Page 432 - Tunable Lasers Handbook

392 Paul Zorabedian

With special mirror coating technology. the finesse of an etalon can be as high as
=10,000, but a finesse of a few hundred is more typically achieved with conven-
tional coatings.

7.2.1.4 Interference Filter
A bandpass interference filter is a multilayer thin-film device [66]. The sim-
plest type is really a Fabry-Perot etalon with d - h. If the thickness of an etalon
is made very small, the orders will be widely separated. This is done by evapo-
rating dielectric-stack mirrors, separated by a half-wave spacer layer, in a contin-
uous coating run on a substrate. Multiple reflector pairs (called cavities) can be
deposited to steepen the passband. Additional metallic-layer blocking stmctures
deposited on another plate are used to eliminate adjacent transmission orders.
The plates are assembled in a sandwich that protects the deposited films. Inter-
ference filters can be made with FWHM bandwidths 2 nm or less in the near
infrared and less than 1 nm in the visible. The peak transmittance can be made
as high as 50 to 70%.
The interference filter is tuned by tilting it in the incident beam. For small
angles (up to 5 to loo). the wavelength of peak transmittance is given by

where 8 is the angle of incidence, izo is the refractive index of the external
medium, and lie is the effective refractive index of the spacer.
7.2.2 Electronically Controlled Filters
7.2.2.1 Birefringent Filter
There are several forms of the birefringent filter [67,68]. They can be tuned
either mechanically or electronically, with electronic tuning being the preferred
means. The basic birefringent filter is called a Lyotfiltel- and comprises an alter-
nating stack of N uniaxial birefringent plates separated by polarizers. The thick-
nesses of the plates vary in a geometrical progression d, 2d, 4d, . . . . 2”-’d. The
transmission axes of the polarizers are all aligned. The light propagates in a
direction perpendicular to the c axis of each of the plates. Transmission through
each segment (plate plus polarizer) will vary sinusoidally, with maxima at wave-
lengths for which the retardation of the plate is a multiple of 2x. For a plate of
thickness d, the free spectral range Ah,,, between successive maxima is approx-
imated by

h
Ah FSR - 1
(dAn/dh - An/h)