Page 429 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 389

direction of the grooves on the grating (Fig. 20). The reader who desires further
details on i:he subject of grating efficiency and polarization effects is advised to
consult the excellent material in [62].

7.2.1.1.6 Wavelength Resolution
The wavelength resolution is obtained by dividing the angular spread of the
beam waist at the grating (waist divergence) by the angular dispersion. The waist
divergence of a Gaussian beam of radius vi!" is given by

The wavelength resolutions for the Littrow and grazing-incidence cases are,
respectively:

(52 j

It is useful to relate grating resolution to thefilled depth of the grating. The filled
depth is the projection of the illuminated region of the grating onto the optical
axis of the cavity. The filled depth Lw is given by

0
r ~ i ~ i ~ r ~ i ~ i r ~ ~ i
0' 10' 20' 30' 40' 50' 60' 90'
First-Order Littrow Diffraction Angle
FIGURE 20 Efficiency versus angle of incidence for Littrow grating (from Palmer [62]).