Page 395 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 355

Assuming neff = 3.5 and Lint = 250-500 pm, we find Avint = 85 to 170 GHz.
Many Fabry-Perot diode lasers, especially long-wavelength InGaAsP lasers,
will oscillate in several axial modes simultaneously in the absence of a wave-
length-selective element in the cavity.

2.4.2 Linewidth
The linewidth of a solitary single-mode laser diode is given by the modified
Schawlow-Townes foimula [5]:

where u, is the group velocity, n for AlGaAs and InGaAsP lasers is about 2.6
SP
and 1.6.;espectively, and a is the linewidth broadening factor.
2.4.3 Linewidth Broadening factor
The semiconductor index of refraction consists of real and imaginary parts

ti = 11' + in" . (12)

The real and imaginary parts are strongly coupled compared to other laser gain
media. The strength of this coupling is characterized by the line*i*idth hr-ocrdeiz-
itig fictor a, defined as

An'
a=- (13)
An" .
The a parameter is the ratio of the changes in the real and imaginary parts of the
refractive index with a change in the carrier density. The linewidth broadening
factor is a positive number with typical values in the range of 4 to 7 near the
middle of the optical gain band and rising steeply to values of 10 to 20 as the
photon energy approaches the band gap [6]. At each wavelength, the value of Q
increases with higher injection current [7]. The degree of dependence of the C!
parameter on device geometry depends on the type of active region [SI. For
index-guided lasers (see discussion later), the a parameter is not strongly depen-
dent on device geometry; that is, it is close to the value for bulk material. For
gain-guided and quantum-well laser diodes a may be geometry dependent and
differ from the bulk value.