光纤激光器被动锁模技术研究进展

董自凯; 宋晏蓉

doi:doi:10.3788/CJL202148.0501006

中国激光, 2021, 48 (5): 0501006, 网络出版: 2021-03-12

光纤激光器被动锁模技术研究进展下载： 3143次特邀综述

Research Progress of Mode-Locked Fiber Lasers Based on Saturable Absorbers

论文大纲

董自凯宋晏蓉 ^*

作者单位

北京工业大学理学部, 北京 100124

激光光学超快光纤激光饱和吸收体可饱和吸收效应 laser optics ultrafast fiber laser saturable absorber saturable absorption effects

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摘要

超快光纤激光是目前激光器研究的一个热点。非线性可饱和吸收效应是光纤激光器被动锁模技术的核心。被动锁模技术主要分为真实饱和吸收体和人造饱和吸收体,真实可饱和吸收体包括:半导体可饱和吸收镜(SESAM)和纳米材料等;人造可饱和吸收体包括:非线性偏振旋转演化(NPE)、非线性光环形镜(NOLM)、非线性多模干涉(NLMMI)和Mamyshev再生器(Mamyshev)等。本文综述了最近各类可饱和吸收效应锁模光纤激光器的发展方向,简要阐明工作原理、技术优势、解决所面临问题的方法以及应用领域。

Abstract

Signature For the past two decades, ultrafast fiber lasers have become fundamental building blocks in many applications, such as optical communications, biomedical imaging, and industrial processing. Passive mode-locking techniques have been investigated. The nonlinear saturable absorption (SA) effect is the core of the passive mode-locking technology of fiber lasers. Passive mode-locking techniques can be categorized into real and artificial saturable absorbers. The real saturable absorbers consist of semiconductor saturable absorption mirrors (SESAM) and nanomaterials. The artificial saturable absorbers consist of the nonlinear polarization rotation evolution (NPE), nonlinear optical loop mirror (NOLM), nonlinear multimode interference (NLMMI), and Mamyshev regenerator (Mamyshev). The abovementioned passive mode-locking technologies have their advantages and disadvantages. In this study, we briefly illustrate their pros and cons and review their recent development in various types of saturable absorption effects in the application of ultrafast pulse generation.

For real saturable absorbers, rising from the extraordinary physical, optical, and electronic properties of graphene in 2004, layered-dependent nanomaterials have attracted significant attention because of their advantages of cost-effectiveness, broadband optical response, high nonlinear, fast relaxation, and flexible compatibility with other photonics structures. The optical modulation effect of nanomaterials provides a pulse shaping mechanism (i.e., reduced absorption with increasing optical intensity); thus, it can support stable pulse generation and operation in a laser system. Most optical modulators are based on the third-order nonlinear optical response of nanomaterials, such as saturable absorption and optical Kerr effects. SA is a process originating from valance band depletion, conduction band filling, and ultrafast intra-band carrier relaxation exhibited by the input power-dependent optical absorption. Various nanomaterial-based SAs have been demonstrated for pulse generation in fiber lasers operating from the visible to mid-infrared regions. For the artificial saturable absorbers, these passive mode-locking technologies including NPE, NOLM, NLMMI, and Mamyshev could generate ultrashort, high repetition ratio, and high peak power pulses with demand. The optical modulation effect of artificial saturable absorbers originates from the operating optical properties: NPE technique using the optical properties of polarization, NOLM technique using optical nonlinear interference, NLMMI technique utilizing nonlinear multimode interference, and Mamyshev mode-locking technique using nonlinear self-phase modulation. These artificial SAs could largely tune optical modulation depths and enable some types of pulse generation.

The above-mentioned passive mode-locking techniques have progressed considerably. NOLM mode-locking fiber laser has been used in frequency comb applications due to its high repetition ratio feature. Mamyshev mode-locking fiber laser-generated ultra-high peak power pulses could be compared to commercial Ti: Sapphire lasers. NPE mode-locking fiber lasers could largely tune operating pulse states, acting as an ideal seed source for laser amplifier systems. Mode-locking fiber lasers made of nanomaterials have wide application prospects due to their flexible features. With increasing demand for fiber lasers, the above-mentioned traditional passive mode-locking techniques need to improve their performances. Thus, we review the recent advancement of these techniques to illustrate how they overcome their disadvantages.

