Page 325 - Tunable Lasers Handbook

6 Transition Metal Solid-state Lasers 285

reflectivity of the grating is lower since the reflectivity of the metal is lower. In
addition, gratings tend to be more damage prone as compared with prisms. Note
that a grating will, in general. polarize a laser. Consequently. the same comments
regarding the losses associated with restricting the laser to operate in a polarized
mode apply. The dispersive characteristics of multiple-prism grating systems are
described in Chapter 2.
Birefringent filters achieve wavelength control by utilizing the variation of the
phase retardation of a wave plate uith wavelength. For normal incidence. the
phase difference CD between the ordinaty and extraordinq wave of ti nave plate is

CD = 274 1ZC, -11, )d/h , (37)

where tio and ne are the ordinary and extraordinary refractive indices. respec-
tively, d is the thickness of the wave plate, and h is the aavelength. If a poly
chromatic polarized wave is incident on the wave plate. only some of the nave-
lengths will have a phase difference. which is an integer multiple of 2x. These
wavelengths will interfere constructively as they exit from the wave plate and
emerge with the same polarization as the incident polarization. If a polarization
discrimination device is used after the wave plate, only the wabelengths that
have the correct polarization will suffer no loss. By using this wavelength vary-
ing loss, a wavelength selective device can be made.
Both birefringent filters and Lyot filters can be made using this principle.
Lyot filters (681 employ several wave plates to achieve better spectral resolution.
Between each wave plate is a polarizer. By using these polarizers, good wave-
length resoliition can be achieved. However, this leads to a filter with high trans-
mission losses. High losses are incompatible with efficient lasers. To obviate
these losses, birefringent filters were created [69,70]. These devices are nave
plates orientzd at Brewster‘s angle. In this configuration, the Brewster’s angle sur-
faces act as the polarizer, eliminating the polarizer as a loss element. Since the
degree of polarization of a Brewster’s angle surface is not as high as that of a
polarizer, the wavelength resolution is not as high as that of a Lyot filter. Phase
difference between the ordinary and extraordinary waves can be calculated for 2
wave plate at Brewster’s angle by taking into account the variation of the refrac-
tive index with orientation and the birefringence. Because birefringent filters con-
sist only of wave plates oriented at Brewster’s angle, they can have low loss.
assuming a polarized laser, and can be damage resistant.
Etalons, like birefringent filters. operate on a principle of constructive inter-
ference. An etalon consists of two parallel reflective surfaces separated by a dis-
tance d. Wavelengths that fill the distance betmeen the mirrors with an integer
multiple of half-wavelengths will be resonant: that is. resonance occurs when