1.School of Science, Changchun University of Science and Technology, Changchun 130022, China 2.Shandong Provincial Engineering and Technical Center of Light Manipulations, Shandong Provincial Key Laboratory of Optics and Photonic Device, School of Physics and Electronics, Shandong Normal University, Jinan 250358, China 3.School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 101407, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11774038, 11874056, 11474039, 11274053) and the Taishan Scholar Project of Shandong Province, China
Received Date:20 September 2019
Accepted Date:24 October 2019
Published Online:05 February 2020
Abstract:Supercontinuum (SC), as one of the most spectacular phenomena occurring in the nonlinear process of intense femtosecond laser-material interaction, has attracted considerable interest. The broadband SC sources have a variety of applications including the spectroscopy, fluorescence microscopy, remote sensing, and generation of few-cycle pulses. Over the last few decades, the SC has been extensively investigated in various optical media, including liquid, gas, and solid. Especially, ultrabroadband SC sources have achieved remarkable development in the photonic crystal and micro-structured fibers. Even so, the generation of the SC with high brightness, high spatiotemporal coherence and good maneuverability, is still a challenging topic. The SC generation from femtosecond filamentation is a unique white light source with high pulse energy, high brightness and high spatiotemporal coherence, whose spectral range spans from ultraviolet to mid-infrared. In recent years, numerous studies have been conducted to optimize the filamentation and SC. The control of filamentation such as the filament length, number and position, as well as the generation of the ultra-broadband spectrum with high spectral energy density has been realized. To date, the optimal control of SC has been realized by the spatial modulation or time-domain shaping of the femtosecond laser pulse. However, there is no report on the control of SC generation and filamentation by spatiotemporally modulating the femtosecond laser pulses as far as we know. In this work, a spatiotemporal modulation for the femtosecond laser pulse is proposed, which combines the spatial modulation by using microlens array (MLA) and the laser pulse shaping based on liquid crystal spatial light modulator. We investigate the control of the SC generation from the filamentation of the spatiotemporally modulated femtosecond laser pulses in fused silica by using the feedback optimal control based on genetic algorithm. In our experiments, with the increase of the iterative generation, the cut-off wavelength in the blue-side extension of the SC becomes shorter gradually, and the spectral intensity of the SC increases significantly. After the eighth iteration, the increase of the spectral intensity slows. With the number of iterations increasing further, the intensity and broadening of SC spectrum will no longer apparently change. Hence, the feedback optimization control of spectral intensity of SC is realized, and the SC with controllable spectral intensity in a certain range is obtained. The maximum intensity variation of SC is more than three times. By integrating the spectral intensities of SC for different iterative generations, we characterize the increase trend of SC conversion efficiency. During the first few iterations, the conversion efficiency increases rapidly. Then it increases slowly after eighth generations and reaches its maximum after several generations (10th generation). The conversion efficiency has a similar evolution to the spectral intensity of the SC. To explain the physical mechanism, the initial envelope of the shaping pulse with typical iteration generation is calculated. It can be concluded that the spatial modulation of MLA allows for higher incident laser energy and for more filaments’ generation, which increases the energy of SC radiation directly. The peak intensity and envelope distribution of time domain pulse are the main factors affecting the spectral intensity and broadening the SC. Keywords:femtosecond filamentation/ supercontinuum/ laser pulse shaping/ feedback control
图 1 实验装置示意图(M, 800 nm高反镜; G, 光栅; CL, 柱面镜; LC-SLM, 液晶空间光调制器; MLA, 微透镜阵列; L, 平凸透镜; FS, 熔融石英; DM, 二向色镜; IS, 积分球)
图 2 (a)时空调制飞秒脉冲成丝的超连续辐射随迭代代数的演化; (b)典型迭代代数对应的超连续辐射光谱
图 3 超连续辐射光谱积分(400?700 nm波段)随迭代代数的变化
图 4 典型迭代代数对应整形激光脉冲初始包络, 图中迭代代数依次为(a) 1, (b) 3, (c) 8和(d) 17