I estimated the frequency noise spectrum of the laser source by multiplying the feedback signal (IMC-SERVO_SLOW_OUT) and the PZT actuator efficiency reported in klog:29194.
The result is shown in Fig.1.
It appears that the frequency noise can be measured very clearly above a few Hz.
I believe that the cutoff at a few kHz is due to some low-pass filters just before this channel.
The slope of the spectrum is slightly more gentle than that of a typical NPRO laser.
Next, I checked the relative intensity noise at IMC-CAV_TRANS and IMC-CAV_REFL (Fig.2) and coherence between these and the feedback signal analyzed above (Fig.3).
The reasonable point is the absence of coherence between TRANS or REFL and PDH at all frequencies.
However, several points remain puzzling.
- The relative intensity noise differes even though there is coherence below 10 Hz.
- The relative intensity noise varies above 10 Hz.
- There is no coherence between TRANS and REFL above 10 Hz.
Potential reasons for these discrepancies include dark noise from these PDs and air turbulence.
I will conduct further investigation to address this inconsistency.
