Page 403 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 363

200 400 600 800 1000 1600 2000
1 I I I I l l I /
Commercially
available H H H H H I
devices

Wavelength range H H H
of commercial AIGalnP!GaAs InGaAsiGaAs InGaAsAnP
technologies 610-690 H880-1100, /i600-2100
AIGaAdGaAs InGaAaPilnP
780-880 1 100-1 600
I
H
New +I Il-V1 and Ill-V InGaAsP,GaAs
materials
compounds 690-880
400-600
I I I I I l l I
200 400 600 800 1000 1600 2000
Wavelength (nm)
FIGURE 7 \Vavelength ranges of diode laser technologies. (Reproduced with permission from
Waarts [1Z].i

away from the band edges, there are more states to fill, and their rate of motion
with respect to pumping rate slows down. This tends to limit band filling. A typi-
cal tuning range for bulk active-region gain media is roughly ~AV~,~~, = 50 meV.
For a more quantitative discussion of the factors determining the gain profile of a
semiconductor laser. see [ 131.
The gain profile is experimentally determined by placing the gain chip in an
external cavity laser and measuring the threshold current versus wavelength. The
tuning curve typically has a "bathtub" shape with a relatively flat central region
and steeply sloped sides (Fig. 8) [14]. The long-wavelength side (approaching
the band edge) is quite abrupt. The roughly constant energy limit of band filling
just mentioned implies a wavelength tuning range that increases roughly as the
square of the center wavelength. This is born out by the data in Table 2. which
gives typical tuning ranges for different active-region materials and center wave-
lengths. From these values the following empirical expression relating tuning
range to center wavelength can be deduced:

AL = 4.2 x 10-' A: (AA and a in nrn) . (16)

2.9.2 Quantum We!/ Active-Region Gain Media
Two major effects are associated with the reduction of the active region
thickness from -0.1 prn to -10 nm. First, the injection current required to
sustain transparency is reduced by about the same factor as the active-region
thickness. Thus, ECLs with quantum-well gain media have lower threshold
currents. Second, the quantum-well density of states is a staircase function of
(hv -E,) that is everywhere lower than the corresponding (hv -E,):? function for