Research on Novel Fiber-optic Strain Sensor Based on Single Micro-channel Cladding-film Fabry-Perot Interferometer

The fiber strain and pressure sensors based on Fabry-Perot (FP) interferometers are widely used owing to their good stability, simple configuration, high resolution, and miniature head. However, the traditional FP interferometer sensor has the drawbacks of low coupling efficiency, difficulty in bonding, calibrating and controlling precisely. As an alternative to the existing strain sensor, a novel single microchannel claddingfilm based FP interferometer strain sensor was proposed and designed in this paper. The configuration of a single microchannel claddingfilm based FP interferometer sensor head for strain measurement was constructed by positioning a microchannel based fiber end with a polymer film closely attached to its periphery on the metal plate. Thus, the polymer film could behave as a main sensing cavity, which is modulated by the applied perturbation, while the fiber cladding behaves as an auxiliary one. Both of them form the FP interferometer cavity. It is the most prominent highlight in the design of the proposed structure. The interface between input fiber core and the microchannel should be placed at a 45° angle relative to the core axis so as to change the beam propagation direction from horizontal to perpendicular. Moreover, the sensor was modeled using a twobeam optical interference equation and its potential application in stress measurement was discussed. The absolute cavity length of a single FP sensor system was determined by fringe counting. While a sensor system that can be capable of multiplexing multiple sensors was discussed. The reflection spectra of the multiple proposed FP interferometer sensors were also calculated at a constant temperature and analyzed in the spatial frequency domain by taking the fast Fourier transform (FFT) algorithm. In conclusion, a novel fiber strain sensor was proposed based on single microchannel claddingfilm FP interferometer, which could be manufactured using femtosecond laser based processing technique, chemical etching, or focused ion beam. Moreover, a sensor system that can be capable of multiplexing multiple sensors was discussed. Eleven proposed sensors were addressed in one system, and instead of the fringe counting technique, a novel frequency multiplexing method was employed to calculate their frequency spectra to demonstrate the feasibility of this multiplexing concept. The system was demonstrated to be capable of multiplexing more proposed sensors with appropriate minimal interval for the thickness of the polymer films. The sensor based on the single microchannel has more remarkable mechanical property and higher stability compared with existing fiberoptic strain sensors, which has revealed potential applications in refractive index measurements.