Experimental Investigation Of Fabricating Optical Waveguides With High Repetition Rate Femtosecond Laser In Fused Silica

As the basic component of integrated optics and optoelectronics, opticalwaveguide has been widely used. Femtosecond laser writing technique has been oneof the most important waveguide fabricating techniques for its unique advantages,such as simpler process, lower cost, applicable to different mediums and able tomicromachine geometrically complex structures in three dimensions. As the lasersource of traditional femtosecond laser micromachining, low repetition rate Ti:Sapphire laser limited the machining efficiency, and limited this technique in actualapplication. The new generation femtosecond laser, photonic crystal fiber laser, hashigh repetition frequency, which can effectively solve this problem. Heataccumulation effects will be notable as the laser pulse repetition rate is increased.How to utilize the heat effects is crucial to high repetition rate femtosecond lasermicromachining. Besides, the waveguides fabricated by femtosecond laser has someweak points, such as poor cross-sectional symmetry, uncontrollable beam mode, largetransmission loss, small mode area.This thesis focuses on utilizing heat accumulation, improving cross-sectionalsymmetry, realizing control of beam mode and magnifing mode area. Experimentalinvestigation of fabricating optical waveguides in fused silica is carried out based on ahigh repetition rate photonic crystal fiber femtosecond laser. Main points including:1. The application of heat accumulation for52MHz femtosecond lasermicromachining in fused silica was researched. Parameters and formation mechanismof two typical structures was analyzed. Single-fine-line and double-fine-linewaveguides were fabricated. The influence of laser parameters on waveguide wasanalyzed, including numerical aperture, focus depth and writing mode. Thesingle-fine-line waveguides had polarization selectivity, and achieved fundamentalmodes for laser of653nm,980nm and1040nm. The refractive index change wasabout5.4×10^-5, and the transmission loss was about4.6dB/cm. The double-fine-linewaveguides could output two beams with different intensity. Formation mechanism ofwater-droplet structure was analyzed based on bubble nucleation theory.2. Femtosecond laser with variable （650kHz-2MHz） repetition rate has beenapplied to microfabricate fused silica. Parameters and formation mechanism of three typical structures was analyzed. Threshold energy for type Ⅱ damage has beenmeasured as a function of the pulse repetition rate and the scan speed. The role ofthermal diffusion and heat accumulation effects in forming waveguide isdemonstrated. Double-line waveguides with the optimal parameters have been written,where the mode guided is fundamental mode and nearly circular. The polarizationsensitivity was verified through simulation. Hexagonal microstructure waveguidewith elliptical cells has been made, where the mode has a nearly Gaussian intensityprofile at1040nm. The microstructure waveguide has a large mode area about247.48μm2and single polarization propagation property with extinction ratio about9.05. The numerical aperture of the waveguide is about0.017.