Page 246 - Photonics Essentials an introduction with experiments

Measurements in Photonics

240 Characterizing Photonic Devices in the Laboratory

10.9 Photon Detectors
There is a choice of device for light detection: photodiodes, charge-
coupled devices (CCDs), photomultiplier tubes, etc. We will limit our
discussion to photodiode detectors, the most widely-used devices for
detecting light. Photodiode detector circuits can be used in two ways:

1. Photovoltaic mode. This means that you plug the leads into a volt-
meter, such as the lock-in amplifier, and measure the voltage de-
veloped by absorbing photons. No power supply is needed.
2. Photocurrent mode. In this case, you connect the photodiode so
that it is reverse-biased in a circuit with a load resistor. The volt-
age drop across the load resistor is then measured by a voltmeter.
The photocurrent is the voltage divided by the load resistance
(Fig. 10.12). You choose the load resistor. It must be less than the
input impedance of the lock-in amplifier, or 100 M . On the oth-
er hand if you are working at f = 1000 Hz, the R · C product
L
must be smaller than 1/f. If C = 1000pF, then R L must be less
than 100 k . However, the larger R L , the larger the signal at the
lock-in. This is the case because the photodiode drives a certain
current through the circuit that is proportional to the number of
photons detected. So the larger the resistance, the larger the volt-
age generated across the resistor. In most cases, 10 k to 100 k
is a good choice for R L .

10.10 Curve Tracer
The curve tracer allows you to get a current voltage trace of your de-
vices. This will enable you to determine the cathode and anode of a
photodiode, light-emitting diode, or laser. This is important because
you can burn out your laser instantly by putting it into substantial re-
verse bias.

Figure 10.12. Photodiode detector circuit—Photocurrent mode.

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