Volatile Organic Compounds Detecting Based On Photonic Crystal Fiber

This paper focuses on the research of volatile organic compounds (VOCs)detecting based on photonic crystal fiber (PCF). Compared with the common opticalfibers, the PCFs have many advantages for VOCs detecting. The PCF is insensitive totemperature, so that it is more stable than the common optical fibers. Moreover, theair holes of the PCF make it can work as a common light waveguide as well as acontainer for tha sample molecules, so that the energy exchange between the light andthe sample is more direct than that of a common fiber. Based on these uniqueadvantages about PCFs, our purpose is to research the properties of the PCF andcombine them with suitable sensing mechanisms to work as VOCs sensing elements.The main contents are described as follows.A chemical vapor sensor based on a fiber loop mirror (FLM) combined with ahigh-birefringent photonic crystal fiber (HiBi-PCF) is proposed and experimentallydemonstrated. Because the resonant dips of the HiBi-PCF FLM are sensitive to thephase difference, a high sensitivity on chemical vapor can be realized by measuringthe wavelength shift of the resonant dips when the chemical vapor sample diffusinginto the air holes of the HiBi-PCF. Experimental results show it reaches up to0.837pm/ppm for ethanol when a5.1cm long PCF is used in the FLM; the outcome of thesimulation analysis agree with the experiment well.A novel volatile organic compound sensor based on the bandgap shift of ahollow-core photonic bandgap fiber (HC-PBGF) is proposed and theoreticallyanalyzed by the Comsol software. Since the bandgap of the HC-PBGF is sensitive tothe periodic structure as well as the refractive index of the material which forms theperiodic structure, we can detect the concentration of the VOC by monitoring thebandgap shift. Applying the full-vector finite-element method (FEM) and theperfectly matched layer (PML), we simulate a series of the bandgap under differentconcentrations of methanol and study the relationship between the bandgap and the methanol’s concentrations in detail. The simulation results show that the proposedVOC sensor is feasible and the sensitivity for methanol reaches up to8.06×10-4nm/ppm.A simple method to work out the multiplexing of tapered fiber based sensors isproposed and demonstrated. By cascading a tapered fiber with a fiber Bragg grating(FBG), the sensor head is provided with a wavelength identification, different FBGsprovide the sensor heads different reflective peaks and they can be distinguished inoptical spectrum. By compositing several such sensor heads with a multi-channelbeam splitter, a star-style topological structure sensor for multipoint sensing isachieved. A sensor for dual-point measurement of the displacement and temperaturesimultaneously is experimentally demonstrated by using a2×2coupler in this paper.Experiment results show that the sensor works well and the largest sensitivities reachto0.11dB/μm for displacement in the range of0to400μm, and~0.0097nm/℃for temperature between20℃to70℃. Combining it with the previous works ofwhich laying films on the tapered fibers for VOCs sensing, we can improve themultiplex sensing system, and use it for the multi-point VOCs detection.