Optical configuration of fiber optic sensor for symmetric dynamic response about the optical null

Abstract

A method of determining the displacement of a fiber optic sensor from a target in which a first lens is spaced from the target at a first distance an a second lens is spaced from said target at a second distance which provides the greatest intensity response at said first distance and thereafter spacing said first lens from said second lens to provide symmetrical slope values of response.

I claim: 
     
       1. A method of determining the displacement of a fiber optic sensor from a target in which a first lens is spaced from the target at a first distance and a second lens is spaced from said target at a second distance which provides the greatest intensity response at said first lens to achieve the largest response slope value for a given distance and thereafter spacing said first lens from said second lens to provide symnmetrical slope values of response. .Iadd. 
     
     
       2.  A method of determining the displacement of a fiber optic sensor from a target comprising the steps of: positioning a first lens at a fixed distance from said target;   positioning a second lens at a first variable distance from said first lens;   positioning a fiber optic sensor at a second variable distance from said second lens; and   selecting values for said first and second variable distances whereby reflected light is guided from said target to receiving fibers in said fiber optic sensor such that a curve of received optical power versus axial motion in a direction perpendicular to a face of said target is symmetrical about a null point which represents an in focus condition. .Iaddend. .Iadd.3. The method of claim 2 wherein said safety step comprises matching an exit pupil of said first lens and an entrance pupil of said second lens, matching an exit pupil of said second lens to an average entrance pupil of said fiber optic sensor, and adjusting angles of incidence of the primary reflected light rays from the target to said receiving fibers. .Iaddend. .Iadd.4. The method of claim 3 further comprising the step of providing an AC signal to a transducer which in turn provides a mangetic force which causes a dithering motion of said fiber optic sensor and said second lens about said null point. .Iaddend.   
     
     
        .Iadd.5.  The method of claim 4 further comprising the step of providing an error control circuit which comprises a dither amplitude sensor, an electronics module connected to the output of said sensor which produces an AC signal proportional to and synchronous with a fundamental motion of said fiber optic sensor, an error detector connected to an output of said electronics module and having an input of an electrical signal based on an optical signal generated in said fiber optic sensor and having twice the dither frequency, said error detector functioning to produce an electrical output which drives a motor so as to move said fiber optic sensor in a direction towards said null point. .Iaddend.