In the last lock, while the calibration measurements were happening and while the IFO was thermalizing before the calibration measurements, we took the squeezer offline to check some things that would ideally be checked after the laser current is changed.
The SHG temperature didn't need to be changed, but its conversion efficiency has dropped, so I reset the minimum conversion efficency threshold.
I adjusted the half wave plate to bring the green power into the fiber closer to 20mW, according to the launch diode. I would like to double check the calibration of the two SHG power monitor diodes. I added a parameter file for the squeezer guardians, so hat we would not have the TEC temperature hard coded in multiple places and can update it more easily. (I also changed the temperature to 33.32 degrees).
I then tried to measure the nonlinear gain at a couple of different green powers, to make this easier I also added parameters that would change with green power to the parameter file. I will plot and post the data soon.
We still need to double check the squeezing angle next time we are locked.
The message of these non linear gain checks is that we get consistent enough results for different green powers and different methods of measuring the nonlinear gain.
Looking at the SHG power launch diode and the OPO reflected diode at a time when the OPO was unlocked earlier today, the transmission from the launched green power to incident power on the OPO is 15.8%. I measured the powers using the launch diode for these measurements, but have multiplied everything by 15.8% to make the plots in terms of green power incident on the OPO.
We have wondered if there is something wrong with our estimation of the nonlinear gains, in part because our estimates of losses and phase noise depend on them, and in part because we have had discrepancies when using different methods to measure the nonlinear gain. We measure the nonlinear gain by locking the OPO with green light, and injecting a low power IR seed beam through the path used for the CLF. We measure the IR power on a diode in the homodyne path, while using a PZT to modulate the phase of the seed between amplification and deamplification. One method for estimating the nonlinear gain is to measure the maximum and minimum of the transmitted IR, and use these to derive the nonlinear gain. Another method is to first measure the unamplified seed by slowly scanning the OPO with no green power.
Here the normalized non linear interaction strength is x = sqrt(P/P thresh) and the maximum amplification (nonlinear gain) = 1/(1-x)^2 while the minimum from deamplification is 1/(1+x)^2. In order to estimate x from the ratio of the max/min we get x = (sqrt(max/min)-1)/(sqrt(max/min)+1)
The first plot shows the amplification and deamplification measured for different green powers, with the expectation for a threshold of 31mW plotted for reference. This seems fairly consistent with expectations. The second attached plot is intended to help compare the three possible methods of estimating the normalized non-linear interaction strength (or the threshold) from each of these measurements. (Based on the ratio of maximum over minimum, or max/ unamplified or minimum / unamplified) The upper subplot shows x, while the lower plot shows the infered threshold based on each measurement. While there is a systematic error between the different techniques, as is most clear from the threshold power plot, it is not large enough to really have much of an impact on the estimated interaction strength.
Note: I measured the dark offset on the diode used to measure the amplified seed while the OPO was scanning, and treated this as a dark offset which is subtracted from all measurements. If I ignore this (set it to 0), I do get large discrepancies between these methods.