Page 201 - Introduction to Information Optics

3. Communication with Optics

Table 3.1

Material Wavelength range (/tm) Band gap energy (eV)

GalnP 0.64-0.68 1.82 1.94
GaAs 0.9 1.4
AlGaAs 0.8 0.9 1. 4- 1.55
InGaAs .1.0-1.3 0.95- -1.24
InGaAsP 0.9-1.7 0.73-1.35

To enhance the performance of the semiconductor laser, a heterostructure is
generally used, as shown in Fig. 3.12. For example, for the AlGaAs-based
laser, a heterostructure consists of a thin layer of GaAs sandwiched between
two layers of p- and rc-doped AlGaAs. With this structure, under the forward-
bias case, a large concentration of accumulated carriers in the thin GaAs layer
can be formed that leads to a large number of carrier recombinations arid
photon emissions so that medium gain can be achieved. In addition, the
refractive index of GaAs is larger than that of AlGaAs, so this thin layer may
also serve as an optical waveguide. The single transversal mode operation can
be achieved when the thickness of the waveguide is thin enough; i.e., the
normalized frequency V satisfies the following condition:
2nd
___ (3.41)

where n { and « 2 are the refractive index of the GaAs layer and the AlGaAs
layers, respectively. Thus, by introducing a thin GaAs layer, a good-quality
optical beam can be achieved.
To further reduce the spectral linewidth of the output laser beam, A/t, so that
lower intramodal dispersion can be achieved, besides employing the single
transversal mode structure, the single longitudinal mode operation is also

1 p-AIGaAs

d — AAAA-*
n-AIGaAs

Fig. 3.12. Basic structure of the heterostructure semiconductor laser.