# Acousto optical modulators

by Niles
Tags: acousto, modulators, optical
 P: 1,818 Hi Say I am looking at an AOM working in the Bragg regime (i.e., only a single diffracted beam). It is easy to show using Bragg's law that the frequency-shift Ω of the diffracted wave is given by $$\Omega = 2n\omega \frac{v}{c}\sin(\theta)$$ Here Ω is *also* the frequency the AOM is driven with, in other words the LHS is constant in the sense that in does not depend on the incoming light (so the frequency-shift imparted on the wave is constant). However, the RHS does depend on the incoming light, since the angle θ of the diffracted beam is equal to the angle of incidence of the incoming beam, so I can change it easily by e.g. turning the AOM. In my book it says that the shift Ω is zero for forward scattering and maximum for backscattering. This is what I don't understand: The shift Ω is the same as the frequency of the phonons in the material, which is *constant*. So how can I change the frequency shift of the diffracted wave by changing the angle on incidence? Best, Niles.
 P: 1,818 OK, I understand my error now.
 Sci Advisor P: 5,401 What book are you using? I have one by Korpel, and it's not working for me...
P: 1,818

## Acousto optical modulators

I am using Boyd's Nonlinear Optics, it has a nice chapter on spontaneous light-scattering including Acoustooptics. I hope it works out.

Best,
Niles.

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