Fiber Low Coherence Interferometry Measurement Technology,

A new technique for optical fiber interferometer sensor use is possible ,namely that associated with the use of low coherence ,broad band spectral sources such as light emitting diodes(LED),multimode laser diodes or halogen lamps. This sensing method is termed "low coherence" or "white light" interferometry. Although this technique of optical fiber sensing was not reported until 1983, firstly for use with single mode fibers and then with multimode fibers, its principle of operation had been demonstrated previously as a potential communication system in 1975.As with all interferometric methods, optical path length changes of the light waves are observed through interferometric fringe pattern analysis. There are two distinct methods in optical fiber sensor system for the observation of such fringe pattern and these can be identified as spectral domain processing, and phase domain processing.In the spectral domain technique, a spectrum analyzer is used for the optical processing element.In the phase domain technique, it takes on the form of Michelson, Fabry-Perot or Mach-Zehnder type. It is necessary in this method that the sensing cavity has an optical path length imbalance greater than the coherence length of source such that under normal conditions no interference effects are observed at its output. The light waves returned from the sensor interferometer cavity are temporally incoherent. It is the function of the second processing interferometer to re-establish interference effects by bring components of the radiation back into temporal coherence.One major difficulty when using an LED or multimode laser diode in the interferometer system is the identification of the central fringe. A relatively simple, but very effective, technique is the use of a centroid algorithm to identify the central fringe. A second method for enhancing the identification process of the fringe is a variant of the two wavelength method. We require two wavelength that have a wide spectral separation in the order of 100nm.Convert the lateral movement of the spatial fringes across the detector plane into temporal fringes by use of grating elements. The relative positioning of the two grating elements is adjusted such that their periodic structures are out of phase by π/2 radians. Since changes in the sensor interferometer cavity length produce corresponding lateral displacement of the spatial fringes over the static grating elements, the π/2 phase relation between the outputs of the two photo-diodesproduces sinφ and cosφ outputs respectively. With such complementary signals itis possible to determine both the change in OPD of the sening cavity as well as the direction of this change.