(Uchiyama, Tanaka, Miyakawa)

We still suspected the slight mode hop, so we changed the laser crystal temperature from 37.0 to 36.0 degrees, which was the same temperature as December reached to DC readout with the neoLASE laser.

Power estimation at the PMC REFL and TRANS was not consistent with the neoLASE laser, so we calibrated the input power and PMC REFL power at DCPD and RFPD, with the actual measurement at the beam dump of PMC REFL. The measured power was 21.16W with 9A current. We changed the conversion gain at K1PSL_PMC_REFL_RFDC from 2.360 to 2.030 and K1LAS_POW_FB from 6 to 7.5. 

In the morning, we compared the free run RIN (relative intensity noise) at ISS PD between the FB laser and the neoLASE laser, and the neoLASE had 10 times higher noise below 10Hz to 1Hz (see Fig. 1).

Then we compared the RIN between upstream and downstream before PMC. The up location was before the AOM, and the down location was the reflected light of PMC (see fig.2). Fortunately, the upper location showed less RIN below 10Hz, which means that the laser itself is OK for intensity noise and noise is added somewhere. The high frequency of blue line is limited by the detector noise because very low power was injected.

We suspect the beam is clipped at the AOM, and actually, we found a clipped shape after the AOM. We moved the closest mirror position to eliminate the clip, and the intensity fluctuation looked a bit better. At that time, we could engage the ISS servo with the integrator for a while, but it did not last long, so we continued the investigation of RIN at many places.

For safety and convenience, we used only the master laser of neoLASE for the investigation. RIN  became worse and worse from upstream to the downstream (see Fig. 3). However, the mode was not matched at all if only the master was ON, and all the beam sizes during measurement were very large. This behavior of growing noise might be due to the clipping at each optics.

At the very last stage, we found a clip at the RFPD in 20W operation. We tried to find the best position on the PD, and the DC power increased by ~5%.

We should find good and safe locations to measure RIN with 9A next.