| Owing to the unique advantages of low propagating loss, low cost, compactness, high sensitivity and accuracy, simultaneous sensing ability, and immunity to electromagnetic interference, fiber-optic sensors have become the research hotspot in sensor fields.Fabry-Perot interferometer, Mach-Zehnder interferometer and long-period fiber grating are widely used for fiber-optic sensors. In this paper, these two different kinds of interferometers and long-period fiber grating sensor have been investigated. We have mainly discussed pressure and temperature characteristics of the dual-cavity Fabry-Perot interferometer based on the silica tube and single mode fiber(SMF). The temperature as well as strain responses of Mach-Zehnder interferometer and long-period fiber grating both based on the suspended-core fiber are analyzed. The main contents are shown as following:1. We make an overview of the characteristic, application as well as classification of fiber-optic sensor. The principles of various types of fiber-optic interferometers and fiber gratings are demonstrated. And the research status of fiber-optic sensor in recent years is cited in detail.2. A dual-cavity Fabry-Perot interferometer based on the silica tube and SMF is proposed for pressure sensing. The device is fabricated by splicing two SMFs to both ends of a section of silica tube with a certain length. Meanwhile, a microchannel is drilled in the middle part of the silica tube from the surface to the core by femtosecond laser pulses, so that outside air can diffuse into the core. Cleaving one of the SMFs to a designed length and ensuring that the free spectral ranges of two cascaded Fabry-Perot cavities are closed. So the optical Vernier effect will be generated in the proposed sensor and the sensitivity can be significantly amplified.The magnification principle of the Vernier effect is analyzed in theory, and pressure as well as temperature characteristics of the dual-cavity Fabry-Perot interferometer sensor are experimentally explored. In the experiments, the sensing properties of two samples with different magnification factors are discussed. As a result, the pressure sensitivities of thesetwo samples are obtained to be-92.76 nm/MPa and-51.38 nm/MPa, respectively. Compared with the single Fabry-Perot interferometer, the sensitivities are respectively magnified with the factors of 23.19 and 12.85.3. A simple and compact Mach-Zehnder interferometer is presented for high temperature measurement. The device is composed of a short segment of suspended-core fiber sandwiched between two SMFs with a misaligned splicing point. It is easily manufactured by using cutting and splicing technologies. In addition, the mode field distribution of the suspended-core fiber is calculated by the commercial software COMSOL Multiphysics with finite element method, and the interference pattern of the proposed Mach-Zehnder interferometer is considered to be induced by the phase difference between LP01 core mode and like-LP01 cladding mode. Meanwhile, temperature and strain measurements of the device are experimentally carried out. From 20 °C to 200 °C, the temperature sensitivity of the device is 24.24 pm/°C, whereas a higher sensitivity of 53.87 pm/°C is acquired in the range of200 °C to 1000 °C. In addition, the response to strain is so weak, with low cross-sensitivity of3×10-3?C/??.4. Characteristics of long-period fiber grating inscribed in the suspended-core fiber by800 nm infrared femtosecond laser pulses are investigated. The grating period and the period number of long-period fiber grating in the suspended-core fiber are 160 ?m and 42,respectively. The resonant dip is obtained to be at the wavelength of 1489.75 nm with high fringe contrast of 12.54 d B. Meanwhile, the 3 d B wavelength width is 6.42 nm, and insertion loss induced in the fabrication is about 1.5 d B. Finally, responses of the long-period fiber grating to strain, temperature and surrounding refractive index are experimentally explored.The strain sensitivity of-1.65 pm/?? is achieved, and the results show that the device is independent to the temperature and surrounding refractive index. |
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