Direct Energy, 2018a

158 7.4 Devices Involving Stimulated Emission
a photon by spontaneous or stimulated emission. Alternatively, other excited
electrons decay to a lower energy level while emitting heat or lattice
vibrations instead. The internal quantum eciency is the ratio of the rate
with which excited electrons decay and produce a photon over the rate at
which all excited electrons decay [10, p. 562]. It depends on temperature,
the concentration of impurities or crystalline defects, and other factors [10,
p. 596].
Eciency is also determined by the laser cavity. A laser cavity reects
photons towards the active material. However, the laser cavity must let
some light exit. In many lasers, the cavity is formed by mirrors. While
these mirrors reect most of the light, some light is absorbed and some light
is transmitted through the mirrors as laser output. Many lasers which use
mirrors include lenses, prisms, and other optical components in the cavity
to focus or lter light to the active material. These components may also
reect or absorb some light and thereby decrease the laser eciency. As
mentioned above, the cavity of many semiconductor lasers is formed by
the interface between the active material and the surrounding air. While
external mirrors can reect over 99% of photons [86, p. 159], mirrors formed
by semiconductor air interfaces are much less ecient. The amount of light
reected depends on the indexof refraction of the material. In gallium
arsenide, for example, the index of refraction is 3.52 which corresponds to
only 31% of light reected at each interface [97].
The inuence on eciency of internal absorption and mirror reectivity
can be summarized in a single relationship [98].
( )
1
ln
R
η eff = η eff−other ( ) (7.22)
αl +ln
In this equation, R is the unitless mirror reectivity, α is the absorption
coecient of the active material in units m −1 , and l is the length of the
active material in m. The term η eff−other represents the eciency due to
all other factors besides absorption and mirror reectivity, and η eff is the
overall eciency. Equation 7.22 can be rewritten with some algebra.
⎛
⎞
1
η eff = η eff−other
⎝1 − ( ) ⎠ (7.23)
1+ 1 ln 1
αl R
These eciency concepts generalize to other energy conversion devices
which produce light. Equation 7.20 also describes the overall eciency of
LEDs and lamps in addition to electrically pumped lasers. The concepts
1
R