Ultrafast Polarization-Maintaining Fiber Lasers: Design, Fabrication, Performance, and Applications
超快保偏光纤激光器：设计、制造、性能和应用

The development of ultrafast polarization-maintaining fiber lasers (UPMFLs) has freed them from the confinement of laboratory environments, allowing them to work under more stringent scientific and industrial conditions. The strong birefringence of the PM fiber forces the polarization state of the ultrashort pulse to be maintained during propagation, thus effectively resisting the disturbance of environmental factors, such as temperature,
超快保偏光纤激光器 （UPMFL） 的发展使它们摆脱了实验室环境的限制，使它们能够在更严格的科学和工业条件下工作。PM 光纤的强双折射迫使超短脉冲的极化状态在传播过程中保持，从而有效地抵抗环境因素的干扰，如温度、

humidity, and pressure variations. The all-PM fiber configurations greatly simplify the laser structure, which not only enhances the reliability of the laser but also improves the reproducibility of the mode-locked state. All the above advantages broaden the application possibilities of UPMFLs and drive forward their commercialization efforts. With improved lasing performance and enhanced environmental stability, the generated ultrashort pulses have enabled plentiful crucial applications such as ultrafine material processing, spectroscopy, and biomedical imaging. In particular, the construction of optical frequency combs (OFCs) based on UPMFLs has continuously refreshed the records for the lowest residual carrier-envelope-offset phase jitter and pulse timing jitter, further promoting rapid progress of high-precision optical metrology. ${}^{[1]}$${}^{[1]}$^([1]){ }^{[1]}
湿度和压力变化。全保偏光纤配置大大简化了激光器结构，不仅增强了激光器的可靠性，还提高了锁模状态的再现性。以上优势拓宽了 UPMFL 的应用可能性，并推动了其商业化工作。随着激光性能和环境稳定性的增强，产生的超短脉冲使大量关键应用成为可能，例如超细材料加工、光谱学和生物医学成像。特别是基于 UPMFL 的光频率梳 （OFC） 的构建，不断刷新最低残余载波包络偏移相位抖动和脉冲定时抖动的记录，进一步推动了高精度光学计量的快速发展。 ${}^{[1]}$${}^{[1]}$^([1]){ }^{[1]}

The lasing performance of UPMFLs depends critically on different pulsepicking and pulse-shaping techniques. The longitudinal mode in the laser cavity can be phase-locked by either active modulators or passive saturable absorbers (SAs) so that a pulse train can be formed in the temporal domain. Although actively mode-locked fiber lasers can achieve harmonic mode-locking for high-repetition-rate pulse generation, their complex frequency or intensity modulation systems and unsatisfactory output pulse performance make them lag behind passive ones. ${}^{[2,3]}$${}^{[2,3]}$^([2,3]){ }^{[2,3]} In contrast, passive mode-locking offers a simpler and more efficient approach to realize ultrashort pulse generation by simply introducing the SA effect into the laser cavity. Moreover, passively mode-locked fiber lasers are more desirable due to their advantages such as low cost, maintenance-free, and compactness. From the perspective of obtaining high-quality pulses, an ideal SA needs to satisfy several key criteria, such as appropriate modulation depth, fast recovery time, broad operational bandwidth, and compatibility with the desired working wavelength. Furthermore, the high damage threshold and low saturation peak power of SA are extremely beneficial to the long-term stable operation and easy self-starting of the laser, respectively. Over the past few decades, numerous passive SAs have been proposed and used in transmissiontype or reflection-type PM fiber oscillators, as shown in Figure 1a,b. Semiconductor materials and low-dimensional materials can be employed as real SAs in UPMFLs due to their intensity-dependent nonlinear responses and unique optical
