Study On Micro-/Nanofiber Devices Based On Evanescent Field

Similar to their electronic counterparts, photonic devices also show the trend for miniaturization. Micro- or nanophotonic devices, which manipulate light on the wavelength or subwavelength scale, demonstrate various impressive functions. Micro-or nanoscale waveguides are one of the fundamental building blocks for micro- or nanophotonic devices, and are hot topics in current photonics research. Being one of the typical micro or nanoscale waveguides, micro- or nanofiber (MNF), featured with simple fabrication and low loss, is attracting more and more attentions.One of the outstanding properties of MNFs is their possibilities to offer high fraction of evanescent fields. In the thesis, we first investigate the mode field properties of MNFs. Using analytical expressions of the mode fields of silica MNFs, we study the electric field and energy distribution, both inside and outside the silica MNFs with linear and circular polarization. Besides, we also investigate the diameter-dependent waveguide dispersions in MNFs.Two MNFs can form a micro coupler by means of side-by-side evanescent coupling, which is very helpful for building MNF devices. The mechanism of evanescent coupling of MNFs are investigated using perturbation model, and the behavior of an evanescent coupler is numerically investigated using a 3D-FDTD method.Started with the taper-drawing fabrication of MNFs from silica fibers and bulk glasses, the thesis puts its emphasis on resonance and interference MNF devices based on evanescent coupling. In resonance devices, MNF knot resonators and microring lasers are investigated. The main efforts are made on the theoretical modeling of MNF ring lasers, which is based on ring resonator equations at the coupling region and rate equations for active materials. Analytical expressions are given for lasing condition, pump threshold and quantum efficiency for three-level and four-level transitions in Er³⁺ and Yb³⁺ doped glasses. It shows that pump resonance can significantly reduce the threshold, increase the pump absorption, and thus greatly increase the quantum efficiency. Besides, we also study the effect of the coupling loss and the ring size at the pump resonance. It is found that highly doped MNFs with large absorption cross sections will facilitate low-threshold, high quantum efficiency microring lasers with ring diameters down to tens of micrometers. These results may offer valuable reference to the realization of MNF-based ring lasers or other kind of microlasers.In the section of interference MNF devices, we present the fabrication and optical characterization of MNF Mach-Zehnder interferometers in detail. By means of micromanipulation under an optical microscope, miniature MNF couplers with different splitting ratios can be easily constructed. Based on two MNF couplers, silica and glass MNF Mach-Zehnder interferometers are assembled, with typical dimensions of tens to hundreds of micrometers. The extinction ratio can reach 10 dB by finely adjusting the coupler length. Besides, the path-length differences can be tuned by micromanipulation, and thus the free spectrum range can be easily adjusted. By virtue of its easy fabrication, compact size, tunability and convenient integration with fiber system, MNF Mach-Zehnder interferometers are promising to find applications in sensors, optical modulators and other miniature photonic devices.