I have been working at relocking the PMC and have been running into a lot of problems.

The table layout when I approached it - the old mode matching lenses were still in place, but the last two the steering mirrors into the PMC after the lenses were gone. I looked at an old elog photo and put the last two mirrors back about at the right place.

Then I

1. Minimized the table power by rotating almost everything into s before the polarizing beam splitter ~100mW of P on the table.
2. Aligned crudely aligned to the PMC until I got fringes out the other side.
3. Aligned more by hand, sweeping the cavity with a funciton generator until I started to see what looks like 00, then try to crudely maximize this by hand/eyeball
4. Then I set up a PDH locking using

•  A Marconi at 10 dBm and 35 MHz into a splitter
•  One output of the splitter into the EOM
•  The other output through a 3 dB attenuator into the L channel of a ZAD-6 Mixer
•  A Thorlabs PDA10A photodiode (not the best for 1064 - Si) on PMC REFL
•  The output of the PD into the RF of the Mixer
• The Output of the Mixer into an SR560 amplifier
• The output of the SR560 into the DAQ

I was unable to get a good error signal /unable to lock by toggling a 1 Hz pole as I swept through resonance with various DC gains and offsets

Things I tried:

• Changing the PD cable length by 1.4m (about .437*pi)
• Changing the gain of the SR560 to get in the middle of the dynamic range of my ADC
• Changing the sign of my feedback to be wrong (desparation?)
• Checked the alignment onto the photodiode on a scope (tens of mV)
• Checked the output of the mixer into an HP4395A to make sure the EOM was working - I saw the 35MHz mixed down sideband beat signal - so yes, the eom works.
• Went to measure the beam waist (maybe my coupling is so bad that even perfectly aligned my error signal would be buried in the noise?) and noticed a serious ghost beam almost coaxial
• Qualitatively compared the modal content of the beam to an old measurement to see if I had a bimodal PSL output
• Replaced the first mirror after the mode matching lenses by one with a wedge in a non circular mount.
• Not using the SR560 and just using an analog passive low pass filter (pole at 1.9 MHz) to kill my mixed down 35 MHz sideband beat
• Putting an offset on my DAQ input based on what I see as the input when I sweep through 00
• Using the 35.5 MHz Wenzel frequency doodad (on principle - not as troubleshooting)