Page 398 - Tunable Lasers Handbook

358 Paul Zorabedian

2.6. 7 Vertical Guiding
Modern diode lasers are double heterostructures in the vertical direction.
A thin active layer is sandwiched between top and bottom cladding layers. the
top layer being y-type and the bottom !?-type. The active layer is composed of
a different semiconductor material having a lower band gap and consequently
a slightly larger index of refraction than the p and 17 cladding layers that lie
above and below it. The layers are comprised of various binary compounds
and their associated lattice-matched ternary or quaternary alloys. The relative
position of the materials in the sandwich depends on whether the band gap of
the binary is larger or smaller than that of the alloy. For example. in the case of
a GaAs/GaAlAs laser, the active layer is composed of GaAs and the cladding
layers are composed of GaAlAs. In the case of an InP/GaInAsP laser, on the
other hand, the active layer is GaInAsP and the cladding layers are InP. In a
double-heterostructure device, the carriers are vertically confined by potential
barriers and the photons are vertically confined by the refractive index gradi-
ents of the slab waveguide formed by the cladding and active layers. The
active layer thickness in conventional lasers is -0.1 pm, while in quantum-well
lasers the active layer thickness is about an order of magnitude thinner-about
10 nm.

2.6.2 Active Region Vertical Structures
Laser diodes can be subdivided into two main categories according to the
thickness of their active regions [9].

2.6.2.1 Bulk Active Region
Conventional lasers have active regions that are about -0.1 pm thick. At this
magnitude, the carriers in the active region material exhibit the same properties
as in bulk material. The active regions of conventional laser diodes are grown
either by liquid-phase epitaxy (LPE) or vapor-phase epitaxy, which is also
known as metalorganic chemical vapor deposition (MOCVD). Conventional
growth methods are the most amenable to low-cost, high-volume production.

2.6.2.2 Quantum-Well Active Region
When the thickness of the active region is reduced by about an order of
magnitude to -10 nm, the carriers exhibit properties that differ from the bulk
because of quantum confinement. Such devices are called quaiiturn-$%>ell laser.
diodes. Quantum-well active regions can be grown by MOCVD or by molecu-
lar-beam epitaxy (MBE). When used as gain media in ECLs, quantum-well
lasers have advantages in terms of lower threshold current and increased tuning
range.