Page 404 - Tunable Lasers Handbook

364 Paul Zorabedian

60 -
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0 8
50- %
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-
- I]
I
U
0
E 40-
E %
3 30- “.e
3
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E 20-
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lZ00 1225 1250 1275 1300 1325 1 io
FIGURE 8 Typical diode laser threshold current versus wavelength curve. (Reproduced with
permission from Zorabedian and Trutna [ 11.1.)

the bulk case. Because of the reduced density of states, for a given pumping
level the quasi-Fermi levels are pushed farther apart than in bulk material, result-
ing in a broader tuning range [ 181.
Another feature of quantum-well gain media is that the thin layers need not
be lattice matched to the substrate because they can sustain elastic strain without
the formation of defects. Lattice-mismatch strain shifts the energy bands and can
be created intentionally in order to obtain a shifted tuning range. Quantum-well
active media can comprise either a single quantum well (SQW) or multiple
quantum wells (MQW). SQW devices have the lowest transparency current. An
MQW device has a higher transparency current but also has higher maximum
gain. Furthermore, spreading the injected electrons into multiple quantum wells

TABLE 2 Tuning Ranges of Conventional Gain Media

Typical tuning range
Nominal wavelength of a single bulk DH laser
Material system (nm) (nm) Reference
InGaAlP/GaAs 670 -15 15
GaAlAs/GaAs 780 -25 15
GaAlAs/GaAs 850 -30 16
InGaAsP/InP 1300 -70 17
InGaAsPInP 1550 -100 17