Summary of the work done today:

Alignment and other work on PSL table

As mentioned in a previous elog, the beatnote amplitude I obtained was tiny - so I checked the alignment of the two beams onto the PD. I did this as follows:

• Checked the alignment of the two beams on the recombination BS. Moved the steering mirror for the PSL beam until the two were aligned, as verified by eye using an IR card
• Turn the steering mirror just before the fast focusing lens and thorlabs PD (kept the fork fixed, just loosened the screw on the post to do this) such that the far-field alignment of the two beams could be checked. I used the BS to tweak this alignment as necessary
• Iterate the previous two steps till I was happy with the alignment
• Return steering mirror before the PD to its original position, tweak alignment until DC level on the PD was maximized (as verified using an oscilloscope) 
• Adjust the HWP just after the lightwave laser such that the power arriving at the PD from the PSL beam and the lightwave beam were approximately equal - verified by blocking each beam and checking change in the DC level

After doing all of this, I found a beatnote at ~-10dBm at a temperature of 45.3002 degrees on the Lightwave. The DC level was ~8V (~4V contribution from each beam). 

PLL and frequency nosie measurements:

Pretty much the same procedure as that described in this elog was followed for setting up the PLL and taking the measurements, except that this time, I used the two SR560s in a better way to measure the open loop TF of the PLL. This measurement suggested a UGF of ~ 10kHz, which seems reasonable to me. I turned the 11MHz marconi off because some extra peaks were showing up in the beat signal spectrum. I judged that the beatnote was not large enough to require the use of an attenuator between the PD and the mixer. I was able to lock the PLL easily enough, and I've attached spectra of the control signal (both uncalibrated and calibrated). To calibrate the spectrum, I did a quick check to determine the actuator gain of the spare Lightwave laser, by sweeping the fast PZT with a low frequency (0.5Hz) 1Vpp sine wave, and looking at the peak in the beat signal spectrum move on the network analyzer. This admittedly rough calibration suggests that the coefficient is ~5MHz/V, consistent with the other Lightwave. Eric suggested a more accurate way to do this would be to match up spectra taken using this method and by locking the PLL by actuating on the FM input of the Marconi - I didn't try this, but given the relatively large low-frequency drifts of the beatnote that I was seeing, and that the control signal was regularly hitting ~2V (i.e shifting the frequency by ~10MHz), I don't think this is viable with a low MHz/V coefficient on the Marconi, which we found is desirable as described here. 

Bottom line:

The spare Lightwave frequency noise seems comparable to the other two measurements (see attachment #2). If anything, it is a factor of a few worse, though this could be due to an error in the calibration? I'm also not sure why the shapes of the spectra from today's measurement differ qualitatively from those in elog 11929 above ~7kHz. 

Some random notes:

• Do we want to do an AM/PM characterization of the spare Lightwave laser as well? It might be easier to do the PM measurement while we have this measurement setup working
• Yesterday, I noticed some peaks in the spectrum of the PD output while only the PSL beam was incident on it, at ~35MHz and ~70 MHz. They were pretty small (~-50dBm), but still clearly discernible over the analyzer noise floor. It is unclear to me what the source of these peaks are.