Page 255 - Photonics Essentials an introduction with experiments

Experimental Photonics: Device Characterization in the Laboratory

Experimental Photonics: Device Characterization in the Laboratory 249

1. The presence of light creates a current in the diode, and therefore
creates a bias voltage. Does the presence of light drive the diode to-
ward forward bias or reverse bias?
2. What is the maximum value of the photovoltage that you were able
to measure?
3. The photodiode is an energy conversion device. The electrical pow-
er generated by the photodiode is equal to the area that the I–V
characteristic creates in the 4th quadrant of the I–V curve. This
can be approximated by the product of the voltage at zero current
and the current at zero voltage. What level of electrical power does
the photodiode generate in your measurements?
4. Compare the diode I–V characteristic to that calculated by the
model equation (Eq. 3.14). The theory we developed says that the
current is proportional to the exponent of the voltage. What about
the experimental result? Is it true? If so, over what range of volt-
age and current does this relationship apply? Where do the largest
differences between theory and experiment occur, in forward bias
or reverse bias? Why does theory fail to give a good account? Is the
theory wrong, or are there external influences to the p-n junction
that need to be considered? What are the physical sources of these
influences? Why might this be the case?

11.2 Detection Using the Lock-in Amplifier

Objectives
In these experiments we provide an introduction to lock-in amplifier
operation and observe the optical absorption properties of various
semiconductors.

Background
Your experiments in the laboratory will be made in the presence of
many sources of noise. You would probably like to take data with the
room lights on, so you can see what is going on. However, you have al-
ready observed in Section 11.1 that stray light from a lamp changes
the photocurrent. Stray light is a source of noise. There is also electri-
cal noise to deal with. The ac line frequency varies widely. The lock-in
amplifier is designed to handle these problems. When used correctly,
you can reduce the level of noise by many orders of magnitude.
The principles of lock-in operation are explained in Section 10.7.
You need to modulate your signal at a frequency that is different from
that where noise occurs. This is called narrow-band amplification. De-
pending on the filter characteristics and on the quality of the modula-

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