Research Progress on All-Polarization-Maintaining Mode-Locked Fiber Lasers

Abstract

This article reviews the research progress of all-polarization-maintaining mode-locked fiber lasers. Owing to their excellent resistance to environmental interference and high stability, all-polarization-maintaining mode-locked fiber lasers hold significant application value in various fields, including industrial processing, communications, medical applications, and military applications. This article provides a detailed introduction to the structures, working principles, and performance characteristics of all-polarization-maintaining mode-locked fiber lasers based on different mode-locking mechanisms, such as SESAMs, two-dimensional materials, nonlinear polarization rotation, nonlinear optical loop mirrors, nonlinear amplifying loop mirrors, and figure-9 cavity. Additionally, this article discusses the challenges faced by all-polarization-maintaining mode-locked fiber lasers and their future development directions, including integration, miniaturization, multi-wavelength output, and the potential applications of new materials.

1. Introduction

With the rapid advancement of optical fibers and related technologies, fiber lasers have shown great application potential across diverse fields, such as industrial processing, communications, medical applications, and military uses, owing to their unique advantages. Among them, ultrashort-pulse fiber lasers, marked by high peak power, narrow pulse width, and high pulse energy, possess significant application value in precision measurement, biomedical applications, scientific research, aerospace, and industrial production.

Mode-locking techniques are an effective way to generate ultrashort pulses. Passive mode-locked fiber lasers relying on saturable absorbers feature a simple structure, easy all-fiber integration, narrow pulse width, and high stability [1]. A saturable absorber’s transmittance varies nonlinearly with the optical field intensity. It has low transmittance in a weak optical field but significantly higher transmittance in a strong one. This property introduces amplitude modulation to the fiber laser’s optical field. Coupled with the fiber’s nonlinear and dispersion effects, it enables the generation of mode-locked pulses.

Traditional mode-locked fiber lasers usually employ conventional single-mode fibers. The fundamental mode (HE11) in single-mode fibers consists of two degenerate polarization modes, HE11x and HE11y, perpendicular to each other. In an ideal scenario, single-mode fibers are made of isotropic materials and have a perfectly symmetric shape, allowing linearly polarized light to maintain its polarization during propagation. However, real fibers are imperfect, breaking the mode degeneracy and causing mode birefringence. When pulses travel through real fibers, their polarization state changes. Especially under stress, the stress-induced refractive index change of the single-mode fiber’s principal axes can reach the order of 10−5, severely altering the optical pulses’ polarization state and leading to unstable mode locking or even unlocking of the fiber laser [2].

Polarization-maintaining fiber introduces strong birefringence through geometric structural design (such as panda type and bow-tie type), resulting in notably different effective refractive indices along the fast and slow axes. When linearly polarized light travels along one of the fiber’s principal axes, minimal coupling occurs into the perpendicular axis, ensuring that the transmitted light’s polarization state remains constant. Since the birefringence caused by external stress is much smaller than the PM fiber’s inherent birefringence, the impact of stress on the incident light’s polarization state is eliminated. All-polarization-maintaining (APM) mode-locked fiber lasers can effectively reduce the random birefringence from mechanical stress, enhancing the laser’s environmental interference resistance and stability [3,4].

Saturable absorbers in mode-locked fiber lasers generally fall into two categories: One is material-based, including semiconductor saturable absorber mirrors (SESAMs) [3,4] and two-dimensional materials like graphene [5], topological insulators, MXenes [6,7,8], and transition metal sulfides [9,10,11]. When excited by intense light, these materials pump carriers from the ground state to the excited state, depleting ground-state ions and partially occupying the excited state, thus causing absorption saturation. The excited state returns to the ground state through thermal stabilization and recombination, enabling the laser’s Q-switching and mode locking. Material-based saturable absorbers such as SESAMs often suffer from high cost and complex fabrication processes. Graphene and topological insulators, on the other hand, may face challenges in large-scale integration and stability. In the future, the development of material-based saturable absorbers may focus on improving the performance–cost ratio, simplifying the fabrication process, and enhancing the stability and compatibility for large-scale applications. The other category is based on the fiber’s nonlinear effects, such as nonlinear polarization rotation (NPR) [12], nonlinear optical loop mirrors (NOLMs) [13], and nonlinear amplifying loop mirrors (NALMs) [14]. Both NOLMs and NALMs use the Sagnac interferometer’s switching characteristics for mode locking. To improve NALMs’ self-starting, non-reciprocal devices with Faraday rotators (FRs) and wave plates are added to the cavity to increase the intra-cavity pulses’ nonlinear phase shift. The improved cavity structure, similar to the Arabic numeral 9, is known as the figure-9 cavity mode-locked fiber laser [15]. NPR makes use of the polarization states in two directions of the optical fiber, and there is a problem of polarization state walk-off between the two polarization states in the PM fiber. For NOLMs and NALMs, they face the problem of difficult startup. The figure-9 cavity mode-locked PM laser solves these problems, but the startup process still needs to be optimized. For this type of laser, the development direction focuses on optimizing the design of the laser to improve its stability.

