There's a correction

1) the beam waist inside the cavity, it seems that the 237 um(which corresponds to frequency shift = 258.29)  I got does not agree with the ROC and the cavity length.

 I calculated the beam waist based on the same ROC and the cavity length, and got 261 um (which correspond to frequency shift = 219.32 MHz)

This plot based on the frequency shift of 219.32 MHz step,

And now I plot from -100MHz to 100 MHz, the 35.5 MHz peak coincides with a peak from one of higher order modes( n+m = 20)

The exact number of that peak is 35.6 MHz.