Progress There are two solutions to improve the stability of the NPE passive mode-locking technique. The first one is to replace traditional fiber with polarization maintaining (PM) fiber (Fig. 3), and the second one is to use an external intelligent algorithm to stabilize laser performance (Fig. 4). All-PM fiber laser cavity construction is not easy since the orthogonal polarization states will induce a walkaway effect when propagating. Szczepanek et al. used the PM fiber segments cross-fused method to solve the intracavity orthogonal polarization walkaway effect. They obtain ultrashort pulses under the condition of all-PM fiber construction. An external intelligent algorithm is a method to actively control fiber laser performance and successfully realize an intelligently controlled fiber laser system. Its advantages include not only stabilizing the laser performance but also switching the pulsed laser operation states. A major advantage of the NOLM mode-locking technique is phase stability. Therefore, a high repetition ratio fiber laser for frequency comb application always employs the NOLM technique (Fig. 7). Besides, NOLM can tolerate larger cavity loss than other mode-locking techniques due to its nonlinear interference mode-locking mechanism. Thus, the NOLM mode-locking technique was the first reported visible waveband mode-locking fiber laser where the cavity loss is very large (Fig. 8). The nanomaterial-based saturable absorption (SA) mode-locking technique has a flexible feature that could extend its application region. However, its mode-locking mechanism is laser intensity-dependent absorption rising from nanomaterials. The mode-locking laser performance mainly depends on the optical properties of nanomaterials. The optical properties of integrated nanomaterials SA cannot be operated. To solve this issue, an external gate-controlled fiber laser can be used to tune the optical properties of nanomaterials, thereby controlling the mode-locking performance (Fig. 11). NLMMI is a novel mode-locking technique inspired by the Kerr effect mode-locking mechanism of a solid-state mode-locking laser. Its advantages are its ultrafast response time and all-fiber structure (Fig. 14). Mamyshev regenerator with self-phase modulation mechanism can generate ultra-high peak power. However, it has certain disadvantages such as the Mamyshev regenerator requires an external laser to excite Mamyshev regenerator operation (Fig. 16). With the above mode-locking techniques, ultrafast fiber lasers can satisfy end-users.

Conclusions and Prospects As the mechanism of saturable absorber effects for mode-locked fiber lasers are clear, researchers will be able to choose appropriate mode-locking mechanisms to satisfy the specific demands of end-users. Finally, the recent progress of ultrafast fiber lasers poses a new challenge; thus, more investigation is required.

1 引言

超快光纤激光在**、科研、工业以及医疗等领域均具有重要应用^[1]。光纤激光器具有光束质量好、热管理效率高、结构灵活紧凑和维护成本低等优势^[2]。光纤作为柔性波导,在赋予光纤激光器以上所述优势的同时,也引入了色散和非线性效应^[3]。脉冲在光纤中传输时,较小的纤芯波导容易产生较高的功率密度,所激发的非线性效应会累积较高的非线性相移导致脉冲分裂为多个脉冲或者塌陷为类噪声脉冲,这极大地限制了超快光纤激光输出的脉冲能量与峰值功率。为了产生超快光纤激光,平衡或者抑制非线性效应成为关键。其中一种有效方案为增加模场面积,抑制非线性响应。因此,拥有较大模场面积的光子晶体光纤技术得到飞速发展。另一种方案是通过控制谐振腔内的色散来平衡非线性效应。在负色散域,由于色散和非线性平衡而形成的脉冲称为孤子,其峰值功率受限于孤子面积理论^[4]。为了打破负色散对脉冲功率的限制,采用色散管理脉冲即腔内包含正色散和负色散两部分光纤,在正色散域拉伸脉冲降低峰值功率,在负色散域压缩脉冲提升峰值功率并输出,可使脉冲输出功率提升一个数量级^[5]。色散管理孤子依然受到腔平均色散孤子面积理论的约束,致使其功率无法进一步提升。去除负色散光纤使光纤激光器工作在全正色散域,同时腔内引入滤波器增加频域损耗可以获得耗散孤子^[6-7]。脉冲能量的典型值为20 nJ,脉冲宽度约为200 fs,进一步提升了峰值功率。另外,自相似子也是获得大能量的方式之一,其特点是脉冲在腔内传输过程中维持抛物线形状并且始终为线性啁啾^[8-9]。为了控制腔内自相似运转,采用两种方式,一种是被动自相似子的方式即从时域控制,在输出位置之后引入色散延迟器件;另一种是放大自相似子的方式即从频域控制,在增益放大之后引入窄带滤波器件。这些方式都能在一定程度上提升脉冲能量,但是依然受限于非线性相移和连续光干扰等因素影响无法进一步提升脉冲峰值功率。