To date, both material-based and equivalent saturable absorbers have been applied in APM mode-locked fiber lasers to generate ultrashort pulses. This article reviews the research progress of such lasers, introduces the structures and working principles of fiber lasers based on SESAMs, two-dimensional materials, NPR, NOLMs, and NALMs, and discusses their development trends and application prospects.

2. All-Polarization-Maintaining Mode-Locked Fiber Lasers

2.1. APM Mode-Locked Fiber Lasers Based on SESAMs

An SESAM is a crucial passive mode-locking device. Its basic structure integrates a mirror with a semiconductor saturable absorber. Typically, the bottom layer is a mirror, on which a thin semiconductor saturable absorber film is grown. The preparation process of the SESAM is relatively complex and involves multiple key steps and technologies. The absorber part usually adopts a quantum well structure to achieve the modulation of the intensity of the laser beam. The growth of the quantum well structure often employs technologies such as Molecular Beam Epitaxy (MBE) or Metal–Organic Chemical Vapor Deposition (MOCVD). The mirror part is based on the Distributed Bragg Reflector (DBR) structure. The DBR is composed of multiple alternating layers of materials with high and low refractive indices. Through the careful design and preparation of the DBR structure, the SESAM can achieve a reflectivity of over 99% at the target wavelength. When growing the quantum well and DBR, by precisely controlling parameters such as the growth temperature and gas flow rate, the lattice mismatch at the interface can be reduced to a very small extent, thus significantly improving the overall performance of the SESAM. The performance parameters of the SESAM determined by the manufacturing process, such as the modulation depth, saturation intensity, recovery time, etc., directly affect the output characteristics of the laser. A higher modulation depth helps to achieve mode locking more effectively and generate stable, short pulses; a lower saturation intensity enables the laser to achieve saturable absorption at a lower power, which is beneficial for reducing the laser’s starting threshold; a short recovery time ensures that the laser can generate pulses with a high repetition frequency. By adjusting the thickness and reflectivity of the saturable absorber, we can tune parameters like the modulation depth and bandwidth of the SESAM. When used in an APM fiber laser, an SESAM enables the generation of stable mode-locked pulses [2,3,4,16,17,18,19,20,21,22,23,24,25].

In Ref. [3], the generation of self-similar pulses from an environmentally stable fiber laser made solely of PM fibers was reported. As depicted in Figure 1, two distinct cavity configurations were studied. In Figure 1a, the output coupler is a fixed-fiber pigtailed coupler. In Figure 1b, the output coupler consists of a bulk polarizer and a quarter-wave plate, allowing for tunable output coupling. Stable pulses could be achieved across a broad range of cavity parameters, such as pump power, output coupling ratio, and net cavity dispersion.

As shown in Figure 1, a pair of transmission gratings were used to provide negative dispersion for compensating the intra-cavity dispersion. In all-normal-dispersion mode-locked fiber lasers (without negative dispersion devices), filters [2,16,17,18,20] are usually needed to assist in pulse shaping. In Ref. [2], an environmentally stable all-normal-dispersion femtosecond fiber laser was reported. The design featured a linear cavity containing a PM fiber and with a very large normal group-velocity dispersion (GVD). A birefringent filter was employed to perform spectral filtering. In Ref. [17], a tilted-fiber Bragg grating was inserted into the cavity as the filtering element, as illustrated in Figure 2a. Figure 2b,c present the auto-correlation trace and spectrum of the output pulse, respectively. As shown in Figure 2c, the spectrum profile had steep edges, which are typically regarded as the characteristic features of dissipative solitons. Since most SESAM saturable absorbers are of the reflective type, SESAM mode-locked fiber lasers often adopt a linear-cavity or sigma-cavity structure.

Table 1. Part of the fiber lasers mode-locked with SESAMs. NA: non-available.

