The Study On Waveguides Fabricated In Transparent Medium By Direct Femtosecond Laser Inscription

In integrated photonics, optical waveguides are basic components that are defined as cores of high refractive index with claddings of low refractive index. The functional guiding devices such as gratings, couplers, splitters, optical amplifiers and laser oscillators can be manufactured on chip-scale wafers of diverse materials based on waveguide structures. Among these devices, optical waveguide power splitters, which can switch optical signal from single input to multiple outputs, play as essential elements for integrated photonic circuits. A few techniques have been applied for waveguides fabrication, including liquid-phase epitaxy, ion-beam implantation/irradiation, ion/proton exchange, and femtosecond laser inscription. Among these techniques, femtosecond laser micro-processing has been developed to be one of the most efficient and low-cost ways for waveguides fabrication in diverse materials. Femtosecond laser micro-processing is widely applied because of its unique advantages such as little damage around the processing area, ultrahigh precision and ability for direct three-dimensional (3D) micromachining. Waveguides can be directly fabricated by focusing the laser beam beneath the sample surface, which will not cause any damage on the surface. Positive changes of refractive index emerge inside or beside the laser track, through which waveguides of diverse structures will be induced. Various optical structures including tubular waveguides, symmetric waveguides, couplers and splitters have been successfully achieved in fused silica and bulk PMMA based on Type-I waveguides.The work of this paper is focused on some aspects underneath:(1) The Type-I waveguide and 1×2,1x4 beam splitters were fabricated in fused silica by direct femtosecond laser inscription. The research studied the impact on waveguide end-face shape and loss with different repetition rates, pulse energy, slit and annealing treatment at scan speed of 5 mm/s. The Type-I waveguide with the lowest insertion loss of 2.84 dB was achieved at the conditions of 50 KHz pulse rate,5mm/s scan speed and 2.28 μJ pulse energy. Based on this condition, the 1 x2 beam splitter with insertion loss of 7.02 dB and the 1 x4 beam splitter with insertion loss of 8.18 dB were produced.(2) The Type-I waveguide and 1x2,1x4 beam splitters were fabricated in bulk PMMA by direct femtosecond laser inscription. The research studied the impact on waveguide end-face shape and loss with different pulse energy and scan speed at repetition rate of 1 MHz. The Type-Ⅰ waveguide with the lowest insertion loss of 4.32 dB was achieved at the conditions of 1MHz reptition rate,4mm/s scan speed and 185 nJ pulse energy. Based on this condition, the 1 ×2 beam splitter with splitting ratio of 1:1 and the 1×4 beam splitter with splitting ratio of 1:0.93:1:0.95 were produced. As different refractive index occurred at different scan speeds, the beam splitter with the splitting ratio of 1:1.96 was achieved, using the scan speed of 4mm/s and 6mm/s, respectively. And the beam splitters with different splitting ratios were demonstrated...