It's a quiet night, so I went down the lab to measure the beat signal. We are getting close. I think I have to review my noise budget calculation and estimate the error in the measurement carefully.
So after a few things Evan and I did a few days ago:
- rotate the stack to get rid off the reflected beam from the window
- fix the insulation so the beam is not clipped on the opening.
- add more modulation depth to RCAV path with the EOM driver (tuned to 14.75MHz)
- Minimize some RFAM, by rotating the half wave plate in front of the sideband EOM
Then I measured the beat signal.
We reduce some noise from scattered light at frequency below 100 Hz, we are limited by some white noise at high frequency ~ above 1 kHz.
fig1: measurement vs noise budget
fig2: zoom in. The slope of the measured signal agrees well with the slope of thermal noise.
ToDo
- Estimate/measure shot noise PD noise and electronic noise in the setup. See if they match up with the measurement.
- Review the noise budget calculation. Looking at the slope of the signal around 1kHz, I think the calculated brownian noise is lower than what it should be.
- noise hunting, seems like scattered light at frequency below 100Hz. There are many mechanical peaks, and harmonic lines at higher frequency.
- add the contribution from RIN induced TO noise in the nb.
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