8.2. COOLING SCHEME FOR LASER MATERIALS 251
In a practical situation, the thermalmodeling of a laser medium is a tedious
and cumbersome task. Variations in boundary conditions and pump energy,
the presence of surface coatings, presence of material impurities, and so forth,
can cause significant deviation from the theoretically predicted results. Since
accounting for all such nonlinearities is not possible, alternate methods are
needed for efficient heat removal.
Current technology in controlling the operation of thermoelectric devices is
restricted to a constant current setting at which optimal performance is ex-
pected. If there are changes in the operating environment, the cooling rate has
to be manually adjusted to the new operating conditions. Some recent advances
in TEC applications have required automatic controllers, which has led to the
design and implementation of classical PID type controllers. There is a need
for the dynamic control of the cooling process to achieve sustained optimality
conditions. It should be noted that over-cooling will cause thermal gradients
that severely affect the laser behavior. In this context, a fuzzy logic-based ap-
proach provides a promising technique for the control and operation of TECs
by providing a wide range of variability. The controller implicitly compensates
for highly nonlinear phenomena.
Fuzzy controller design Before proceeding to the design of a controller, it
is useful to understand the operating principle of a thermoelectric device. A
thermoelectric deviceis a solid-state energy converter that contains arrays
ofn-type andp-type semiconductors thermally joined in parallel and electrically
joined in series at both ends to form a couple. Then-type semiconductor has ex-
cess electrons whereas thep-type is electron deficient, so these convert electrical
energy to thermal energy and thermal energy to electrical energy, respectively.
When a thermoelectric device is converting thermal energy to electrical energy
it is called a thermoelectric generator (TEG). When a thermoelectric device is
converting electrical energy to thermal energy it is called a thermoelectric cooler
(TEC).
TEGs operate on the principle of Seebeck Effect, a solid-state theory that
explains current generation through a pair of dissimilar semiconductors due to
temperature gradient. Such a heat engine is relatively inefficient and produces
power that is suited for systems that have low power requirements. One possible
application for such devices is for deep space applications.
TECs, on the other hand, operate on Peltier Effect. As noted above, the
n-type andp-type semiconductors are electrically connected in series and ther-
mally connected in parallel between two ceramic plates to form a couple. As
current passes through the couples, from then-type to thep-type, it creates a
temperature gradient across the TEC when heat energy is absorbed from the
cold junction, transported through the semiconductors by electrons (n-type) and
holes (p-type) and transferred to the hot junction. If the direction of current is
reversed, heat is transported from the hot junction to the cold junction. The
rate of cooling is proportional to the current through the circuit and the number
of TEC couples. By appropriately controlling the current input to the TEC,