基于二硒化铪可饱和吸收体的被动调Q掺铒光纤激光器的研究 
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1 引言
调Q光纤激光器具有光束质量好,结构灵活,成本低和稳定性好等优点[1-4],已经被广泛应用于材料加工处理、光纤通信、雷达和生物医学诊断等领域中[5-8]。调Q技术主要分为主动调Q和被动调Q。被动调Q技术主要依赖于可饱和吸收体(SAs),需要在谐振腔内加入可饱和吸收体来控制腔内损耗从而实现调Q脉冲输出,具有方法简单、操作方便、稳定性好的优点。其中以半导体可饱和吸收镜(SESAMs)[9]的应用最为广泛,但是其价格昂贵、损伤阈值低、制作工艺复杂。石墨烯[10-11]、碳纳米管[12]、黑磷[13]、过渡金属硫化物(TMDs)[14-17]、拓扑绝缘体[18]等具有独特的结构和非线性光学特性,近年来作为可饱和吸收体在激光器中得到了广泛应用。其中,过渡金属硫化物因具有典型的MX2(M为Ti、Hf、Re、Mo、W, X为S、Se) 结构、独特的能带结构和光学性能而得到了极大的关注[19]。TMDs具有典型的三明治层状结构,两层X原子夹着一层M原子,通过范德瓦尔斯力连接在一起,其种类比较丰富。目前,二硫化钨(WS2)[20]、 二硒化钼(MoSe2)[17]、 二硫化钼(MoS2)[21-23]、 二硫化锡(SnS2)[24-25]、二硫化镍(NiS2)[26]、二碲化钨(WTe2)[27]和二硒化钛[28]作为可饱和吸收体已经在激光器1 μm、1.5 μm等波段实现了调Q或者锁模脉冲输出[29]。
二硒化铪(HfSe2)作为过渡金属硫化物的一种,具有典型的层状结构,已经吸引了越来越多研究人员的关注,每层HfSe2都是由两层Se原子夹着一层Hf原子组成,原子键的强作用使每层原子连接在一起,而层与层之间通过较弱的范德华力连接,因此比较容易实现少层或者单层的剥离,从而形成2D层。HfSe2能带间隙宽度大约为1.1 eV,属于间接带隙的半导体材料,具有很好的光谱响应宽度和可饱和吸收特性等[22],在超短脉冲光纤激光器中具有广阔的应用前景。
本文主要报道了一种新型二维材料HfSe2-SA的制备及其表征。该材料具有较高的调制深度,损伤阈值高,制作简单。研究了基于HfSe2-SA的被动调Q掺铒光纤激光器,在实验上产生了中心波长为1532.3 nm的稳定的调Q脉冲输出。随着泵浦功率的变化,获得了脉冲重复频率在42.3~88.2 kHz内可调,脉冲宽度为3.3~1.8 μs的稳定的激光脉冲输出。
2 二硒化铪可饱和吸收体的制备及表征
采用机械剥离法制备HfSe2薄片,首先利用Scotch胶带从HfSe2晶体[如
3 实验装置
图 1. AFM和拉曼光谱仪的测量结果。(a) HfSe2薄片的原子力显微镜图;(b)直线A的高度剖面图;(c) HfSe2的拉曼光谱图;(d) HfSe2可饱和特性吸收曲线
Fig. 1. Measurement results by AFM and Raman spectrometer. (a) AFM image of HfSe2 flake; (b) height profile along straight line A; (c) Raman spectrum of HfSe2; (d) saturable absorption curve of HfSe2
4 分析与讨论
在未接入HfSe2-SA时,通过改变泵浦功率和旋转偏振控制器,均未发现脉冲信号。当HfSe2-SA接入环形腔内时,通过控制PC来调节偏振状态和逐渐增加泵浦功率,当泵浦功率为140 mW时,实验中产生了稳定的调Q脉冲,此时激光器的输出功率为0.68 mW。泵浦功率为180 mW时在示波器上观察到的脉冲序列如
图 3. 调Q激光器的脉冲特性。(a)脉冲序列;(b)脉冲宽度;(c)输出光谱;(d)频谱
Fig. 3. Pulse characteristics of Q-switched fiber laser. (a) Pulse train; (b) pulse duration; (c) output spectrum; (d) frequency spectrum
为了进一步得到光纤激光器的性能,测量了HfSe2调Q光纤激光器的重复频率和脉冲宽度随泵浦功率的变化关系,结果如
图 4. 基于HfSe2-SA的光纤激光器性能。(a)脉冲重复频率和脉冲宽度随泵浦功率的变化; (b)输出功率和单脉冲能量随泵浦功率的变化
Fig. 4. Performance of fiber laser based on HfSe2-SA. (a) Repetition rate and pulse duration versus pump power; (b) pulse energy and output power versus pump power
5 结论
报道了一种基于新型二维材料HfSe2的被动调Q掺铒光纤激光器。采用机械剥离法制备HfSe2薄片并以其为可饱和吸收体,其调制深度为18%,非饱和损耗为62.2%。随着泵浦功率的增加,获得了中心波长为1532.3 nm的脉冲激光输出,且当泵浦功率从140 mW增加到360 mW时,重复频率从42.3 kHz增加到88.2 kHz,脉冲宽度从3.3 μs减少到1.8 μs。调Q脉冲输出的最大功率为6.04 mW,最大单脉冲能量为68.5 nJ。该研究结果扩宽了二维材料的光学调制范围,而且证实了新型二维材料HfSe2可应用于1.5 μm波段脉冲激光器。
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Article Outline
张凯龙, 陈浩伟, 陆宝乐, 白晋涛. 基于二硒化铪可饱和吸收体的被动调Q掺铒光纤激光器的研究[J]. 光学学报, 2020, 40(13): 1314001. Kailong Zhang, Haowei Chen, Baole Lu, Jintao Bai. Passively Q-Switched Erbium-Doped Fiber Laser Based on HfSe2 Saturable Absorber[J]. Acta Optica Sinica, 2020, 40(13): 1314001.
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