The Property Investigation Of Microstructured Optical Waveguides Based On Femtosecond Laser Writing

Due to better guiding properties and design degrees of freedom, microstructuredoptical waveguides (MOWs), which are based on photonic crystals, have been widelyused in advanced front research of optical communications and optoelectronics.Compared with traditional techniques, femtosecond direct laser writing (FDLW) hasthe advantages of simple operation and high machining accuracy. Thus it is verysignificant to fabricate MOWs by FDLW and investigate their properties.Based on the summary analysis of theoretical research methods and thefabrication process of MOWs, we have investigated the mode characteristics of37-core waveguide with large mode area by numerical simulations and experiments.Then the bandgap property of anti-resonant reflecting optical waveguide (ARROW)was studied by using the plane wave expansion method, which provides theoreticalbasis for further developing femtosecond laser writing. The specific accomplishmentsof this thesis are as follows:1. In the range of refractive index contrast of the type I waveguide fabricated byfemtoscond laser, we analyzed the mode characteristics of evanescently coupled37-core waveguide in depth by the vectorial finite element method which hassatisfactory numerical precision and calculational efficiency. And the effects of thesingle core’s numerical apertures (NAs) and spacing on the mode characteristics of the37-core waveguide were investigated. It was found that the mode field areas of thefundamental mode LP01were all larger than577μm2which is the effective area whenthe NA equals0.1with the diameter of each single core of2μm, the core spacing of5μm and the operating wavelength of980nm.2. We fabricated37-core waveguides by writing37parallel type I (a type ofwaveguide with laser-induced soft refractive index changes) lines inside bulk fusedsilica with low-repetition-rate (1kHz) femtosecond laser under different writingconditions and different spacing. The measured near-field profiles showed that thewaveguide supported both a single mode (LP01) and two modes (LP01and LP11) andthe effective areas of LP01mode at a scan velocity of600μm/s with5μm spacing are1190μm2(2.5μJ pulse energy) and790μm2(5.0μJ pulse energy) respectively, whichverify the simulation results of37-core waveguides.3. We calculated and analyzed the influence of the parameters on the bandstructure and the bandgap in the ARROW by the plane wave expansion method. Numerical simulations show that the bandgap properties of ARROW are related withthe single track’s diameter and refractive index, the filling ratio and the pitch. With thefixed pitch8μm and refractive index of laser-induced single track2.418, the bandgap islarger when the single track’s diameter is in the range of3.0~7.0μm in a GLS glasswith refractive index of2.398. The study may be useful in femtosecond laser writingARROWs in GLS glass.