Ultrashort Laser Pulse Phenomena, 2nd Edition

Sensors Based on fs Lasers 601

detector (ϕ). In contrast to standard techniques that convert a phase difference

into an amplitude difference in an interferometer, this method converts the phase

difference into a beat frequency. As we saw in Fig. 5.6, even with an unstabilized

laser it is possible to record a change in v of 1 Hz, which, according to Eq. (13.7)

corresponds to a phase sensitivity of 10

−7

in a cavity of τ

RT

= 16 ns. If—as is

often the case—the phase difference is because of a differential change in cavity

length or perimeter P: ϕ = k



P, then Eq. (13.7) can be written:

v

v

=

P

P

. (13.8)

With v ≈ 3 · 10

14

Hz, v ≈ 1 Hz and a cavity perimeter ...