Page 392 - Tunable Lasers Handbook

352 Paul Zorabedian

To the best of my knowledge. no work has been published so far with the intent
of achieving tunability because their low gain will not support much insertion
loss for external cavity components. However. if VCSELs continue to grow in
importance as some predict. greater adaptation to their use in external cavities
may follow.

2 SEMICONDUCTOR OPTICAL GAIN MEDIA

2.1 Laser Diode Basics

A semiconductor laser diode (Fig. 1) serves as the gain medium of an ECL.
The laser diode is a semiconductor device about 250 to 500 pm long by about 60
ym thick mounted on a copper or ceramic heat sink. Current is injected through
a top ohmic contact. Photons are generated and guided by the epitaxial layers of
the structure. The thin layer in which electrons and holes recombine to produce
light is called the acth?e region. Stimulated emission in the active region forms
the basis for laser action driven by optical feedback from the facets or from the
external cavity. We start by reviewing some of the basic properties of laser
diodes, which are important for the design of ECLs.

2.2 Light Output versus Current Curve
The light output versus current (L-Z) curve (Fig. 2) is characterized by the
threshold current Z,, and the quantum efficiency q. Saturation at high current is
caused by ohmic heating and Auger recombination. The linear portion of the L-Z
curve is explained by the laser diode gain model.

2.3 Gain Model

2.3. 7 Gain
The optical gain g varies nearly linearly with injected carrier density N:

where o is the differential gain cross section and NT is the carrier density
required for transparency.

2.3.2 loss
The active region contains optical losses such as free-cmier absorption,
scattering, and other possible effects. These factors make up the active-region