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4 Light Sources for Fiber Links
Fig. 4.11 Bandgap energy
and output wavelength as a
function of aluminum mole
fraction x for Al x Ga 1−x As at
room temperature
1.4
Al mole fraction, x
1.6
1.8
0.7
0.8
0.2
0.3
Wavelength ( μm)
0.1
Bandgap energy (eV)
λ = 810 nm at x = 0.08
Direct bandgap
region for
Al x Ga 1-x As at 300°K
of the peak emitted radiation. This is illustrated in Fig. 4.11. For values of x greater
than about 0.37, the bandgap changes from a direct to an indirect bandgap. The value
of x for the active-area material is usually chosen to be around 0.1 for an emission
wavelength of 800−850 nm. An example of the emission spectrum of a Ga 1−x Al x As
LED with x = 0.08 is shown in Fig. 4.12. The peak output power occurs at 810 nm.
The width of the spectral pattern at its half-power point is known as the full-width
half-maximum (FWHM) spectral width. For LEDs this parameter varies between 20
Full-width
half-maximum
FWHM
Fig. 4.12 Spectral emission pattern of a representative 810-nm Ga 1−x Al x As LED with x = 0.08,
which gives a spectral pattern width at its half-power point of 36 nm