DetectorResponse

Detector response

Objective: To find the property of the incident electromagnetic field that determines the electrical output of an EM detector. For example, is the detector output proportional to the incident electric field or the intensity, or something else within experimental error? Once you answer this then you could, in principle, put the detector up to a crack in your microwave oven and from the detectors voltage output you could then determine how much microwave power is leaking through the crack. If you don’t know what property of the EM field the detector responds to, then there is little modeling or science that can be done with it. The objective of this lab is to calibrate the response of the detector.

Model: We need to generate an EM field, with a known variation in it, with which to illuminate the detector. This could be done by putting attenuators in front of the source and measuring the voltage output of the detector for different attenuation of the EM wave and then plot detector voltage output vs EM field incident on the detector.

You will, however, use polarization to generate a known variation in the EM on the detector. See https://en.wikipedia.org/wiki/Polarization_%28waves%29 You will use a polarized source and send its EM field through a wire grid polarizer. The EM wave which traverses this wire grid polarizer has its amplitude, E0, attenuated by a known amount (see the Malus’s law section and the wire grid polarizer section of this link https://en.wikipedia.org/wiki/Polarizer). However, the direction of E0 is also changed.

Let’s assume that the detector responds equally to any polarization of the EM wave incident upon it. The magnitude of this EM wave is then E0 cos[theta] where theta is the angle shown in the figure of the link above. If the detector responds to the magnitude of this electric field then the voltage output from the detector will be E0 cos[theta].

The detector may respond to the intensity of the incident EM field and/or respond to a component of the incident electric field. That is, it could also act as a polarizer, responding only to a component of the incident electric field. Here are some links on the microwave diode

Method: Use the rotating polarizer apparatus to generate the known variation in the EM wave incident on the detector. Use both a microwave and HeNe EM source with a microwave diode detector and photodiode respectively. Measure the output voltage as a function of time while the polarizer rotates. How do you account for error in this measurement?

Discussion and questions:

A description of the microwave diode is found here http://www.allaboutcircuits.com/textbook/semiconductors/chpt-2/junction-diodes/ http://www.elprocus.com/crystal-diode-circuit-working-with-applications/ -How does a circuit with such a diode in it respond to an EM wave at 10^10 Hz? -Can your scope display oscillations at this frequency? -Wire grid polarizers for the infrared are described here https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1118 -Why do these grids not behave like gratings (as are used in the spectroscopy lab)? -Use the Agilent voltmeter to display the 60 Hz oscillation in intensity of the room lights. Does setting the PLC on the instrument mitigate this noise.

Second week team projects: -Replace the microwave source and detector with a HeNe laser and photodiode. Tape a polarizer for visible light on the microwave polarizer. Collect photodetector response data for the He-Ne laser. -show how the PLC (power line cycle averaging) feature works on the Agilent voltmeter by letting room light into the photodiode and mitigating its signal using the PLC feature.