Page 426 - Tunable Lasers Handbook

386 Paul Zorabedian

a [ sin oi + sin cp,) = n7~ , (45)

where a is the groove spacing (pitch). is the incident angle, 'p, is the diffracted
angle of the m'th order, and n7 is the order of diffraction. The diffracted light is
dispersed according to its spectral content. with different wavelengths appearing
at different angles. Differentiating the grating equation gives the angular disper-
sion D, which describes how much the diffraction angle changes as the wave-
length varies:

Diffraction gratings are usually used in first order in ECLs, that is. with ni = 1. The
zeroth-order (specular reflection) beam is sometimes used for output coupling.
The wavelength resolution of a grating-tuned external cavity is determined
by the angular dispersion multiplied by the acceptance angle for coupling back
into the gain medium active region. The angular dispersion can therefore be used

FIGURE 1 8 Types of plane diffraction gratings. (a) Reflection grating. (b) Transmission grating
(reproduced with permission from Palmer [62]).