Reconfigurable directional coupler in lithium niobate crystal fabricated by three-dimensional femtosecond laser focal field engineering  

作　　者：Qian Zhang Meng Li Jian Xu Zijie Lin Haofeng Yu Min Wang Zhiwei Fang Ya Cheng Qihuang Gong Yan Li 

机构地区：[1]State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University and Collaborative Innovation Center of Quantum Matter, Beijing 100871, China [2]State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China [3]Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China

出　　处：《Photonics Research》2019年第5期503-507,共5页光子学研究（英文版）

基　　金：National Key R&D Program of China(2016YFA0301302,2018YFB1107205);National Natural Science Foundation of China(NSFC)(11474010,11627803,61590933)

摘　　要：For crystals, depressed cladding waveguides have advantages such as preservation of the spectroscopic as well as non-linear properties and the capability to guide both horizontal and vertical polarization modes, but fabrication is always quite time consuming. In addition, it is usually difficult to couple modes propagating in different depressed cladding waveguides through evanescent field overlap, so it is often required to dynamically reconfigure photonic waveguide devices using external fields for classical or quantum applications. Here, we experimentally demonstrate the single-scan femtosecond laser transverse writing of depressed cladding waveguides to form a 2 × 2 directional coupler inside lithium niobate crystal, which is integrated with two deeply embedded microelectrodes on both sides of the interaction region to reconfigure the coupling. By focal field engineering of the femtosecond laser, we specially generate a three-dimensional longitudinally oriented ring-shaped focal intensity profile composed of 16 discrete spots to simultaneously write the entire cladding region. The fabricated waveguides exhibit good single guided modes in two orthogonal polarizations at 1550 nm. By applying voltage to the deeply embedded microelectrodes fabricated with the femtosecond laser ablation followed by selective electroless plating, we successfully facilitate the light coupling from the input arm to the cross arm and thus actively tune the splitting ratio. These results open new important perspectives in the efficient fabrication of reconfigurable complex three-dimensional devices in crystals based on depressed cladding waveguides.For crystals, depressed cladding waveguides have advantages such as preservation of the spectroscopic as well as non-linear properties and the capability to guide both horizontal and vertical polarization modes, but fabrication is always quite time consuming. In addition, it is usually difficult to couple modes propagating in different depressed cladding waveguides through evanescent field overlap, so it is often required to dynamically reconfigure photonic waveguide devices using external fields for classical or quantum applications. Here, we experimentally demonstrate the single-scan femtosecond laser transverse writing of depressed cladding waveguides to form a 2 × 2 directional coupler inside lithium niobate crystal, which is integrated with two deeply embedded microelectrodes on both sides of the interaction region to reconfigure the coupling. By focal field engineering of the femtosecond laser, we specially generate a three-dimensional longitudinally oriented ring-shaped focal intensity profile composed of 16 discrete spots to simultaneously write the entire cladding region. The fabricated waveguides exhibit good single guided modes in two orthogonal polarizations at 1550 nm. By applying voltage to the deeply embedded microelectrodes fabricated with the femtosecond laser ablation followed by selective electroless plating, we successfully facilitate the light coupling from the input arm to the cross arm and thus actively tune the splitting ratio. These results open new important perspectives in the efficient fabrication of reconfigurable complex three-dimensional devices in crystals based on depressed cladding waveguides.

关 键 词：lithium NIOBATE CRYSTAL fabricated THREE-DIMENSIONAL FEMTOSECOND laser FOCAL field engineering vertical polarization modes 

分 类 号：O4[理学—物理]