-
当抽运功率设定在0 mW~53 mW范围时,激光器在窄线宽连续光下运转;将抽运功率调至78 mW(激光锁模阈值为53.5 mW),同时旋转或挤压腔内的PC,可以获得稳定的锁模脉冲运作,测得输出功率为4.1 mW。图 3是该抽运条件下脉冲激光器的输出特征。激光器谐振在1060.72 nm,光谱带宽为0.22 nm,如图 3a中蓝色实线所示。输出光谱右侧具有明显的凯利边带,证明激光器工作在反常色散区内的典型传统孤子。这种不对称的凯利边带主要归因于啁啾光纤光栅左侧强的滤波效应[24-25]。图 3b是示波器轨迹图,相邻脉冲序列间隔为53.59 ns,对应的重频为18.66 MHz。实验中激光器谐振腔分为两部分,腔长为5.5 m,左侧单模光纤约1.5 m为LP01模式谐振,右侧约4 m长的两模光纤为LP11模式谐振。故锁模激光器的基重频f与腔长的关系可以表达为:
图 3 a—光谱图 b—示波器轨迹图 c—自相关轨迹图 d—射频频谱图
Figure 3. a—optical spectrum b—pulse trace c—autocorrelation trace d—radio frequency spectrum
$f=\frac{c}{2\left(n_{\mathrm{LP}_{01}} L_{\mathrm{LP}_{01}}+n_{\mathrm{LP}_{11}} L_{\mathrm{LP}_{11}}\right)}$
(1) 式中,c为光速,nLP01为单模光纤LP01模式的有效折射率;nLP11为两模光纤中LP11模式的有效折射率;LLP01为LP01模式在腔内传输的长度,约为1.5 m;LLP11为LP11模式在腔内传输的长度,约为4 m;通过计算得到基重频f≈18.66 MHz,这与5.5 m腔长相匹配。输出脉冲的强度自相关轨迹如图 3c所示,自相关仪的扫描范围设定为150 ps,测得其半峰全宽为10.9 ps。采用射频谱分析仪在4 MHz范围内进行扫描,得到的射频图如图 3d所示,射频光谱位于18.66 MHz,说明激光器此时工作在基频锁模,其信噪比高达65 dB。此外,频谱仪扫描范围设定为500 MHz时,也没有观察到明显的强度调制。以上实验结果表明,该锁模激光器具有很好的工作稳定性。
图 4为锁模脉冲激光器输出功率与抽运功率的对应关系。基于线性拟合计算出激光器的效率为8.6%。当抽运功率调至140 mW时, 9.5 mW的输出功率被获得。值得一提的是,保持腔内偏振控制器状态不变,设定抽运功率高于53.5 mW时, 激光器能实现稳定的自启动。
为了观察激光器输出的光场特性,本文作者利用一个近红外CCD相机监测锁模激光器输出端光斑的强度分布。在输出端加一个偏振控制器去消除LP11模式的简并度,通过仔细地调节输出端的偏振控制器,径向偏振光束(TM01)和角向偏振光束(TE01)输出能被实现。环形中空状的强度分布被清楚地观察到,如图 5a和图 5f所示。本文作者在输出端与CCD之间放置一个起偏器作为检偏器,通过旋转起偏器的方向,输出模式的偏振特性被表征,环形强度分布的输出光斑通过检偏器后变为两瓣分布,两瓣的旋转方向和起偏器的旋转方向一致,若两瓣分布方向与检偏器轴的方向平行,则为径向偏振光,如图 5b~图 5e所示;若两瓣分布方向与检偏器轴的方向垂直,则为角向偏振光,如图 5g~图 5j所示。为了获得这两个输出模式的纯度,基于光纤弯曲法[26]进行纯度测量,经计算,径向偏振光的模式纯度为97.4%,角向偏振光的纯度为97.6%。
图 5 a~e—径向偏振光及其通过起偏器后的图案 f~j—角向偏振光及其通过起偏器后的图案
Figure 5. a~e—the pattern of radially polarized beam and its pattern after passing througha linear polarizer f~j—the pattern of azimuthally polarized beam and its pattern after passing througha linear polarizer
当保持78 mW的抽运功率不变,仅仅通过调节腔内的偏振控制器,腔内各个模式的损耗发生改变,在啁啾光纤光栅的共同作用下,可使得脉冲柱矢量激光器谐振在其它波长。如图 6a所示, 激光器的谐振波长能够从原先的1060.72 nm灵活地调谐到1061.17 nm、1061.96 nm、1062.80 nm、1064.13 nm、1065.30 nm、1066.04 nm,其波长调谐范围为5.32 nm,它被CFBG的反射带宽限制,可以使用更大带宽的CFBG提高波长调谐的范围。图 6b所示是锁模脉冲的重复频率与中心波长的关系。随着中心波长向长波方向移动,激光脉冲的重复频率从18.66 MHz减小到18.60 MHz,这种现象是由色散引起的,不同波长在腔内的群速度是不同的,在这个反常色散腔内,长波在腔内往返一次时间更长,即长波对应的重复频率更小。
波长可调谐的锁模柱矢量光纤激光器
Wavelength-tunable mode-locked cylindrical vector beam fiber laser
-
摘要: 为了在光纤激光器中获得具有中心波长可调谐的锁模脉冲柱矢量,采用半导体可饱和吸收镜和高反的啁啾光纤光栅作为腔镜,搭建了直腔掺镱脉冲光纤激光器,腔内插入长周期光纤光栅作为模式转化器件,进行了实验验证,取得了波长可调谐的柱矢量脉冲数据。结果表明,激光器工作在1060.72 nm时,光谱带宽0.22 nm,输出斜率效率为8.6%,锁模脉冲宽度为10.9 ps,重频18.66 MHz,锁模脉冲信噪比高达65 dB,同时获得了模式纯度超过97%的柱矢量光束;调节腔内的偏振控制器来改变腔内波长的损耗,可以实现锁模柱矢量脉冲的谐振波长在1060.72 nm~1066.04 nm连续可调。该研究为可调谐脉冲柱矢量光纤激光器研制提供了重要的参考价值。