where a1, a2 are the intensity attenuation factors due to the mode
mismatch and surface imperfection of M1 and M2, respectively;
c1, c2 are defined as the optical transmission loss factors of FP
Cavity1 and Cavity2, respectively; U1, U2 are the light phase delays
(round-trip) induced by Cavity1 and Cavity2, and can be expressed
as