Experimental Investigation Of Femtosecond Laser Direct Writing Of Stressed Optical Waveguides

Optical waveguide is one of the most important basic components in integratedoptics, and widely used in optical communication and signal processing as opticalsignal propagation channels and bridges connected with other devices. Theresearchers have dedicated themselves to developing the effective methods tofabricate waveguides with excellent performances. Femtosecond laser direct writing(FLDW) is a new technology of waveguide fabrication developed in the past tenyears. Femtosecond laser pulses, when focused inside transparent materials, canachieve high intensities to give rise to nonlinear multi-photon absorption andavalanche ionization. Energy is deposited in the vicinity of the focal spot, andpermanent modifications of the structure of the transparent materials are occurred.Such photo-induced modifications often result in a change in the refractive index ofthe materials that is highly confined to the focal volume of a lens or microscopeobjective. By translating the transparent materials with respect to the focal volume itis possible to fabricate optical waveguides with high spatial precision in a variety ofoptical materials. This technique enables the reliable, fast, and low-cost fabrication ofphotonic devices without the design and fabrication of a complex mask in contrast tomost conditional techniques such as ionic exchange, diffusion or lithographic. In thisdissertation, the main contents are classified as follows:Phosphate glasses can incorporate large concentrations of rare earth ions, makingthem an ideal host material for fabricating compact high-gain waveguide lasers andamplifiers. However, many phosphate glasses will typically exhibit or favor negativechanges to the index of refraction inside the fs-laser irradiated region. As aconsequence, high quality single-mode waveguides cannot be easily fabricated byfemtosecond laser direct writing. Raman microscopy was used to analyze the laserwritten track in neodymium-doped phosphate glass. The Raman peak associated withP-O bonds exhibits a negative shift in wavenumber, which reveals longer P-O bondsand an expansion of the glass network in the fs-laser irradiated region. We inscribed a buried type-II waveguide in neodymium-doped phosphate glass by writing twoparallel lines in close separation in view of the stress-induced increase in refractiveindex neighboring the track. Microluminescence spectra reveal that the Nd3+fluorescence property is not significantly affected by waveguide formation process,which indicates that the inscribed waveguide is a good candidate for activewaveguide.Terbium gallium garnet has twice the Verdet constant of Terbium-doped glass, andis widely used because of its Faraday rotation. The rotation of plane of polarizationcannot be occurred in double line waveguide under applied magnetic field due to thephase mismatch of TE mode and TM mode. In order to fabricate Faraday rotatingwaveguides, the circular depressed-cladding structure is adopted. The double line andcircular depressed-cladding waveguides are candidates for waveguide polarizer andmagneto-optic device respectively.Waveguide laser has low laser threshold ascribed to good overlap between pumpand signal light. A dielectric crystalline waveguide laser has high optical gain.Nd:YVO4is one of the most efficient laser host crystal. We inscribed waveguide inNd:YVO4crystal and demonstrated waveguide laser.Depressed circular cladding waveguide consists of a number of parallel tracks. Inthose structures, multiple scans are necessary to form the tubular closed-formstructures. In order to improve efficiency, we introduced a novel method of single-stepdirect writing of stressed-induced optical waveguide with circular cladding inphosphate glasses by the use of focused fs hollow laser beams. We demonstrated thewriting of waveguides with different size by changing beam parameters.