Sheila, help from Marc Pirello

This morning I went to ISCT6 to look for potential laser feedback which might explain the laser difficulties we've had over the last few months (repeated mode hopping).  Cleaning up a few non-dumped beams and adding a second Faraday in the SHG path doesn't seem to have fixed the power fluctations we've had. 

There were some things on the table that look like potential problems (drawing here: D1201210)

Right at the output of the laser, the quarter wave plate labeled QW1A is upstream of the lens L1, which is mounted on the same base plate as HW1.  The reflection off of HW1 is brighter than the other reflections (I didn't get a power measurement), and was hitting the barrel of MR1.  Since this is mounted on the same base plate as the lens which I didn't want to move, I wasn't able to adjust this alignment much, but did slightly change it.  This reflection was landing on a PDA100A with an ND filter on it, I am not sure why but the PDA was not powered on, so I removed it and replaced it with a razor blade.  

The two rejected beams from the Faraday which go down were not dumped but hitting the table,  I mounted a dump for the one on the output side of the Faraday, but there isn't a way to actually mount one for the beam on the input side which goes directly down, so I placed a razor blade on the table catching that beam.  

I check that most of the power rejected by the Faraday (beam going up on the input side) was a back reflection from the SHG by blocking the path downstream in different places. Peter King helped me find a Faraday yesterday in the optics lab, with polarizers attached.  While there is room to mount this Faraday between BS1 and L2, but when I placed the Faraday there everything downstream (SHG and AOMs, fiber coupling) was mis-aligned.  Because I wouldn't have had time to realign all of that in the maintence window, I instead installed the Faraday in the path to the SHG only, between M22 and DS1.  I had to shim the Faraday to get the height right. This reduced the beam rejected by the first Faraday to something that is not a backreflection.  I realgned the SHG, and adjusted the waveplate HW9 to get the power into the SHG to match what it was before adding the Faraday (around 70mW).  We must be dumping a lot of power on the Faraday dumps, so we might want to replace it with a razor blade.  

The attached screenshot shows the symptom that we are hopping to address by getting rid of possible laser feedback.  At around -3e6 seconds in the attached screenshot and at around -2.6e6 seconds the laser current was adjusted to avoid mode hopping and each time the green power became more stable for a few days (weeks) before it became unstable again.  When we have adjusted the laser current, we have been watching for mode hopping by scanning the SHG, where we can see the additional mode come and go as the current is changed. Today I didn't see evidence for mode hopping when scanning the SHG.  It does not look like my work has resulted in a more stable green power, so as far as that is a reliable diagnostic of the laser problems, this hasn't fixed the problems.