Lab #1 - Introduction, Safety and Measurements
ECE 482/582
Goals:
1. To understand the safety issues and hazards of working in the Optical Electronics
Laboratory.
2. To see various laser sources and photodetectors in action to appreciate the wide variety
in the field.
3. To develop skill in making basic optical and electrical measurements.
4. To develop an understanding of the theory, operation, and application of the lock-in
amplifier to measure small, noisy signals.
Reading:
Laser Safety
Kuhn, Laser Engineering, pp.441-449
[http://www.dermatology.org/laser/eyesafety.html – other information]
 Rule #1 - DO NOT LOOK INTO A LASER BEAM!
Lock-In Amplifiers
http://www.cpm.uncc.edu/programs/lia.pdf
http://www.thinksrs.com/downloads/PDFs/ApplicationNotes/AboutLIAs.pdf
Preparatory Questions:
1. What are the three main safety hazards in the Optical Electronics Laboratory?
a.
b.
c.
2. What can you do to prevent accidents from these 3 hazards?
a.
b.
c.
3. You should be familiar with Rule #1 above. Let’s compare a 60 W light bulb to a 5
mW laser pointer. It might seem that the laser pointer is 12,000 times less damaging
than the light bulb. However, you can stare at a 60 W light bulb from 1 meter away
with little more discomfort than an after-image. If you stare at a 5 mW laser pointer too
long, your vision will be permanently damaged. The laser is more damaging because it
is a narrow confined beam and your eye can gather most of the emitted energy.
1
However, the light from the light bulb spreads out in all directions and your eye
intercepts only a small fraction of the emitted energy. The eye also focuses the laser
light to a smaller spot size on the retina than it does with the light from the light bulb
(see Kuhn Eq. 4.65).
LASER
Example 1: Staring at the 60 W light bulb at a distance of 1 meter.
The light is spread equally over a sphere of radius 1 meter.
SurfaceArea  4 (1m) 2  12.6m 2
LightBulbPower 60Watts
CorneaIntensity 

 4.8W / m 2
2
SurfaceArea
12.6m
The light will enter through the pupil, which is about 2mm in diameter.
PupilArea   (1mm)2  3.1106 m2
The total power entering the eye is,
EyePower  CorneaIntensity  PupilArea  1.5 105W
The eye focuses the image of the light bulb onto the retina to a spot about 300 µm in
diameter. Therefore, the intensity at the retina is,
RetinaIntensity 
EyePower 1.5 105W

 212W / m2
SpotSize  (150 m)2
Question: Calculate the retinal intensity for a 2 mW laser beam entering the eye with a
collimated beam diameter of 2 mm. The eye will focus this laser beam to a spot diameter
of 10 µm.
2
NOTE** You will be using lasers that can cause irreparable damage to the human
eye. You must be alert at all times and use proper safety techniques while working in
the lab. Anyone intentionally misusing a laser will be barred from this laboratory
and receive a grade of F.
4. Many times in an optics laboratory, it is necessary to measure very small light levels.
One important instrument that makes this possible is the “Lock-in Amplifier.”
Describe in basic terms how a “lock in amplifier” works and what kind of signal is
needed for its use? [2 or 3 short sentences, not a book.]
Experiment:
1. Using the PIN photodiode detector demonstrated in the lab and an oscilloscope, make a
measurement of the detected fluorescent lighting in the lab room. [Trigger the scope on “line”
or a frequency of 60 Hz.] Sketch the signal or signals you measure. Try pointing the detector
at another light. Does the signal look the same or different? What is the frequency of the
observed signal or signals?
2. Using the setup below of an LED modulated at 1 kHz with a square-wave signal (in our
case by an optical chopper), move the LED closer to the detector until you see a 200 mV peakto-peak modulated on/off signal on the scope with a 1 MΩ input impedance. Sketch the signal
you see in your lab notebook noting the approximate noise level of the signal. Adjust the
lock-in amplifier (LIA) to give a stable signal and note the measured RMS voltage level.
[Am. J. Phys., Vol. 71, No. 11, November 2003, p 1210]
Now move the LED away from the detector a distance of ~1 m. Again sketch the signal you
see on the oscilloscope with the room lights on. [You will have to adjust the vertical
3
sensitivity to a much lower scale to see the signals.] Now block the signal by putting your
hand in front of the LED. Does the lock-in signal go to zero as you would expect?
Questions to Answer:
1. How do the retinal intensities of the 60 W light bulb and the 2 mW (e.g., a laser
pointer) compare? Which is more likely to cause permanent retinal burning?
2. What was the frequency observed with the PIN photodiode when looking at the room
lights? Give an explanation for this. Explain briefly how a fluorescent light works
(look it up).
3. With the LED 1 m from the detector, what was the signal-to-noise ratio as measured
from the oscilloscope? Could you easily see the modulated LED signal? How did
using the lock-in amplifier help in measuring this signal? What was the RMS detector
voltage you measured?
What happened to the output when you blocked the LED with your hand just in front of
the LED? What would you think if you blocked the LED and the signal remained the
same on the lock-in scale? [More often, the signal is reduced greatly but maybe not to
zero. This can happen often if you are not careful with grounding and other stray light
pickup.]
4. Name all the different lasers you saw in the lab this week. For each laser, tell if they
were a gas, liquid, or solid state laser; their primary output wavelength; if they were
CW or pulsed.
4
LASER SAFETY RULES
ECE 482/582
The following rules will help prevent any accidents in the laboratory when working with lasers.
If the safety of a device or object is in question, ask the instructor or lab TA for assistance.
1. Be aware of all operating lasers and electronic equipment in the laboratory.
2. Notify others in the laboratory before approaching or operating laboratory equipment.
3. Do not look directly into a laser beam no matter what the circumstances.
4. When working with laser beams, control and confine them so they are not a hazard to
yourself or others in the lab.
5. Do not wear a watch or jewelry with reflective surfaces when working with high-power
lasers.
6. Keep laser beams in a horizontal plane below eye level.
7. Keep your eyes above the level of the laser beams.
8. Leave the room light on when possible. This helps you to see what you are working on.
Also, the eye pupils open wider when it is dark, presenting a larger target for the laser
beam. Use black cloths as much as possible to isolate components from light.
9. Wear the appropriate laser safety goggles. [Goggles are not needed for mW level
lasers as long as you don’t look into the beam.] IR and UV lasers are the worst.
10. Turn off power supplies before working with laser electrical contacts.
11. Use common sense and be alert.
12. Respect both lasers and electricity at all times.
Since we also use glass optical fibers in this lab, be careful not to use your bare hand to sweep
across flat surfaces. You might accidentally jamb a “glass splinter” into your hand or finger.
These are quite painful and very difficult to remove.
5