Page 105 - Optical Communications Essentials

Light Sources and Transmitters

Light Sources and Transmitters 95

Figure 6.8. Possible photon emission and absorption processes for a
two-level atomic system.

by stimulation, thereby emitting a photon. The corresponding photon generation
processes are called spontaneous emission and stimulated emission, respectively.
Spontaneous emission occurs randomly “at the will” of the excited electron.
Consequently, spontaneously generated photons have random phases and frequencies
(or equivalently, random wavelengths), since the electrons can return to the ground
state from any level in the conduction band. Therefore, this type of light has a broad
spectral width and is called incoherent.
Stimulated emission occurs when some external stimulant (such as an incident pho-
ton) causes an excited electron to drop to the ground state. The photon emitted in this
process has the same energy (i.e., the same wavelength) as the incident photon and is in
phase with it. Recall from Chap. 2 that this means their amplitudes add to produce a
brighter light. Thus this type of light is called coherent. Under normal conditions the
number of excited electrons is very small, so that stimulated emission is essentially neg-
ligible. For stimulated emission to occur, there must be a population inversion of carri-
ers. This fancy term simply means that there are more electrons in an excited state than
in the ground state. Since this is not a normal condition, population inversion is
achieved by supplying additional external energy to pump electrons to a higher energy
level. The “pumping” techniques can be optical or electrical. For example, the bias volt-
age from a power supply provides the external energy in a semiconductor device.
Laser action normally takes place within a region called the gain medium or laser
cavity. To achieve lasing action within this region, the photon density needs to be built
up so that the stimulated emission rate becomes higher than the rate at which photons
are absorbed by the semiconductor material. A variety of mechanisms can be used
either at the ends or within the cavity to reflect most of the photons back and forth
through the gain medium. With each pass through the cavity, the photons stimulate
more excited electrons to drop to the ground state, thereby emitting more photons of
the same wavelength. This process thus builds up the photon density in the gain
region.
If the gain is sufficient to overcome the losses in the cavity, the device will start to
oscillate at a particular optical frequency. The point where this oscillation occurs is
called the lasing threshold. Below this point both the spectral range and the lateral
beam width of the light output are broad, like that from an LED. Beyond the lasing
threshold the device behaves as a laser, and the light output increases sharply with bias
voltage, as shown in Fig. 6.9. As the lasing transition point is approached, the spectral
width and the beam pattern both narrow dramatically with increasing drive current.

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