1.College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China 2.College of Mechatronics and Control Engineering, Shenzhen University, Shenzhen 518060, China 3.College of Liberal Arts and Sciences, National University of Defense Technolog, Changsha 410073, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11704260, 61775146, 61773266, 11904240), the Science and Technology Research and Development Foundation of Shenzhen, China (Grant No. JCYJ20170818144254033, JCYJ20190808141011530), the Natural Science Foundation of Guangdong Province, China (Grant No. 2018A030310637) and the Start-up project of scientific research for new teachers of Shenzhen University, China (Grant No. 2017020)
Received Date:30 December 2019
Accepted Date:15 February 2020
Published Online:05 May 2020
Abstract:We demonstrate a bismuth (Bi) saturable absorber (SA) for generating ultrafast pulse. The Bi SA is fabricated by the Bi film deposited on the surface of microfibers through using magnetron sputtering. Its nonlinear optical properties are investigated. The as-prepared Bi SA has outstanding nonlinear absorption property demonstrated by the open aperture (OA) Z-scan system at 1500 nm and balanced twin-detector method at 1560 nm. The nonlinear optical property of Bi SA shows that the modulation depth, the nonsaturable losses, and the saturable intensity at 1.5 μm are 14% and 79%, and 0.9 MW/cm2, respectively. Besides, the closed aperture (CA) Z-scan measurement is also implemented to estimate the nonlinear refractive index of Bi film. The Bi film shows that the typical CA/OA curve possesses the feature of peak-valley profile, meaning that the sample with a negative nonlinear refractive index is self-defocusing. In our experiments, the parameters of the nonlinear absorption coefficient β and the nonlinear refractive index n2 are estimated at about 2.38 × 10–4 cm/W and –1.47 × 10–9 cm2/W according to the actual experimental data points, respectively. To further investigate its nonlinear optical property, the microfiber-based Bi SA is embedded into an erbium-doped fiber laser with a typical ring cavity structure. Based on the Bi SA device, the stable ultrafast pulses are generated at 1.5 μm with the pulse width of 357 fs, the output power of 45.4 mW, corresponding to the pulse energy of 2.39 nJ, and the signal-to-noise ratio is 84 dB. The stable soliton pulses emitting at 1563 nm are obtained with a 3-dB and 6-nm spectral bandwidth. The experimental results suggest that the microfiber-based Bi SA prepared by magnetron sputtering deposition (MSD) technique can be used as an excellent photonic device for ultrafast pulse generation in the 1.5 μm regime, and the MSD technique opens a promising way to produce high-performance SA with a large modulation depth, low saturable intensity, and high power tolerance, which are conducible to the generation of high power and ultrafast pulse with high stability. Keywords:saturable absorber/ fiber laser/ magnetron sputtering deposition/ bismuth
