| As the fundamental element of integrated photonics, micro/nano opticalwaveguides (MNOW), including micro/nano optical fiber (MNOF) andsilicon-on-insulator (SOI) waveguide, have attracted widely attention, inrecent years. Due to the high refractive index contrast between siliconwaveguide core and air/silica substrate, the light field is confined in a verysmall size, which offers an excellent foundation for high density photonicintegration. And the MNOF, featured with low-loss, strong evanescent fieldand the compatibility with single-mode fibers, is the most promising methodfor the optical interconnects.A key issue for on-chip or inter-chip optical interconnection is thecoupling between waveguides. In this dissertation, we focus our research onthe analysis and design of highly-efficient coupling structures.First, we classify the coupling structures according to the couplingmechanism and technique, etc. The research progress is also reviewed.Secondly, we analyze theoretically the mode fields of several kinds ofmicro/nano optical waveguides with different structure or materials. Themode field distribution and the evanescent field characteristics for the slabwaveguide, the silicon strip waveguide and micro/nano fiber waveguide havebeen discussed in this section. The coupling mechanisms between micro/nanooptical waveguides are also introduced in detail. Several factors, determiningthe coupling efficiency of couplers, are discussed, which pave the way for thenext sections on the design of coupling structures. In addition, thetransmission-line model is proposed to analyze the coupling between two micro/nano wagveguides.Next, we study the coupling characteristics of micro/nano opticalwaveguides by using experiment and simulation method respectively. Acoupling structure consisting of two conical micro/nano fibers (CMNFs) withthe same structure and material is experimentally demonstrated. Distinct fromuniform micro/nano fibers (UMNFs), the coupling efficiency depends on notonly the overlapping length between two CMNFs but also the tapering angleof the CMNFs. With the increase of overlapping length, the couplingefficiency gradually converges to a stable value, with its convergence speeddetermined by the angle of the CMNFs. Experimental result shows theconvergent coupling efficiency can be larger than95%. And experimentalresults also show that the coupling characteristics of CMNFs are dependenton overlapping length and taper angle, which verify the simulationconclusions. Meanwhile, the coupling characteristics of the couplerconsisting of two waveguides with different materials and structure are alsosimulated. The proposed structure is characterized by high couplingefficiency, wavelength insensitivity, large misalignment tolerance, and easyfabrication. Theoretical analysis and numerical simulation results show thatcoupling efficiency of>90%can be achieved with a taper length of~4.5μm.With these advantages, we believe the proposed coupling structure can beused for optical packages of silicon photonic chips.And then, using the prior results of the theory, experiment andsimulation, we propose a novel multi-beam power combiner which is anevanescent field-based coupling structure by using tapered micro-nano fibers.Traditionally, it is difficult to realize the power combiner by using opticalfiber coupler. However, in our experiment, we find a novle structure tocombine optical power from two or more fibers into one fiber by usingtapered micro-nano fiber. It shows that the maximum power combingefficiency can be higher than90%. The combining efficiency is overlaplength dependence. As long as the overlap length is long enough (~7mm), astable high combining efficiency can always be achieved. The presented optical power combiners with the advantages of easy fabrication, low-loss,low-cost, and wavelength insensitivity can find potential applications inmicro/nano photonic devices, optical communications and opticalinterconnects.Finally, a conclusion and expectation are made for the dissertation...
|
PDF Full Text Request