Year	Gain Fiber	Laser Configuration	Total Dispersion	Duration	Energy	Repetition Rate	Ref.
2005	Yb	Linear cavity	0.03 ps2	280 fs	1 nJ	17 MHz	[3]
2008	Yb	Sigma cavity	All-normal dispersion	750 fs	25 nJ	63 MHz	[16]
2008	Yb	Linear cavity	0.17 ps2	310 fs	2.2 nJ	33 MHz	[2]
2012	Yb	Sigma cavity	All-normal dispersion	475 fs	1.36 nJ	38.1 MHz	[17]
2018	Yb	Linear cavity	All-normal dispersion	20.6 ps	1.2 nJ	29 MHz	[18]
2022	Yb	Sigma cavity	−1.06 × 104 fs2	109 fs	0.44 nJ	553 MHz	[19]
2022	Yb	Sigma cavity	All-normal dispersion	758 fs	18.2 μJ	495 KHz	[20]
2022	Yb	Linear cavity	NA	486.5 ps	0.45 mW	13.2325 MHz	[21]
2023	Er	Linear cavity	−1500 fs2	303 fs	NA	1.049 GHz	[22]
2024	Er	Linear cavity	NA	30 ps	6 nJ	32.07 MHz	[23]
2024	Er	Linear cavity	NA	299 fs	1.71 pJ	87.3 MHz	[24]
2022	Nd	Linear cavity	−0.05 ps2	1.8 ps	130 pJ	51 MHz	[25]

summarizes some of the previous reports on SESAM mode-locked fiber lasers.

SESAMs can effectively achieve mode locking in fiber lasers. They enable the generation of stable, mode-locked pulses across the 1.55 μm, 1 μm, 980 nm, and 2 μm wavelength bands. When integrated with dispersion management components within the laser cavity, they can produce various types of pulses, including conventional solitons, dispersion-managed solitons, and self-similar pulses. Moreover, SESAMs’ compatibility with fiber integration simplifies the process of achieving all-fiber lasers. However, SESAMs have their drawbacks, such as non-saturable losses, a low damage threshold, and the introduction of a certain degree of group-velocity dispersion within the laser cavity. With the continuous progress of research, there is great potential for further optimizing the performance parameters of SESAMs. By integrating them with novel fiber materials and micro/nano processing technologies, the performance of lasers is expected to be elevated to a new level.

2.2. APM Mode-Locked Fiber Lasers Based on Low-Dimensional-Material Saturable Absorbers

In addition to SESAMs, other low-dimensional materials have also served as saturable absorbers. These materials encompass graphene, topological insulators, transition metal dichalcogenides, black phosphorus, and metal–organic frameworks. These two-dimensional nanomaterials feature high optical nonlinear polarization coefficients, ultrafast carrier dynamics, wide operating wavelength ranges, and easy fabrication and integration. Their key advantages as saturable absorbers include the ability to generate ultrafast laser pulses across the visible–mid-infrared spectral range, along with simple fabrication and fiber integration compatibility. To date, carbon nanotubes [26,27,28,29,30], graphene [5,31,32,33,34,35], and WSe₂ have all been successfully used in APM mode-locked and Q-switched fiber lasers [36] to produce stable laser pulses.

Carbon nanotubes (CNTs) are one-dimensional nanostructured materials, which can be seen as hollow cylinders formed by rolling carbon atoms. In the nonlinear optics field, CNTs have shown great application potential and unique optical properties because of their special electronic structure and one-dimensional nanostructure. A common way to fabricate saturable absorbers is to disperse CNTs in a polymer matrix. The interaction between light and CNTs can be easily achieved by inserting the polymer film between two fiber connectors. Figure 3a [26] and Figure 3b [28] present the absorption spectrum and normalized nonlinear absorption, respectively, of CNT-based SA reported in the literature.

Most of the reported APM CNT mode-locked fiber lasers are Er-doped fiber lasers operating in the 1.5 μm band [26,27,28,29] and Tm-doped fiber lasers operating near 2 μm [30]. In these two bands, optical fibers exhibit negative dispersion. Consequently, most of these lasers output conventional solitons with Kerr sidebands. In 2008, N. Nishizawa et al. [26] presented an APM Er-doped ultrashort-pulse fiber laser using a single-walled CNT polyimide nanocomposite saturable absorber. By using a PM anomalous dispersive fiber, the 314 fs output pulse was compressed to 107 fs. In 2023, a 1.7 μm femtosecond Tm-doped fiber laser was demonstrated [30], as depicted in Figure 4. With two backward-pumped amplifiers, the average power of the laser was amplified to approximately ~458 mW.

