EE 471 Lab 1
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EE471 Lab 1: Properties of Semiconductor Photodetectors
Introduction
The goal of this lab is to become familiar with the various modes of operation of
semiconductor photodetectors. The photodetector circuit will be operated in the three different
modes that have been described in class. A special board for this purpose has been constructed
and is described at the back (Appendix) of this write-up. You will calibrate your photodetector
circuit using a commercial optical power meter, and put it to use by measuring the beam profile
of a semiconductor laser.
Equipment Checklist
 Photodiode (PD), photodetector circuit board, and power supply (adaptor)
 670 nm semiconductor laser diode (LD) and power supply (adaptor)
 Lens kit, Neutral density filters (in white 35 mm slide holders)
 Optical bench equipment including translation stage (ask for more parts if required)
 Digital Multimeter (DMM) and leads
1. Preliminary Set-up and Measurements
Check that you have all the equipment on your list and set up your apparatus by
examining the demo set-up in the lab. Use a 100 mm focal length PCX (Plano-ConveX) lens to
reduce the beam of the laser diode to a small spot on the photodetector. Approximately, 100 mm
~ 4” = 4 holes on the optical bench so place your lens about 4 holes from the photodetector.
Ideally, we need the spot to be smaller than the PD area to measure all of the beam power and
alleviate alignment problems. (Please avoid touching the lens surface with your fingers,
otherwise a hard-to-remove spot will be left on the lens!)
Before taking measurements, you should get a feel for how the equipment operates so
check that all the components are working normally and note what sources of error there may be
in the measurements. Check to see if the photodetector circuit is functioning in all three modes.
(Refer to the Appendix for descriptions of the 3 modes of operation). Increasing the load resistor
in each circuit should increase the output voltage unless saturation is reached. Covering the
EE 471 Lab 1 2
photodiode with your hand, misaligning the laser source from the PD or putting a filter in the
light path should cause a measurable change in voltage on the DMM.
Record the following measurements on the data sheet provided at the back. You will
need these values when you do calculations in your report.
 Using the DMM, accurately measure the supply voltage VCC (see the Appendix for pin
numbers) and the 3 resistors in each of circuits A and B.
 Place one of the shared Newport optical power meters in the beam path and record an
accurate reading of the laser power at the spot. Make sure the wavelength on the meter is
set to 670 nm. (Ask a TA for help with the power meter.)
2. Photodetector (PD) Measurements
To use your PD to make optical power measurements, it must be calibrated against known
power levels. By taking voltage readings at known power levels you can build a calibration
curve that is specific to your photodetector. In this section, you will measure the output voltage
from your PD when the beam is attenuated by known amounts using the filters provided.
Make sure the focused beam is accurately positioned on the active area of the
photodetector. Fill up Section 2 of the Data Table column-wise in the following way. (You
could leave the filters in the beam while the table data was recorded row-wise but there is a lot
more jumper switching if you do.)
 Start with Cct A, Resistor R1 (3rd column on the data sheet). Record the output voltages
and the filter values for all the filter combinations shown on the data sheet. [Note:
Unfortunately, the filters seem to respond to the light so take the measurements
immediately after placing the filter in the light path. Remove dust from the filter to
improve repeatability.]
 Switch to R2 and repeat, then R3 and repeat.
 Repeat the whole procedure with Cct B and all 3 resistors. You will need to change the
positions of the multimeter leads when you switch to a different configuration.
 Repeat the procedure in the Open Cct mode.
Have a TA check that your numbers are reasonable.
3. Beam Profile Measurements
One of the characteristics of a laser source is its beam profile, which is the variation of
intensity across the beam. For this measurement, we will expand the beam using a concave lens
so that the diameter of the photodetector is a small fraction of the beam diameter. Make an
estimate of the area of the active region of the PD for later calculations.
