3.3. SEMICONDUCTOR LASERS 97
Figure 3.12: A broad-area semiconductor laser. The active layer (hatched region) is sandwiched
betweenp-type andn-type cladding layers of a higher-bandgap material.
conductor with a higher bandgap. The resultingp–nheterojunction is forward-biased
through metallic contacts. Such lasers are calledbroad-areasemiconductor lasers since
the current is injected over a relatively broad area covering the entire width of the laser
chip (∼ 100 μm). Figure 3.12 shows such a structure. The laser light is emitted from
the two cleaved facets in the form of an elliptic spot of dimensions∼ 1 × 100 μm^2 .In
the direction perpendicular to the junction plane, the spot size is∼ 1 μm because of
the heterostructure design of the laser. As discussed in Section 3.1.2, the active layer
acts as a planar waveguide because its refractive index is larger than that of the sur-
rounding cladding layers (∆n≈ 0 .3). Similar to the case of optical fibers, it supports
a certain number of modes, known as the transverse modes. In practice, the active
layer is thin enough (∼ 0. 1 μm) that the planar waveguide supports a single transverse
mode. However, there is no such light-confinement mechanism in the lateral direction
parallel to the junction plane. Consequently, the light generated spreads over the entire
width of the laser. Broad-area semiconductor lasers suffer from a number of deficien-
cies and are rarely used in optical communication systems. The major drawbacks are
a relatively high threshold current and a spatial pattern that is highly elliptical and that
changes in an uncontrollable manner with the current. These problems can be solved
by introducing a mechanism for light confinement in the lateral direction. The resulting
semiconductor lasers are classified into two broad categories
Gain-guided semiconductor lasers solve the light-confinement problem by limit-
ing current injection over a narrow stripe. Such lasers are also calledstripe-geometry
semiconductor lasers. Figure 3.13 shows two laser structures schematically. In one
approach, a dielectric (SiO 2 ) layer is deposited on top of thep-layer with a central
opening through which the current is injected [33]. In another, ann-type layer is de-
posited on top of thep-layer [34]. Diffusion of Zn over the central region converts
then-region intop-type. Current flows only through the central region and is blocked
elsewhere because of the reverse-biased nature of thep–njunction. Many other vari-
ations exist [2]. In all designs, current injection over a narrow central stripe (∼ 5 μm
width) leads to a spatially varying distribution of the carrier density (governed by car-