Today I worked on the alignment for AOM.
The following is the procedure what I did.
- Adjusted the lens position before AOM in order to locate the beam waist around the AOM.
- Played with STMs and AOM stage and maximized diffracted beam power.
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Then adjusted RF level to maximize diffracted beam power.
At that moment, the beam power input to AOM was 8.6mW and that of diffracted beam was 4.6mW.
So the efficiency was about 55%. - Then put a lens after the AOM to make collimated beam.
- Put an iris to eliminate 0 order diffracted beam and pick -1st order one.
- Put a mirror to make the reflection beam enter the AOM again.
- Played with the end mirror to see the double-passed 1st order diffracted beam.
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Eventually I found 2 beams are picked off by PBS.
One is 0 order double-passed beam and the another is -1st order double-passed one.
I mean the former one is frequency shifted only once, and the latter one is shifted twice. - Measured the double-passed beam power and it was 2mW.
Double-passed beam has the same beam path as input beam.
I think this may be reasonable for double-pass AOM configuration.
I think the next step is to optimize the alignment to maximize double-passed beam and minimize the beam jitter.
Also I should check the spec sheet to confirm the diffraction efficiency and estimate the requirement for beam jitter.
In addition, I connected the tube between scroll vacuum pump and cryostat chamber (attached).
According to the spec sheet, the diffraction efficiency of AOM is 87.7% at 1060nm and 500um beam diameter with 1.50W RF power.
This morning, I tweaked the STMs to increase the diffracted beam power.
However the diffraction efficiency was about 50%.
I'm suspecting the beam diameter is so small that the efficiency is low.
Since I have another double-pass AOM, I will input larger diameter beam to another AOM and see the effect of beam diameter on efficiency.
Also I will check the RF power.