My attempt at measuring the FSR of the cavity is off by ~20% of the expected value.

I scan the frequency of the laser and record the time between two 0,0 modes. The voltage between the two modes can be calculated from the scan amplitude and frequency. Then, the FSR of the cavity from experimental data should be:

t_start = -48.12e-3 #sec

t_end = 13.8383232e-3 #sec

scan_amp = 2 #volts

scan_freq = 5 #Hz

period = 1/scan_freq #sec

miliamp_per_V = 2e-3

meters_per_miliamp =  2.625e-10 #5e-12/20e-3

cav_len = 2.4

lambda_1 = 1550.08e-9

c = 3e8


delta_t = t_end - t_start

delta_V = delta_t/(period/2) * 2 * scan_amp


FSR = delta_V*(miliamp_per_V)*(meters_per_miliamp)*(c/lambda_1**2)

This gives an FSR of 162 MHz which is clearly way off. I think the discrepancy comes from the uncertainty in the number used for the wave wavelength change of the laser as a function of the pump current. I was eyeballing this before, but here i used the number from the data sheet orginally emailed along with our purchase of the laser. See Attached. He did not provide the raw data. I used a web plot digitizer to get the raw data. The increasing/decreasing.csv are the results of this. The slope of these give the number used in the above calculation.

TLDR: I think the frequency of the laser is a poor way to calibrate the FSR.