I tried the idea that the PRC can resonate f1 and f2 at the same time if the arm gives the reflection phase to f1 and f2 with the ratio of 1 vs 5.

The details are described on wiki. The point is this removes all of the PRC/SRC/asymmetry mumbo jumbo.

The calculated cavity lengths for f_mod of 11.065399MHz are:

• Arm Length: 37.7974 [m]

• PRC Length: 6.7538 [m]

• SRC Length: 5.39915 [m]

• Asymmetry (lx-ly): 0.0342 [m]

Here is the actual values derived from the photos.

• Arm Length: 37.54 [m] (0.26m too short)

• PRC Length: 6.760 [m] (6mm too long)

• SRC Length: 5.415 [m] (16mm too long)

• Asymmetry (lx-ly): 0.0266 [m] (8mm too long)