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飞秒激光成丝超连续辐射具有高强度和高时空相干性等优点, 作为一种超宽带光源在很多领域都具有广泛的应用前景. 本文提出一种结合微透镜阵列的空间调制和基于液晶空间光调制器的时域整形的飞秒激光脉冲整形方式, 利用基于遗传算法的反馈优化控制, 实现了飞秒激光在熔融石英中成丝产生的超连续辐射强度的调制, 得到了在一定范围内光谱强度可控的超连续辐射光谱; 光谱的能量密度可以从0.03 μJ/nm调制到0.09 μJ/nm, 其能量密度变化达到了初始值的3倍. 计算了典型迭代代数对应的整形脉冲时域包络, 分析了超连续光谱随迭代代数的演化趋势, 结果表明, 脉冲包络的峰值强度和波形分布是影响超连续光谱展宽和强度的主要物理原因.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.
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Keywords:
- femtosecond filamentation /
- supercontinuum /
- laser pulse shaping /
- feedback control
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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)
Fig. 1. Schematic diagram of experimental setup. M, 800 nm HR mirror; G, grating; CL, cylindrical lens; LC-SLM, liquid crystal spatial light modulator; MLA, microlens array; L, plano-convex lens; FS, fused silica; DM, dichroic mirror; IS, integrating sphere.
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
Fig. 2. (a) Evolution trend of supercontinuum generation by the spatiotemporal modulation femtosecond pulse filamentation with the increasing of iterative generation; (b) supercontinuum spectra of several typical iterative generations.
图 3 超连续辐射光谱积分(400−700 nm波段)随迭代代数的变化
Fig. 3. Spectral intensity integration (400−700 nm) of the supercontinuum as a function of the iterative generation.
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
Fig. 4. Initial envelopes of shaped laser pulse with typical iteration generations of (a) 1, (b) 3, (c) 8 and (d) 17, respectively.
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[1] Chin S L, Hosseini S A, Liu W, Luo Q, Théberge F, Aközbek N, Becker A, Kandidov V P, Kosareva O G, Schroeder H 2005 Can. J. Phys. 83 863
Google Scholar
[2] Alfano R R, Shapiro S L 1970 Phys. Rev. Lett. 24 584
Google Scholar
[3] Alfano R R, Shapiro S L 1970 Phys. Rev. Lett. 24 592
Google Scholar
[4] Petersen C R, Prtljaga N, Farries M, Ward J, Napier B, Lloyd G R, Nallala J, Stone N, Bang O 2018 Opt. Lett. 43 999
Google Scholar
[5] Dupont S, Petersen C, Thøgersen J, Agger C, Bang O, Keiding S R 2012 Opt. Express 20 4887
Google Scholar
[6] Poudel C, Kaminski C F 2019 J. Opt. Soc. Am. B 36 A139
Google Scholar
[7] Riedle E, Bradler M, Wenninger M, Sailer C F, Pugliesi I 2013 Faraday Discuss. 163 139
Google Scholar
[8] Petersen C R, Moselund P M, Huot L, Hooper L, Bang O 2018 Infrared Phys. Technol. 91 182
Google Scholar
[9] Kasparian J, Rodriguez M, Méjean G, Yu J, Salmon E, Wille H, Bourayou R, Frey S, Mysyrowicz A, Sauerbrey R 2003 Science 301 61
Google Scholar
[10] Dudley J M, Genty G, Coen S 2006 Rev. Mod. Phys. 78 1135
Google Scholar
[11] Price J H V, Feng X, Heidt A M, Brambilla G, Horak P, Poletti F, Ponzo G, Petropoulos P, Petrovich M, Shi J, Ibsen M, Loh W H, Rutt H N, Richardson D J 2012 Opt. Fiber Technol. 18 327
Google Scholar