为了进一步提升超快光纤激光的输出性能以满足光频率梳、太赫兹、生物光子学等应用的需求,对于锁模光纤激光器来说,深入理解并合理利用非线性效应成为关键所在。从被动锁模光纤激光器产生锁模脉冲的机理来看,波导介质引起的群速度色散、非线性效应,滤波器的频谱滤波效应,以及可饱和吸收体(SA)引起的自振幅调制效应等物理过程之间的相互平衡是形成稳定脉冲的关键因素^[10],本文将重点论述基于可饱和吸收效应被动锁模技术的研究进展。可饱和吸收体是利用非线性效应产生超快光纤激光的被动光调制器件,其光调制作用一般是指衰减光强的能力随光强的增大而降低^[11]。可饱和吸收体的实现方式分为真实饱和吸收体和人造饱和吸收体。真实饱和吸收体包括半导体可饱和吸收镜(SESAM)^[12]和纳米材料^[13];人造可饱和吸收体包括非线性多模干涉(NLMMI)^[14]、非线性偏振演化(NPE)^[15]、非线性光环形镜(NOLM)^[16]和Mamyshev再生器(Mamyshev)^[17],可饱和吸收体的不同实现方式如图1所示。体现可饱和吸收体性能的参数包括:响应时间、饱和功率、调制深度、线性损耗、损伤阈值等。响应时间快慢与脉冲宽度之间的关系定义了快饱和或者慢饱和吸收体,在较高功率条件下,慢饱和吸收体会因为对脉冲前后沿部分的调制不同,导致脉冲不稳定^[18]。另外,慢饱和吸收体还会限制激光器系统的工作带宽,不利于激光器的高重复频率产生。饱和功率的高低直接影响超快光纤激光器的锁模阈值,低饱和功率条件下容易自启动锁模^[19]。调制深度大有利于锁模的建立和稳定,并且可以窄化脉冲^[20]。此外还存在反饱和调制,即衰减光强的能力随光强的增大而增大并对脉冲输出类型产生影响。例如,反饱和效应会使脉冲从耗散孤子转变为耗散孤子共振^[21]。可饱和吸收体作为光调制器件,低线性损耗和高损伤阈值是获得高性能超快光纤激光的基本保障。

光纤激光器被动锁模技术研究进展 下载： 3143次特邀综述

1 引言

图 1. 可饱和吸收体分类图

Fig. 1. Classification diagram of saturable absorbers

2 非线性偏振演化锁模技术

图 2. 非线性偏振演化可饱和吸收体原理图[26]

Fig. 2. Mechanism of NPE saturable absorption effect[26]

图 3. 全保偏光纤NPE锁模激光器实验装置图以及可饱和吸收效应原理图[24]

Fig. 3. All-polarization-maintaining fiber NPE mode-locking fiber laser setup and the mechanism of saturable absorption effect[24]

图 4. 智能NPE锁模光纤激光器实验装置[30]

Fig. 4. Experimental setup of intelligence NPE mode-locking fiber laser[30]

3 非线性光环形镜锁模技术

图 5. 非线性光环形镜可饱和吸收效应原理图[38]

Fig. 5. Mechanism of NOLM saturable absorption effect[38]

图 6. 全保偏8字型锁模光纤激光器实验装置图[32]

Fig. 6. Experimental setup of all-polarization-maintaining figure 8 mode-locking fiber laser[32]

图 7. 高重复频率9字腔锁模光纤激光器实验装置图[34]

Fig. 7. Experimental setup of high-repetition figure 9 mode-locking fiber laser[34]

图 8. 可见光波段NOLM锁模光纤激光器实验装置图[58]。(a)原理图;(b) 635 nm全光线结构8字型DSR光纤激光器照片

Fig. 8. Experimental setup of the visible-wavelength NOLM mode-locked fiber laser[58]. (a) Schematic; (b) photograph of 635 nm all-beam structure figure 8 DSR fiber laser

4 纳米材料类饱和吸收体锁模技术

图 9. 纳米材料类可饱和吸收效应原理图[13]

Fig. 9. Schematic diagram of saturable absorption effect of nanomaterials[13]

图 10. 新型纳米材料类可饱和吸收体[69]

Fig. 10. Novel nano-materials saturable absorbers[69]

图 11. 电控全光纤石墨烯器件的示意图和图片[78]

Fig. 11. Schematic and image of gate-controlled all-fiber graphene devices[78]

5 非线性多模干涉锁模技术

图 12. 非线性多模干涉可饱和吸收体原理图[80]

