A Low Cost Scanning Fabry Perot Interferometer for Student Laboratory K.T.Satyajit, Suresh Doravari*, T.E.Kanakavalli ** , Sharath Ananthamurthy Department of Physics, Bangalore University, Jnanabharati Campus, Bangalore-560056 *Indian Institute of Astrophysics, Koramangala, Bangalore – 560036 ** Department of Physics, M.S.Ramaiah College, Bangalore-560054 We have built a low-cost Scanning Fabry Perot Interferometer for use in student laboratory. Our setup consists of two highly reflecting concave mirrors facing each other and mounted on a home built chassis. One of the mirrors is attached to a piezo buzzer extracted from a motorcycle horn and is driven with a triangular wave from a signal generator, or through a 50 Hz AC output from a variac. Light from a laser source to be diagnosed is made to enter the Fabry Perot cavity. The beam that undergoes multiple reflections emerges from a hole through the mirror-PZT combination and is made incident on a fast photodiode. A simple amplifier circuit optimizes the signal monitored on an oscilloscope. The Free Spectral Range (FSR) of the device is 5 GHz, and much larger than the FSR of the laser under test. Using this we were able to clearly resolve the longitudinal modes of a He-Ne 3mW laser. The FSR of the laser is measured to be 838 MHz. The Fabry-Perot setup alone costs less than Rs. 1500 to build. The design is simple and can be built as student projects, and is an ideal tool for laser diagnostics. It can be used to make accurate measurements without having to resort to commercially available interferometers that are expensive. 1.INTRODUCTION 1.1 Basics of Fabry Perot Cavity (Etalon) The principle behind a Fabry Perot Interferometer (FPI) is explained in many textbooks and we will briefly summarize the relevant parts here 1,2,3,4 . FPI relies on multiple beam interferometry. Because of multiple beam interference, a sharp contrast between dark and bright fringes is seen and this results in excellent wavelength resolution. In the etalon (cavity) multiple reflections between two closely spaced partially silvered surfaces occur. Part of the light is transmitted each time the light ray reaches the second surface, resulting in multiple offset beams, which can interfere with each other. The large number of interfering rays produces an interferometer with extremely high resolution 3 . A Fabry-Perot Interferometer, has a resolvance given by, r r m - = Δ 1 π λ λ -------------(1) Where m = order of interference λ d 2 ≈ for small angles, r = reflectance of etalon surfaces. Which means the wavelength separation of two spectral lines is r m r π λ λ ) 1 ( - = Δ -------------(2) Its free spectral range (FSR), the change in wavelength necessary to shift the fringe system by one fringe, can also characterize the interferometer: