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Exp_6_fa09 - Physics 3330 Experiment#6 Fall 2009 Photometer...

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Physics 3330 Experiment #6 Fall 2009 Photometer and Optical Link Purpose You will design and build a photometer (optical detector) based on a silicon photodiode and a current-to-voltage amplifier whose output is proportional to the intensity of incident light. First, you will use it to measure the room light intensity. Then you will set up and investigate an optical communication link in which the transmitter is a light emitting diode (LED) and the receiver is your photodiode detector. Introduction Experiment 6 demonstrates the use of the photodiode, a special p-n junction in reverse bias used as a detector of light. The incoming radiation energy excites electrons across the silicon band gap, producing a current or a pulse of charge proportional to the incident energy deposited in the detector. In this experiment we will introduce a number of "photometric" quantities that are widely used in opto-electronics. At the end of the lab, you will use a lock-in amplifier to extract a weak signal from noise. Readings For general background on opto-electronics, see H&H Section 9.10. For a general discussion of lock-in detection, see H&H Section 15.15. All of the detailed information you will need for this experiment is given below. Data Sheets for the PD204-6C silicon photodiode and the MV5752 GaAsP light emitting diode are available at the course web site. The manual for the SR510 lock-in amplifier is also posted at the web site. New Apparatus and Methods PHOTODIODE The PD204 photodiode used in this experiment is a p-intrinsic-n (PIN) silicon diode operated in reverse bias. A sketch if the photodiode structure is shown in Figure 6.1. The very thin p-type conducting layer acts as a window to admit light into the crystal. The reverse bias voltage maintains a strong electric field throughout the intrinsic region forming an extended depletion layer. The depletion layer should be thicker than the absorption length for photons in silicon in order to maximize the efficiency. Any incident photon whose energy exceeds the band-gap energy is Experiment #6 6.1 Fall 2009
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absorbed to produce an electron-hole pair by photoelectric excitation of a valence electron into the conduction band. The charge carriers are swept out of the crystal by the internal electric field to appear as a photocurrent at the terminals. The photocurrent is proportional to light intensity over a range of more than 6 orders of magnitude. Figure 6.1 Structure of the photodiode. + incident light + p-type layer n-type layer intrinsic region photocurrent electron-hole pair LIGHT EMITTING DIODE The MV5752 light emitting diode acts electrically just like any diode. It emits light when forward- biased due to direct radiative recombination of electrons and holes. The forward voltage drop is about 1.7 V rather than 0.6 V because the LED is made of GaAsP instead of silicon. LOCK-IN AMPLIFIER
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Exp_6_fa09 - Physics 3330 Experiment#6 Fall 2009 Photometer...

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