Page 394 - Tunable Lasers Handbook

354 Paul Zorabedian

2.3.5 Output Power and Quantum Efficiency
Below threshold, the carrier density is proportional to the injection current.
Once the laser diode begins to oscillate. the carrier density is clamped at the
threshold value given by

The threshold current It,, is given by

where q is the electronic charge. is the volume of the active region, and tc is
the carrier lifetime. Above threshold, the relation between output power Po,, and
injection current Z is given by

The differential quantum efficiency q,,, is given by

- hv
r\ exr - 7 a mir
'l
int
a mir + a inr
where q,,, is the probability of radiative recombination for carriers injected into
the active region, which is close to unity for most semiconductor lasers [4].

2.4 Spectral Properties of Output
2.4.7 Diode Laser Axial Modes
The phase part of the threshold condition specifies the axial modes of the
diode laser. The frequencies V, and wavelengths hq of the Fabry-Perot modes of
the solitary diode laser are given by

where q is an integer. c is the velocity of light, ne% is the index of refraction, and
Lint is the physical length of the active region. The frequency spacing between
diode laser axial modes is thus given by