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Industrial Sensors and Contr ol
The silicon APDs have three different values of ionization ration: 367
• The Super-Low k (SLiK) APDs used in photon counting
modules
• The high-performance reach-through structures with k = 0.02
for applications requiring extremely low noise APD and high
gain
• The low-cost silicon epitaxial APD with k = 0.06 ideal for high
SNR applications.
In germanium and InGaAs APDs, the k-value is generally quoted
at M = 10, which somewhat overestimates F at M < 10 and underesti-
mates F at M > 10.

7.24 The Geiger Mode
In the Geiger mode, an APD is biased above its breakdown voltage
5
6
(V > V ) for operation at very high gain (typically 10 to 10 ).
R BR
When biased above breakdown, an APD will normally conduct a
large current. However, if this current is limited to less than the APD’s
“latching” current, there is a strong statistical probability that the cur-
rent will fluctuate to zero in the multiplication region, and the APD
will then remain in the “off” state until an avalanche pulse is trig-
gered by either a bulk or photogenerated carrier. If the number of
bulk carrier–generated pulses is low, the APD can therefore be used
to count individual current pulses from incident photons. The value
of the bulk dark current is therefore a significant parameter in select-
ing an APD for photon-counting, and can be reduced exponentially
by cooling.

7.25 Crack Detection Sensors for Commercial, Military,
and Space Industry Use
Accurate and precise detection of crack propagation in aircraft com-
ponents is of vital interest for commercial and military aviation and
the space industry. A system has been recently developed to detect
cracks and crack propagation in aircraft components. This system
uses optical fibers of small diameter (20 to 100 μm), which can be
etched to increase their sensitivity. The fibers are placed on perforated
adhesive foil to facilitate attachment to the desired component for
testing. The fiber is in direct contact with the component (Fig. 7.42).
The foil is removed after curing of the adhesive. Alternatively, in
glass-fiber-reinforced plastic (GFRP) or carbon-fiber-reinforced plastic
(CFRP), materials that are used more and more in aircraft design, the
fiber can be easily inserted in the laminate without disturbing the
normal fabrication process. For these applications, bare single fiber or