Yuhang, Marc, Michael

We tweaked a lot of the infrared mode cleaner control loop, from optics to electronics.

Details

First of all, the power to the IRMC_REFL PD was decreased using an ND1 filter, since the voltage was near saturation. A lens was also placed to reduce the beam size on the PD.

We investigated a lot of connections coming from the photodetector and going to the servo board, cables, servo settings, DC blocks. The reference level for the IRMC error signal was about 1.2 V before the replacement of the main laser EOM, and right now we have 120 mV. By comparison, the SHG error signal only went down by about a factor of 4 after the EOM change. They each use the same PD (Thorlabs InGaAs RFPD PDA05CF2) so it seems that there was some other source of reduction. We scanned and fixed all of the settings of the IRMC servo board, particularly the servo gain, detection threshold and PZT scanning speed. Halfway through we had some trouble locking, so decided to check with SR560 instead of the servo, and indeed it could lock with quite low gain. However, for the servo the gain is not sufficient to provide a good signal. By slowing the scan speed on the servo and moving the scan window around using the PZT offset knob on the high voltage driver, it could be seen that the servo is attempting to lock with a flashing red light, so it is detecting the PDH signal and the reflected peak crossing the locking threshold.

Eventually we decided to consult Pierre Prat regarding the electronics board. We propose to replace resistor R33 of figure 1 which determines the amplification of the gain potentiometer.