| Refractive index (RI) is an important parameter closely related to temperature, concentration etc. Micro RI probes have been widely used in various fields such as bio-chemistry, iatrology, food safety requires with characteristics of high precision, high stability, anti-corrosion, fast response. Fiber-optic refractive index sensor has been developed rapidly because of the above advantages. Among various fiber RI sensors, fiber optics Fabry-Perot sensors which have become one of the hot research spots due to its high precision, wide measurement range.In order to resolve the disadvantages of present fiber RI sensor, such as complicated structure, big size, short life-span, and poor mechanical stability, this dissertation explores high performance micro fiber optics Fabry-Perot (FP) RI probe which can provide potent test means for area like bio- chemistry. This paper is based on three kinds of novel micro fiber FP RI sensors and the main works and results are listed as follow:1. Micro fiber optics FP RI sensor based on contrast interrogation. Theoretical calculation reveals the sensor can test any RI range larger than 1. Experimental results show a resolution of ~4×10-5 and good linearity is obtained when RI is in range of 1.3333~1.4712. Etched FP has a big transmission loss. Comparing to laser machining, ething is low cost and easy to handle to fabricate FP, though it is difficult to control the size of micro FP.2. Dual FPs fiber RI sensor based on wavelength interrogation. The demodulation method is converted into wavelength-based from the intensity-based through reducing the length of second F-P. Theoretical resolution of 127 nm/RIU is obtained and corresponding resolution is up to ~10-6. Experimental results show the pit wavelength of reflection spectrum has a red-shift of 6.27 nm when n0 increases from 1.3333 to 1.4719 which can be regarded as linear range. Average sensitivity of 45.26 nm/RIU is achieved, and the system provides a resolution of 2.21×10-5 RIU. The performance of sensor is determined by the length of FP cavity. High precision can be obtained in given linear RI range by optimizing the length of FPs.3. Micro fiber optics FP Interferometric RI sensor with micro-channel. A in-line FP cavity is also included in this structure. A micro channel is fabricated on the reflection face of fiber by polishing and works as the entry of gas or liquid. The variation of RI is demodulated from the extreme wavelength in reflection spectrum. Sensor provides a sensitivity of 1068 nm/RIU when the tested RI range from 1.3333 to 1.3899. A RI resolution of 1.488×10-7 is achieved and corresponding wavelength resolution of optical spectrum analyzer is 1 pm. The sensor is applied to study the variation of water RI when the water temperature changes from 29℃~61℃. This novel structure resolves the disadvantages such as big size, poor stability in traditional FP RI sensors. More analysis can be done on the diameter of micro channel.Above-mentioned micro fiber FP RI sensors have good characteristics such as simple structure, small size, high stability, and high resolution. They are easy to be made by etching and polishing, but the reflection surface of micro FP is coarse. Improvement can be accomplished if high precision technique is adopted. |
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