Fig. 12. Schematic diagram of NLMMI saturable absorption[80]

图 13. SMF-SIMF-GIMF-SMF非线性多模干涉掺Tm光纤激光器[80]

Fig. 13. SMF-SIMF-GIMF-SMF nonlinear multimode interference Tm-doped fiber laser[80]

图 14. NCF-GIMF非线性多模干涉光纤激光器[82]

Fig. 14. NCF-GIMF NLMMI nonlinear multimode interference fiber laser[82]

6 Mamyshev锁模技术

图 15. Mamyshev再生器可饱和吸收效应原理图[3]

Fig. 15. Schematic diagram of Mamyshev regenerator saturable absorption effect[3]

图 16. 环形Mamyshev振荡器示意图[24]

Fig. 16. Schematic diagram of the ring Mamyshev oscillator[24]

图 17. 理论结果说明190 nJ 脉冲输出[24]。(a)腔内脉冲的光谱演化;(b)脉冲宽度演化(圆圈)和均方根带宽(方块)

Fig. 17. Numerical simulation results for 190 nJ output pulse[24]. (a) Spectral evolution of the pulse in the cavity; (b) evolution of the pulse duration (circles) and RMS bandwidth (squares)

7 结束语

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光纤激光器被动锁模技术研究进展下载： 3143次特邀综述

图 2. 非线性偏振演化可饱和吸收体原理图^[26]

Fig. 2. Mechanism of NPE saturable absorption effect^[26]

图 3. 全保偏光纤NPE锁模激光器实验装置图以及可饱和吸收效应原理图^[24]

Fig. 3. All-polarization-maintaining fiber NPE mode-locking fiber laser setup and the mechanism of saturable absorption effect^[24]

图 4. 智能NPE锁模光纤激光器实验装置^[30]

Fig. 4. Experimental setup of intelligence NPE mode-locking fiber laser^[30]

图 5. 非线性光环形镜可饱和吸收效应原理图^[38]

Fig. 5. Mechanism of NOLM saturable absorption effect^[38]

图 6. 全保偏8字型锁模光纤激光器实验装置图^[32]

Fig. 6. Experimental setup of all-polarization-maintaining figure 8 mode-locking fiber laser^[32]

图 7. 高重复频率9字腔锁模光纤激光器实验装置图^[34]

Fig. 7. Experimental setup of high-repetition figure 9 mode-locking fiber laser^[34]

图 8. 可见光波段NOLM锁模光纤激光器实验装置图^[58]。(a)原理图;(b) 635 nm全光线结构8字型DSR光纤激光器照片

Fig. 8. Experimental setup of the visible-wavelength NOLM mode-locked fiber laser^[58]. (a) Schematic; (b) photograph of 635 nm all-beam structure figure 8 DSR fiber laser

图 9. 纳米材料类可饱和吸收效应原理图^[13]

Fig. 9. Schematic diagram of saturable absorption effect of nanomaterials^[13]

图 10. 新型纳米材料类可饱和吸收体^[69]

Fig. 10. Novel nano-materials saturable absorbers^[69]

图 11. 电控全光纤石墨烯器件的示意图和图片^[78]

Fig. 11. Schematic and image of gate-controlled all-fiber graphene devices^[78]

图 12. 非线性多模干涉可饱和吸收体原理图^[80]

Fig. 12. Schematic diagram of NLMMI saturable absorption^[80]

图 13. SMF-SIMF-GIMF-SMF非线性多模干涉掺Tm光纤激光器^[80]

Fig. 13. SMF-SIMF-GIMF-SMF nonlinear multimode interference Tm-doped fiber laser^[80]

图 14. NCF-GIMF非线性多模干涉光纤激光器^[82]

Fig. 14. NCF-GIMF NLMMI nonlinear multimode interference fiber laser^[82]

图 15. Mamyshev再生器可饱和吸收效应原理图^[3]

Fig. 15. Schematic diagram of Mamyshev regenerator saturable absorption effect^[3]

图 16. 环形Mamyshev振荡器示意图^[24]

Fig. 16. Schematic diagram of the ring Mamyshev oscillator^[24]

图 17. 理论结果说明190 nJ 脉冲输出^[24]。(a)腔内脉冲的光谱演化;(b)脉冲宽度演化(圆圈)和均方根带宽(方块)

Fig. 17. Numerical simulation results for 190 nJ output pulse^[24]. (a) Spectral evolution of the pulse in the cavity; (b) evolution of the pulse duration (circles) and RMS bandwidth (squares)