Yuhang, Pierre, Eleonora P, and Aritomi

In the beginning, Matteo and Eleonora modified the TAMA PD by apply two resistors and this amplifies the AC signal. This activity increased the SNR and make it possible to lock the coherent control loop.

Then I and Chien-Ming make this amplification even larger again, but the amplification increased both signal and noise. After this, we found there is a bandwidth limitation of AC channel op-amp when we increase the gain.

So we decide to use a higher speed op-amp and try to increase a bit the gain and to see if we can increase SNR furthermore. This replacement was did based on the modified PD improved by Chien-Ming. There were problems while Pierre was soldering the new op-amp. The copper board pin was fallen! But, fortunately, the lost pins are the pins which are not connected to anything else. Also, the day when we just finished the modification, we tested it on the bench but we didn't see any coherent control error signal. Actually, we found the very similar thing(cannot see BAB be amplified and de-amplified after OPO transmission) on the next workday.   I and Aritomi-san we did the measurement of parametric (de)amplification measurement again while scanning the green phase with frequency slowly. After that, we could see everything well. We also realized at that time, the characterization of best OPO temperature/p-pol PLL locking frequency should be done maybe once a month.

After the replacement, we characterized the dark noise and compare it with the old dark noise before replacement. We found an increase of dark noise, we think this may come from the rise of a large current bias on the op-amp.

Then we measured the free-running noise and error signal noise spectrum (residual locking noise). The results are shown in the attached figures.

In the future,

we will also try to play a bit with the amplification of the AC signal. And also try to reduce the current bias of op-amp.