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MIF (ITF Control)
yutaro.enomoto - 18:02 Wednesday 25 April 2018 (4831) Print this report
Fine adjustment of f1 freq. and application of a whitening filter.

[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.

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