We wish to study the coherence of the two NPROs i.e. PSL and the X-end-NPRO by locking both of them to the X-arm and then observing the green beat frequency fluctuations.
What we did:
a) locked the PSL to the X-arm as described in 4153
b) locked the x-end-NPRO to the X-arm with a PDH lock to the reflected green from the ETMX
c) Obtained the green beat signal with a spectrum analyser as described in 3771
Please see the attached screen shots from the spectrum analyser. They are taken with different BW and sweep range settings. They give a estimate of the width of the green beat signal and the range of the frequency fluctuations of the beat-note.
a) width of the beat note is less than 6KHz if measured over time scales of a few milli seconds
b) the frequency fluctuations of the beat note are about 100KHz over time scales longer than 100ms
We wish to record the beat note frequency as a function of time in order to establish the stability over time scale of a day.