UPMFL 的激光性能在很大程度上取决于不同的脉冲拾取和脉冲整形技术。激光腔中的纵向模式可以通过有源调制器或无源可饱和吸收器 （SA） 进行锁相，以便在时间域中形成脉冲序列。尽管主动锁模光纤激光器可以实现谐波锁模以产生高重复率脉冲，但其复杂的频率或强度调制系统以及不令人满意的输出脉冲性能使它们落后于无源激光器。 ${}^{[2,3]}$${}^{[2,3]}$^([2,3]){ }^{[2,3]} 相比之下，被动锁模提供了一种更简单、更高效的方法，只需将 SA 效应引入激光腔即可实现超短脉冲生成。此外，无源锁模光纤激光器因其成本低、免维护和紧凑等优点而更受欢迎。从获得高质量脉冲的角度来看，理想的 SA 需要满足几个关键标准，例如适当的调制深度、快速恢复时间、宽工作带宽以及与所需工作波长的兼容性。此外，SA 的高损伤阈值和低饱和峰值功率分别对激光器的长期稳定运行和易于自启动极为有利。在过去的几十年里，许多无源 SA 被提出并用于传输型或反射型 PM 光纤振荡器，如图 1a、b 所示。半导体材料和低维材料由于其强度依赖性的非线性响应和独特的光学特性，可以用作 UPMFL 中的真实 SA

Figure 1. Different pulse-picking SAs (a) and pulse-shaping mechanisms © in transmission-type or reflection-type UPMFLs (b) (the output coupler and pump in the laser are omitted for simplicity). SESAM: semiconductor saturable absorber mirror; NOLM/NALM: nonlinear optical loop mirror or nonlinear amplifying loop mirror; NPE: nonlinear polarization evolution; SPM: self-phase modulation; OSF: offset spectral filtering; 1D/2D materials: one-dimensional or two-dimensional materials.
图 1.传输型或反射型 UPMFL （b） 中不同的脉冲拾取 SA （a） 和脉冲整形机制©（为简单起见，省略了激光器中的输出耦合器和泵浦）。SESAM：半导体可饱和吸收镜;NOLM/NALM：非线性光学环镜或非线性放大环镜;NPE： 非线性极化演化;SPM： 自相位调制;OSF： 偏移光谱滤波;1D/2D 材料：一维或二维材料。
characteristics. Meanwhile, artificial SAs based on the Kerr nonlinear effect have attracted continued research enthusiasm and have become the most prevalent mode-locking techniques in UPMFLs. Pulse shaping is influenced significantly by either the dispersion map or the presence of intra-cavity nonlinear attractors. Different pulse-shaping mechanisms (Figure 1c), such as soliton, stretched pulse, similariton, dissipative soliton, and cascaded Mamyshev regeneration have been developed in UPMFLs. The combination of pulse-picking SAs and pulse-shaping mechanisms, as well as the all-PM laser configuration, results in enhanced lasing performance and heightened resistance to environmental fluctuations. Moreover, the fusion of distinct laser architectures and mode-locking techniques paves the way for advancements in optical imaging, ranging, and spectroscopy through the development of wavelength-tunable and dual-comb UPMFLs.
特性。同时，基于 Kerr 非线性效应的人工 SA 吸引了持续的研究热情，并已成为 UPMFL 中最普遍的锁模技术。脉冲整形受色散图或腔内非线性吸引子的存在有显著影响。在 UPMFL 中已经开发了不同的脉冲整形机制（图 1c），例如孤子、拉伸脉冲、相似子、耗散孤子和级联 Mamyshev 再生。脉冲拾取 SA 和脉冲整形机制以及全 PM 激光器配置相结合，可增强激光性能并提高对环境波动的抵抗力。此外，通过开发波长可调和双梳 UPMFL，不同的激光架构和锁模技术的融合为光学成像、测距和光谱学的进步铺平了道路。

The current review summarizes the design, fabrication, performance, and applications of UPMFLs. The structure of the paper is the following: Section 2 introduces the principal passive mode-locking techniques employed in the design and fabrication of UPMFLs. Furthermore, it presents a summary of accomplishments in diverse dispersion regimes, focusing on pulse duration, spectral bandwidth, repetition rate, and pulse energy. Section 3 is dedicated to highlighting the significance of wavelength-tunable and dual-comb PM fiber oscillators, owing to their vast range of potential applications. Section 4 delves into an examination and analysis of various chosen representative applications of ultrafast fiber lasers. Section 5 offers a concise overview of the future advancements expected in UPMFLs, along with fresh perspectives and strategies to address the existing obstacles and limitations.
