[ericq, gautam]

Summary of work done tonight:

• The PDH setup at the Y-end has been restored after I had pulled the whole thing apart some weeks ago to see that nothing was obviously wrong with the uPDH box
• Adjusted the temperature of the Y-end laser such that a beatnote was obtained - I did this using the IR beat (the end laser temp wasn't updated after the PSL temp was changed recently)
• The Y green beatnote was found easily, there was no alignment on the PSL table necessary, though there is room to improve this situation (beatnote amplitude was ~ -35dBm though we are used to more like -25dBm)
• The X green beat remains elusive - I played around with the alignment onto the green beat PD at the PSL table for some time, and the two beams are aligned as far as I can tell given the constrained area available in that area. It may be that I have to clear some optics, do a rigorous near-field/far-field alignment of the two beams and then try again
• Since we had two strong (-5dBm for Y, -9dBm for X) IR beatnotes, we decided to take the ALS noise spectra for these. So as to not overload the amplifiers, a -10dB attenuator (-6dB) was placed directlty after the Y (X) IR beat PDs, before routing these signals through the usual green beat signal chain. Attached is the measured spectrum. The new values of the temperature sliders at which beatnotes can be found are : 1700 for X and -5990 for Y (spectra taken at these values).

To do:

• For both ends, find the three temperatures at which we have beatnotes, and choose the middle one
• PM characterization of AUX X laser - it may be that the excess noise in the X spectrum is due to sub-optimal PDH
• Align the Y green better at the endtable, also take an OLTF measurement for the Y PDH loop
• Re-check the alignment onto the green beat PD for the X beat

Remarks:

• The Lightwave laser controllers differ from the Innolight ones in that it is not possible to directly set the signal to the SLOW control BNC to 0, and have that as the new reference point. Rather, there seems to be some setpoint which is saved as a reference, and the moment any signal is applied to the SLOW control BNC, it adjusts the actual temperature w.r.t. this saved setpoint. I believe it is possible to update this setpoint (it is also possible to update the calibration of the power readout, this is an additional issue at the X end), but since this wasn't critical, I've left it as is for the moment...
• The ALS nosie spectrum for the Y arm IR beat is surprisingly good!