[Fujimoto, Ushiba, Hirose]

Summary

It was confirmed that the POP RF90 signal was reduced at PRMI(klog37667). Currently, as the IRX failed to lock, the LO output is being operated at 10 dBm instead of 13 dBm (klog #37656). It has been suggested that the input-output characteristics of the harmonic generator may be non-linear, and we considered the possibility that the modulation index had decreased in line with the reduction in LO output. We therefore set the LO output to 10 dBm and 13 dBm and compared the sideband modulation index using a cavity scan. As a result, although the measurement result increased from 0.0038 to 0.006, It decreased from the previous reading of 0.01 (klog33306). At present, the cause of this reduction has not been identified. After measurement, the LO output was returned to 10 dBm(Fig1). We plan to continue investigating the cause from tomorrow onwards.

Details

• The measurement results of cavity scan are as follows. (Although we had initially planned to use IRX_NORM for the evaluation, the sideband signals were buried in noise; therefore, we used the p-pol DCPD signal from TMSX.)

  	sideband/TEM00
  LO output 10 dBm	0.18/47=About 0.0038(TEM00:Fig2, sideband:Fig3)
  LO output 13 dBm	0.3/48=About 0.006(TEM00:Fig4, sideband:Fig5)

  ** However, we have not yet been able to identify the peaks at each angular frequency.
  The measurement at 13 dBm is only about 60 per cent of the previous measurement of 0.01, and remains low. At present, we have been unable to identify the cause of the reduction in the modulation index to approximately 60 per cent of previous measurement values, occurring along the signal path from the LO through the various circuits to the EOM crystal. Furthermore, if this were due to variations in the interferometer’s alignment, one would expect to observe changes in the peak intensity of TEM00. However, no significant changes were observed in the TEM00 peak during these measurements. Accordingly, it is considered difficult to explain the current decrease in the modulation index solely by alignment variations. At present, the cause of the decrease in the modulation index compared to previous measurements remains unknown.

• Once the measurements were complete, the LO output was reset to the current operating value of 10 dBm. From tomorrow onwards, we plan to continue investigating the cause of the reduction in the modulation index. In particular, with regard to the TMSX’s DCPD signal, we are considering whether adjusting the gain might reduce the impact of noise, thereby improving the measurement accuracy of the sideband signals.

Note

• We followed the procedure for the cavity scan as described in klog36322.

• If you request ALS_LOCKED whilst the IMC is locked, the low-frequency fluctuations in the MCL are fed back to the IR laser’s frequency. As these frequency fluctuations are also passed to the GR laser via the PLL, the GR laser’s frequency fluctuates. In order to maintain the GRX in a resonant state under these conditions, the feedback to the AOM increases, which exceed the control range of the AOM’s VCO. Consequently, the GRX lock becomes unstable.  To prevent this, the GRX’s RFPD signal must be fed back to the IMC’s MCL (MCE) on the low-frequency side(in the ‘Note’ section of klog36322). In particular, the steps outlined in the ‘Note’ section must be carried out immediately after requesting ‘ALS_LOCKED’ in Step 2 of the ‘Detail’ section of klog36322. Please note that any delay in taking this action will cause the GRX lock to become unstable for the reasons stated above.

• In this cavity scan, measurements were taken under the following conditions:
  IMC output laser power: 10 W or more
  PRM state: MISALIGNED_FOR_LOCKAQ

• The OFFSET application channel and ramp time for the cavity scan were based on klog33306. For this measurement, ‘-1’ was set for FM1 in the filter bank, and the OFFSET was applied by specifying the ramp time in diaggui. The filter bank settings can be checked by clicking on the relevant channel in diaggui. Whilst the sweep time should ideally be determined by taking cavity poles into account, in this instance I referred to the settings in klog33306. As the peak of TEM00 in IRX_NORM remained at a nearly constant value across each scan, I concluded that the sweep time set for this experiment was appropriate.