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Message ID: 1703     Entry time: Sun Aug 7 23:27:01 2016
Author: awade 
Type: DailyProgress 
Category: Other 
Subject: Recovering alignment of north refcav and optimisation of PDH error signal 

After misalignment the north path (PSL:1697) today I installed some irises and painstakingly walking the beam back into alignment.  Then, scanning the laser PZT and stepping through crystal temperature, I found the TEM00 mode and realigned.  The visibility attained was 20.2 % (2.41 V off res., 1.60 V on res.): this is marginally better than the 13% earlier reported.

Power going into the north refcav now is measured to be 1.679 mW with 1.489 mW returning when off resonance, this is a 12 % round trip loss.  Not sure what is happening there, something to check on later.

Transmission signal on the Thorlabs PDA10CS transmission PD is 3.00V on resonance and 9.40 mV off resonance (there is a dark offset of 9.40 mV).  Side bands (the two 14.75 MHz PM side bands) were 37.8 mV, this is 28.4 mV above the dark voltage. Thus an estimate of the modulation conversion to sidebands is 0.9% of the carrier.

I found the error signal at the monitor port of PDH control box to be -346 mV to +432 mV (~Vpp=770 mV).  It was also not very symmetrical, especially for the side band components. (sorry no picture, I don't have a working floppy disk).

As the RF PD for the PDH loop have been move in distance from the refcavs since the last implementation by Tara and Evan, we should expect the effective delay between RF and LO to have changed. However, the cabling has been kept the same. Using a SRS DB64 delayer box in the LO electrical path, the error signal could be improved to -832 mV to +684 by delaying the signal by 20 ns: this effectively doubled the error signal peak-to-peak. This may explain some lost gain in the control loops.  It is good practice to start with the best possible error signal anyway, so I made some adjustments. There was a 3 m SMA cable in the PD RF line that I replaced with a 1 m cable. I then swapped out a short 20 cm SMA in the LO line for a 2.2 m BNC (not ideal changing types, by all I had for a single length for now).  Thus I affected a change of 4 m between the paths, increasing the phase delay by ~20 ns (waves travel~2/3 c in coaxial cables). The resulting error signal was -800 mV to 552 mV. This is not as much as I'd hopped, by I think the mounts may have relaxed in the time it took me to optimize cable length.

Tomorrow I will try to look at south path PDH optimization.


-awade, Sun Aug 7 23:26:49 2016



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