Experimental Investigation Of Optical Waveguides Fabricated By Femtosecond Laser In Glass And Laser Crystals

As one of the basic components in integrated optics and optoelectronics, optical waveguide has been applied in many fields such as optical telecommunications and optical signal processing. Femtosecond laser writing optical waveguide has been one of the most powerful techniques, as it shows a number of advantages such as simpler device production equipment, avoiding photolithography and intrinsically a three-dimensional (3D) technique.The fabrication of waveguide in glasses and crystals has been paid more attention in these years. Silicate glass is excellent candidate for various passive and active optical devices applied in optoelectronics, as its refractive index is similar with optical fibers. And miniaturized waveguide lasers and amplifiers with high optical gain and output power can be obtained in rare-earth doped crystalline structures with high optical damage thresholds.In this thesis, optical waveguides are fabricated in the silicate glass, neodymium-doped KGd(WO₄)₂ (Nd:KGW) and neodymium-doped yttrium lithium fluoride (Nd:YLiF₄). The formation mechanisms of the waveguides are analyzed, the index profile reconstruction is numerically calculated.The propagation of light in waveguides array is investigated. In this dissertation, the research works have been summarized as below.Optical waveguides are fabricated in the silicate glass. Combined with the atomic force microscopy (AFM) experiments, the formation mechanism of the waveguide is discussed. The origin of the waveguides asymmetry is analyzed. And the spatial intensity distribution of the femtosecond laser with different NA and focusing depths has been calculated. This will be helpful in the femtosecond laser micromachining. One dimensional 1×5 waveguides array and two dimensional 3×3 waveguides array are also fabricated in the silicate glass. According to the output pattern in waveguides array, the coupling constant in one dimensional waveguides array and the horizontal, vertical directions and diagonal directions coupling constants are calculated in the two dimensional waveguide arrays based on the coupled-mode theory.Neodymium-doped KGd(WO₄)₂ crystal exhibits a low lasing threshold with a broad and strong absorption band, as well as strong Raman conversion properties which makes it exceptionally suitable as an excellent gain medium for effective laser media.Waveguides with low propagation loss and symmetric near field mode distribution are fabricated in neodymium-doped KGd(WO₄)₂ by using double lines method. The affects of pulse energy, writing speed and distance between the lines on the near field mode distribution symmetry and loss are analyzed. Combined with the atomic force microscopy (AFM) experiments, micro-luminescence and micro- Raman spectra, the mechanisms for femtosecond laser induced structural change and the luminescence and Raman characteristic of the waveguide are discussed. Micro-luminescence and micro-Raman spectra are also performed to reveal the preservation of the fluorescence and Raman properties in the waveguides, which implies strong potential for high-power integrated laser actions and the integrated self-Raman laser sources based on the Nd:KGW waveguides. A new method of index profile reconstruction is presented, and the mode distribution is numerically calculated by the beam propagation method, which shows reasonable agreement with the experimental modal distributions.Neodymium-doped yttrium lithium fluoride (Nd:YLiF₄) is an excellent candidate for low-threshold continuous-wave (CW), mode-locked laser operation due to its many advantages, such as large fluorescence linewidth, natural birefringence and low thermal lensing.1×11 planar waveguides array is first fabricated in Neodymium-doped yttrium lithium fluoride (Nd:YLiF₄) by using double lines method. Based on the output pattern in waveguides array, the coupling constant is calculated.Based on the AFM experiments, the interaction between femtosecond laser and Nd:YLiF₄ crystal are demonstrated, the formation of the periodic voids are analyzed. According to the near field mode distribution, the index profile of the waveguide is reconstructed. Micro-luminescence spectra show that there is no Nd3+ fluorescence quenching in the waveguides and waveguids array during the formation processing, which indicates a fairly good potential for further laser actions and integrated multi-beam laser source in a compact device.