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Unformatted text preview: FP  1 Department of Physics, Indiana University (HOM 12/21/99) FabryPerot Interferometer Goal: Understand how a FabryPerot Interferometer works and use it to observe the hyperfine splitting of spectral lines. Equipment: FabryPerot etalon, diode laser, Mercury lamp, photo multiplier, traveling stage, filters... 1. Introduction A FabryPerot spectrometer is simply an arrangement of two parallel glass plates, called an etalon . Only the inner surfaces play a role. They have a reflective coating, and form a cavity in which light can be reflected back and forth. The outer surfaces are not quite parallel to the inner surfaces, so that they do not contribute to the backandforth reflection. 2. The Parameters of the Etalon There are two parameters that govern the performance of the FP interferometer, the first is the separation d between the reflecting surfaces, and the second is the reflectance R , or the transmittance T =1 R (assuming that here is no light absorbed). The reflectance R is the fraction of the intensity that is reflected back on each of the two mirror surfaces. Look at the figure on the left. There is a light ray of intensity I i incident on the etalon at an angle . The ray passing through (labeled t0 ) thus has the intensity I = I i T 2 , the next ray, reflected twice has the intensity I 1 = I i T 2 R 2 , the next one I 2 = I i T 2 R 4 , etc. If we shine a laser through the etalon at a screen placed on the downstream side, choosing an angle that is large enough, we observe a series of spots. We can place the etalon on a goniometer table and use the traveling stage with the photomultiplier behind a vertical slit to measure the position x k of these spots and determine their intensity (an example of such a measurement is given in App.1). Convince yourself that the intensity of the k th spot is I k = I i T 2 R 2k , and that its position is given by x k = 2kd sin . Applying this to your measurement, determine d and R of the etalon. Since it is difficult to determine the zero point =0, take a measurement at either side, i.e., for 1 and 2 . In the analysis of the data you vary the zero point until the two measurements give consistent results. TASK 1: derive the two equations for x k and I k . TASK 2: using a grey wedge to attenuate the light entering the photomultiplier in known increments, establish the appropriate operating voltage such that the response (anode current) is linear....
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 Fall '09

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