[aaron, shruti]

I observed an unexpected behavior this afternoon that I still can't explain. I managed to get the cavity locked using the LB box servo in LFGL mode. When I turned off the servo box by switching to 'lock off' mode, the cavity maintained lock and the PDH error signal was passed through to the current modulation point. Only the LB servo was driving the modulation point, and the temperature tuning was also off. My understanding from the LB manual is that in 'lock off' mode, no control signal is summed into the output signal... so why was the PDH error signal passed through?

Later, we started controlling the temperature of the laser diode using some slow epics channels.

1. Lock the cavity using the LB box (in LFGL mode) to modulate the current
2. Turn on the python PID loop. The script and configuration we used are in controls@spirou:~/cryo_lab/scripts/temp_control/.
   • Operated the temperature control loop at 5 Hz in pure integrator mode, with K_I = 0.1-0.5       
   • I have the PDH setpoint at 0, and checked that any DC offset on the ADC is small (< 10 counts)
3. Next, we tuned the input and sweep offsets of the LB box to optimize our dynamic range. We need the temperature control loop on for this operation, to avoid railing the current modulation input at the laser driver
   • first, tuned the LB box input offset until the error monitor was centered on 0 V
   • Next, tuned the LB box sweep offset until the current control was centered on 0 V
4. With the low frequency gain limited to 20 dB, the PDH error signal was wandering at a few Hz. We turned up the gain limit to 40 dB, though we could have increased the gain of the temperature PID until it took over at those frequencies.

We observed a 100 kHz oscillation in the noise spectrum after this procedure. We weren't able to change the oscillation by tuning the laser current (within 10 mA) or servo gain (while maintaining lock).

We measured the open loop transfer function of only the LB servo box (feeding back its output to -B), and didn't see a feature at 100 kHz or an oscillation in the noise spectrum. We measured the transfer function in both 'lock on' and 'LFGL' modes. We did observe a broad peak near 4.5 MHz in the noise measured at the LB error monitor (attachment 1, 2. The sharp peaks are artifacts from the Moku present even with no input connected).

Data are available in the ligo.wbridge google drive. Attachment 7 shows the broad 100 kHz oscillation on the PDH error signal in purple.

Attachment 3: Updated OLTFs with LB1005 measured separately

We measured the OLTF of the LB1005 servo by feeding back to itself with the setting 'LFGL' (Low Frequency Gain Limit) set to 40 dB, INT (pure integrator until it hits LFGL), and gain of 5.1 at 300 kHz.

 I (Shruti) think the gain setting was slightly different today which makes the green curve slightly higher in magnitude than the orange curve, but otherwise it seems to track it and do nothing strange. The orange curve is the TF of the LB1005 derived when the servo is used to lock the laser to the PSOMA cavity as in elog 2759. The phase for both measurements also seems to be the same up to 1 MHz.

Data for LB1005 in Attachment 4 and remaining data used for the plots in Attachments 3 and 4 of elog 2759.

Attachment 5: Measured OLTFs again with new settings

After these changes we measured the loop TFs again. It is strange that the phase of the open loop first increases and then decreases.

Also there is a weird dip at 80 kHz (right above the UGF) in both the Plant TF and the full open loop TF.

The LB1005 measured separately and in the full closed loop differ by ~4dB, the full loop setting resulting in the lower curve, even with the same gain setting. Otherwise the two run almost parallel, at least below 1 MHz.

The data for this is in Attachment 6.