Lab 7 : Detection of Light
I. Objective
The purpose of this experiment is the following:
1. to introduce to the student a method of quantitatively measuring light from a light source. You
will use two methods: An optical power meter and a photodetector.
2. to illustrate the need of operating a light detector in the “linear” range. In the linear range, the
voltage output of the photodetector is proportional to the incident light power.
II. Introduction
Various types of light sources are used extensively in today’s research environments. This includes
everything from x-rays, ultraviolet, infrared and visible light sources. However, the most sophisticated
light source is of little use to any researcher or scientist if it cannot be measured or manipulated to suit
the users needs. In this experiment, a Helium-Neon laser or a diode laser will be used as the light source.
The student will become familiar with Neutral Density filters and how they can be used to alter the
power of the light source. Finally, this lab will examine the method of quantitatively measuring the light
from the He-Ne laser using a power meter. The power meter will be used to calibrate a more sensitive
Photodetector.
A. Operation of Neutral Density Filters
A Neutral Density Filter (NDF) is used to attenuate or reduce the power in an optical beam. Darker filters
attenuate more light then less-dark filters. The NDF is characterized by its ND value according to the
following formula:
P  Po 10 ND
(1)
Where Po is the incident power on the filter and P is the transmitted power, and ND is the neutral density
value of the filter. As an example an ND=1 filter attenuates the power by a factor of 10. An ND=2 filter
attenuates the power by a factor of 100. The ND=0.3 filter is very useful for calibration purposes because
it attenuates the power 50.12% or by nearly a factor of two. Some of the ND filters are labeled by part
number. Use the following table to identify the corresponding ND value.
ORIEL ND FILTER PART NUMBERS:
Model # ND value Model # ND value
50790
0.1
50830
0.5
50800
0.2
50832
0.6
Model #
50840
50845
ND value
1.0
1.5
50810
0.3
50834
0.7
50850
2.0
50820
0.4
50836
0.8
50860
3.0
B. Operation of a Photodetector
The photodetectors that are used in this lab convert the TOTAL light power incident on the detector to a
corresponding voltage. The voltage can be measured using a voltmeter, oscilloscope, or some other
device that can convert the voltage into a number to be used in calculations.
C. Operation of an Oscilloscope
An oscilloscope is an instrument that produces a graphical plot of a measured voltage as a function of
time. For this lab, the measured voltages will be constant and therefore not depend on time. If you are
unfamiliar with the operation of the oscilloscope, ask your instructor for a quick lesson. The simplest
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method for reading the voltages on the oscilloscope is to use the “Measure Voltage” button on the
Oscilloscope’s front panel. Under the collection of “MEASURE” buttons on the front panel, push
“Voltage”. On the buttons below the Oscilloscope screen choose “V average” and a text line should
appear on the bottom of the screen that displays a numeric value of the voltage.
III. Procedure
The apparatus must be completely set up according to the attached schematic diagram in order to perform
the following experiments. The purpose of the two mirrors is to adjust the height and direction of the
laser beam. Rather than manipulating the laser mount to adjust the height and direction of the beam, you
can use the fine adjustment knobs on the mirror mounts.
The NDF mount is a semicircular holder. The photodetector (PD) should be connected to the
oscilloscope Channel 1 using a co-axial cable.
Figure 1: Experimental Setup.
A. Measuring PD voltage using an oscilloscope. Does Background light affect the
measurement?
1. Place the PD on the table with the diode facing the lights on the ceiling. Trigger the scope on
auto with a timebase of roughly 20 msec/division. Make sure to DC couple the input. You should
observe a small DC signal. If you do not observe any detectable signal, please ask for help.
2. Cover the PD with your hand and observe any changes. What is the observed change in the
detected signal due to?
3. Place the PD on the table in a mount. Do no direct the laser light onto the PD yet. Increase the
sensitivity of the oscilloscope until you observe a non-zero voltage response from the detector.
Can you still observe the DC signal observed in Part 1 above?
B. Determine Optical Power of laser
1. Using the Optical Power Meter, determine the power of the laser beam (in mW) AFTER the two
mirrors. There are only two power meters in the lab, so if one is not available right now, go on to
the next task and measure the power sometime later.
C. Determine the Linearity of the Diode
1. At this point, you can use your calibration from III.B to determine the optical power on the
photodetector for different ND filters. Before EACH measurement with the detector, you should
adjust the position of the light on the detector to maximize the signal. This optimization of the
light on the detector is necessary because if the beam is deviated off of the detector, the apparent
reading from the photodetector will change.
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2. It is most likely that the direct light from the laser will saturate the detector (i.e. the detector
becomes “sunblind”- it does not measure the “true” intensity of the light). However, it may not.
The result of saturating a detector is that is does not operate in a linear fashion. ie. The voltage
produced by the detector is not proportional to the power of incident light. Therefore it is
necessary to determine what Neutral Density (ND) filter(s) (if any) should be placed before the
photodetector so that the detector responds linearly to detected light. You can quickly determine
if the detector is linear by placing a 0.3 ND filter between in from of the detector, the output
signal from the detector should drop by 50% (not 10%, 20% etc.). With no ND filters in the laser
path, record the voltage from the detector. Next place the 0.3 ND filter in front of the detector.
Record the new voltage from the detector. Compare the two voltages that you just measured. Is
the second 50% of the first value? Is the detector is linear. Is it linear? Why do you conclude so?
3. Remove the 0.3 ND filter. Choose an assortment of ND filters to calibrate of the Detector. Be
sure to include the 3.0 ND filter and cover the range from 0 ND (ie. no filter) to 3 ND. For
example, you could use the 0.1, 0.3, 0.5 and 1.0 ND filters. Then use the 1.0 AND the 0.3 ND
filters together for an effective attenuation of 1.3 ND. For each ND filter or combination of
filters, record the corresponding voltage from the oscilloscope.
4. You should also make a measurement by blocking the laser light completely. NOTE YOU WILL
MEAURE A NON-ZERO VOLTAGE when you block the laser light. This is a CONSTANT
offset value that you will use to develop a calibration curve.
IV. Summary of Results
1. Develop a calibration curve for the photodetector voltage (volts) as a function of Power (milliwatts).
To calculate the voltage at a particular power level, make sure that you subtract the BACKGROUND
voltage that you measured in Part III.4 from each voltage value.
2. Below what power level is the photo diode linear?
3. Plotting only the data identified as being in the linear range from your answer to 2 above, fit your
data (using EXCEL for example) to a linear trend and determine the sensitivity (Volts per mW) of
your detector.
4. Using your calibration curve, determine if a ND filter is required to enable the photodetector to
operate linearly. If a filter is required, state what value of filter would be required and why.
5. Does room light generally affect your measurement of the photodetector calibrate curve? Why?
6. Does room light affect the calibration point for the 3.0 ND filter? Why?
7. Assuming that a 1.0 ND filter is required to make the photodetector linear, where in Figure 1 relative
to the photodetector, should the filter be optimally placed. Why?
Required equipment:
He-Ne or Diode Laser with mount
Mirrors (2) with mounts
ND filter holder with mount
ND filter set
Photodetector with mount
Oscilloscope
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