Fiber optic multi-sensitivity sensor employing modal ring technique in a graded index multimode fiber

Mixed modes in a graded index multimode fiber can be separated into multi-separated mode rings by using the Modal Ring Technique. Mode rings exit the fiber in ring patterns with different propagation constants and spatial separations. In the past, only single mode fibers could be used in fabrication of the interferometric sensors. Now, the Mach-Zehnder interferometric strain sensor can be built by using graded index multimode end etched fibers in which two different mode rings of reference and sensing fibers can be combined outside the fiber over a photodetector and yield an interference signal as a number of fringes at the output sensor. Farther more, combining two sets of different mode rings from reference and sensing fibers yields two interference signals with two different numbers of fringes. A matrix table of the predicted numbers of fringes for different sets of interference signals and the fringe visibilities can be calculated. The result is that numbers of fringes for interference signals from theoretical data matched the experimental data.;Analysis of interference signals between two different mode rings yields different number of fringes, with ranging from one fringe to twenty fringes (limited by mode power distribution) with 750 mustrain applied to fiber. Successfully extended etching technique to a graded index multi-mode fiber to separate modes into more than hundred mode rings. Characterized mode rings reveal that mode rings of the multi-mode graded index etched fiber all have the same linear polarization; the smaller the emission angle/radius, the lower the mode order and the larger the propagation constant.