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--> --> --> 1.引 言超快光纤激光器在激光医学、吸收光谱、激光大气通信、遥感和高分子材料加工等方面具有广阔的应用前景[1]. 可饱和吸收体(SA)作为谐振腔内脉冲的启动器和整形器, 可从腔内噪声序列中选择出能量最高的脉冲, 是实现被动锁模运转的关键光学元件. 目前唯一商用的SA是半导体饱和吸收镜(SESAM)[2,3], 然而, 受半导体带隙宽度和反射镜衬底的限制, SESAM的工作带宽通常只有几十纳米, 且SESAM的制备复杂, 价格昂贵. 近年来一些低维材料, 如石墨烯[4-6]、拓扑绝缘体[7,8]、过渡金属二硫化物[9-12]、黑磷[13-15]等, 被制作成SA并应用于超快脉冲产生, 推动了新型SA的研制. 其中, 石墨烯和拓扑绝缘体虽然具有超快的载流子响应, 但其Dirac点附近较低的电子态密度限制了其非线性吸收特性(调制深度低)[16-18]; 过渡金属二硫化物具有较大的带隙, 其光学响应波段主要位于可见光区域; 尽管不同层数黑磷的带隙在0.3—2.0 eV可调[15], 可实现从可见光到中红外波段的光学响应, 但黑磷的化学稳定性差. 近年来, 科研工作者致力于研制新型的高性能SA, 以弥补现有SA存在的缺点. 其中, 铋烯因其独特的性质引起了人们极大的兴趣. 根据第一性原理理论计算, 铋烯具有高载流子迁移率[19]、优异的热导率[20]、良好的自旋电子性质和应变诱导带跃迁[20-22]等卓越的光电特性. 另外, 块体铋属于半金属, 当层数减小到22层以下时将表现出拓扑绝缘体性质, 当层数小于8层时将表现出量子自旋霍尔相位, 最终在单层情况下表现出半导体性质, 其带隙为0.74 eV并拥有高载流子迁移率[19,20,23]. 已有报道采用机械剥离法[24]和外延生长法[25]制备铋烯, 通常将铋烯和聚合物做成薄膜来作为SA, 由于聚合物容易被激光损伤, 这种薄膜类SA不利于高能量超短脉冲产生[24]. 机械剥离法存在厚度不可控以及不均匀等问题, 随机性大, 不利于SA的精准制备, 直接影响SA的表现性能. 与常见的机械剥离法与外延生长法相比, 磁控溅射沉积(MSD)法可在目标衬底上批次性、可重复、可控地制备薄膜, 是一种精准制备SA的有效方法[12,26]. 本文采用MSD方法在微纳光纤表面沉积纳米级厚度铋膜制备铋SA. 将其应用到掺铒光纤激光器中, 获得稳定的锁模脉冲输出. 脉冲中心波长为1563 nm, 输出功率为45.4 mW, 脉冲宽度为357 fs, 重复频率为19 MHz. 实验表明, 微纳光纤-铋结构可以作为一种有效的倏逝波耦合型SA器件. 2.微纳光纤-铋SA的制备与表征在光纤拉锥机上对单模光纤(SMF-28e)进行熔融拉锥处理, 制备出锥区束腰直径约为13 μm的微纳光纤. 将铋靶材、石英片和微纳光纤置于磁控溅射仪的真空室中, 将真空度降至9 × 10–4 Pa, 再将氩气注入到腔内. 由于铋为单质半金属材料, 使用直流驱动的方式镀膜, 参数为: 氩气流量为15 sccm (1 sccm = 1 mL/min), 电流为0.2 A, 持续时间为2 min, 通过控制时间来控制薄膜厚度. 图1(a)是在扫描电子显微镜下观察到的微纳光纤-铋SA的锥区位置, 锥区直径为13 μm, 插图为镀在锥区表面铋薄膜的表面形态, 可见薄膜十分致密地覆盖在微纳光纤的表面上. 图1(b)为镀膜光纤端面, 图1(c)是图1(b)中红色框内的放大结果, 可以看出铋薄膜在光纤表面沉积的厚度约为39.7 nm. 由514 nm激光激发的50—500 cm–1范围内铋的拉曼光谱如图1(d)所示, 铋薄膜在约72 cm–1处显示出面内Eg模的显著峰, 在约97 cm–1处显示出面外A1g模的相对弱峰, 此结果与先前的工作[27]中报道的值一致, 证实了我们所制备的铋薄膜的高质量. 为了进一步评价铋薄膜的质量, 对所制备的铋薄膜进行了X射线衍射(XRD)分析, 图1(e)是石英片上铋膜的XRD图谱, 将其与铋的XRD标准PDF卡片(PDF#97-005-3796)比对, 表明铋膜的XRD图谱中仅有纯铋元素的峰, 进一步证实通过磁控溅射法制备了高质量薄膜. 图1(f)为石英片上铋膜在1400—1800 nm之间的线性透过率, 在1563 nm处的平均透过率为22.2%. 图 1 铋薄膜表征结果 (a)覆盖铋薄膜拉锥光纤的锥区扫描电子显微镜图像, 插图为铋薄膜的表面形貌; (b)镀铋膜的光纤端面; (c)铋薄膜沉积在光纤上的厚度; (d)铋薄膜的拉曼光谱; (e)铋薄膜的XRD图; (f)铋薄膜的线性透过率 Figure1. Bi film characterization results: (a) Scanning electron microscope images for the taper region of the microfiber coated with the bismuth film (the inset shows the surface morphology of the bismuth film); (b) optical fiber end face with bismuth coating; (c) thickness of bismuth thin film deposited on optical fiber; (d) Raman spectrum of bismuth film; (e) XRD diagram of the bismuth film; (f) linear transmittance of bismuth thin film.
为了确认光与覆盖在锥区表面铋膜的渐逝波相互作用, 将波长为650 nm的红光导入镀膜锥区, 图2(a)所示为未通光前的光学图像, 图2(b)显示了通光后的锥区图像, 通过泄漏的红光可以明显地看出光与物质的相互作用. 采用自制的锁模光纤激光器, 在中心波长为1550 nm、脉冲宽度为270 fs、基频为40.5 MHz的条件下, 测量了基于铋SA的非线性光学吸收特性. 非线性饱和吸收曲线如图2(c)所示, 测得调制深度(αs)、非线性饱和损耗(αns)和饱和强度(Isat)分别约为14%, 79%和0.9 MW/cm2. 图 2 微纳光纤-铋SA的非线性表征 (a)没有和(b)具有650 nm引导光时样品腰部区域的光学显微镜图像; (c) SA的饱和吸收特性 Figure2. Nonlinear characterization of micro-nano fiber-bismuth SA: Optical microscope images of the waist region of the sample (a) without and (b) with the guiding 650 nm light; (c) saturable absorption property of SA.
图 1 铋薄膜表征结果 (a)覆盖铋薄膜拉锥光纤的锥区扫描电子显微镜图像, 插图为铋薄膜的表面形貌; (b)镀铋膜的光纤端面; (c)铋薄膜沉积在光纤上的厚度; (d)铋薄膜的拉曼光谱; (e)铋薄膜的XRD图; (f)铋薄膜的线性透过率
图 2 微纳光纤-铋SA的非线性表征 (a)没有和(b)具有650 nm引导光时样品腰部区域的光学显微镜图像; (c) SA的饱和吸收特性
图 3 (a)不同激发功率下的标准开孔Z扫描曲线; (b)标准化的闭孔/开孔Z扫描曲线
图 4 实验装置图
图 5 1.5 μm锁模特性 (a)锁模光谱; (b)基频为19.0 MHz、分辨率为10 Hz的射频频谱, 插图显示了2 GHz跨度的射频频谱; (c)具有sech2拟合的脉冲持续时间为357 fs输出脉冲的自相关轨迹, 插图是输出脉冲的时间序列图; (d)输出功率/脉冲能量随着输入功率的变化