
INTERFERENCE 211
where f
0
is the fundamental resonant frequency. The spacing of adjacent resonant frequencies
is also f
0
and this frequency is often called the free spectral range or FSR:
FSR =
c
2
. (7.92)
When the source light is turned on, if it is at a resonant frequency, the recirculating power
inside the interferometer will increase on every pass, until steady state is reached. Neglecting
losses, the small fraction of this power that is extracted in steady state will be equal to the input
by conservation of energy. Therefore, the recirculating power must be
P
recirculating
=
P
out
T
2
=
P
out
1 −R
2
=
P
0
1 −R
2
. (7.93)
If the output mirror reectivity is R
2
= 0.999, ...