Page 200 - Introduction to Information Optics

3.3. Critical Components 185
A semiconductor laser consists of a forward-biased p-n junction. When the
forward current through the diode exceeds a critical value known as the
threshold current, optical gain in the resonator due to stimulated emissions
overcomes the losses in the resonator, leading to net amplification and
eventually to steady-state laser oscillation, as described in detail in the
following.
Under the forward bias, free electrons and holes will move back to the
depleted region. Thus, there is a chance for "recombination." This recombina-
tion process releases the energy in the "light" form -> generate light. In terms
of band gap theory, there are two allowed energy levels existing in the material,
which are called the conduction band and the valence band. In the conduction
band, electrons are not bound to individual atoms so that they are free to
move. In the valence band, unbound holes are free to move. The generated
photon energy is determined by the band gap between the conduction and
valence bands. Mathematically, this can be written as

£, (3.40)

34
where h is the Planck's constant h = 6.63 x 10 ' J • s, v is the output light
frequency, and E c and E v are the energy of conduction and valence bands,
respectively. Figure 3.11 shows the basic structure of a semiconductor laser.
The polished side surfaces form the resonant cavity so that a particular
wavelength can be amplified.
Table 3.1 lists several types of materials used to fabricate semiconductor
lasers and their corresponding operating wavelength range.

Current /

200 Mm
L / 300 Mm / /-v
Y

P
/ M /\ Cleaved
t 100 Wn
facet
Saw-cut
facet

Fig. 3.11. Basic structure of the semiconductor laser.