目前的综述总结了 UPMFL 的设计、制造、性能和应用。本文的结构如下：第 2 节介绍了 UPMFL 设计和制造中采用的主要无源锁模技术。此外，它还总结了不同色散机制下的成就，重点关注脉冲持续时间、频谱带宽、重复频率和脉冲能量。第 3 节专门强调了波长可调和双梳状 PM 光纤振荡器的重要性，因为它们具有广泛的潜在应用。第 4 节深入探讨了超快光纤激光器的各种选定代表性应用的检查和分析。第 5 节简要概述了 UPMFL 预期的未来进步，以及解决现有障碍和限制的新视角和策略。

Among real SAs, semiconductor SA mirror (SESAM) consists of a semiconductor material that acts as both a mirror and an
在真正的 SA 中，半导体 SA 镜 （SESAM） 由一种半导体材料组成，该材料既是镜子又是
absorber. The absorber section of the SESAM is responsible for modulating the intensity of the laser beam and achieving modelocking. A quantum well structure is utilized to confine the motion of electrons and holes in two dimensions. Typically, the mirror section is based on a distributed Bragg reflector (DBR) structure. A DBR is composed of multiple alternating layers of high and low refractive index materials, such as semiconductor materials or dielectrics. The reflection layers are precisely designed to create constructive interference for the desired laser wavelength, resulting in high reflectivity. The combined action of the highly reflective mirror and the SA in the SESAM allows for efficient coupling of the laser beam into the ultrafast fiber or solid-state oscillators and the generation of stable femtosecond or picosecond pulses.
吸收。SESAM 的吸收器部分负责调制激光束的强度并实现锁模。量子阱结构用于限制电子和空穴在二维中的运动。通常，反射镜截面基于分布式布拉格反射器 （DBR） 结构。DBR 由多个交替的高低折射率材料层组成，例如半导体材料或电介质。反射层经过精确设计，可针对所需的激光波长产生相长干涉，从而产生高反射率。高反射镜和 SA 在 SESAM 中的联合作用允许激光束有效地耦合到超快光纤或固态振荡器中，并产生稳定的飞秒或皮秒脉冲。

The research conducted in 1993 has been the first to employ a SESAM as a mode-locker in an Erbium (Er)-doped UPMFL. By utilizing a bulk InGaAsP SA grown on an InP substrate, the proposed laser delivers nearly transform-limited 320 fs pulses with a pulse energy of $40\mathrm{pJ}{.}^{[4]}$$40\mathrm{pJ}{.}^{[4]}$40pJ.^([4])40 \mathrm{pJ} .^{[4]} The initial demonstrations of UPMFLs primarily operate in the $1.55\mu \mathrm{m}$$1.55\mu \mathrm{m}$1.55 mum1.55 \mu \mathrm{~m} band, influenced by the widespread availability of communication band (C-band) devices, making it convenient for early experimental setups. Since then, SESAM has gained intensive usage as a real SA in Ytterbium (Yb)-doped UPMFLs operating in the $1\mu \mathrm{m}$$1\mu \mathrm{m}$1mum1 \mu \mathrm{~m} band. The extensive adoption is mainly attributed to their industrial applications and the proven effectiveness of SESAM for pulse-picking. Fiber lasers commonly contain an intra-cavity anomalous group velocity dispersion (GVD) section, as a crucial component for dispersion compensation and the generation of femtosecond