PHOTODIODES

Photodiodes are one of the most widely used transducers for measuring the intensity of laser light. You will design and build a circuit which produces a signal from a photodiode which is linear in intensity, you will test the linearity, and then you will use this circuit to perform an unrelated experiment.

LIBRARY WORK. Find out what you can about the physics of photodiodes. How do they work? For what wavelengths would you expect a silicon diode to work? Germanium?

CIRCUITRY. If we think of the photodiode as a device with a resistance which depends on light intensity, and if the resistance is inversely proportional to the intensity, how would we design a circuit to get a signal that is proportional to intensity? (not by measuring the resistance) If we put a voltage across the diode and measure the current flow, would that do it? Put a resistor, R, in series with the diode, and put a voltage, E , across this series. The resistance of the photodiode equals r=A/x, where x is the intensity of the incident light. Check whether the voltage across either r or R is nearly linear in x. What are the limits of linearity, in terms of r and R? In terms of Vout? Connect such a circuit, using a prototyping breadboard and an IC socket, so that you can easily change resistors.

LINEARITY. The light reflected off a glass slide at normal incidence is given by Fresnel's equation,
,
,
with n=1.50. Reflect a laser off one, then two slides, and compare the measured intensity (from your diode) to the original beam. Is your transducer linear? What do you calculate for n?

ELECTIVES: Do #1, plus do either #4, or both #2 and 3. 1Ù (4Ú (2Ù 3))
1. Compare the AC and DC voltages registered by your transducer sitting in the lab room. Explain why there are both types of signals. (Use an oscilloscope to find the frequency of the AC component.)
2. Compare the AC and DC voltages registered by your transducer in a darkroom with a tungsten-filament bulb. Explain.
3. Verify the inverse-square law for a standard light source (no lasers!).
4. Soap suds: Take a test tube, construct a holder for it, put a little detergent and some water in the test tube, and produce a rich foam of bubbles. Shine the laser through the foam. See whether it is possible to measure any output signal through the foam. Calculate the number of bubble surfaces, and thus the mean size of the bubbles, from the output intensity, given Fresnel's equation, with n=1.33 (water).

Two experiments you might want to conduct with this system are the following: (a) After the suds have settled a little, measure the unreflected light through the test tube as a function of height (Correlate this to what you measure directly for the size of the bubbles.), or (b) Leave the system stationary and measure the unreflected light at a fixed height as a function of time. This second experiment will take much longer, but it may tell you how the average size of the bubbles evolves with time.

NEXT TIME:
We will do independent projects, so sit down and have a chat with your lab instructor well in advance of then to figure out what project to do.