Page 390 - Tunable Lasers Handbook

350 Paul Zorabedian

CAVITY
\ LENGTH
CONTROL

PZT

DIODE
FIGURE 1 Generic tunable extended-cavity diode laser.

between the phase and amplitude of the intracavity optical field. Laser diodes are
somewhat tunable by varying temperature or current. but these methods are awk-
ward and have limited ranges, which do not fully exploit the broad semiconduc-
tor gain bandwidth.
I .2.2 Advantages of External-Cavity Lasers over Solitary Diode lasers
ECLs retain in large measure the compactness and ease of use of solitary
cavity diode lasers and in addition provide a number of performance enhance-
ments. A typical semiconductor ECL has a volume of -1000 cm3. A properly
designed ECL will operate on a single external-cavity longitudinal mode. The
density of accessible modes is increased by the ratio of the external to solitary
cavity lengths. Truly phase-continuous tuning without mode hops is also pos-
sible. The linewidth of ECLs is greatly reduced in comparison to solitary diode
lasers because of the longer photon lifetime of an external cavity. The use of an
external filter allows tunability across the wide gain bandwidth of the semicon-
ductor gain medium.
7.2.3 Comparison with Other Types of Tunable lasers
Compared to other types of tunable lasers, external-cavity semiconductor
lasers are compact, are easily pumped by direct injection current excitation, have
high wallplug efficiency, are air cooled, and have long lifetimes. However, their
output power is generally lower (typically -1 to 10 mW, although up to 1 W has
been reported).