pulses. For Yb-doped UPMFLs mode-locked by SESAMs, components such as grating pairs, photonic crystal fibers (PCFs), and chirped fiber Bragg gratings (CFBGs) have been incorporated into the laser cavity to implement dispersion management. ${}^{[5-8]}$${}^{[5-8]}$^([5-8]){ }^{[5-8]} Consequently, the presence of the intra-cavity anomalous GVD segment in fiber lasers gives rise to diverse pulse-shaping mechanisms, which leads to the generation of conventional solitons, stretched pulses, and similaritons. In 2005, the pioneering work of I. Hartl et al. have introduced the first fully integrated Er and Yb-doped UPMFLs, which adopt Fabry-Perot structures. The
1993 年进行的研究首次将 SESAM 用作掺铒 （Er） UPMFL 的模式锁柜。通过利用在 InP 衬底上生长的体 InGaAsP SA，拟议的激光器可提供几乎受变换限制的 320 fs 脉冲，脉冲能量为 $40\mathrm{pJ}{.}^{[4]}$$40\mathrm{pJ}{.}^{[4]}$40pJ.^([4])40 \mathrm{pJ} .^{[4]} UPMFL 的初始演示主要在 $1.55\mu \mathrm{m}$$1.55\mu \mathrm{m}$1.55 mum1.55 \mu \mathrm{~m} 频带中运行，受通信频带（C 波段）器件广泛使用的影响，使其便于早期实验设置。从那时起，SESAM 作为真正的 SA 在频 $1\mu \mathrm{m}$$1\mu \mathrm{m}$1mum1 \mu \mathrm{~m} 带中运行的掺镱 （Yb） UPMFL 中得到了广泛使用。其被广泛采用主要归功于其工业应用以及 SESAM 在脉冲采集方面的成熟有效性。光纤激光器通常包含腔内异常群速度色散 （GVD） 部分，作为色散补偿和产生飞秒脉冲的关键组件。对于由 SESAM 锁模的掺镱 UPMFL，光栅对、光子晶体光纤 （PCF） 和啁啾光纤布拉格光栅 （CFBG） 等组件已被整合到激光腔中，以实现色散管理。 ${}^{[5-8]}$${}^{[5-8]}$^([5-8]){ }^{[5-8]} 因此，光纤激光器中腔内异常 GVD 段的存在产生了不同的脉冲整形机制，从而导致产生常规孤子、拉伸脉冲和相似子。2005 年，I. Hartl 等人的开创性工作推出了第一个采用法布里-珀罗结构的完全集成的 Er 和 Yb 掺杂 UPMFL。这
design employs a chirped PM fiber grating to provide anomalous GVD for dispersion compensation. Through the integration with a PM amplifier system, both Yb and Er master oscillator power amplifier (MOPA) configurations have demonstrated the capability to generate high-energy ultrashort pulses with 100 fs pulse duration. ${}^{[9]}$${}^{[9]}$^([9]){ }^{[9]} Furthermore, an ultra-compact Er-doped fiber frequency comb has been constructed based on the PM fiber oscillator design. ${}^{[10]}$${}^{[10]}$^([10]){ }^{[10]} Soon after that, self-similar pulses featuring a parabolic top along with steep edges in their spectra, have been obtained in a SESAM-based UPMFL. The manipulation of net cavity dispersion is achieved through the utilization of a pair of transmission gratings. ${}^{[7]}$${}^{[7]}$^([7]){ }^{[7]} The similaritons are characterized by linear chirp, which is advantageous for subsequent amplification and compression. Following direct preamplification, the parabolic pulses are amplified in a single-polarization Yb-doped PCF to the microjoule level, without requiring any additional temporal stretching. The linear chirp accumulated by