Fiber-Optic Sensors - Applied Optics Fundamentals and Device Applications - page 176

4.17.1 Laser Operation

Some of the basic requirements for laser operation include the use of direct
bandgap semiconductor properties, low photon absorption in the bulk mate-
rial, high probability of stimulated emission through pumping, the existence
of an intense flux of photons at the diode junction, and the existence of an
inverted electron population. With these requirements, the semiconductor
junction diode will operate as a laser source with the photon amplification
being collimated and the emitted energy in the 1-25 Å wavelength range.
For direct bandgap materials such as GaAs, by definition, the conduction
band minimum and valence band maximum occur at the same point in “ k ”
space (see Figure 4.17). In this case, only a photon is required to cause an
electron transition. For indirect bandgap materials, an electron transition
absorbs a photon and absorbs or creates a phonon. The direct transition is
more probable, so that direct bandgap materials are more optically active
than indirect materials.
Light emission in semiconductors is of two types: spontaneous emission
and stimulated emission. With spontaneous emission, a hole and an electron
randomly combine. With stimulated emission, a hole and an electron are
stimulated to recombine by an existing photon. When an electron absorbs
a photon, the electron will make a transition to a higher energy state. When
an electron falls to a lower energy state, a photon is emitted. The emission
of one photon can cause other electron transitions. The light-emitting transi-
tions occur in the bandgap, and the wavelength of the emitted light roughly
corresponds to the absorption edge wavelength of the material. Note that
the bandgap decreases with increasing temperature so that the energy of the
emitted photon also decreases [20] (see Figure 4.18).
A requirement for an emissive transition is that there must be a filled upper
(initial) energy state and a corresponding empty lower (final) energy state,
so, to obtain significant light emission, increased concentrations of holes and

E

E

(a) k OR (b) k

FIGURE 4.17
Direct absorptive electronic transitions: (a) in a direct bandgap material and (b) in an indirect
bandgap material.