Research On Nonlinear Optical Effects In Microfiber Devices

Recently,optical micro fiber(MF)has received more and more attention due to its great potential in micro/nano-photonic devices.MF is a kind of optical fiber with waist diameter of several hundred nanometers to several micrometers.However,it shows quite different optical and mechanical properties from traditional optical fibers because of huge reduction in physical size.MF possesses outstanding advantages of strong light confinement,large evanescent fields,high mechanical strength,great flexibility and low insertion loss,and the strong light confinement significantly reduces the threshold of nonlinear optical effects.Moreover,nonlinear optical effects in MF show some novel characteristics compared to those in conventional optical fibers.In this thesis,we mainly focus on surface second-harmonic generation and multi-peaked Brillouin scattering in MF,and radiation-pressure-dominated nonlinear optical effects in MF Bragg grating.The outline of our research is as follows:(1)Surface second-harmonic generation(SHG)in MF.Second-order nonlinear polarization originating from surface nonlinearity will lead to surface SHG in MFs,which differs a lot from the traditional SHG in conventional optical fibers.We introduced the mechanism for this kind of nonlinear optical effect,and discussed the phase-matching method and coupled-amplitude equation for surface SHG in MF.Based on coupled-amplitude equation,we theoretically demonstrated that higher surface SHG conversion can be achieved by introducing slot structure into MF.Our calculation results showed that the maximum conversion efficiency of slot-MFs is about 25 times of that in circular MFs.(2)Multi-peaked Brillouin scattering in MF.Due to the tight confinement of both photons and phonons,guided acoustic waves(GAWs)arise and induce multi-peaked Brillouin scattering in MFs,which is also quite different from what happens in standard single-mode fibers(SMFs).We investigated the physical mechanism for this phenomenon,and discussed the fundamental concepts related to Brillouin scattering in MFs,including photo-elastic effect,driving forces of stimulated scattering,acoustic modes,Brillouin coupling coefficient and phase-matching condition.On this basis,we experimentally studied the backward Brillouin scattering in silica MFs,presenting the spectrum evolution and the spatial distribution of Brillouin signals along the fiber.Furthermore,the strain sensing characteristics of Brillouin scattering in MF were investigated.The strain sensitivities for scattering peaks of waist acoustic TR21 and R02 modes were measured to be 0.008609 and 0.02004MHz/??,respectively.This work contributes to further comprehension of photon-phonon interactions in micro and nanoscale photonic devices,and the results may pave the way to potential applications for fast optical sensing and detection using Brillouin scattering of guided acoustic waves in MFs.(3)Nonlinear optical effects in microfiber Bragg grating(MFBG).In this thesis,unique nonlinear properties of MFBG have also been investigated.First,the manufacturing method for MFBGs was developed.By focused ion beam(FIB)milling technique,MFBGs with strong refractive index modulation(10-2)and fine performance have been fabricated.Next,the force sensing characteristics of the fabricated MFBG were investigated.The experimental results showed that MFBG of 2.5?m in diameter has a force sensitivity of?3146nm/N,which is 3 orders of magnitude higher than that of conventional fiber Bragg grating.The ultrahigh force sensitivity indicates that MFBG may response to weak optical force like radiation pressure.Thus,the nonlinear optical effects caused by both Kerr effect and radiation pressure were analyzed.By solving the coupled-mode equation,it has been demonstrated that radiation-pressure-induced strain dominates the nonlinear processes in MFBG and leads to novel nonlinear all-optical control effect.By this effect,bandwidth and group delay of MFBG can be all-optically reconfigured by incident light.In conclusion,we investigated the unique nonlinear optical effects in MF devices,including surface SHG,multi-peaked Brillouin scattering and nonlinear all-optical control effect.These effects provide new opportunities for the development of micro/nano-photonic devices,and have great potential in miniaturized fiber-based lasers,sensors and all-optical devices.