self-phase modulation (SPM) during the amplification process can be compensated by using standard dispersive elements. The laser system is capable of generating $240\mathrm{fs},1.2\mu \mathrm{J}$$240\mathrm{fs},1.2\mu \mathrm{J}$240fs,1.2 muJ240 \mathrm{fs}, 1.2 \mu \mathrm{~J} pulses at a repetition rate of $17\mathrm{MHz}{}^{[11]}$$17\mathrm{MHz}{}^{[11]}$17MHz^([11])17 \mathrm{MHz}{ }^{[11]} By utilizing FBGs as intra-cavity components, it becomes feasible to design all-fiberized lasers without the need for any free-space coupling. The all-fiber format eliminates the complexities associated with aligning and coupling optical elements in free space. Therefore, the laser systems can be simplified to achieve compact and robust structures with enhanced stability. CFBGs for dispersion and nonlinearity engineering are employed in SESAM-based all-PM and all-fiber lasers, which operate in the wave-breaking-free, stretched-pulse, and chirpedpulse regimes. ${}^{[12,13]}$${}^{[12,13]}$^([12,13]){ }^{[12,13]} Besides, a narrow-band FBG can serve a dual purpose by simultaneously stabilizing and dispersion-managing the laser cavity. ${}^{[14]}$${}^{[14]}$^([14]){ }^{[14]} It is worth noting that in the stretched-pulse regime where the intra-cavity dispersion approaches zero, modelocked pulses may become unstable, influenced by the modulation depth of the SA. Both experimental findings and numerical simulations have demonstrated that a higher modulation depth of the SA enables stable mode-locking across the entire dispersion regime. ${}^{[8]}$${}^{[8]}$^([8]){ }^{[8]} All-normal-dispersion (ANDi) fiber lasers are demonstrated to deliver high-energy ultrashort pulses without the need for dispersion management. The spectral filtering cuts away the spectral structure and temporal wings of the highlychirped pulses, which is found to be critical to setting the boundary conditions and stabilizing the pulse evolution in the ANDi cavity. The first ANDi UPMFL with a SESAM has been invented to emit 300 fs and 2 nJ dissipative solitons. The pulse-shaping is ensured by the spectral-filtering mechanism, which is provided by a free-space birefringent filter. ${}^{[15]}$${}^{[15]}$^([15]){ }^{[15]} The SESAM-based oscillator, equipped with a fiber-pigtailed narrow-band filter, features an all-fiber configuration and produces environmentally stable picosecond pulses. ${}^{[16]}$${}^{[16]}$^([16]){ }^{[16]} In addition, a limited gain bandwidth, a PM fiber loop mirror, and various FBGs (such as CFBGs, tilted-FBGs, and uniform FBGs) can also offer