Optimizing Beam Size for Increased Diffraction Efficiency

[cks]

In order to increase the diffraction efficiency of the AOM, we need to reduce the beam size of the laser. According to the manual of my AOM, it needs to have the beam size of 220um to get more than 90% diffraction efficiency.

So, I decided to do so by putting a lens in front of it and another one at the back of AOM.

However, I'm not sure what is the focal length I should use for both the lenses?

I remember my professor used to tell me that longer focal length is better, but I forgot why is it the case? Is 50mm considered ok?

 

[Andy Resnick]

Long focal length singlets generally suffer from less aberration, that's the reason for the rule of thumb.

It's been a while since I used an AOD/AOM, but I was never able to get more than about 60% diffraction efficiency. Polarization control of the beam can help, also. And IIRC, I needed to nearly fill the aperture of the AOD to maximize the diffraction efficiency, not minimize the beam size- the spatial density grating within the crystal worked best when the beam size covered many periods.

 

[cks]

The aperture of the AOM is too small for my laser beam spot size, that was one of the reasons why I needed to focus the laser. The AOM is only able to pass through 87% of laser beam while it's not on. From the manual, it should have more than 95% transmission.