Fiber optic sensor having dual simultaneous sensitivities employing mode ring technique

Fiber optic interferometric sensors have an extremely high maximum sensitivity when compared to fiber optic intensity based sensors. Their disadvantage is the complex and expensive signal demodulation techniques that are employed due to their multi-valued output signal in addition to zero-sensitivity at some measurand values. Employing two interferometric sensors of different sensitivity allows an extended range of operation with relatively high sensitivity, however; the two sensors can never be placed at the same exact spatial location. This dissertation documents the dual-sensitivity fiber optic strain sensor which provides two simultaneous outputs of different sensitivity enabling the unambiguous use of interferometric sensors over an extended range. The dual-sensitivity fiber optic strain sensor has one sensing region and therefore the measurand is sensed at one location. The dual-sensitivity fiber optic strain sensor combines two fiber optic sensor architectures: Two-Mode fiber optic interferometric strain sensor (low sensitivity) and the Mach-Zehnder fiber optic interferometric strain sensor (high sensitivity). The dual-sensitivity fiber optic strain sensor also employs the end-etched fiber technique that enables the separation of modes. This technique allows the separated modes to be recombined in inter-mode (different mode order interference yielding low sensitivity output) and intra-mode (same mode order interference yielding high sensitivity output) interference configurations. One sensing region, dual simultaneous sensitivities and the use of end-etched fiber for sensing applications are some of the unique features of the dual-sensitivity fiber optic strain sensor.