Komori, Tanaka

### Abstract

We obtained the IMC cavity pole frequecny 6.71 +/- 0.09 kHz. It is 1.1 times larger than previous measurement. so the finesse is roughly estimated to be about 418

### What we did

• Following the klog18016 procedure of previous measurement, we connected MC CMS and Moku:lab as follows
  • CMS's Input Section's OUT2 --> Moku ch2; note that there has been a 20 dB attenator before IN1, and the gain between IN1 and this OUT2 was 5 dB (-20 dB + 15 dB (IN1GAIN)).
  • CMS's SlOW OUT1 --> divided by a T --> Moku ch1, while connecting the output to the NPRO PZT.
  • Moku EXC1 --> CMS's EXC B -> SLOW path .
• To set up the configuration similar to the previous measurement, we manually turned off all common boost filters and disconnected the EOM path with the IMC locked.
• We measured the TF from SLOW OUT1 to OUT2. Fig. 1 shows the result. we obtain the cavity pole frequency 6.71 +/- 0.09 kHz from fitting about the phase TF (fig.2).
  • In fitting, we used the data from 2 kHz to 10 kHz where the gain fluctuation seemed to be less than 10 %. This fitting also takes into account the time delay.
•  I used the pole frequency obtained from the phase transfer function to superimpose the gain transfer function. At this time, I adjusted the gain by eye. In this case, it turned out to be about 63 times.
• If the round-trip cavity length is 53.3 m, the finesse (F=c/(L_round*f_pole*2)) is about 418.