Naoki, Vicky, Nutsinee

Recently we noticed that squeezing level at homodyne depends a lot on CLF power. To investigate this, we measured squeezing at homodyne with different CLF power. 

We changed CLF power by rotating HWP before PBS in SQZT0 and measured squeezing at homodyne. We also monitored CLF launch power (H1:SQZ-CLF_LAUNCH_DC_POWERMON), CLF OPO REFL (H1:SQZ-SHUTTER_I_TRIGGER_OUT_DQ), and CLF6 (H1:SQZ-CLF_REFL_RF6_DEMOD_RFMON). Since the optimal squeezing phase is different for different CLF power, we tweaked the squeezing phase for each CLF power. The gain of CLF and LO loops should be changed for different CLF power to have the same loop gain, but we did not change them for this quick measurement. During this measurement, we removed the CLF ISS from guardian since we need to tweak the setpoint of CLF ISS for different CLF power.

The attached figure shows the squeezing at homodyne with different CLF power. The squeezing level with 40uW of CLF launch power is about 3.5dB, while it is 2dB with 120uW of CLF launch power. The squeezing level did not improve by reducing the CLF power to 20uW.

Note: It seems that there is a discrepancy between CLF launch power and CLF6. For example, the difference of CLF6 between 40uW and 79uW of CLF launch power should be 6dB, but it is 9dB. Vicky pointed out that one of the reasons could be due to the junk light at CLF launch PD (non-optimally polarized light). The non-optimally polarized light at CLF launch PD will not reach at OPO REFL since it will be rejected by in-vac PBS. In fact, when we reduced the CLF launch power from 79uW to 40uW by changing the drivepoint of CLF ISS (reduce the CLF power generated by AOM without rotating HWP), CLF6 is -28. This value makes more sense than -32.3 when we reduced the CLF power by rotating HWP. We need more investigation.