Page 400 - Tunable Lasers Handbook

360 Paul Zorabedian

2.6.3.2 Index-Guided Buried-Heterostructure Devices
In an index-guided buried-heterostructure device, a stripe of active-layer
material about 0.1 to 0.2 pm thick and 1.0 to 2.0 pm wide is completely buried
in lower index cladding material. This guiding structure requires careful control
of the fabrication process. However, almost all commercial laser diodes com-
prise some sort of buried heterostructure because the resulting strong index guid-
ing results in the lowest threshold currents, typically 10 to 30 mA, good trans-
verse mode stability, and negligible astigmatism. Typical differential quantum
efficiencies are in the range of 40 to 60% for both facets. The near-field spot size
is typically 0.5 to 1.0 x 2.0 pm. Beam divergence angles are typically in the
range of OH = 30" to 40" and 8,. = 40" to 50".

2.6.3.3 Ridge-Waveguide Laser Diodes
In a ridge-waveguide laser, thickness variation of the upper cladding layer
provides optical confinement. This type of structure is simpler and easier to fab-
ricate than a buried heterostructure. Current confinement is not as tight as in a
buried heterostructure, so the threshold is also somewhat higher. Typical L-Z
characteristics are Zth = 20 to 30 mA and q = 50%. The higher threshold is com-
pensated by the fact that the lower current density allows higher power opera-
tion. The spot is somewhat larger than in buried heterostructure lasers. so the far-
field beam is less divergent. The less divergent output beam makes it easier to
couple efficiently a ridge-waveguide laser to an external cavity.
Some typical laser diode characteristics are given in Table 1.

2.7 Gain Stripe Structures
Several types of patterning of the active stripe are used in gain media for
ECLs (Fig. 6j.
2.7. I Fabry-Perot Single Stripe

The basic laser diode top structure is a single contact stripe perpendicular
to the cleaved-facet mirrors. The active region thus lies within a Fabry-Perot

TABLE 1 Typical Laser Diode Characteristics

Threshold Differential Divergence
Guiding current quantum efficiency OH x Ov
mechanism (mA) (%o) FWHM Astigmatism

Oxide stripe >50 40 10' x 10" 5-50 pm
Buried heterostructure 10-20 >50 30" x 40" 0 (nominal)
Ridge waveguide 70-30 -50 25" x 35" 0 (nominal)