Kiwamu and Satoshi

related log: 17275

As pointed out previous klog entries, sometimes PLL X and Y show larger noise level. We searched noise sources to investigate this issue. Especially, we searched the sources in the PSL room by tappig each mirror mount, but could not find any suspicious optics.

After several trials, we found that the noise levels became larger when we tapped PROMETHEUS's laser heads or optical bench around the laser heads. Moreover, some pedestals or clamps could higly excite the peaks around 3.4 kHz. In order to mitigate the excitation of resonant peaks around 3.4 kHz, we removed unnecessary clamps left on the optical bench as shown in attached pictures. After this cleaning work, coupling to the peaks in PLL X around 3.4 kHz was mitigated, but that in PLL Y was still peaky. Measured spectra are shown in below.

When we tapped each laser head, its mechanical resonances were excited and showed up in the beat error signals.

Resonant frequency is different for each laser, and this may be due to the difference in the types of pedestals beneath laser heads. From this results, the bump in PLL X around 250 Hz derives from the mechanical resonance of laser head.

Then we tapped the optical bench, and the below is the result.

As one can see, resonant peaks around 3.4 kHz in both X and Y were excited by tapping the optical bench. In fact, these peaks were so sensitive that they could be excited by clapping hands, or so.

Hypothesis

Small opto-mechanical components on the PSL optical bench behave as antennae which excite resonant peaks around 3.4 kHz. Further investigations on the PSL table may lead to the reduction of noise sources.