spectral filtering effects. ${}^{[17-20]}$${}^{[17-20]}$^([17-20]){ }^{[17-20]} More importantly, the adoption of the FBG for narrow spectral filtering in a picosecond ANDi laser with a SESAM has a crucial impact on the noise performance. By effectively eliminating the Gordon-Haus jitter, the FBG helps achieve few-femtosecond timing jitter. ${}^{[21]}$${}^{[21]}$^([21]){ }^{[21]} In contrast to conventional solitons, whose pulse energy is limited by the soliton area theorem, higher pulse energy can be achieved in stretched-pulse, self-similar, or ANDi regimes.
设计采用啁啾 PM 光纤光栅为色散补偿提供异常 GVD。通过与 PM 放大器系统集成，Yb 和 Er 主振荡器功率放大器 （MOPA） 配置都展示了产生具有 100 fs 脉冲持续时间的高能超短脉冲的能力。 ${}^{[9]}$${}^{[9]}$^([9]){ }^{[9]} 此外，基于 PM 光纤振荡器设计构建了超紧凑的掺铒光纤频率梳。 ${}^{[10]}$${}^{[10]}$^([10]){ }^{[10]} 不久之后，在基于 SESAM 的 UPMFL 中获得了具有抛物线顶部和陡峭边缘的自相似脉冲。净腔色散的操纵是通过使用一对透射光栅来实现的。 ${}^{[7]}$${}^{[7]}$^([7]){ }^{[7]} 相似子的特征是线性啁啾，有利于后续的扩增和压缩。直接预放大后，抛物面脉冲在单极化 Yb 掺杂 PCF 中被放大到微焦耳水平，无需任何额外的时间拉伸。自相位调制 （SPM） 在放大过程中积累的线性啁啾可以通过使用标准色散元件进行补偿。激光系统能够以重复 $17\mathrm{MHz}{}^{[11]}$$17\mathrm{MHz}{}^{[11]}$17MHz^([11])17 \mathrm{MHz}{ }^{[11]} 频率产生 $240\mathrm{fs},1.2\mu \mathrm{J}$$240\mathrm{fs},1.2\mu \mathrm{J}$240fs,1.2 muJ240 \mathrm{fs}, 1.2 \mu \mathrm{~J} 脉冲 通过使用 FBG 作为腔内组件，设计全光纤激光器变得可行，而无需任何自由空间耦合。全光纤格式消除了在自由空间中对准和耦合光学元件的复杂性。因此，可以简化激光系统，以实现紧凑而坚固的结构，并增强稳定性。 用于色散和非线性工程的 CFBG 用于基于 SESAM 的全 PM 和全光纤激光器，它们在无波断、拉伸脉冲和啁啾脉冲范围内运行。 ${}^{[12,13]}$${}^{[12,13]}$^([12,13]){ }^{[12,13]} 此外，窄带光纤光栅可以同时稳定和色散管理激光腔，从而达到双重目的。 ${}^{[14]}$${}^{[14]}$^([14]){ }^{[14]} 值得注意的是，在腔内色散接近零的拉伸脉冲范围内，锁模脉冲可能会变得不稳定，这会影响 SA 的调制深度。实验结果和数值仿真都表明，SA 的较高调制深度可以在整个色散范围内实现稳定的锁模。 ${}^{[8]}$${}^{[8]}$^([8]){ }^{[8]} 全正常色散 （ANDi） 光纤激光器被证明可以传输高能超短脉冲，而无需色散管理。光谱滤波去除了高啁啾脉冲的光谱结构和颞翼，这对于设置 ANDi 腔中的边界条件和稳定脉冲演化至关重要。第一个带有 SESAM 的 ANDi UPMFL 已经发明，可发射 300 fs 和 2 nJ 耗散孤子。脉冲整形由自由空间双折射滤波器提供的频谱滤波机制确保。 ${}^{[15]}$${}^{[15]}$^([15]){ }^{[15]} 基于 SESAM 的振荡器配备了光纤尾纤窄带滤波器，采用全光纤配置，可产生环境稳定的皮秒脉冲。 ${}^{[16]}$${}^{[16]}$^([16]){ }^{[16]} 此外，有限的增益带宽、保偏光纤环镜和各种 FBG（如 CFBG、倾斜 FBG 和均匀 FBG）也可以提供光谱滤波效果。 ${}^{[17-20]}$${}^{[17-20]}$^([17-20]){ }^{[17-20]} 更重要的是，在带有 SESAM 的皮秒 ANDi 激光器中采用 FBG 进行窄光谱过滤对噪声性能有至关重要的影响。通过有效消除 Gordon-Haus 抖动，FBG 有助于实现几飞秒的定时抖动。 ${}^{[21]}$${}^{[21]}$^([21]){ }^{[21]} 与脉冲能量受孤子面积定理限制的传统孤子相比，在拉伸脉冲、自相似或 ANDi 状态下可以获得更高的脉冲能量。

In these scenarios, the pulse undergoes significant spectral and temporal variations during one round trip in the laser cavity, resulting in a reduced average peak power. Nevertheless, the accumulated Kerr nonlinearity in the small-core single-mode fiber (SMF) continues to impose limitations on further energy scaling.
