340 CHAPTER 8. MULTICHANNEL SYSTEMS
Figure 8.8: Four kinds of filters based on various interferometric and diffractive devices: (a)
Fabry–Perot filter; (b) Mach–Zehnder filter; (c) grating-based Michelson filter; (d) acousto-optic
filter. The shaded area represents a surface acoustic wave.
A Fabry–Perot (FP) interferometer—a cavity formed by using two mirrors—can act
as a tunable optical filter if its length is controlled electronically by using a piezoelec-
tric transducer [see Fig. 8.8(a)]. The transmittivity of a FP filter peaks at wavelengths
that correspond to the longitudinal-mode frequencies given in Eq. (3.3.5). Hence, the
frequency spacing between two successive transmission peaks, known as thefree spec-
tral range, is given by
∆νL=c/( 2 ngL), (8.2.1)wherengis the group index of the intracavity material for a FP filter of lengthL.
If the filter is designed to pass a single channel (see Fig. 8.7), the combined band-
width of the multichannel signal,∆νsig=N∆νch=NB/ηs, must be less than∆νL,