Optical Methods for Detection of Very Small Vibrations and the Design of a Laser Diod System using Optical Feedback
Abstract
The aim of this work is to measure very small vibrations.Coherent light is used and its properties are L11.vestigated.First, the theory of interferometry and speckles are dealt with.The speckles, often seen as optical noise, are here regarded as vibration information carriers~ Secondly, an active interferometer, based on a V-channel double heterostructure laser diode of Fabry-Perot type, is designed and tested.The frequency of the coherent light emitted from the laser diode is controlled by the injection current.Interference, inside the laser diode, between the emitted light and the scattered light from the vibrating surface give rise to intensity fluctuations.This is opti'cal feedback used in a constructive way.The principles and limitations of an optical feedback interferometer are discussed.Furthermore, design of the electronic circuits that amplify the fluctuating signal and drive the laser diode is presented.
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