Optical Fiber Low Coherence Interferometry For Absolute Measurement Without Scanning Action

This paper proposed an optical fiber low coherence interferometry for absolute measurement without scanning action. In the process of measurement, the information of the measurand is obtained with a fiber Michelson interferometer which is sourced by a broadband light source with center wavelength of 850nm and spectral width of 44.16nm. The broadband spectrum interferometric beam is dispersed with a bulk dispersing grating to be an optical plate in which the wavelengths are continuously distributed. The optical plate is detected with a linear array charge coupled device (CCD). By transforming the wavelength spectrum of the interferometric signal into wavenumber spectrum, absolute optical path difference of the interferometer can be measured precisely by measuring the wavenumber difference between two neighboring peaks of the wavenumber spectrum. Therefore, high precision absolute measurement of physical quantities can be realized by using this measurement system. Unlike the normal low coherence interferometric measurement systems (LCIMS) which have to scan the optical path difference (OPD) of the interferometer or the wavelength of light source in order to demodulate the measurand, there is no need of scanning action during the measurement procedure, which not only simplifies the measurement system but also improves the measurement speed greatly. A fiber Bragg grating (FBG) in the measurement system is employed to produce a feedback signal in a feedback control circuit which is used to stabilize the optical path difference of Michelson interferometer. By using this feedback control circuit, the environmental disturbances are depressed and high measurement precision is obtained, and the system can be suitable for online measurement. The measurement resolution is as high as 6.03nm. A step height with the calibrated value of 50μm which is configurated with two gauge blocks is measured by the system. The standard deviation of 10 times measurement results is 6.8nm.Based on the above work, the thesis also presented an improved measurement system by replacing the optical fiber Michelson interferometer with an optical fiber F-P interferometer which is packaged with a pipe and used as a sensor head which can solve the unnecessary measurement errors of the above measurement system when determining the position of peaks of the wavenumber spectrum. The simulation study of the improved measurement system by MATLAB is discussed.