Page 26 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Source: Photodetection and Measurement

Chapter
2

Amplified Detection Circuitry

2.1 Introduction

In this chapter we progress from photodetectors to photodetection, simple elec-
tronic circuits that allow observation and measurement of static and varying
optical signals with a voltmeter, on an oscilloscope, or as part of an optoelec-
tronic product. We saw in Chap. 1 that the signal output from a photodiode is
strongly influenced by the circuitry connected to it. In particular, unless the
load voltage is much smaller than a few tens of millivolts, simple connection to
a resistive load leads to a logarithmic or at least highly nonlinear response,
while reverse biasing into the third quadrant of the diode’s IV characteristic
generally leads to a current-output response that is linear over many orders of
magnitude of incident power. This reverse bias connection is the basis of much
optical measurement technology. In fact, the simple circuit seen in Fig. 1.8 can
be called our first “instrument.”

2.2 The Bias Box
Figure 2.1 shows a photodiode reverse biased by a small battery, with a series
load resistor whose imposed voltage can be read with a high impedance volt-
meter or oscilloscope. With the addition of a multiway switch to change load
resistors over a wide range, say from 100W to 1MW, and perhaps a changeover
switch to swap the detector polarity, this “equipment” should be in every optical
researcher’s kit bag. I try to keep several available. A 3.3-V lithium cell or 6-V
camera battery is unlikely to damage the majority of silicon photodiodes
through reverse breakdown, but check the photodiode’s voltage ratings first to
be sure. However, it is possible to pass excessive current if the photodiode is
brightly illuminated on the low load resistance settings, so connect it first,
monitor the voltage, and then illuminate. The bias box can be used with almost
any detector that is handy. It has an easily varied sensitivity and responds lin-
early to power changes. It never oscillates, and the battery voltage is almost

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