[Yokozawa, Michimura, Enomoto]
We did two things to decrease the floor noise level of MICH from 100 Hz - 1 kHz.

1. Fine adjustment of f1 frequency to 3 x IMC FSR
= Summary =
We changed the f1 frequency from 16.88 MHz to 16.87293 MHz

= How we did =
- We injected a line in IMC frequency control loop at 7130 Hz (from K1:IMC-SERVO_EXC_B), to swing the laser frequency relative to IMC resonance.
- We monitored line height in the spectra of REFLAIR_A_{I,Q}_ERR.
- We changed f1 frequency so that the height was minimized.

=Results =

- See this plot. AM was reduced by a factor of ~100 by setting f1 to be 16.872923 MHz
- The optimal frequency was 16.872923(2) MHz, which corresponds to IMC FSR of 5.624308(1) MHz
- 616 says the FSR was 5.624357(6) MHz, and this difference corresponds to IMC roundtrip length change of 0.5 mm, even though re-installation was held after iKAGRA!
- We re-adjusted the mixing angle of REFLAIR_A_I and Q from 60 deg to 165 deg
- Optical gain of MICH got larger by a factor of ~ 1.5, thanks to this f1 freq. adjustment.
 -- note that cavity pole freq. of IMC is 5.5 MHz, and so f1 amplitude should have got larger by a factor of sqrt( 1 + ( (f1_old-f1_optimal)/f_IMC )^2 ) = 1.6; this improvement factor is reasonable.

2. Whitening filter is applied to REFLAIR_A_I and Q signals 
1st order whitening filter (zero: 1Hz, pole: 10Hz) is applied, and corresponding anti-whitening filter is also applied in foton filter module.

As a result, ADC noise was improved.