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.
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.
Double-Ended External-Cavity
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
~: -=!FILTER
g " i ....... V- W I I ~,"
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
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.

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