Research On Preparation And Characterization Of Femtosecond Laser Direct Writing Near-infrared Optics Devices

Laser is one of the most important inventions of mankind in the twentieth century.Its birth has promoted human progress in various fields including lighting,communication,astronomical ranging,microscopic measuring,precision processing,medical treatment and military defense.Among them,ultra-fast laser micro processing has attracted much attention as a favorable tool to promote the development of integrated photonics.This thesis focuses on the theoretical simulation and experimental research of near-infrared nonlinear optical devices fabricated by femtosecond laser direct writing technology.Specifically,we have designed and fabricated waveguides for supercontinuum broadening in halide-doped GGSI chalcogenide glasses,and volume phase holograms for nonlinear beam shaping in lithium niobate crystals,which have expanded the application of these two materials in the field of near-infrared and nonlinear integrated optics.The work presented in this thesis mainly includes the following parts:1.Fabrication and transmission characteristics of femtosecond laser direct-written waveguides in sulfur-based glass 75GeS2-15Ga2S3-10CsI(GGSI).A writing laser beam with an operating wavelength of 800 nm,pulse width of 50 fs,and pulse repetition frequency of 1 kHz was used to write single-line waveguides with single-pulse energy of 0.4 μJ,0.6 μJ,0.8 μJ,and 1 μJ,respectively.The speed of the translation stage was kept unchanged at 60μm/s during the experiment.The waveguide transmission loss measured by the self-built loss measurement system shows that the waveguide written with 0.6 μJ pulse has the lowest transmission loss of 1.53 dB/cm.The numerical change of the refractive index of the waveguide is Δn≈9.1×10-4 measured by the numerical aperture method.2.Design and simulation of polygonal GGSI optical waveguide.In order to adjust the zero-dispersion point of the laser inscribed waveguide to the vicinity of 2 μm(wavelength of pump light source in the lab)for better supercontinuum spectral broadening effect,we designed a polygonal waveguide structure,which consists of 7 GGSI single-line waveguides with width of 2 μm and height of 2.25 μm.The finite element method was used to numerically obtain its zero-dispersion point at 2.05 μm.The fundamental mode Gaussian beam shows good propagation characteristics in the polygonal waveguide simulated by the beam propagation method.3.Numerical simulation of supercontinuum broadening of polygonal GGSI optical waveguide.A GNLSE numerical simulation program was written using MATLAB software to simulate the generation of supercontinuum in a polygonal waveguide.The effects of waveguide dispersion,waveguide nonlinearity,waveguide length,initial peak power of the pump pulse,and initial pulse width on the supercontinuum spectral broadening are systematically analyzed.A flat supercontinuum broadening spans from 1.2 μm to 3.6 μm was obtained by pumping a polygonal GGSI waveguide,whose total length,nonlinear coefficient and zero-dispersion-wavelength is 5 mm,0.51 W-1m-1 and 2.05 μm,respectively,using laser pulses with a wavelength of 2 μm,width of 50 fs,and a peak power of 40 kW.4.Theoretical and experimental investigation of laser written volume phase hologram for nonlinear beam shaping in lithium niobite crystals.Photorefractive structures inscribed by laser writing can be directly used for beam shaping without using any external optical elements or structures,and therefore are stable and good for integration.However,beam shaping in these structures are based on linear diffraction,therefore no efficient nonlinear beam shaping can be obtained.To solve this problem,we first experimentally investigated the effects of laser writing parameters on the resolution and maximal depth of photorefractive structures in lithium niobite crystals of different cuts.Then,as a proof of concept,cross-shaped second harmonic vortex beams were obtained by illuminating a fundamental Gaussian beam through a 3D fork hologram in x-cut lithium niobate.Finally,it was theoretically demonstrated that phase-matched nonlinear beam shaping can be generated by utilizing the Bragg diffraction of fundamental beam.We also present the theoretical results on phase-matched vortex second harmonic generation in the wavelength range between 1074-3718 nm by adjusting the period of the hologram and the incident angle of the fundamental beam.