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 [[http://nodus.ligo.caltech.edu:8080/40m/986|elog]]).

We got the design parameters for the reference cavity from [[http://www.ligo.caltech.edu/docs/D/D980670-00.pdf|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]].