Made measurements of the oscillator power noise with a few photodiodes.
PowerNoise.png shows the free-running oscillator relative power noise
measured before the acousto-optic modulator. This is more than 10 times
noisier than when the laser was installed in the H1 enclosure. The
other trace in the plot is the out of loop of the relative power noise.
It is also about a factor of 10 higher than it should be.
Whilst the power stabilisation was locked, I looked at the AC coupled
output of the photodiode and did not observe any oscillations. The maximum
peak-to-peak variations were ~40 mVpp.
If I am reading this plot correctly, the ~37kHz rep-rate seen in last week is probably represented in this spectrum by the peak which the ISS is adding at that frequency. (The 500kHz oscillation is too high to see here.) It might be very informative to see what is going on above 100kHz since the ISS seems to be adding a lot of noise at 100kHz (about a factor of 10 above its input).
Is this plot really calibrated into RIN?
The digital RIN readback for the OOL inner-loop sensor appears to be 20 dB lower than the trace shown here (26773). Same for the digital readback for the HPO transmission.
Look at alog 26893.
Peter's "relative power noise" agrees well with the raw voltage spectrum of Rick and mine. In other words, Peter's plot seems to overestimate RIN by 15 dB or so for the HPL monitor (DC level is about -6V), and about 20dB for 1st loop sensor (DC level -9 to -10 Volt).