Graphene is a two-dimensional material composed of carbon atoms arranged in a sp²-hybridized lattice structure. Due to its unique physical and chemical properties, graphene has been widely applied in mode-locked fiber lasers. In 2012, G. Sobon et al. [5] reported a graphene-based passively mode-locked fiber laser. The saturable absorber was formed by mechanical exfoliation of graphene flakes from pure graphite and then deposited on a fiber connector. This was the first demonstration of an APM Er-doped fiber laser mode-locked by a graphene saturable absorber. In 2015, the same team [31] reported an all-fiber ultrafast Tm-doped fiber laser mode-locked by a multilayer graphene-based saturable absorber. The Raman spectrum and photograph of the graphene/PMMA foil used as the saturable absorber are shown in Figure 5a,b, respectively. Figure 5c shows the experimental setup of the APM Tm-doped fiber, and the measured 603 fs pulse auto-correlation trace is illustrated in Figure 5d. Similar to the case of carbon nanotubes, most of the pulses generated from graphene mode-locked APM fiber lasers are conventional solitons with Kerr sidebands [5,31,32,33]. In 2017, J. Sotor [34] demonstrated an APM stretched-pulse Tm-doped fiber laser. This was the first demonstration of stretched-pulse operation of a graphene-based fiber laser at 2 μm. The performance of the fiber lasers mode-locked with low-dimensional materials is summarized in

Table 2. Summary of the fiber lasers mode-locked with low-dimensional materials. NA: non-available.

Year	Gain Fiber	Material	Total Dispersion	Duration	Energy/Power	Repetition Rate	Ref.
2008	Er	Carbon nanotubes	−0.104 ps2	107 fs	4.8 mW	41.3 MHz	[26]
2009	Er	Carbon nanotubes	−0.149 ps2	504 fs	585 pJ	21.6 MHz	[27]
2016	Er	Carbon nanotubes	NA	6.94 ps	63 pJ	28.94 MHz	[28]
2021	Er	Carbon nanotubes	NA	1.085 ps	11.26 pJ	17.77 MHz	[29]
2023	Tm	Carbon nanotubes	NA	206 fs	8.8 nJ	52 MHz	[30]
2012	Er	Graphene	NA	590 fs	0.91 mW	45.88 MHz	[5]
2012	Er	Graphene	NA	570 fs	24.5 pJ	114.1 MHz	[35]
2016	Er	Graphene	0.0172 ps2	148 fs	25 pJ	101 MHz	[32]
2017	Er	Graphene	0.066 ps2	402 fs	96 pJ	60.64 MHz	[33]
2015	Tm	Graphene	−0.34 ps2	603 fs	1.5 mW	41.46 MHz	[31]
2017	Tm	Graphene	−0.024 ps2	205 fs	220 pJ	58.87 MHz	[34]

.

Low-dimensional material saturable absorbers such as graphene can be prepared by various methods, including mechanical exfoliation, chemical vapor deposition, and liquid-phase exfoliation, and can be compounded with other materials. Moreover, these materials can achieve saturable absorption over a wide range, which is conducive to realizing multi-wavelength mode locking. However, material-based saturable absorbers generally have the problem of a low damage threshold, restricting their application in high-power laser systems. Additionally, the preparation methods of liquid-phase exfoliation or post-transfer make it difficult to control the crystallization quality of the materials, resulting in poor reproducibility. It is necessary to improve the damage threshold, expand the range of saturable absorption parameters, and enhance the parameters’ controllability through material optimization, structure design, and compounding with other materials.

2.3. All-Polarization-Maintaining Mode-Locked Fiber Laser Based on NPR

2.3.1. APM Mode-Locked Fiber Laser Based on NPR Without Phase Bias

Saturable absorbers based on materials offer broad operating wavelength ranges, compact structures, and modulation effects that are relatively insensitive to environmental influences. However, these material-based saturable absorbers often have low damage thresholds, and their long-term stability needs improvement. In contrast, equivalent saturable absorbers realized through the nonlinear effects in optical fibers can address the vulnerability of material-based saturable absorbers to damage. NPR is one such equivalent saturable absorber mechanism.

