TABLE OF CONTENTS In this chapter we progress from photodetectors to photodetection, simple electronic circuits that allow observation and measurement of static and varying optical signals with a voltmeter, on an oscilloscope, or as part of an optoelectronic 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."
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 voltmeter or oscilloscope. With the addition of a multiway switch to change load resistors over a wide range, say from 100 ? to 1M ?, 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...
