Page 256 - Photonics Essentials an introduction with experiments

Experimental Photonics: Device Characterization in the Laboratory

250 Characterizing Photonic Devices in the Laboratory

tion, significant noise reduction can be obtained. However, the lock-in
goes even further and allows you to choose only the signal that is in
phase with the modulation. This results in even more discrimination
power and subsequent improvement in signal-to-noise ratio.

Recommended Equipment
1. Photodiode(s)
2. Silicon wafer
3. GaAs wafer
4. Tungsten light bulb, a low voltage flashlight bulb with incorporat-
ed lens and small filament size (< 2mm) is a convenient choice
5. Lenses
6. Mounts to hold the photodiodes, lenses and light.
7. Chopping wheel
8. Oscilloscope
9. Lock-in amplifier

Procedure
a) Optical Setup. Set up the light source on one side of the chopping
wheel and the photodiode on the other. Connect the photodiode cable
to the oscilloscope amplifier. Set the amplifier to the dc coupling
mode. Turn on the oscilloscope and increase the sensitivity so that
you can tell the difference in the position of the trace on the screen
when you block the room light from the photodiode. The room light is
a major source of background noise. Take a measurement of the back-
ground noise level.
Next increase the current to the light bulb until the signal from the
light source on the oscilloscope screen is greater than the noise level.
You may need to remove the chopping wheel from the path to accom-
plish this. Pick a convenient frequency (try around 200 Hz) for the
chopping wheel, and start it up. Check that the wheel is in the light
path between the lamp and the detector. You should observe a modu-
lation of the signal on the oscilloscope screen that corresponds to the
frequency of the chopping wheel. If you switch the amplifier of the os-
cilloscope to the ac coupling mode, you can eliminate some of the
background noise and better resolve the modulation. This is some-
what similar to narrow-band detection. Note the amplitude of the
modulated signal at the chopping frequency.

b) Lenses. Lenses may be used to improve the signal-to-noise ratio by
controlling the flow of light. Treat the light bulb as a point source and

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