|Title:||Gas detection using photonic crystal fibers|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||Department of Electrical Engineering|
|Pages:||xv, 171 leaves : ill. ; 30 cm|
|Abstract:||Optical fiber gas sensors based on the evanescent wave absorption have attracted considerably attention over the last two decades. Compared with open-path sensors, the evanescent wave sensors have the advantages of easier alignment and much longer interaction length that allow for distributed measurement over an extended area. However, the sensitivity of the evanescent wave sensor is much lower than that of the open-path sensor per equal length, because of the very small percentage of light power (<0.2% for D-fiber based sensors) in the evanescent wave region that interacts with gas sample. Photonic crystal fibers (holey fibers) which have a cladding region comprising of air holes running along the full length of the fiber are expected to have a much higher fraction of evanescent power in the holes and hence allow for evanescent wave sensors with higher sensitivity to be developed. The objective of this project is to investigate theoretically and experimentally the use of the photonic crystal fibers (PCFs) for evanescent wave gas detection. A Finite Element Method (FEM) was used to analyze the relative sensitivity as a function of wavelengths for two types of PCFs that were obtained respectively from Crystal Fibers A/S and Lucent Technologies. At 1531nm, the acetylene absorption wavelength, the relative sensitivity of the Crystal Fibers A/S's fiber that is designed for non-linear optics applications was found to be 13% of the open-path and the relative sensitivity of the Lucent fiber is ~3.5%. These theoretical results agree well with the values obtained from experiments. The relative sensitivity at 1650nm, the methane absorption wavelengths are theoretically predicated to be ~15% and ~4.5% for the two types of PCFs respectively. The relative sensitivity was found to strongly depend on the PCF structural parameters, i.e., the diameter d and pitch Λ of holes in the cladding. For example, by keeping the same d/Λ value of 0.9, reducing Λ from 1.44um to 1.33um increases the relative sensitivity at 1530nm from ~3.5% to ~6.5%. The response time of the PCF sensor was determined by the time taken for gas to diffuse in/out into the hole columns. The gas transport phenomena along the capillary structure (the hole column) was studied theoretically and experimentally. It was found that the diffusion coefficient of acetylene in air for Crystal Fiber A/S's PCF is only slight smaller (by ~8%) than for the continuum state. To realize a PCF sensor with approximately 60 seconds response time, the PCF length should be limited to less than 7cm and both ends of the PCF should be open for gas diffusion. This implies that if a longer length of PCF is needed for the purpose of improving detection sensitivity, periodic opening should be introduced along the sensing PCF every 7cm or less length of fiber. Otherwise, the response time could be impractically long. To be practically useful, PCFs must be connected to other conventional optical fibers. The loss between SMF28 single mode fiber/PCF with various parameters was investigated. The joint loss between PCF and SMF28 was estimated by using FEM and the overlap integral method. The joint loss between the PCF and SMF28 fiber at 1550nm as a function of the separation and the size of the holes is presented. It was found that reasonable low loss can be achieved for a range of Λs and d/Λs. However, the loss is unreasonably high for PCFs suitable for evanescent wave gas detection, which typically require large d/Λs and small Λs. Several techniques for reducing the coupling losses, including tapering the SMF by chemical etching, adiabatically tapering of Microstructure fiber with Ge-doped core and using of lens with suitable aperture and focal length are discussed in the thesis and are believed to be suitable for connecting the gas-sensing PCF with SM fibers. Based on our analysis on the relative sensitivity, response time, current fabrication and connection technology of PCF, and the state of the art source/detection technology, a PCF acetylene gas detection system was designed. The system uses ~42m of the Crystal Fibers A/S's Fiber along which side openings are introduced every 7cm interval. The response time and the detection sensitivity are expected to be 1 min and ~0.6ppm respectively. Novel holey fibers for gas detection were designed. The advantage of these fibers is that it is easier to access the evanescent field from the side of the fibers and hence faster response time. The relative sensitivities and confinement losses of the novel holey fibers at wavelength 1531mm can be up to ~9% and ~10-2dB/m, respectively. Apart from the gas detection, the PCF can also be used in the gas diffusion analysis. PCF has uniform air-hole columns along the fiber length that provides the basis for the study of gas diffusion based on the capillary method. The gas concentration within the air-hole columns can be monitored by measuring the attenuation of light through the PCF caused by the evanescent wave absorption of light by the gas sample. Hence the diffusion process can be studied by analyzing the variation of concentration inside the holes columns, which allows the binary diffusion coefficient between the sample gases being estimated.|
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