在这些情况下，脉冲在激光腔中的一次往返过程中会发生显著的光谱和时间变化，从而导致平均峰值功率降低。然而，小芯单模光纤 （SMF） 中累积的 Kerr 非线性继续对进一步的能量缩放施加限制。
The adoption of low-nonlinearity large-mode-area (LMA) fibers provides a more straightforward approach to scaling up the pulse energy. Recently, the well-known stress-applying parts (SAP) technique has been used to fabricate LMA PCFs with low nonlinearity and single polarization properties. ${}^{[22-24]}$${}^{[22-24]}$^([22-24]){ }^{[22-24]} Using a segment of Yb-doped LMA PCF as the gain medium, a sigma ( $\sigma$$\sigma$sigma\sigma ) laser cavity has been constructed with anomalous dispersion. The mode-locking is guaranteed by the combination of a SESAM and the nonlinear polarization evolution (NPE) technique. Sub$500\mathrm{fs},16.5\mathrm{nJ}$$500\mathrm{fs},16.5\mathrm{nJ}$500fs,16.5nJ500 \mathrm{fs}, 16.5 \mathrm{~nJ} pulses at a repetition rate of 53.33 MHz are directly produced in the developed fiber oscillator. ${}^{[25]}$${}^{[25]}$^([25]){ }^{[25]} Song et al. have reported a linear-cavity laser operating in the soliton-like regime, and a single-polarization LMA PCF is employed as the active medium. The generated ultrashort pulses with sub-600 fs pulse duration and 19-nJ pulse energy are used for broadband terahertz radiation generation. ${}^{[26]}$${}^{[26]}$^([26]){ }^{[26]} Subsequently, the seed oscillator together with an LMA PCF amplifier is constructed by the same research group. The whole laser system is designed to yield wavelength-tunable pulses in a high-nonlinearity PCF, serving the purpose of fast micromachining. ${}^{[27]}$${}^{[27]}$^([27]){ }^{[27]} The Yb -doped LMA PCF oscillator with a $\sigma$$\sigma$sigma\sigma configuration can operate in the vicinity of zero cavity dispersion, enabling the generation of stretched pulses and similaritons. ${}^{[28]}$${}^{[28]}$^([28]){ }^{[28]} To pursue high pulse energy, dispersion compensation elements are eliminated from the laser cavities, making them operate in the ANDi regime. ${}^{[29,30]}$${}^{[29,30]}$^([29,30]){ }^{[29,30]} Figure 2a illustrates a dispersion compensation-free Yb -doped LMA fiber laser, which achieves a pulse energy of 927 nJ , exhibiting a highly competitive lasing performance when compared to the solid-state counterparts. The autocorrelation traces of the uncompressed pulses and compressed pulses, together with the mode-locked spectrum, are depicted in Figure 2b. ${}^{[31]}$${}^{[31]}$^([31]){ }^{[31]} The self-consistency of pulse evolution is ensured by a delicate balance between various factors, which include SESAM nonlinearity, gain filtering, and SPM-induced spectral broadening. The interplay of these mechanisms leads to the remarkable output of the laser, making it possible to achieve high pulse energies without the need for an additional spectral filter. ${}^{[32]}$${}^{[32]}$^([32]){ }^{[32]}
