Page 324 - Tunable Lasers Handbook

284 Norman P. Barnes

a\

Curved
?--
output
Mirror
output Curved
Reflecting I

FIGURE 3 3 Littrow and Grazing-incidence grating configurations. (a) Littrow configuration.
(b) Grazing-incidence configuration.

where N is the order of the reflection. For gratings used in a laser resonator, the
orders are limited to 1 so that the losses associated with the higher orders are
avoided. In the following, we assume that the first-order reflection is always uti-
lized. If a grating is used in the Littrow configuration, the incident and reflected
angles are equal. In this case, the variation of the angle with wavelength is

(35)

Using the same expression for the beam divergence, the single-pass spectral
bandwidth is
-I
A& = (274 "rJ cos (8 j)] . (36)
h
Since d, cos(8J can be much larger than dnldh, the spectrz narrowing achieved
with a &rating can be much larger by employing a grating rather than a prism.
Although greater spectral resolution can be achieved with a grating, the
losses of a grating tend to be higher. Losses are associated with both finite
reflectivity of the coating, usually a metal, and less than unity grating efficiency.
Higher losses are particularly pronounced at shorter wavelengths where the