations? Is the direction of applied stain another important difference between the two?


I think you are correct that in an AOM we have a traveling wave of phonons. I know experimentally that laser diffracted in the same(opposite) direction as the RF input will be up(down)-shifted in frequency. This makes sense if one thinks about the momenta of the photon and phonon adding or subtracting. Most AOMs also consist of an RF transducer bonded on one side of the crystal with a piece of rubber damping material placed at the other end, which is also often cut on an angle to reduce RF reflections from the opposite crystal face.


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