Gas detection using photonic crystal fibers.

摘要

Optical fiber gas sensors based on the evanescent wave absorption have attracted considerably attention over the last two decades. 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.; 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/Λ s. However, the loss is unreasonably high for PCFs suitable for evanescent wave gas detection, which typically require large d/Λ s 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.; 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 1531nm can be up to ∼9% and ∼10-2dB/m, respectively. (Abstract shortened by UMI.)