Study On Novel Optical Fiber Gas Refractometer

Measurement of gas refractive index is required in a variety of areas, such as in modern laser technology, optical instruments for photolithography system, and radiation detector in high energy experiment. Conventional gas refractometers, which are generally based on optical interferometers, are bulky and complex. On the other hand, optical fiber sensor has advantages of miniature size, high sensitivity, immunity to electromagnetic interference, resistance to radiation, capability of multiplexing and distributed sensing. In this dissertation, we focus on investigation of novel optical fiber sensors for gas refractive index measurement, including Fabry-Perot interferometric sensors and microfiber coupled whispering gallery mode microsphere resonator sensing scheme. The main contents of this research include:First, we presented a novel optical fiber intrinsic Fabry-Perot interferometer and its application as a gas refracterometry. The interferometer is based on a concave well on single mode fiber endface, which is simply fabricated by chemical etching in hydrofluoric acid in several minutes. The sensor is low cost and has potential in batch fabrication. The sensitivity to refractive index change of nitrogen is441nm/RIU.Secondly, we proposed three gas refractometers based on optical fiber extrinsic Fabry-Perot interferometers with open cavity design. The fabrication of the interferometers only involves fusion splicing. The first interferometer is fabricated by sandwiching a short section of capillary tube between a single mode fiber and a photonic crystal fiber. A sensitivity of974RIU/nm in the refractive index range of1.00054to1.00298was achieved. The second interferometer is also based on similar strucutre but offers the capability to measure the gas refractive index and temperature simultaneously. Finally we present a graded-index fiber collimator based fringe visibility enhanced interferometer. The improvement of fringe visibility is theoretically analyzed using ABCD matrix method.Finally, we demonstrated a robust and integrated whispering gallery mode microsphere resonator system and its application as a gas refractometer. A robust microfiber based Fabry-Perot interferometer is proposed for whispering gallery modes coupling. The interferometer is simply fabricated by chemical etching away the holey area of a photonics crystal fiber, leaving a free-standing solid core enclosed in a silica housing. Both asymmetric and symmetric lineshapes are observed in the reflective spectrum and the resonance’s lineshape is determined by its relative position in reference to the FP background. By selecting the wavelength tuning range, the lineshape can be switched for different applications. This compact and integrated coupler is used to measure the refractive index of nitrogen, demonstrating its potential for gas refractive index measurement with low detection limit.The sensors proposed in this dissertation show advantages over conventional gas refractometers in many aspects such as fabrication process, sensitivity and size. Preliminary laboratory results shown potential to meet not only the needs for gas refractive index measurement but also for other sensing applications.