Highly birefringent photonic crystal fibers and sensors

Abstract

A novel class of material, known as photonic crystal, has opened up new ways to guide the flow of light. In the early 1990s, Photonic Crystal Fibers (PCFs), an optical fiber using photonic crystal cladding, were developed. The pioneering experimental works on these fibers showed that they have inherently unprecedented properties and overcome many limitations of conventional optical fiber, i.e. guiding light in a hollow core, being endlessly single mode, having anomalous dispersion in the visible region, and possessing high nonlinear coefficients, etc. Among the many unique properties of PCFs, this thesis is most concerned with the polarization and modal properties of highly birefringent (Hi-Bi) PCFs with asymmetric core and different hole-sizes along the two orthogonal axes. The thesis starts with a short review of conventional optical fibers and then proceeds to a discussion on the guiding mechanisms of solid core and hollow core PCFs. The main properties of solid core PCFs, i.e. confinement loss, fundamental mode cutoff and dispersion, are reviewed. A special section is devoted to Hi-Bi PCFs and their corresponding properties. The basic properties of an asymmetrical core PCF are theoretically investigated by using the full-vector finite element method (FEM). The calculated birefringence is in good agreement with the measured value. The influence of fiber structural parameters on modal birefringence, mode field diameter (MFD), and half divergence angle are investigated in detail. The group velocity dispersions for the two fundamental modes are also calculated and found to be significantly different for the two orthogonal polarizations. The full-vector FEM is also used to calculate the electrical fields and to evaluate the equivalent MFD of endlessly single mode PCFs. It was found that the MFD increases approximately linearly with pitch ^ and decreases with an increase in air-hole diameter to pitch ratio d/^. An empirical formula is proposed for estimating the MFD. The results calculated by using the formula deviates less than 1% from those obtained from FEM for 0.25 <= d/^ <= 0.45. With the help of the MFD, the connection loss between a single mode fiber and a PCF can be evaluated by using the classical method based on the MFD. Through the analysis of a Hi-Bi PCF by FEM with anisotropic perfectly matched layers (PMLs), we proposed a general design methodology for an asymmetrical core PCF to achieve single polarization single mode (SPSM) operation at an arbitrary operating wavelength. Specifically we optimized the PCF structure for SPSM operation around l.30um and 1.55um. The bandwidths of the SPSM PCFs are respectively 84.7nm and 103.5nm for l.30um and 1.55um, within which one polarization state is attenuated by at least 30dB/m while the orthogonal state suffers a confinement loss of less than IdB/m. The cutoff wavelengths of these fibres are further validated by calculating the effective mode area of each polarization, which deviates less than 4% from that found by the confinement loss calculation using FEM. The coupling losses between the proposed SPSM fibers and single mode fibers were also calculated by using the overlap integral method and found to be ~78% and ~77% at 1.55um and l.30um, respectively. A similar Hi-Bi PCF but with different parameters is found to support only the LP01 and LP11(even) modes from 543nm to 1310nm. The LP11(odd) mode is unsupported within this broad range, and the supported LP11(even) mode has a stable intensity lobe orientation. The very broad two-mode wavelength range will allow a number of novel two-mode devices to be developed. The examples of these devices include acousto-optic frequency shifters, tunable filters, modal filters, optical switches, etc. With the special modal properties of the Hi-Bi PCF, we experimentally demonstrated a two-mode PCF interferometer based on the modal interference between the LP01 and LP11 (even) modes propagating in the same length of PCF. The responses of the interferometer to axial strain and temperature were experimentally investigated over a wavelength range of from 600nm to 1310nm. For the strain sensor, the fiber elongations needed to produce 2 pi phase change decrease with the wavelength, indicating higher strain sensitivity at longer wavelengths. The strain sensitivity is also polarization dependent. The temperature sensitivity of the two-mode PCF sensor was measured and it showed a non-monotonic dependence on the operating wavelength. A mathematical model was developed to explain the non-monotonic temperature dependence, and found to agree in trends with the experimentally measured results. The unique wavelength dependence of the temperature/strain sensitivity would allow temperature insensitive strain measurement to be performed by operating the sensor at two selected wavelengths.