98 CHAPTER 3. OPTICAL TRANSMITTERS
Figure 3.13: Cross section of two stripe-geometry laser structures used to design gain-guided
semiconductor lasers and referred to as (a) oxide stripe and (b) junction stripe.
rier diffusion) in the lateral direction. The optical gain also peaks at the center of the
stripe. Since the active layer exhibits large absorption losses in the region beyond the
central stripe, light is confined to the stripe region. As the confinement of light is aided
by gain, such lasers are called gain-guided. Their threshold current is typically in the
range 50–100 mA, and light is emitted in the form of an elliptic spot of dimensions
∼ 1 × 5 μm^2. The major drawback is that the spot size is not stable as the laser power
is increased [2]. Such lasers are rarely used in optical communication systems because
of mode-stability problems.
The light-confinement problem is solved in theindex-guidedsemiconductor lasers
by introducing an index step∆nLin the lateral direction so that a waveguide is formed in
a way similar to the waveguide formed in the transverse direction by the heterostructure
design. Such lasers can be subclassified as weakly and strongly index-guided semicon-
ductor lasers, depending on the magnitude of∆nL. Figure 3.14 shows examples of the
two kinds of lasers. In a specific design known as theridge-waveguide laser, a ridge is
formed by etching parts of thep-layer [2]. A SiO 2 layer is then deposited to block the
current flow and to induce weak index guiding. Since the refractive index of SiO 2 is
considerably lower than the centralp-region, the effective index of the transverse mode
is different in the two regions [35], resulting in an index step∆nL∼ 0 .01. This index
step confines the generated light to the ridge region. The magnitude of the index step is
sensitive to many fabrication details, such as the ridge width and the proximity of the
SiO 2 layer to the active layer. However, the relative simplicity of the ridge-waveguide
design and the resulting low cost make such lasers attractive for some applications.
In strongly index-guided semiconductor lasers, the active region of dimensions∼
0. 1 × 1 μm^2 is buried on all sides by several layers of lower refractive index. For
this reason, such lasers are calledburied heterostructure(BH) lasers. Several different
kinds of BH lasers have been developed. They are known under names such as etched-
mesa BH, planar BH, double-channel planar BH, and V-grooved or channeled substrate
BH lasers, depending on the fabrication method used to realize the laser structure [2].
They all allow a relatively large index step (∆nL∼ 0 .1) in the lateral direction and, as