Page 409 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 369

a

b

FIGURE 9 Classes of external cavities for diode lasers. (aj Extended-cavity laser. (b: Double-
ended extemal-cavity laser. (cj Ring external-cavity laser.

reduction) on both facets. Each extended-cavity section retroreflects into its
respective facet. One of the extended-cavity sections might contain all of the
wavelength-selective elements, whereas the other might contain only coupling
optics and a retroreflector. The most well-known example of this implementation
is a linear external cavity with a Littrow-mounted diffraction grating on one end
and a mirro'r on the other [38]. Alternatively, both extended-cavity sections could
contain wavelength-selective elements such as acousto-optic tunable filters. The
primary advantage of the double-ended external cavity configuration is increased
suppression of diode cavity resonances obtained by reducing the reflectance on
both facets. The disadvantages are the increase in the number of optical compo-
nents, increased alignment difficulty, and the additional coupling loss associated
with the second extended-cavity section.

3.3 Ring-External-Cavity Lasers

A ring-cavity (Fig. 9c) laser contains a semiconductor gain medium (with
reflection suppression on both facets) and an external feedback path that cross-
couples the outputs of the two facets. This is the most difficult type of external
cavity to align. Like The double-ended external cavity, it has the advantage of
increased solitary-resonance suppression because of the use of reflectance sup-
pression on both facets. It can also be made unidirectional by inserting an optical
isolator into the cavity.