采用低非线性大模面积 （LMA） 光纤提供了一种更直接的方法来扩大脉冲能量。最近，众所周知的应力施加部件 （SAP） 技术已被用于制造具有低非线性和单极化特性的 LMA PCF。 ${}^{[22-24]}$${}^{[22-24]}$^([22-24]){ }^{[22-24]} 使用一段掺镱 LMA PCF 作为增益介质，构建了一个具有异常色散的 σ （ $\sigma$$\sigma$sigma\sigma ） 激光腔。锁模由 SESAM 和非线性偏振演化 （NPE） 技术的组合来保证。重复频率为 53.33 MHz 的子 $500\mathrm{fs},16.5\mathrm{nJ}$$500\mathrm{fs},16.5\mathrm{nJ}$500fs,16.5nJ500 \mathrm{fs}, 16.5 \mathrm{~nJ} 脉冲直接在开发的光纤振荡器中产生。 ${}^{[25]}$${}^{[25]}$^([25]){ }^{[25]} Song 等人报道了一种在孤子状区域工作的线性腔激光器，并使用单偏振 LMA PCF 作为活性介质。产生的脉冲持续时间低于 600 fs、脉冲能量为 19 nJ 的超短脉冲用于宽带太赫兹辐射的产生。 ${}^{[26]}$${}^{[26]}$^([26]){ }^{[26]} 随后，种子振荡器与 LMA PCF 放大器一起由同一研究小组构建。整个激光系统旨在在高非线性 PCF 中产生波长可调脉冲，用于快速微加工。 ${}^{[27]}$${}^{[27]}$^([27]){ }^{[27]} 掺镱的 LMA PCF 振荡器具有某种 $\sigma$$\sigma$sigma\sigma 配置，可以在零腔色散附近工作，从而能够产生拉伸脉冲和相似子。 ${}^{[28]}$${}^{[28]}$^([28]){ }^{[28]} 为了追求高脉冲能量，从激光腔中去除了色散补偿元件，使它们在 ANDi 模式下运行。 ${}^{[29,30]}$${}^{[29,30]}$^([29,30]){ }^{[29,30]} 图 2a 显示了一种无色散补偿掺镱 LMA 光纤激光器，其脉冲能量为 927 nJ，与固态激光器相比，表现出极具竞争力的激光性能。未压缩脉冲和压缩脉冲的自相关轨迹以及锁模频谱如图 2b 所示。 ${}^{[31]}$${}^{[31]}$^([31]){ }^{[31]} 脉冲演化的自洽性是通过各种因素之间的微妙平衡来确保的，这些因素包括 SESAM 非线性、增益滤波和 SPM 诱导的频谱展宽。这些机制的相互作用导致了激光器的显着输出，从而可以在不需要额外光谱滤光片的情况下实现高脉冲能量。 ${}^{[32]}$${}^{[32]}$^([32]){ }^{[32]}

Chirped pulse amplification (CPA) technology is utilized to significantly increase the single-pulse energy using a $40-\mathrm{MHz}$$40-\mathrm{MHz}$40-MHz40-\mathrm{MHz} SESAM-based UPMFL as a seed oscillator, reaching levels in the tens of microjoules. One essential step in the amplification process involves using an acousto-optic modulator (AOM) to lower the pulse repetition rate, resulting in higher single-pulse energy. ${}^{[33]}$${}^{[33]}$^([33]){ }^{[33]} However, the employed AOM introduces losses and decreases energy efficiency, requiring the inclusion of an extra amplifier in the laser system to restore the pulse energy. A more direct approach to simplify the amplification system is to obtain pulses with a lower repetition rate by extending the cavity length of the laser. A low-repetition-rate SESAM mode-locked UPMFL has been demonstrated to deliver narrow-bandwidth picosecond pulses. Various cavity lengths equipped with different uniform FBGs have been explored, leading to the realization of pulse repetition rates as low as $0.7\mathrm{MHz}{}^{[34]}$$0.7\mathrm{MHz}{}^{[34]}$0.7MHz^([34])0.7 \mathrm{MHz}{ }^{[34]} Additionally, a Yb-doped UPMFL mode-locked by a transmissive fiber pigtailed semicon-
啁啾脉冲放大 （CPA） 技术用于使用基于 $40-\mathrm{MHz}$$40-\mathrm{MHz}$40-MHz40-\mathrm{MHz} SESAM 的 UPMFL 作为种子振荡器来显着增加单脉冲能量，达到数十微焦耳的水平。放大过程中的一个重要步骤是使用声光调制器 （AOM） 来降低脉冲重复率，从而获得更高的单脉冲能量。 ${}^{[33]}$${}^{[33]}$^([33]){ }^{[33]} 然而，所采用的 AOM 会引入损耗并降低能源效率，需要在激光系统中包含一个额外的放大器来恢复脉冲能量。简化放大系统的更直接方法是通过延长激光器的腔长度来获得具有较低重复率的脉冲。低重复率 SESAM 锁模 UPMFL 已被证明可以提供窄带宽皮秒脉冲。已经探索了配备不同均匀 FBG 的各种腔长，从而实现了低至 $0.7\mathrm{MHz}{}^{[34]}$$0.7\mathrm{MHz}{}^{[34]}$0.7MHz^([34])0.7 \mathrm{MHz}{ }^{[34]} 此外，由透射式光纤尾纤半导体锁模的掺杂 Yb UPMFL -