Page 402 - Tunable Lasers Handbook

362 Paul Zorabedian

2.7.5 Tapered Gain Stripe
Tapered-stripe gain media [ 1 11 are used as optical amplifiers and in ECLs to
generate high output power in a single spatial mode. The gain region is typically
-4 to 10 pm at the narrow end and tapers linearly up to -130 to 200 pm at the
wide end over a length of -2 mm. Both ends of the amplifier are antireflection
coated. The narrow end acts as a transverse-mode-limiting spatial filter. The
taper allows the beam to expand by diffraction without mode conversion. The
wide output end allows for the extraction of large output powers without damage
caused by heat generation due to optical absorption at the output facet.

2.8 Wavelength Ranges of Laser Diode Technologies
2.8.7 Commercially Available laser Diodes
The availability of semiconductor gain media for ECLs is for the most part
dictated by the commercial availability of laser diodes. At present, commercial
laser diode technologies provide optical gain over most of the wavelength range
from 600 to 2000 nm (Fig. 7) [12]. The development of laser diode technologies
has, in turn, been driven by several mass market applications.
The main commercial technologies and their respective applications are
AlGaInP/GaAs, -600 to 670 nm (digital optical storage and retrieval); AlGaAs/
GaAs, -750 to 870 nm (780 nm for laser printing, 850 nm for data communica-
tions); and InGaAsPnnP, -1.1 to 1.65 pm (two separate bands at 1.3 and 1.55
pm for optical communications).
2.8.2 laser Diode Materials at the Research Stage
In addition to the technologies presently available commercially, intensive
research efforts are being carried out on new materials for shorter wavelength laser
diodes in the 400 to 600-nm range, driven by desire for higher optical storage den-
sities. These research materials are based on II-VI selenide compounds and III-V
nitride compounds. In most cases, the wavelength ranges of the technologies
extend beyond the main wavelengths where the applications are centered.

2.9 Gain Bandwidth of Individual Semiconductor Lasers
2.9. 7 Bulk Active-Region Gain Media
Electrons and holes injected into the active region respectively begin to fill
the bottom of the conduction band and the top of the valence band. The level of
filling of each band is determined by the quasi-Fermi levels EFc and EFL,. The
tuning range is roughly determined by the separation of the quasi-Fermi levels
minus the band gap (hAv - E,= - E,,, - E,,). As pumping increases, the quasi-
Fermi levels are pushed farther apart. The rate of movement of the quasi-Fermi
levels is determined by the density of states. In a bulk active region, the density
of states is proportional to (hv - Eo)”. Therefore, as the quasi-Fermi levels move