Page 427 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor lasers 387
as a figure of merit, but it must be remembered that the parameter of ultimate
importance is the grating resolution divided by the axial mode spacing of the
external cavity. (For a detailed description of multiple-prism grating dispersion.
see Chapier 2.)
7.2.1.1.4 Common Mountings
Diffraction gratings in external cavity lasers combine the functions of the fil-
ter and external mirror. In extended cavities, the light from the grating must be
retroreflected back into the gain medium. Two common retroreflecting mounting
geometries for diffraction gratings in extended-cavity lasers are the autocollima-
tion (Littrow) configuration and the grazing-incidence (GI) configuration.

7.2.1.1.4.1 Littrow Moztnting In the Littrow configuration [Fig. 19(a)], [he
angles of incidence and diffraction are equal: Oj = 'pl. The grating equation becomes

In this case the angular dispersion of the retroreflected beam is identical to that
of the diffracted beam and is given by

A typical angle of incidence for the Littrow configuration is Oi - 50".

7.2.1.1.4.2 Grazing-Zncidence Mounting In the grazing-incidence con-
figuration (Fig. 19b). the intracavity beam makes two passes at the grating. The
diffracted light from the second pass is a retroflection of the incident light from

FIGURE 1 c' Diffraction grating mountings. (a) Littrou. (b) Grazing incidence.