Goals

This laboratory will help you become familiar with the operation and characteristics of two detectors commonly used for optical radiation: silicon photodiodes, and photomultipliers.

Background

Quantitative optical detectors generally fall into three classes: thermal detectors, photoconductive or photovoltaic detectors, and photomultipliers, which actually multiply electrons, not photons. In addition, there are several techniques used for special experimental situations, such as photoacoustic and optogalvanic detection. The basic theory of optical detectors can be found in many texts; a particularly good reference is Detection of Light: From the ultraviolet to the submillimeter , by G.H. Rieke. Other sources are Verdeyen, Wilson and Hawkes, Yariv, or Swindell. Also, useful practical information can often be found in manufacturer's catalogs and application notes. [6, 7, [8] ] Thermal detectors have the advantage of relatively simple absolute calibration that is constant over a very broad spectral range. They have a very slow response time and are used primarily either for low power cw radiation, e.g., solar radiation, or for single-shot energy measurement of low repetition rate lasers. Sensitive disk thermopile detectors can be quite fragile, and are often reserved just for calibrating other detectors. You will not be using a thermal detector in these experiments.

Photodiodes

When a photon is absorbed within a pn-junction, an electron-hole pair is created. If the generation takes place within the junction depletion region, or if the carriers diffuse to the depletion region before recombining, the electron and hole move to opposite sides of the junction under the influence of the intrinsic field in the depletion region, resulting in a current. If the device has no external connections, or only a very high impedance connection, this current must be balanced by a diffusion of majority charge carriers against the intrinsic field. This results in a lowering of the voltage across the junction that can be measured. The voltage change is a logarithmic function of the photon-induced current, and the response is slow. This method of photodiode operation is called the photovoltaic mode; the Newport 818 detector head operates in photovoltaic mode.

If an external reverse bias is applied, the additional field suppresses the majority carrier flow, and a net current flows through the bias circuit. The photon-generated carriers produce a current in the external bias circuit that can be measured, usually by measuring the voltage across a load resistor. This is called the photoconductive mode of operation. Personally, I think this is a poor name, because there are true photoconductive detectors: resistors whose value changes with illumination. A better term might be photocurrent mode, since it is the actual photon-produced current that is measured.

The time constant of a biased photodiode depends on both the internal structure and the external circuit. A simple model of the system is shown in Fig. 1. The current source represents the photo-induced current. The reverse-biased diode can be modeled as an ideal diode plus a parallel plate capacitor with the area of the junction, and a separation equal to the depletion region. The time response of the measured voltage is just the RC time constant of the external load resistor and the junction capacitance. Lowering the load resistance decreases the time response of the circuit, but at the cost of a lower voltage for a given light input, since the diode is a current device. Thus, low junction capacitance is an advantage. Small active areas reduce capacitance, but at the cost of reduced response (smaller sensitive volume) and experimental difficulty (harder to align the small area). Higher reverse bias (within limits) results in a wider depletion region and a lower capacitance. Also, the width of the depletion region is often enhanced in photodiodes by including an intrinsic region between the p- and n-doped regions; these are referred to as PIN diodes. The larger active volume of the PIN structure also increases sensitivity. Thermally generated carriers in a large junction volume can produce measurable noise current in the circuit, called dark current. Photodiodes are available in a wide variety of configurations that make different trade-offs in these parameters for different applications.

Finally, the spectral response of photodiodes depends on the optical properties of the semiconductor used, such as the energy gap and absorption. We will be using silicon photodiodes, which have a broad response covering the visible and near ir, but other materials are available for other regions.

Photomultipliers

Photomultipliers rely on the photoelectric effect to convert photons into electrons at a photocathode. The work function of the photocathode material determines the spectral response of the detector, and the formulation of broadband, sensitive photocathode materials is an art. Electrons from the photocathode are accelerated and impact on the next element, a dynode. Impact ionization at the dynode produces several electrons, which in turn are accelerated into the next dynode. The dynodes are biased and shaped in order to accelerate and focus the electrons through the tube, from element to element. The supply voltage, usually several kilovolts, is divided by a resistor chain to provide the dynode bias voltages.

Typical photomultipliers have from 8 to 14 stages of electron multiplication, leading to total current gains of 10^6 or more at the anode. Electron multiplication is probably the lowest noise method of obtaining gain, which makes photomultipliers the best choice for low level light detection. The equivalent circuit for a photomultiplier is the same as for a photodiode (see Fig. 1, with a gain much greater than 1; the capacitance results from the internal dynode structure. The dynodes are spaced relatively far apart, but there are a lot of them. The end result is that photomultipliers usually have a somewhat slower time response than photodiodes. The fastest tubes have rise times of a few nanoseconds.

Photodiode Experiments

In these experiments you will investigate the properties of two similar silicon PIN photodiodes. The size of the active area of the two diodes are different, as you can see just by looking at them. The large one has an area of 13.7 mm^2, and the small one has an area of 1 mm^2. Since the detector capacitance is a function of area, very high speed photodiodes often have much smaller active areas; the New Focus 25 GHz photodiode is only 25 micron in diameter. The diodes are mounted in a Thor mount that consists simply of a mounting socket, a 22 V bias battery, and a switch; you have to supply the load resistor to complete the bias circuit and to convert the photocurrent into a voltage. We have constructed convenient load boxes for this purpose with a selection of resistors. (Obviously, this is not the optimum configuration of a load for high speed measurements.)

Calibration

Calibration of a photodiode consists simply of applying a known optical power to the active area and measuring the resulting current/voltage. There are two general ways to do this: we could flood the diode with an optical field having a known power density, or we could focus a beam of known power onto the active area. We will use the first technique since it corresponds more closely to the usual experimental situation. Plus, focusing the beam to a small area on the detector can cause local saturation, and there is always some uncertainty in whether all the focused beam is actually collected by the active area.

