[Kambara, Nakano, Michimura]

We have measured the FSR of IMC by tuning the sideband frequency.
The measured FSR was 5.624357(6) MHz, and the roundtrip length was 53.30253(5) m.

What we did
 1. Tuned the sideband frequency for the IMC so that there's no PDH signal. We have repeated this three times independently by each person to estimate the error.

 2. Divide that sideband frequency with 9 to get FSR (since that sideband frequency is an integer multiple of FSR).

Results
 The sideband frequency to null the PDH signal was 50.61921(5) MHz. The error here is the standard deviation of the three independent measurements.
 5.624357(6) MHz was the measured FSR.
 5.6269873 MHz is the designed value from MIF.
 53.30253(5) m was the measured roundtrip length.
 53.28708 m is the designed value from MIF.

Discussion
 There is 1.5 cm significant difference between the designed and the measured.
 Our positioning accuracy of the IMC mirrors were roughly that much. How do you guys think?