Two-dimensional (2D) materials are crystals with one to a few layers of atoms and are being used in many fields such as optical modulator, photodetector, optical switch, and ultrafast lasers. Their exceptional optoelectronic and nonlinear optical properties make them as a suitable saturable absorber for laser cavities. This review focuses on the recent progress in ultrafast laser use 2D materials as a saturable absorber. 2D materials traditionally include graphene, topological insulators, transition metal dichalcogenides, as well as new materials such as black phosphorus, bismuthene, antimonene, and MXene. Material characteristics, fabrication techniques, and nonlinear properties are also introduced. Finally, future perspectives of ultrafast lasers based on 2D materials are also addressed.
In this work, a model based method is applied to phase measuring deflectometry, named modal phase measuring deflectometry. The height and slopes of the surface under test are represented by mathematical models and updated by optimizing the model coefficients to minimize the discrepancy between the reprojection in ray tracing and the actual measurement. The pose of the screen relative to the camera is pre-calibrated and further optimized together with the shape coefficients of the surface under test. Simulations and experiments are conducted to demonstrate the feasibility of the proposed approach.
Based on the avalanche effect of avalanche transistor, a kind of ultra-wideband nanosecond pulse circuit has been designed, whose frequency, pulse width and amplitude are tunable. In this paper, the principle, structure and selection of components' parameters in the circuit are analyzed in detail. The circuit generates periodic negative pulse, whose pulse full width is 890ps and pulse amplitude is -11.2V in simulation mode. By setting up circuit for experiment and changing parameters properly, a kind of ultra-wideband pulse with pulse width of 2.131ns and pulse amplitude of -9.23V is achieved. With the features such as simple structure, stable and reliable performance and low cost, this pulse generator is applicable to ultra-wideband wireless communication system.
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