Acousto optical modulators

[Niles]

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? Niles.

 

[Niles]

OK, I understand my error now.

 

[Andy Resnick]

What book are you using? I have one by Korpel, and it's not working for me...

 

[Niles]

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

 

[sardar]

Hello Niles,

Thank you for bringing up this interesting topic. Acousto-optic modulators (AOMs) are devices that use acoustic waves to modulate light. The Bragg regime, as you mentioned, refers to the condition where only a single diffracted beam is present. In this regime, the frequency shift Ω of the diffracted wave is indeed constant, as it is determined by the material properties and the angle of incidence.

However, the angle of incidence can be changed by turning the AOM, as you mentioned. This means that the angle θ of the diffracted beam can also be changed. And since the frequency shift Ω is dependent on θ, it can also be changed. This is why the frequency shift can vary for different angles of incidence, even though it is a constant value for a fixed angle.

In terms of forward and backscattering, this has to do with the direction of the acoustic wave in the AOM. In forward scattering, the acoustic wave propagates in the same direction as the incident light, resulting in a smaller angle of incidence and therefore a smaller frequency shift. In backscattering, the acoustic wave propagates in the opposite direction, leading to a larger angle of incidence and a larger frequency shift.

I hope this helps to clarify your confusion. AOMs are fascinating devices that have many applications in optics and photonics. If you have any further questions, please don't hesitate to ask. Thank you.