The CC1 PD is used to detect RF signal at 14MHz. While checking this 14MHz signal peak on the spectrum analyzer, I found another peak that is moving in the frequency domain, and its peak height also changes.

This moving peak can reach a level of almost about -63dBm (before amplification). The frequency of this moving peak seems to be +/- 10MHz around 14MHz. Besides, it seems to become even larger when it approaches the 14MHz peak. More importantly, the RF signal from CC1 PD is only -51.3dBm (before amplification), which means that this moving peak may introduce non-negligible noise into the CC1 loop. There is a movie in this link shows the situation.

I also found this peak still exists even when all optical cavities are unlocked. This situation is in the movie in this link (after 20dB amplification). At around 8 second, I switched off CC PLL, the moving peak disappeares. So it seems this moving peak is related with PLL loop.

I also confirmed that this peak is not RF signal cross talk. Because when I block the light incident on CC1 PD, this moving peak disappears.