Page 25 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Photodetection Basics

18 Chapter One

5X1, providing a responsivity of 93A/W at 750nm with a bias voltage in the
range of 1700 to 2000V. APDs in InGaAs are also available for long wavelength
(1.55mm) use.
Note that these detectors are not inherently more sensitive nor do they offer
lower noise than conventional pin-diodes. They all offer about the same wave-
length response, capacitance, and quantum efficiency and so have essentially
the same limitations. APDs are also inherently noisier because of the presence
of excess noise caused by fluctuations in the gain mechanism. However, where
the provision of sufficient external electronic gain and bandwidth is problem-
atic, leading to limitations in receiver performance, the internal gain of the APD
can be very useful. This is generally the case only at higher speeds (say 1MHz
and above). They are rarely superior for low light level detection at low fre-
quencies. They are also much more difficult to use than pin diodes, owing to
the precise regulation of the high bias voltage that is needed, which is itself
temperature-dependent. However, complete detection modules are available
from Hamamatsu and others to ease the design difficulties.
Similar tradeoffs occur with photomultipliers (PM tubes), which generate
photoelectrons in an inorganic semiconductor phosphor. Like the APDs, these
use an internal cascade multiplication process to achieve very high external
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responsivities, typically 10 to 10 A/W. However, their intrinsic sensitivity is
rather low, as their photocathodes exhibit quantum efficiencies much less than
peak semiconductor detector values, for example, 0.1A/W. They also require a
stable, low noise high voltage supply at the kilovolt level. Advantages of PM
tubes are their wide range of sensitivity obtained through a choice of photo-
cathode materials, large detection areas compared with semiconductor photo-
diodes, and low dark currents. They are also relatively insensitive to ionizing
radiation. Both APDs and PM tubes are available with low enough dark cur-
rents to be used in photon-counting mode. Photon counting is a sensitive detec-
9
tion method for use at low incident powers (typically <10 photoelectrons/
second), where instead of averaging photoelectrons into a continuous current,
individual electron charges are counted. This brings the advantage of a kind of
separation between thermal noise and photoevents through the use of carefully
set up threshold discriminators. Large gains in performance are sometimes pos-
sible using this mode of detection.

1.9 Summary
In this chapter we have discussed the basics of photodetection using junction
photodiodes and shown how they can be represented by a simple current-
generator model. This is adequate in most cases to predict performance in a
wide variety of externally connected circuits. Different materials may offer
advantages in special situations, but the majority of laboratory and instru-
mentation applications are dealt with using silicon and InGaAs devices. With
this information, we will now investigate how to use photodiodes in practice to
perform useful measurements.

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