Tunable Lasers Handbook

Tunable ExternaI-Cavily

Semiconductor Lasers

Paul Zorabedian

Photonics Technology Department

Hewlett-Packard Laboratories

Palo Alto, California

1. INTRODUCTION

1.1 What Is an External-Cavity Laser?

A tunable external-cavity laser (ECL) (Fig. 1) comprises an optical gain

medium (a laser diode with antireflection coatings on one or both facets), optics

for coupling the output of the gain-medium waveguide to the free-space mode of

the external cavity, one or more wavelength-selective filters, and one or more

mirrors for defining an external feedback path, possibly with a piezoelectric

translator (PZT) for fine tuning. The external cavity may also contain additional

components such as polarization optics, beamsplitters, prisms, telescopes, etc.

1.2 Why Apply External Feedback to Laser Diodes?

1.2.1 Limitations of Diode Lasers

Semiconductor Fabry-Perot diode lasers are compact and easy to use, but

they suffer from a number of performance limitations that are potentially serious

in many applications: Solitary laser diodes are often multimode, and they exhibit

large linewidths due to a short photon cavity lifetime and strong coupling

Tunable Lasers Handbook

Copyright 9 1995 by Academic Press, Inc. All rights of reproduction in any form reserved.

349

350 Paul Zorabedian

PZT

CAVITY

LENGTH

CONTROL

WAVELENGTH

~

... CONTROL

MIRROR

TUNABLE INJECTION

FILTER "~ (~U .RRENT

COUPLING ~ ~'~~.,. ......

OPTICS

....

.I ~~

ou,PuT

...... ANTI- /'J~'~

REFLECTION

COATING LASER

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.

1.2.2 Advantages of External-Cavil/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 cm 3. 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.

1.2.3 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).