[12] Labruyère A, Tonello A, Couderc V, Huss G, Leproux P 2012 Opt. Fiber Technol. 18 375
Google Scholar
[13] Lefort C, O’Connor R P, Blanquet V, Magnol L, Kano H, Tombelaine V, Lévêque P, Couderc V, Leproux P 2016 J. Biophotonics 9 709
Google Scholar
[14] Budriunas R, Stanislauskas T, Adamonis J, Aleknavi Cius A, Veitas G, Gadonas D, Balickas S, Michailovas A, Varanavicius A 2017 Opt. Express 25 5797
Google Scholar
[15] Hakala T, Suomalainen J, Kaasalainen S, Chen Y 2012 Opt. Express 20 7119
Google Scholar
[16] Chin S, Petit S, Borne F, Miyazaki K 1999 Jpn. J. Appl. Phys. 38 L126
Google Scholar
[17] Alfano R R 2006 The Supercontinuum Laser Source: Fundamentals with Updated References (New York: Springer) pp473–480
[18] Garejev N, Tamošauskas G, Dubietis A 2017 J. Opt. Soc. Am. B 34 88
Google Scholar
[19] Dubietis A, Tamošauskas G, Šuminas R, Jukna V, Couairon A 2017 Lith. J. Phys. 57 113
[20] Lu C H, Tsou Y J, Chen H Y, Chen B H, Cheng Y C, Yang S D, Chen M C, Hsu C C, Kung A H 2014 Optica 1 400
Google Scholar
[21] He P, Liu Y Y, Zhao K, Teng H, He X K, Huang P, Huang H D, Zhong S Y, Jiang Y J, Fang S B, Hou X, Wei Z Y 2017 Opt. Lett. 42 474
Google Scholar
[22] Camino A, Hao Z Q, Liu X, Lin J Q 2014 Opt. Lett. 39 747
Google Scholar
[23] Li D W, Xi T T, Zhang L Z, Tao H Y, Gao X, Lin J Q, Hao Z Q 2017 Opt. Express 25 23910
Google Scholar
[24] Li D W, Zhang L Z, Xi T T, Hao Z Q 2019 J. Opt. 21 065501
Google Scholar
[25] Curtis J E, Koss B A, Grier D G 2002 Opt. Commun. 207 169
Google Scholar
[26] Weiner A M 2011 Opt. Commun. 284 3669
Google Scholar
[27] Mendoza-Yero O, Carbonell-Leal M, Doñate-Buendía C, Mínguez-Vega G, Lancis J 2016 Opt. Express 24 15307
[28] Hong Z F, Zhang Q B, Ali Rezvani S, Lan P F, Lu P X 2016 Opt. Express 24 4029
Google Scholar
[29] Li P P, Cai M Q, Lü J Q, Wang D, Liu G G, Qian S X, Li Y, Tu C, Wang H T 2016 AIP Advances 6 125103
Google Scholar
[30] Heck G, Sloss J, Levis R J 2006 Opt. Commun. 259 216
Google Scholar
[31] Chen A M, Li S Y, Qi H X, Jiang Y F, Hu Z, Huang X R, Jin M X 2017 Opt. Commun. 383 144
Google Scholar
[32] 常峻巍, 许梦宁, 王頔, 朱瑞晗, 奚婷婷, 张兰芝, 李东伟, 郝作强 2019 光学学报 39 0126021
Google Scholar
Chang J W, Xu M N, Wang D, Zhu R H, Xi T T, Zhang L D, Li D W, Hao Z Q 2019 Acta Opt. Sin. 39 0126021
Google Scholar
[33] Zhdanova A A, Shen Y J, Thompson J V, Scully M O, Yakovlev V V, Sokolov A V 2018 J. Mod. Opt. 65 1332
Google Scholar
[34] Ackermann R, Salmon E, Lascoux N, Kasparian J, Rohwetter P, Stelmaszczyk K, Li S, Lindinger A, Wöste L, Béjot P 2006 Appl. Phys. Lett. 89 171117
Google Scholar
[35] Thompson J V, Zhokhov P A, Springer M M, Traverso A J, Yakovlev V V, Zheltikov A M, Sokolov A V, Scully M O 2017 Sci. Rep. 7 43367
Google Scholar
[36] Zhan L D, Xu M N, Xi T T, Hao Z Q 2018 Phys. Plasmas 25 103102
Google Scholar
[37] Xu M N, Zhan L D, Xi T T, Hao Z Q 2019 J. Opt. Soc. Am. B 36 G6
Google Scholar
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