In the past, we always demodulate the reflection of GRMC at 78MHz to get PDH and lock GRMC. However, the 15.2MHz modulation on the main laser which is well within the linewidth of SHG, which makes this 15.2MHz modulation present also on green. To check what is the quality of this modulation, we did a demodulation of GRMC reflection at 15.2 MHz today.

The PDH demodulated at 15.2MHz and 78MHz are shown in attached figures. We can see that the PDH at 15.2MHz is even larger than the signal at 78MHz. This indicates that the SHG process seems not to degrade the sideband of green. This could make the lock of GRMC easier in the future.