Based On The Hollow Fiber Fabry-Perot Interferometer And Its Sensing Application Research

The F-P cavity length and refractive index of a fiber Fabry-Perot (F-P) interferometer will change when interferometer is subject to axial stress, temperature, bending and other external actions,which causes the changing of interference optical path difference. This character can be used widely in sensing measurements. Today, all-fiber, integration and multi-parameter measurement have become the main direction of the optical fiber F-P sensor technology development. This paper will theoretically and experimently analysis the sensing principles and characteristics of the F-P interferometers based on hollow fibers.Firstly we introduce the research developments and applications of hollow fibers and fiber F-P interferometers. Then it describes the principles of the traditional optical F-P interferometers and optical fiber F-P interferometers,and it also analysises the sensing characteristics of fiber optic F-P interferometers. Considering all of above, we design and produce two new fiber F-P interferometers based on hollow fibers (Air cavity and quartz cavity cascaded optical fiber F-P interferometer and Air cavity F-P interferometer based on eccentric optical fiber). Finally we measure and analysis the two interferometers' sensing characteristics.Air- and silica-cavity cascaded optical fiber F-P interferometer (FPI) is produced by several arc discharges, making it collapsed and forming a cascaded air cavity and quartz cavity.In addition, this hybrid structure is appropriate for axial strain sensing because the hybrid FPI is located at the middle rather than the end of the transmitting fibers. The spectrum was transformed to the spatial frequency domain by fast Fourier transform (FFT) to obtain the detailed information of the spectrum, and the spatial frequency peak of the air- and silica-cavity can be extracted by filtering other frequencies. Then the other components are filtered and by Inverse FFT processing, we get two F-P cavities's interference fringes. We analysis the experimental data of this fiber F-P interferometer (FPI)'s temperature and stress sensing. It turns out that air-cavity-based FPI is sensitive to axial strain and insensitive to temperature, whereas the silica cavity-based FPI has a higher temperature sensitivity and lower strain sensitivity relative to the air-cavity-based FPI.Therefore, such a multi-cavity FPI structure is feasible for simultaneous measurement of strain and temperature.Air cavity F-P interferometer based on excentric core fiber is made by splicing excentric core fiber, hollow fiber and multimode fiber to form a air F-P cavity. By analysising the temperature and bending experimental data, we find that because the air F-P cavity's length is long and the the thermal optical coefficient of air is low, its temperature sensitivity is low. And because the excentric core fiber's radial stress direction is different in every bending measurement, this interferometer has different bending sensitivities,so the experimental results show its good temperature stability and bending sensitivity. It show that this interferometers has good temperature stability and higher bending sensitivity.