These photodiodes will measure the ambient light as well as the input beam, just like the Newport detector, and you must subtract the ambient signal manually. It is convenient to use a chopper wheel when making measurements, since it provides alternate on and off periods for easy determination of the minimum and maximum voltage levels. The difference corresponds to the light you are trying to measure. Place the chopper either right after the laser or in between the steering mirrors, so it blocks the laser but not ambient light that may enter the detector during a measurement.

Linearity and Dark Current

Check the linearity of the detectors by inserting attenuators in the beam and measuring the response. You should be able to add attenuations of 3 OD (small area detector) to 4 OD (large area detector) and still have a measurable signal into a 1 MegaOhm load. Since these light levels are very low, you will need to use a custom light shield, a.k.a. a cardboard box. Place the box over the detector and put the attenuators right against the input hole of the box.

At the highest oscilloscope sensitivities you should see random voltage spikes generated by thermal electron-hole pairs. You will have to turn off or dim the room lights, even with the shield. Confirm the source of these spikes by covering the input hole of the box; notice the effect of shining a flashlight near or at the box edges. Are the spikes simply electrical noise picked up by your cables? What is the minimum detectable signal, both in terms of watts and photons per second?

Time Response

In many applications photodiodes are not used to make quantitative power measurements, but rather to determine the time behavior of an optical source. In such cases, it is important that the time response of the detection system be faster than any variation of the measured signal, otherwise an integration takes place. For example, the detector in a 1 Gbps optical communications link must have a sub-nanosecond response time. The purpose of the next set of measurements is to determine the time response of the detector and compare it to our simple model of Fig. 1. We will need a much faster light source than the chopper to test our detectors. We will use a light emitting diode (LED) driven by a fast pulse. You can turn off the HeNe laser and the chopper.

In summary, low load resistances are necessary to get fast response times, but they can result in signals that are too small to measure reliably. One needs either more light or more detector gain. You can try using lenses to collect more light from the LED and to focus it onto the detector. This is not easy; I was only able to gain about a factor of two, but sometimes two is a lot. You can also try driving the LED harder to get more light: the connectors on the back panel of the SRS pulse generator produce 1 microsec--long pulses with ten times the amplitude. You might destroy the LED (I have not tried this), but they are cheap. Next, we will investigate a more sensitive detector.

Photomultiplier Measurements

The 1P28 photomultiplier (PM) is described in the handout. It is a convenient general purpose tube with a response throughout the visible. Like most things made of glass, photomultipliers are fragile. The mechanical shock from, for example, dropping the tube on the table or knocking the mount over, can damage the internal components, even if the glass envelope is not broken. So please use care handling the PM. The tube is mounted in a housing that provides some mechanical protection, shields the tube from stray light, and makes electrical connections to the base of the tube. You can remove the top of the housing to examine the tube itself. High voltage for the tube is generated in the housing base by a dc-dc converter, so there is no external high voltage. There are two connections on the housing base; the multiconductor cable connects the tube to the control box and provides power and monitors the tube voltage. The supply voltage applied to the PM is adjusted using a knob on the front of the control box, and the voltage on the tube is indicated on the digital display. The gain of the PM is approximately an exponential function of supply voltage.

The BNC connector on the housing base is the signal output. The outer shell is grounded and the center conductor is connected to the anode of the tube. (The BNC connectors on the rear and front of the control box are connected directly together, and are just for convenience of mounting.) Like silicon photodiodes, photomultipliers produce a photocurrent proportional to the input light and require a load resistor. Since electrons flow out of the anode to ground, the signal voltages produced across the load resistor will be negative.

Photomultipliers are much more sensitive than photodiodes, and can be damaged by excessive light levels; actually, it is the resulting current that damages the tube elements by overheating them. The maximum average output current for this tube is 0.1 ma . NEVER expose a PM directly to room light when the voltage is on, and always begin measurements with the minimum supply voltage (minimum sensitivity) and then turn up the voltage while monitoring the current. Note that some tubes also have a maximum photocathode current, which is independent of the gain or voltage. Exceeding this value damages the photocathode material and reduces the response, or destroys the tube. Photomultipliers designed for very low noise, low light level detection should be shielded from strong light even when off to protect the photocathode.

Response Calibration

In the next set of experiments you will determine the response of the PM as a function of operating voltage. The photocathode has sufficient area to accept the entire HeNe laser beam, so you will not need the negative lens and iris. Just measure the HeNe power with the Newport power meter. You will want to use the chopper again to provide a convenient reference level.

Time Response

You will again use a pulsed LED as a fast light source to test the time response of the PM. Set up the LED as described in Sec. 3.3.

As with the photodiode, you should determine the time response (rise and fall times) of the detection system as a function of load resistance, but now you will also vary the PM supply voltage and the pulse length. It will probably be convenient to collect your data in the form of a rough table in your notebook with columns for the variables, data, and any notes.

Of course, we have no way of knowing if the light from the LED is really following the input pulse for nanosecond pulses. Obviously, we can't measure it with this detection system. At the least, the LED efficiency is probably reduced for short pulse lengths.

If you wish, you can check the ability of this detection system to resolve two closely spaced pulses. The pulse generator will produce two pulses of arbitrary length and separation, one on the AB output, the other on the CD output. Experiment with combining them to drive the LED, and observe the resulting detected signal.

These experiments should give you some appreciation of the detection problems involved in building a 1GHz optical communications system. Current optical telephone links operate at 2.5, 5, and even 10Gbps rates. What kind of detector do you think they use? Check some references to find out.