Stefan, Evan, Yuta, Kiwamu

We looked at the infrared PDH signal when the PSL frequency was stabilised by the ALS technique. 

The residual noise of the PSL frequency with respect to the arm cavity was measured to be 90 Hz in RMS, integrated from 7 kHz to 10 mHz.

Noise Meausrement:

After a smooth ALS's handoff, we shifted the COMM VCO frequency to place the infrared on top of a 00 resonance. Then we measured the PDH signal at the REFL port while maintaining the resonance only with the ALS beatnote.

Right now, the performance is limite by 80 mHz bump and acoustic-looking forest between 100 and 1000 Hz. The plot above is calibrated in Hz for a 1064 nm laser and cavity pole was removed by applying a zero at 42 Hz. The red curve is REFLAIR_RF9 divided by TRX to have a wider linear range. Because the fluctuation is bigger than the linewidth REFLAIR_RF9 without the TRX normalization underestimated the actual noise.

Linearization of the PDH:

In order to expand the linear range of the PDH signal, we did the legacy technique where the PDH signal was divided by the intracavity power. We used LSC_X_TR_A_LF for the denominator. The attached screenshot shows the waveforms with and without the division. The red curve is the one with the division. The pink curve shows the one without the division. You can see that the red curve can go to bigger numbers.

Also, there were glitches which seemed to happen roughly once in 30 seconds. The attaches shows an example screenshot of it. Typically it somehow drops the IMC transmitted light at the same time. As you can see, LSC_CARM_IN1 shows a sharp spike.