Research On Crystal Optical Waveguide Based On Femtosecond Laser Inscription Technology

Optical waveguide is a medium that guides light waves through its structure.It is also called a dielectric waveguide.According to the principle that total reflection occurs due to different refractive indices between different interfaces,optical waveguides limit light to the order of microns,and the high density generated in the cavity results in no radiation transmission of light waves only in the waveguide structure.Optical waveguide is also a basic component to realize integrated optical circuits with different functions,such as optical couplers,attenuators,modulators,switches,beam splitters,amplifiers,etc.Rare-earth doped laser crystals such as yttrium aluminum garnet（YAG）crystals,vanadate crystals,sapphires havee been proved to be excellent substrate materials for optical waveguides fabrication.Ion implantation is a traditional method to fabricate optical waveguides,during which a certain dose of ions is injected into the substrate materia,leading to refractive index change and the forming the waveguide structure.Femtosecond laser inscription is a kind of optical waveguide preparation technology developed in recent years.Focused femtosecond laser pulse is used to scan substrate material and induce local refractive index change of substrate material to form waveguide structure.In our work,femtosecond laser inscription technology and ion implantation technology are used to prepare different types of optical waveguides.The performance of the produced waveguides,including transmission loss,micro-fluorescence and micro-raman characteristics,the laser characteristics of the optical waveguide and the up-conversion of the optical waveguide are thoroughly investigeted.According to thefabricating methods and waveguide structures,the main works can be summarized as follows:Cladding waveguides in Nd,Gd:CaF₂ crystals by femtosecond laser inscription technology:Four sets of circular cladding waveguides with diameters of 20μm,25μm,30μm,and 35μm were fabricated by using different laser energies.The transmission modes,propagation losses and refractive index of all cladding waveguide structures have been tested.The minimum transmission loss is calculated to be 0.87 dB/cm,corresponding to a refractive index contrast of7×10^-3.The micro-fluorescence and micro-Raman characteristics of the waveguide were measured with a confocal microscope.By analyzing the changes in fluorescence intensity,fluorescence shift and fluorescence peak width,it was found that the core guiding region is composed by high quality unirradiated crystalline network,which preserves the original properties for laser and nonlinear optics applications.At the same time,the output couplers with reflectivity of 9%and 70%were used for waveguide laser generation.The laser threshold was as low as 98.8 m W and 114.2 m W for TM and TE polarization,respectively.The up-conversion spectra under different polarizations are obtained by rotating the half-wave plate,and the intensities of the emission line centered at 485 nm and 585 nm are measured with the angle of the polarizer.By using the direct femtosecond laser micro-machined depressed cladding waveguides in Pr:CaF₂crystal with different parameters:The lowest propagation loss of waveguides is found to be around 0.71 dB/cm under measuring at a wavelength of 633 nm with TE polarization.The relationship between waveguide losses and written parameters is experimentally determined,indicating that the most appropriate written power is about 150 m W.Additionally,single-mode transitions mode are achieved.The micro-photoluminescence mappings show that the original fluorescence properties in the waveguide region are very well preserved during the femtosecond laser processing.Planar optical waveguides in bonding crystals of Nd:YAG/Cr:YAG by C³⁺ion implantation method:The 633nm laser was used to test the transmission losses and transmission modes of the optical waveguide by end-coupling technology.A confocal microscope was used to test the micro-fluorescence and micro-Raman spectra of the planar optical waveguide,and to analyze the difference between the micro-fluorescence and micro-Raman properties of the waveguide region and the substrate region.