Tunable Lasers Handbook

368 Paul Zorabedian

facet. The beam expands inside the buried-facet region since there is no wave-

guiding. Therefore, the reflection at the semiconductor-air interface does not

couple strongly back into the waveguide. The reflectance decreases with increas-

ing length of the buried-facet region. However, if the nonguiding region is too

long, the internal beam will hit the top-surface metallization, creating a multiple-

lobed far-field output and spoiling the ability to couple efficiently to the mode of

the external cavity. This limits the length of the buried facet to <-15 ~tm and the

corresponding reflectance back into the waveguide to >--20 dB. Therefore,

buried-facet gain media would probably give poor performance in a simple

extended-cavity laser, but they might be useful in either a double-ended external

cavity or ring laser.

3. CLASSES OF EXTERNAL-CAVITY LASERS

The term external-cavity laser is often used generically to describe any con-

figuration in which the feedback path extends beyond one or both of the facets of

the gain medium. However, it is useful to distinguish three distinct classes of

external cavities: the extended cavity, the double-ended cavity, and the ring cav-

ity. The following briefly describes each type.

3.1 Extended-Cavity Lasers

The extended-cavity laser (Fig. 9a) comprises a semiconductor gain chip

with an antireflection coating on one facet, optically coupled through the coated

facet to an external optical system that includes a retroreflecting end mirror. This

configuration has also been called a pseudo external cavity [37]. The opposite

facet, which is either uncoated or coated as a high reflector, serves as an end mir-

ror of the cavity and is often the output coupler. The extended cavity is the most

common configuration for the following reasons: (1) It requires only one antire-

flection coating operation. (2) An extended cavity can be built using commercial

diode laser packages in which the output of only one facet is accessible. (3) The

extended-cavity laser is relatively easy to align because the subthreshold emis-

sion from the gain chip is strong enough to provide an adequately bright refer-

ence beam. (4) Excellent optical performance can be obtained provided an excel-

lent AR coating is applied. However, even with a high-quality facet coating,

effects of the residual diode cavity resonances are still observable and are some-

times the cause of nonideal behavior.

3.2

Double-Ended External-Cavity

Lasers

The double-ended external cavity laser (Fig. 9b) contains a semiconductor

optical amplifier with antireflection coatings (or some other type of reflectance

8 Tunable External-Cavity Semiconductor Lasers 369

a

~: -=!FILTER

FILTER i FILTER

2T!

g " i ....... V- W I I ~,"

FIGURE 9

Classes of external cavities for diode lasers. (a) Extended-cavity laser. (b) Double-

ended external-cavity laser. (c) 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 mirror 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.