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EE 471 Lab 4
EE471 Lab 4: Wave Optics and Diffraction
Equipment Checklist
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EE 471 Lab 4
· Optical bench equipment
· HeNe laser, GaAs laser diode
· 50 mm Plastic ruler
· Rotational stage and paper screen
· Diffraction grating
· 50 mm and 100 mm pinholes
· Halogen desk lamp
· Commercial Spectrometer, Camera, Computer
· Image screen
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EE 471 Lab 4
Preliminary alignment
As in lab 2, align the HeNe laser, iris and a target at opposite ends of the optical bench. Refer to the demonstration apparatus already set up. Fix the iris in place in front of the laser in case the laser gets moved out of position later on.
1. Diffraction through a pinhole
Setting up:
· Position the 100 mm pinhole in the path of the HeNe laser beam. Some effort will be required to get it positioned optimally so be patient. The best alignment will be when the diffraction pattern is brightest.
· Place a white screen in the light path and observe the pattern at 30-40cm from the pinhole. Record the distance from the pinhole to the image. Your lab notes should include a sketch of the set-up.
Measurements:
· Using the plastic ruler, measure the positions of the maxima and minima on a line across the center of the circular ring pattern. Measure as many rings as you can. It may be easier to fix the screen to the optical table and then mark with pencil the locations of the rings, then measure your markings.
· Capture an image of the pattern using the CCD camera. (The TA will help here.)
· Repeat the whole procedure using the 50 mm pinhole. You should move the screen closer to the pinhole for best results (10-15cm)]
Report:
Include the formula for the expected pattern from a circular pinhole. Calculate the positions of the maxima and minima across the center of the pattern and compare to the values measured with the ruler.
Use Matlab (or Mathcad, Mathematica, Excel, etc.) to make 3D surface plots and contour plots of the theoretical intensity patterns. Normalize to the center intensity so maximum values are 1.0. Plot the measured intensities using the red data from the CCD camera bitmaps and detailed knowledge of the camera array dimensions (ask for help for these steps, if required). From the contour plots, briefly compare the theoretical and measured patterns. Do this for both pinholes. The CCD camera specifications are:
· 640´480 pixels
· pixel size 7.5 x 7.5mm
· Sensing area is 4.8´3.6mm
· CCD array is sunken ~20mm in from the front of the camera
Sample data for diffraction through a pinhole.
2. Diffraction gratings
Setting up: (Refer to the demo set-up when doing the following)
· Notes on handling the gratings:
· Never touch the grating surface (the shiny side)
· Never set the grating down such that the grating surface is against anything.
· Touching (oil from your fingers), scratches, and pits in the grating cause irreversible damage.
· Set up the rotational stage and diffraction grating on a post and adjust the height so the beam impinges on the grating approximately at its center.
· Set the stage to 90º degrees and place the grating on the stage such that the incoming laser beam is perpendicular to the grating surface (the primary reflection should travel back along the path of the laser). Make sure that the grating is centered on the stage, so that when it is rotated, the beam is always reflected from the center of the stage. There is an arrow marking in pencil on one edge of the grating, which should be facing upwards. Ask the TA to check your grating setup if you are unsure.
· Attach the homemade measuring screen to the stage. Refer to the demo setup for clarification if necessary. Fix the paper screen with tape so that it does not move when the stage is rotated. Turn on the HeNe and observe the various diffraction orders on the cylindrical screen.
Measurements:
· At 0º, record the angular positions of all the orders on the screen. Note that each division on the rotary stage is 2 degrees. Make measurements for the following angles of incidence: -20°, -10°, 0°, 10°, 20°
· What do you notice about the relative intensities of the diffraction orders? Record the relative intensities of the orders as they appear to you.
Report:
For the HeNe measurements, construct tables of the predicted and measured angular positions of the diffraction orders. Matlab, Excel, etc. is convenient for comparing the measured values and those calculated from the grating equation.
· From your measurements determine the grating frequency (lines/mm).
· Which diffraction order is the grating design for? How is this accomplished? (No need to be too specific here, just identify the concept.)
· Would the grating still work if the blaze angle were a different value?
· What could be said about the angular dispersion (dq/dl) if the #lines/mm of the grating were doubled?
3. Commercial spectrometer
With the help of the instructors, use the Ocean Optics spectrometer and software to capture the spectra of the following light sources:
· room lights (use the optical fibre as the input to spectrometer)
· desk lamp (use the optical fibre)
· desk lamp combined with the 2 lasers (bounce some laser light into the spectrometer opening without overwhelming the spectrometer) This will take some teamwork with your lab partner.
Notes on using the Ocean Optics Software:
· Set the view to autoscale (View à Spectrum Scale à Autoscale)
· Use the snapshot button to freeze the spectra sample
· Save the sample (File à Save à Sample). This is a text file that can be easily processed in MATLAB or Excel.
Report:
Include plots of the spectra you captured and comment on any features observed (lines, continuous features, etc.). For the lamp spectrum, estimate the effective filament temperature by comparing to a blackbody spectrum (Ch. 13 in Hecht, use the Wien Displacement Law).