The Study Of The Enhancement Of Second-Harmonic Wave Generation In Fiber And It’s Application

Silica fiber is prominent for its role in nonlinear optical research, since it can squeeze optical light in a small cross-section with a very long distance. However, due to the centrosymmetry of silica molecules, nonlinearities in optical fibers are mostly limited in third-order processes and inelastic scatterings. The advent of micro-structured fiber, also called as photonic crystal fiber, and tapered fiber further shrinks the cross-section of the optical fiber by two orders of magnitude and enables more convenient manipulation to the dispersion of the optical fiber. More importantly, these microfibers open the door to implement the second-order nonlinearities in optical fibers, which are of vital importance to fiber laser technologies and all-fiber quantum optical techniques. The former is related to the laser frequency doubling and originates from a second-order harmonic generation; the latter is related to the entangled photon pair generation and is from a spontaneous parametric down conversion. However, before implementing practical usages of these two effects, their conversion efficiencies need to be improved significantly, which is the main goal of this proposal. First, we will modify the conventional fiber tapering technique by taking into account and compensating viscosity effects of quartz materials during the heating-and-drawing process. We plan to manufacture much longer fiber tapers with uniform diameter and to enhance nonlinearity via the phase-matching principle. Second, a slab waveguide made of Li Nb O3 crystal will be attached to the fiber taper and give rise to much higher nonlinear coefficient because of its overlap with light. This ‘fiber-Li Nb O3 crystal’ hybrid structure is supposed to raise the nonlinear coefficient by two orders of magnitude. Altogether, by enhancing the interaction length and coefficient, we aim to boost the conversion efficiency of the second-order nonlinear process, which is squarely proportional to the product of the length and the coefficient, more dramatically. Large number of theoretical calculations about three kinds of waveguide are discussed by this paper. Through this research, we hope to develop a brand-new optical device based on second-order nonlinearities in tapered fibers.