Study On Fiber-optic Gas Refractive Index And Temperature Sensor Based On Mach-Zehnder Interferometry

In the last decade,R&D of fiber-optic sensors for gas refractive index and temperature detection has been the popular topic in academic and industrial fields.This thesis proposes three novel fiber-optic sensors using Mach-Zehnder interferometry for sensing of gas refractive index and surrounding temperature.First,we demonstrate a high-precision gas refractometer using a capillary fiber.The hollow core and cladding of the capillary constitute the sensing arm and reference arm of a M-Z interferometer.Coherent lights coupled into the hollow core and cladding would excite modes propagating therein independently.The output light from the core and cladding would interfere and generate interference fringes.Changes in the refractive index in the hollow core of the capillary would modify the phase difference between the core modes and cladding modes,thus shifting the interference fringes.By measuring the shifts of the interference fringes or detecting the variations in optical intensity caused by the fringe shift,we could characterize performance of the sensor.Using similar methodology,we also proposed a gas refractometer based on a dual-core fiber（DCF）,which has an air core and a Ge-doped silica core.Coherent light is coupled into the air core and silica core simultaneously.The output light from the two cores would generate interference.Changes in the refractive index of the air core would thus shift the interference fringes.The experimental resolutions of the two sensors are 5.7×10^-8RIU and 1×10^-8RIU,respectively,which are comparable to those of the top-notch gas refractometers.Besides,these two sensors have the advantages including low cost,small footprint,ease of fabrication,fast response,and straightforward sensing mechanism,and they are suitable for precise measurements of minute changes in gas refractive index and concentration,and air pressure.Moreover,in this thesis,we also propose a fiber-optic temperature sensor based on a dual core fiber.This DCF has eccentric core and a suspended core that is suspended in the embedded fluidic channel within the fiber.Silicone oil is filled in the fluidic channel.We use a SMF-MMF-DCF-MMF-SMF structure to enable the in-and out-coupling of the DCF.Temperature variations would modify the refractive index of the silicone oil,thus leading to shifts of the interference spectra output from the sensor.We characterize the sensor by correlating the spectral shift as a function of temperature changes.Experimental results indicate that sensitivity of the sensor is as high as-1.42 nm/RIU,comparable to those of the fiber-optic SPR sensors.The dynamic range of the sensor is greater than 120℃.As comparable to existing sensors fabricated using fiber post-processing techniques（such as fiber polishing,tapering,and fs-laser milling）,the proposed sensor has enhanced mechanical strength.The proposed temperature sensor would find its niche applications in environment temperature detection and architecture temperature monitoring.