KAGRA Logbook

Aso, Nishino

Abstract:

We did three things.

1) We've closed the OMC-ASC loops and centred the beam positions. The filter parameters were slightly tuned to make it stable.

2) We've locked the cavity by dithering

3) We've roughly estimated the transmissivity of the OMC.

Details

1) First we added a booster filter to OMC-ASC control servo (zpk([1],[0.01],100,n)) instead of the integrater. We also added a phase bubble to the (#21841) LeadLag filter.

We could fix the beam positions onto almost center of each QPD.

We could see a oscillation at the frequency of ~850Hz and its harmonics. Espescially the 5th harmonics around 4250 Hz appears around 5 kHz, the dithering frequency. This peak dissapears after we changed the clock gain from 2000 to 20000. (see the picture below. K1:OMC-LSC_ERR_OUT has a peak around 750 Hz. It doesn't come from dithering, but from the breadboard oscillation.)

2) We successfully locked the OMC. The lock point was around 30 mW in the sum of the DC output. Since the OMC chanber is still in air, we need a lot of PZT range and the feedback signal is completely saturated (the range is -10000 to 10000). The lock was unstable but last for tens seconds.

Then we managed to introduce the transmittance of the OMC4 outside of the chamber and into the GigE camera.

3) We roughly estimated the transmissivity. First we need to reconstruct the voltage.

The incident power: 75 [mW]

Transmitted power: 30 [mW]

count2mW (O3GK): 1.7e-3 [mW/count]

voltage2count (ADC, 16bit): 610 [uV/count]

-> V_trans = 10.8 [V]

According to the conversion factor $\alpha$ [V/mW] measured in NAOJ (#21078), and also taking into account that we changed the transimpedance from 100 Ohm to 400 Ohm (see Dan Chen's report), the estimated transmittance power was:

V_trans* $\alpha$ *(100/400) $\simeq$ 38 [mW]

Transmissivity $\simeq$ 51 %

This result is not consistent with the value ~97% measured in NAOJ(#20764). Now the lock is unstable and we cannot scan the frequency, so maybe it's better to seal the OMC chamber in vacuum and measure the finesse to check the loss.

And also we need more investigation of the total gain of the readout both A/D circuit.