The reference cavity was locked using a 21.5 MHz signal from a rack mounted board (FSS REF - Crystal Frequency Reference http://www.ligo.caltech.edu/docs/D/D980353-C.pdf). That turned out to have many problems and we removed it completely from the system circa 2008 with a fixed 21.5 MHz OCXO from Wenzel and an RF Amplifier (see elog).
We got the design parameters for the reference cavity from D980670
Spacer Material |
Fused Silica (Corning?) |
Mirror Material |
Fused Silica |
Mirror T_HR |
300 +/- 30 ppm |
Mirror R_AR |
< 500 ppm |
Mirror RoC |
500 mm |
Cavity Length |
8" (203.2 mm) |
Cavity Finesse |
10000 |
Cavity g-factor (g1*g2) |
0.352 |
Cavity waist |
0.218 mm |
Cavity FSR |
737.7 MHz |
Cavity pole |
35.2 kHz |
Some FEA of the LIGO/40m reference cavity using COMSOL: /FEA
See also Frequency Stabilization Servo.