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?