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AY 105 Lab Experiment #1: PhotometryAs discussed in class, in this lab you will be using a silicon photodiode and a setof interference filters to determine the intensity Iλof a quartz tungsten halogen (QTH)lamp. This lab will also introduce you to working on an o ptical table, and in Part Bto using a laser with a spatial filter and collimator.When using silicon diodes as photo detectors light incident on the diode producesa current flow from the cathode to the anode, proportional to the number of incidentphotons.In this week’s experiment, you will use a transimpedance amplifier to measurethis current flow very accurately. This the important thing to understand is: how doesa measurement of the current flow from the photodiode translate into the number ofincident photons? Astronomers typically quantify the sensitivity of a detector usingwhat is called the “quantum efficiency” (QE), which is the ratio of the number ofdetected photoelectrons to the number of incident photons. An ideal detector thushas a QE of 1.0, but of course in practice the QE is always less than 1. The QEdepends on the wavelength of the incident photons, a nd can range over many or dersof magnitude for various detectors: for example, photographic plates have QE < 0.01,photomultiplier tubes have QE < 0.2, and photodiodes and CCDs can have peak QEranging up to 0.9 or so. Engineers, however, typically measure a quantity called the“responsivity” for photodio des, in units of amperes/watt . This makes sense, as we aremeasuring the rate of generated photoelectrons (in amps, or coulombs/sec) in terms ofthe rate of incident photon energy on the detector (in watts, or joules/sec). Thus theQE is a unitless version of the responsivity, and an expression relating the two is:R =qηλhc. (1)Note that λ appears explicitly in this relation—it’s needed to convert an energyrate into a photon rate (as blue photons carry more energy than red ones).This manua l also contains a plot of the responsivity of your photodiodes as afunction of wavelength, and a table of these values (with estimated uncertainties, F ig ure1 and 2).Part AIn the first part of this week’s lab you will set up the quartz tungsten halogenlamp, relay lens, and photodiode in roughly the arrangement just discussed in class.The main difference will be that instead of one biconvex lens to reimage the filament1Ay 105 Spring 2008 Experiment 1 2of the QTH la mp onto the silicon photodiode, you will use two (more-or -less) plano-convex lenses separated by a short distance. In this manner you can establish a parallelbeam between the two lenses (in optics t erminology a parallel beam is oft en called a“collimated” beam). The best location for the interference filter which defines thespectral bandpass is in the collimated beam: interference filters are comprised o f many“thin film” layers of a dielectric material. As you know from basic physics, constructiveand/or destructive interference occurs as light passes through thin films, and the “inter-ference filter” exploits this phenomenon using many layers having thicknesses designedso that only a narr ow range in wavelengths is transmitted. But what matters opticallyis not the thickness of the layer but rather t he pa th length (measured in wavelengths)of the light through the layer; the sp ectral bandpass will therefore be different fromthe design bandpass if the filter is not normal to the incoming light, and so the bestlocation for the filter is in the parallel beam between the two lenses rather than in adiverging or converging beam (whose rays will not be everywhere normal to the filter).A second twist on the r adiometry method is the suggestion that you also placean iris in the collimated beam b etween the two lenses. You can adjust the diameter ofthis iris to a specified diameter independent of (but not greater than, obviously) t helens diameters. The experimental setup for Part A should therefore look somethinglike Figure 1.Figure 1: Experimental setup for Part A.What follows are some instructions and suggestions on how to create this setup.You’re free to do things differently, but please ask either the instructor or the T.A.if you’re not sure how to implement your idea or to make sure that your approachdoesn’t risk damage to the equipment; at the very least you will have someone to takethe blame instead of you if such damage should occur, and there’s even the chance thatthe risk will be identified ahead of time.Optical Bench. You will notice that the optical table has 1/4-20 (i.e. 1/4-inchAy 105 Spring 2008 Experiment 1 3diameter, 20 threads per inch) holes spaced 1-inch apart over almost its entire surface1.While often it is convenient to use t hese holes directly, for this lab yo u will probablyfind it better to first bolt the shorter of the two “optical rails” to the front left sideof the table, and then mount your optics to “carriers” which can slide over the rail.You can then focus without being limited by the 1-inch hole spacing on the table itself.The lab drawers underneath the tables contains optical mounting hardware, and inparticular a black box of 1/4-20 bolts (and a n Allen ball-end screw-driver) to use whenbolting the rail to the table.Post Holders. These accept the 12 mm diameter posts which carry optics, andare screwed into “bases” which will attach both to the table directly and to the topsof the carriers. It’s best to use flat washers underneath the bolts when attaching thebases, since the holes in the bases a r e slotted. Since moving the carriers along therails gives you motion in the x-direction (say), you should orient the slotted holes inthe base in the y-direction to allow you t o align each component to the optical axis.Note that the unusually shaped hole in the post holder does not achieve this alignmentautomatically.Translational Stage. Since you will be asked to make fine focus adjustments ofthe detector position, it is recommended that you attach the large translational stagedirectly to the optical table at the right end of the optical rail, with the translationdirection aligned with the direction of the rail. Turning the micrometer adjustment t oits extremes will reveal countersunk holes in the base for 1/4-20 screws to a tt ach it tothe table. Note that there are tapped holes in the top on which to bolt a post holderand base to support the photodiode assembly. Once the base is attached to the ta ble,mount the photo diode assembly and set


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CALTECH AY 105 - Photometry

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