Note that the beam from the laser diode is somewhat elliptical. Rotate it in the holder so
that the long axis is vertical. Replace the convex lens with a concave lens and position the lens
in the beam so that at the PD, the visual beam diameter is in the range 1-2 cm (or larger). You
may have to try different lenses to achieve this. The PD should be mounted on a post on the 100
mm translation stage so that you can move it across the entire beam. Try to align the beam and
the PD so that the PD will pass through the centre of the beam.
Use the detector in reverse bias mode (Cct A) and make sure the output is less than ~1
volt at maximum so the system should be in the linear range (output voltage proportional to
optical power). If the voltage exceeds 1 volt, change the jumper to a smaller resistor. If the
voltage gets too low (i.e. the DMM is no longer accurate), use a larger resistor value.
EE 471 Lab 1 3
 Do a test run by using the translation stage to move the PD across the beam to find the
maximum intensity (i.e. voltage) position and the points on the edges where the intensity
drops to approximately 1% of the maximum.
 On the data sheet, record the output voltage and position on the stage for 11 evenly
spaced points including and between the two 1% points (so the distance between points is
1/10 of the total). If the voltage gets too low, switch to a higher resistor.
 Rotate the beam 90º and repeat the measurements.
When you are satisfied with your measurements, have the TA initial your lab sheets and clean up
your workspace.
4. Report:
 From your data recorded in Section 2, produce I-V (diode) curves of the form shown
below. Also draw the load lines for the photoconductive mode, as shown here (i.e.
R1,R2,R3).


For Ccts A and B, produce curves of detector current vs. incident optical power.
Calculate the optical power from the known unattenuated power and the filter value. For
the photoconductive mode case, calculate the detector responsivity in A/W. Estimate the
quantum efficiency of each photodetector. How do the values compare with expected
values? (These can be found in texts.)
On a single graph, normalize (divide all values by largest central value) and plot the 2
profiles of the LD beam in dB units (so the largest value is 0 dB). Indicate the FullWidth-Half-Maximum (FWHM) beam widths, i.e., the –3 dB points.
EE 471 Lab 1 4
Appendix - Photodiode Circuit Board
Take a moment to familiarize yourself with the circuit board. The circuit schematic, board
layout, and output pins/pads are given below. (In Cct B and Open Cct, the diode polarities are
reversed.)
The switch in the middle of the board determines the mode of operation. Note left switch
means right circuit and vice versa!
 Switch right:
Cct A (left on board), photoconductive (reverse bias) mode
Vout = Pin 1 to ground (Pin 5 or backplane)
 Switch left:
Cct B (right on board), short circuit mode (diode voltage = 0)
Vout = Pin 4 to ground
 Switch centre:
Open circuit mode (diode current = 0)
Pin 1 to Pin 2
The short circuit and reverse bias modes each have three different resistors for determining
the range of the output voltage depending on the incident light levels. Each set has a jumper for
selecting one resistor. If you have very small voltage readings, the jumper may be moved to a
higher resistance to increase the output voltage. The pins/pads to probe with the DMM for the
various modes are summarized in the table below.
Pin 2
VCC
Idiode
+
Vdiode
Pin 1, Vout
Cct A
Pin 2
Pin 4
Pin 1
Cct B
EE471 Data Sheet:
Open Cct
Group: _________________ TA: ___________________
EE 471 Lab 1
Section 1
Cct A:
R1 = _______ kΩ
R2 = _______ kΩ
Cct B:
R1 = _______ kΩ
R2 = _______ kΩ
Supply Voltage:
VCC _____________ Volts
Beam Power: POPT _____________ mW
Section 2:
R3 = _______ kΩ
R3 = _______ kΩ
Output Voltages for various optical powers
Filter(dB)
Cct A
Nominal
Actual
0
0
R1
Open
Cct
Cct B
R2
R3
R1
R2
R3
2
4
5
7
10
infinity
Section 3:
Run 1
Position
blocked
Beam profile measurements
R (kΩ)
Vout
Run 2
Position
k (KΩ)
Vout
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