The Polarization Characteristics Of Evanescent Coupling Between Two Optical Micro/nano Fibers

Micro/nano fiber with nanometer-and micrometer-order diameter has interestingoptical properties as tight optical field confinement and large evanescent fields. Highefficiency energy exchanged can be acquired by evanescent coupling between twoMicro/nano fibers, which is also used to make fiber microcouplers, microresonators,compact and sensitive fiber sensors and hybrid integrated optoelectronic devices. Themicro/nano fiber mentioned in this thesis is directly fabricated by drawingsingle-mode silica fibers in fusion.1、 In order to discuss the characteristics of the polarization coupling of twomicro/nano optical fiber, TE and TM polarization modes are analyzed at overlappingzone firstly, by using3D Full Vector Beam Propagation Method (3D-FVBPM) withBeamprop module of Rsoft-CAD software, and the analytical results of characteristicsof the polarization coupling show that a polarization beam splitter (PBS) based oncoupled micro/nano optical fiber can be constructed.2、On the condition that polarization extinctions are more than15dB simultaneously,the geometric parameters of the PBS are designed by3D-FVBPM, which is asimplified model based on straight waveguide. Numerical simulations show that whenthe optimal parameters are as follows, the diameter of about0.9μm, the gap of about0.5μm and the length of218μm, we can acquire fabrication tolerances for thepolarization splitter at a wavelength of1550nm when the extinction ratio ofpolarization of both output port of the splitter are larger than15dB, such as about10nm bandwidth, overlapping length of±2μm, and about-3nm~2nm for the diameterand gap respectively.3、 The numerical model with two bent output waveguides based on straightwaveguide is analyzed, which impact parameters of polarization beam splittingperformance, including inside spacing of two outputs, wavelengths and fabrication tolerances. The new three groups of optimized combination parameters are acquiredby adjusting overlapping length, corresponding the inside two outputs spacing of6μm,6.5μm, and7μm. Numerical simulation results show that fabrication tolerances for thepolarization splitter about3nm bandwidth, about±1nm for gap and about±0.1μm forinside spacing of two outputs respectively, when the extinction ratios for bothpolarizations are larger than15dB. Compared with simplified straight waveguidemodel, the model adding two bent output waveguides is more close to the realstructure and more theoretical guidance on the experiment.The innovations of this thesis are:1． A new numerical model of polarization beam splitter based on opticalmicro/nano-fiber is designed, by using Beamprop module of Rsoft-CAD software.2．The micro-metre polarizer based on micro/nano optical fiber coupling structure isproved in theory.3．The geometrical parameters optimized are calculated, of which the polarizingdevice can obtain minimum scale. The device tolerance in fabrication and bandwidthare also analyzed.