Page 323 - Tunable Lasers Handbook

6 Transition Metal Solid-state Losers 83

For a Brewster's angle prism at the angle of minimum deviation, the variation of
the angle of deviation can be readily related to the variation of the refractive
index with 1:he wavelength. Thus. the above expression becomes

&=&o+2-A . (32)
di1
dh
An estimate of the allowable variation of the angle of deviation can be obtained
from the beam divergence. For a TEM,, mode Gaussian beam, the beam diver-
gence 8, is given by h/nivO where vt', is the beam radius. Thus, for a single pass,
the spectral bandwidth can be estimated as

For a material like fused silica. the variation of the index of refraction with wave-
length, that is. the dispersion, is on the order of 2.0 x 203 m-1. However. maten-
als with higher dispersion are available. For example, the dispersion of SF6 glass
is 9.8 x 1Ot m-1. Materials such as SF6 glass are satisfactory for low-power
applications, but they are susceptible to laser induced damage at higher power
densities. On the other hand. the simple fused silica prism can be extremely
damage resistant.
Gratings can produce significantly smaller spectral bandwidths than prisms
since they can have a greater variation of the angle of deviation with wavelength.
Gratings can be used in either a Littrow or a grazing-incidence configuration
166.671. In a Littrow configuration, the radiation at the selected wavelength is
retroreflected. As such, the grating can be used as one of the mirrors of the res-
onator. In most cases, plane gratings are used. In this configuration, the output
mirror is often a curved mirror so that a stable resonator can be formed. In the
grazing-incidence configuration, the grating is used as a mirror internal to the
resonator. One of the incident angles associated with the grating is a grazing
angle, as shown in Fig. 33. A grazing-incidence angle produces two useful
results. One result is that the radiation is spread over larger area of the grating,
reducing the energy density on the grating and consequently the probability of
laser induced damage. hother result is the increase in the number of illumi-
nared grooves. With a higher number of illuminated grooves, the wavelength
selectivity of the grating is enhanced.
Single-pass spectral bandwidth can be estimated by calculating the variation
of the reflected angle with wavelength. Using the grating equation, the angles of
incidence and reflection Or are related to the groove spacing dg by the relatim

sin (8;) + sin (0, ) = Nh/d, , (34)