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Unformatted text preview: FP - 1 Department of Physics, Indiana University (HOM 12/21/99) Fabry-Perot Interferometer Goal: Understand how a Fabry-Perot Interferometer works and use it to observe the hyperfine splitting of spectral lines. Equipment: Fabry-Perot etalon, diode laser, Mercury lamp, photo multiplier, traveling stage, filters... 1. Introduction A Fabry-Perot 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 back-and-forth 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