Abstract: To obtain wavelength-tunable pulse cylindrical vector beam, a linear cavity mode-locked doped-Yb fiber laser was experimentally constructed by using a chirped fiber grating and a semiconductor saturable absorption mirror. A long-period fiber grating was inserted into the cavity to serve as mode conversion device. The results show that when the mode-locked fiber laser operates at 1060.72 nm, the spectral width, slope efficiency, pulse width, fundamental repetition frequency, signal-to-noise ratio, and the cylindrical vector beam mode purity is 0.22 nm, 8.6%, of 10.9 ps, 18.66 MHz, 65 dB, >97%, respectively. Besides, the operating wavelength of the mode-locked fiber laser can be continuously tuned from 1060.72 nm to 1066.04 nm due to the changing of the cavity loss by adjusting the polarization controller. This study may provide an important reference for the development of wavelength-tunable pulsed cylindrical vector beam fiber laser.
-
Key words:
- lasers /
- wavelength tuning /
- mode-locked /
- cylindrical vector beam /
- ytterbium doped fiber laser
-
-
[1] ZHAN Q. Cylindrical vector beams: From mathematical concepts to applications[J]. Advances in Optics and Photonics, 2009, 1(1): 1-57. doi: 10.1364/AOP.1.000001 [2] NIZIEV V G, NESTEROV A V. Influence of beam polarization on laser cutting efficiency[J]. Journal of Physics, 1999, D32(13): 1455-1461. [3] BOZINOVIC N, YUE Y, REN Y X, et al. Terabit-scale orbital angular momentum mode division multiplexing in fibers[J]. Science, 2013, 340(6140): 1545-1548. doi: 10.1126/science.1237861 [4] HUANG L, GUO H, LI J, et al. Optical trapping of gold nanoparticles by cylindrical vector beam[J]. Optics Letters, 2012, 37(10): 1694-1696. doi: 10.1364/OL.37.001694 [5] YAO H, SHI F, WU Z Y, et al. A mode generator and multiplexer at visible wavelength based on all-fiber mode selective coupler[J]. Na-nophotonics, 2020, 9(4): 973-981. [6] ZHANG L, ZHANG W D, LU F F, et al. Azimuthal vector beam exciting silver triangular nanoprisms for increasing the performance of surface-enhanced Raman spectroscopy[J]. Photonics Research, 2019, 7(12): 1447-1453. doi: 10.1364/PRJ.7.001447 [7] KUGA T, TORII Y, SHIOKAWA N, et al. Novel optical trap of atoms with a doughnut beam[J]. Physical Review Letters, 1997, 78(25): 4713-4716. doi: 10.1103/PhysRevLett.78.4713 [8] GROSJEAN T, COURJON D, and SPAJER M. An all-fiber device for generating radially and other polarized light beams[J]. Optics Communications, 2002, 203(1/2): 1-5. [9] LIU T, CHEN S P, HOU J. Selective transverse mode operation of an all-fiber laser with a mode-selective fiber Bragg grating pair[J]. Optics Letters, 2016, 41(24): 5692-5695. doi: 10.1364/OL.41.005692 [10] LI H X, YAN K, ZHANG Y M, et al. Low-threshold high-efficiency all-fiber laser generating cylindrical vector beams operated in LP11 mode throughout the entire cavity[J]. Applied Physics Express, 2018, 11(12): 122502.1-122502.3 doi: 10.7567/APEX.11.122502 [11] PELEGRINA-BONILLA G, HAUSMANN K, SAYINC H, et al. Analysis of the modal evolution in fused-type mode-selective fiber couplers[J]. Optics Express, 2015, 23(18): 22977-22990. doi: 10.1364/OE.23.022977 [12] ZHAO Y, LIU Y, ZHANG L, et al. Mode converter based on the long-period fiber gratings written in the two-mode fiber[J]. Optics Express, 2016, 24(6): 6186-6195. doi: 10.1364/OE.24.006186 [13] ZHANG Y, LI H, DAI C, et al. All-fiber high-order mode laser using a metal-clad transverse mode filter[J]. Optics Express, 2018, 26(23): 29679-29686. doi: 10.1364/OE.26.029679 [14] MEIER M, ROMANO V, FEURER T. Material processing with pulsed radially and azimuthally polarized laser radiation[J]. Applied Physics, 2006, A86(3): 329-334. [15] MALKA V, FRITZLER S, LEFEBVRE E, et al. Electron acceleration by a wake field forced by an intense ultrashort laser pulse[J]. Science, 2002, 298(5598): 1596-1600. doi: 10.1126/science.1076782 [16] MARCEAU V, APRIL A, PICHE M. Electron acceleration driven by ultrashort and nonparaxial radially polarized laser pulses[J]. Optics Letters, 2012, 37(13): 2442-2444. doi: 10.1364/OL.37.002442 [17] ZHOU Y, LIN J, ZHANG X, et al. Self-starting passively mode-locked all fiber laser based on carbon nanotubes with radially pola-rized emission[J]. Photonics Research, 2016, 4(6): 327-330. doi: 10.1364/PRJ.4.000327 [18] WAN H, WANG J, ZHANG Z, et al. High efficiency mode-locked, cylindrical vector beam fiber laser based on a mode selective coupler[J]. Optics Express, 2017, 25(10): 11444-11451. doi: 10.1364/OE.25.011444 [19] TAO R X, LI H X, ZHANG Y M, et al. All-fiber mode-locked laser emitting broadband-spectrum cylindrical vector mode[J]. Optics and Laser Technology, 2020, 123: 105945. doi: 10.1016/j.optlastec.2019.105945 [20] TALAVERANO L, ABAD S, JARABO S, et al. Multiwavelength fiber laser sources with Bragg-grating sensor multiplexing capability[J]. Journal of Lightwave Technology, 2001, 19(4): 553-558. doi: 10.1109/50.920854 [21] ZHU J, YANG Y, ZHANG Z, et al. Weakly-coupled MDM-WDM amplification and transmission based on compact FM-EDFA[J]. Journal of Lightwave Technology, 2020, 38(18): 5163-5169. doi: 10.1109/JLT.2020.3001008 [22] YE M, YU Y, ZOU J, et al. On-chip multiplexing conversion between wavelength division multiplexing-polarization division multiplexing and wavelength division multiplexing-mode division multiplexing[J]. Optics Letters, 2014, 39(4): 758-761. doi: 10.1364/OL.39.000758 [23] ALLEGRE O J, PERRIE W, EDWARDSON S P, et al. Laser microprocessing of steel with radially and azimuthally polarized femtosecond vortex pulses[J]. Journal of Optics, 2012, 14(8): 085601. doi: 10.1088/2040-8978/14/8/085601 [24] HAN D D, LIU X M. Sideband-controllable mode-locking fiber laser based on chirped fiber Bragg gratings[J]. Optics Express, 2012, 20(24): 27045-27050. doi: 10.1364/OE.20.027045 [25] LI J, WANG Y, LUO H, et al. Kelly sideband suppression and wavelength tuning of a conventional soliton in a Tm-doped hybrid mode-locked fiber laser with an all-fiber Lyot filter[J]. Photonics Research, 2019, 7(2): 103-109. doi: 10.1364/PRJ.7.000103 [26] SUN B, WANG A T, XU L X, et al. Low-threshold single-wavelength all-fiber laser generating cylindrical vector beams using a few-mode fiber Bragg grating[J]. Optics Letters, 2012, 37(4): 464-466. doi: 10.1364/OL.37.000464