Logic gates are circuits that perform logical operations such as OR, AND, NOT, NOR and NAND. Any complex logic circuit can be composed of these logic gates. The future communication network will develop towards the goal of the all-optical network. All-optical logic gate devices are important components in fast and high-capacity all-optical information processing such as all-optical addressing identification, optical packet switching, and photon computation. At present, all-optical logic gates are mainly made of waveguide fiber, which is difficult to be integrated on small integrated chips. The defect waveguide in the photonic crystal has the characteristics of small size and strong light control ability, which makes it a key method to design a miniaturized optical logic gate. So far, all-optical logic gates are mostly designed and integrated on silicon-based photonic crystal materials. However, two-photon absorption and third-order nonlinearity of silicon will lead to nonlinear loss and signal crosstalk, resulting in low extinction of silicon logic gate. Lithium niobate has unique electro-optic, acousto-optic, piezoelectric and other physical properties, so it is an ideal material for developing integrated optical path in the future. The development of all-optical logic gate based on lithium niobate optical platform is of great significance to integrated optics and all-optical networks.

Figure:  Schematic diagram of half-adder for lithium niobate thin film photonic crystal based on all-optical logic gate

Based on cylindrical two-dimensional lithium niobate photonic crystal, We have studied the dual-light interference process of a W1 defect waveguide and the influence of L2 defect micro-cavities of lithium niobate photonic crystal on the waveguide light. A novel design of an all-optical logic gate is proposed, which combines a two-dimensional photonic crystal defect waveguide with a photonic crystal defect micro-cavity. This is a new type of all-optical logic gate, which is based on the existing silicon-based photonic crystal all-optical logic gate. It is designed by taking advantage of lithium niobate material in photonic integrated design and based on double optical interference as well as combining with micro-cavity structure. The defect cavity is used to improve the extinction ratio of different logic output signals. The designed all-optical logic gate has a maximum extinction ratio of 23dB. Then, according to the new basic logic gate device, the structure of all-optical half adder is designed, and the basic functions of logic operation are successfully realized.

Future research work will be extended in the design of thin film photonic crystals, and it is expected to realize new all-optical logic gates and logic operations on lithium niobate thin film photonic crystals in the future.

Article：Chenghao Lu, Bing Zhu, Chuanyi Zhu, Licheng Ge, Yi’an Liu, Yuping Chen* and Xianfeng Chen. All-optical logic gates and a half-adder based on lithium niobite photonic crystal micro-cavities. Chinese Optics Letters. 17(7), 072301(2019) （Cover Paper）

Link：https://www.osapublishing.org/col/abstract.cfm?uri=col-17-7-072301