Page 165 - Optical Communications Essentials

Passive Optical Components

Passive Optical Components 155

called the free spectral range (FSR). This is given by

λ 2
FSR = (9.10)
2nD
Another important parameter is the measure of the full width of the passband at its
half-maximum value, which is designated by FWHM (full-width half-maximum). This
is of interest in WDM systems for determining how many wavelengths can lie within
the FSR of the filter. The ratio FSR/FWHM gives an approximation of the number of
wavelengths that a filter can accommodate. This ratio is known as the finesse F of the
filter and is given by
π R
F = (9.11)
1 − R

In practice one uses a multilayer filter consisting of a stack of several dielec-
tric thin films separated by cavities, as shown in Fig. 9.9. The layers typically

Figure 9.9. A multilayer filter consists of a
stack of several dielectric thin films sepa-
rated by cavities.

Figure 9.10. Transmission characteristics of three
different thin-film filter structures relative to the
peak wavelength.

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