Ushiba, Tanaka
## Abstract
Frequncy noise seems to sometimes become noisy and it makes IMC lock acquisition difficult. We changed the temperature of the laser crystal but the situation is not changed. We need more investigation.
## What we did
we found that IMC lock acquisition becomes difficult when PMC error signal (K1:PSL-PMC_MIXER_MON_OUT_DQ), which reflects laser frequency noise, becomes noisy. Also, the frequency noise sometimes becomes noisy. fig.1 shows the spectra and time-series of PMC related signals at the moment when the PMC error signal becomes noisy. As for spectra, blue lines show each spectra when the PMC error signal was relatively quiet, and red lines show each one when the PMC error signal became noisy like the time-series in the upper left panel in the ndscope window. As you can see, the spectra of the PMC error singal (K1:PSL-PMC_MIXER_MON_OUT_DQ) and the feefback signal (K1:PSL-PMC_PZT_HV_MON_OUT_DQ) above 100 Hz becomes large. On the other hand, Intensity spectra of a laser source (K1:LAS-POW_FIB_OUT_DQ) and a PMC transmitted light (K1:LAS-POW_PMC_OUT_DQ) seem not to be changed significantly.
Last night, we swept the setpoint of the temperature control for the laser crystal by sweeping K1:IMC-SERVO_NPRO_TEMP_BIAS_OFFSET from 0 to 1 to find the temperature region where the PMC error signal seemed to be relatively quite. This time, I set the offset value to 1 and turned on the null filter with 10000 ramp time. As long as I remember, when Miyakawa-san and I changed the BIAS_OFFSET value from 0.4 to 0, the setpoint of temperature control was changed by 2 degrees, that is, we scanned the laser temperature within 5 degrees. (Miyakawa-san, please give some comment if you remember)
Anyway, Fig.2 shows the overview of the sweep. Unfortunately, at last ~4 mins, PMC lock was lost due to the actuator satutaion. Moreover, around "-15h" PMC seemed to be lost. Fig. 3 shows the time-series around "-15h". According to this figure, the PMC trans. power drastically decreased before the lock loss even though actuators were not saturated and the FIB_OUT power did not decrease. This may indicate that the mode-hop occurred aroung this temperature.
Using this result, we found the temperature region where not only the PMC error signal but also PLLX/Y error signals seemed to be relatively quite. Fig. 4 shows the region where the BIAS_OUTPUT value is from -6200 to -4200 cnts. From this, we set the BIAS_OFFSET value to 0.3175 so that the OUTPUT value becomes -5200 cnts (= 0.3175*(5*3276), (5*3276) comes from the gain value labeld "cali" in FM1 of the K1:IMC-SERVO_NPRO_TEMP_BIAS filterbank). Also, the feedback amplitude from IMC is allowed within +/-0.06 (= +/-1000/(5*3276)) so that the temperature keeps in this range.
After today's finesse measurements for X/Y arms, we locked only PMC and monitored the PMC and PLL error signals in this state. Unfortunately, however, after ~40 mins, all of errror signals became noisy (fig.5).
We changed the BIAS_OFFSET value to 0.425 by obtaining it with the same procedure. But this value also seems to be bad (fig.6).
We need more investigation.
## Next to do (plan)
- monitor the PMC error/feedback signals with Moku:lab to measure their higher frequency region
- change the applied current to the power amp or the master laser?
## Note
We found that the error signal behaviour seems to be often changed suddenly like fig.7
