Daniel, Naoki, Nutsinee
Using the original The way of the beast method we determined the FC cavilty length down to +-14um precision. We added 41.89 MHz sideband to the 40MHz EOM and added a 1kHz line from SR785 to this sideband. We locked the filter cavity on the 40MHz as usual. With the help of SR560 (3kHz double pole, 100 gain) we were able to see the injected 1 kHz clearly on the scope. While the cavity was locked, we tune the 41.89 MHz until the 1kHz line disapeared from the scope in both I and Q. We then fine tuned the frequency using SR785 (I-MON/1kHz excitation). We had about 2 Hz precision. In the end we settled with 41.781168 MHz as our FSR.
Let
f_mea = 41.781168 MHz
n = 83 (number of FSR)
c = speed of light
FC cavity length is
(c*n)/(2*f_mea) = 297.774993 m +- 14um
The length given by the survey team (alog 65509 and alog 62917) was 297773mm+-16.6mm
filter cavity FSR = 503.38757 kHz ± 0.02 Hz
Dhruva, Vicky
In operation, the filter cavity length held at finite detuning, which changes FC length very slightly, within error of this. We can use the RLF frequency to estimate filter cavity length, when it is IR locked.
With FC-IR locked for sqz data taken 2/19/23, we calculate a filter cavity operating length of L_fc = 297.772(3) m.