Page 408 - 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 pm 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 esternal-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