We propose and analyze a novel orientation-free pressure sensor based on a π-shifted all-single-mode-fiber (SMF) Sagnac interferometer. Compared with a conventional pressure sensor based on high-birefringence polarization-maintaining fiber, this all-SMF structure is more simple, stable, and economical. Because an initial π shift is introduced into the Sagnac loop by adjusting actively the polarization controller, the intensity response of the sensor is independent of the birefringence orientation associated with the device under test (DUT). This configuration not only eliminates the effect of an uncertain birefringence orientation in the fiber loop, it also exhibits excellent repeatability and high sensitivity, which will ease the following demodulation of the measurand. The Jones matrix method is used to analyze the orientation-free characteristic of this sensor, and some theoretical and experimental results are also given.
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