The structure of a mode-locked laser based on nonlinear polarization rotation is shown in Figure 6. Its working principle is as follows: After the pulse passes through the polarization-dependent isolator, it becomes linearly polarized light. Subsequently, the second polarization controller (PC2) converts it into elliptically polarized light. As pulses propagate through an optical fiber, the nonlinear phase shifts induced by self-phase modulation and cross-phase modulation act on the two orthogonal polarization components of elliptically polarized light, causing the polarization state of the pulses to change. Since the nonlinear phase shift depends on intensity, the evolution of the polarization state is also nonlinear, resulting in different polarization states at various positions within the optical pulse. By adjusting another polarization controller (PC1), when the pulse passes through the polarization-dependent isolator again, the central part of the pulse with high intensity has less loss, while the edge part has more loss, thus playing the role of saturable absorption. In 1991, Hofer et al. demonstrated the pioneering work of NPR, proposing a highly nonlinear laser cavity based on self-phase and cross-phase modulation [37].

NPR works on the principle that two orthogonal polarization components, each acquiring different nonlinear phase shifts, induce the rotation of a polarization ellipse. As a result, the light within NPR resonators must exhibit elliptical polarization. Due to the significant birefringence of PM fiber, the two polarization components propagating along the two principal axes of the PMF experience large group-velocity differences. After multiple round trips, these components will gradually separate from each other [38], eventually turning into continuous waves (CWs) or noise. Thus, for a long time, it was commonly believed that the NPR technique was only applicable to non-PM or partially PM fiber cavities [39]. However, thermal and mechanical disturbances in optical fibers can cause random birefringence, which can destabilize NPR mode-locked lasers. To enable these lasers to operate outside the laboratory environment, significant progress has been made in developing NPR mode-locked lasers using APM fibers.

One way to compensate for the walk-off between two orthogonal polarization states caused by birefringence is to cross-splice PM fibers [40,41,42,43,44,45,46]. As shown in Figure 7a [39], a pulse with a certain polarization state can be split into two pulses parallel to the fast and slow axes of the PM fiber. After propagating a certain distance in the PM fiber, the group-velocity mismatch due to birefringence makes the two pulses completely separate in time. If these two pulses then pass through another section of PM fiber of the same length, which is rotated by 90° along the axis relative to the first section, the walk-off caused by the group-velocity mismatch will be compensated, and the two pulses will recombine into one pulse. In 2016, Wang et al. [40] theoretically and experimentally studied the feasibility of this method. In an APM mode-locked fiber laser, a stable dissipative soliton was obtained. At a pump power of 460 mW, the pulse energy was 2.1 nJ, with a spectral width of 17.5 nm and a pulse width of 11.7 ps. In this compensation method, if only two sections of PM fibers are used, the spectral and temporal profiles may become uneven, and in extreme cases, multiple pulses may occur. To avoid this, the PM fiber can be divided into three [40] or more segments for cross-splicing, as shown in Figure 7b. In this way, the two pulses swap their roles as fast and slow pulses in each consecutive segment. Each pulse travels an equal distance along the slow and fast axes, thus compensating for the group-velocity mismatch and the phase shift caused by the refractive index difference between the slow and fast axes. In 2023, Jiang et al. presented an APM mode-locked fiber laser operating around 976 nm [45]. The mode locking was achieved using a special section of the laser comprising three pieces of PM fiber with specific deviation angles between the polarization axes and a polarization-dependent isolator, as shown in Figure 8. By optimizing the NPR section and adjusting the pump power, stable dissipative solitons were generated.

In addition to cross-splicing, a Faraday mirror (FM) can also be used to achieve a 90° fiber rotation in a reflective configuration [47,48,49,50,51,52]. When the pulses pass through the PM fiber for the first time, walk-off happens (Figure 7c). After reflecting off the FM, the pulses propagating along the fast and slow axes are interchanged, compensating for the walk-off (Figure 7d). This method eliminates the need for precise matching of the fiber lengths. In 2020, M. Yu et al. [52] reported an all-fiber, APM, NPR, mode-locked, dispersion-managed fiber laser that used an FM to compensate for the group-velocity mismatch caused by birefringence. This laser generated dispersion-managed solitons, dissipative solitons, bound-state pulses, and noise-like pulses across a wide range of dispersion values. Combining an FM with multiple cross-spliced fiber segments can improve the temporal overlap of pulses and enhance the cross-phase modulation effect, thereby further increasing the environmental stability of the laser [39,53,54]. G. Ye et al. [54] reported such an APM NPR mode-locked fiber laser. As shown in Figure 9a, a PM Faraday rotation mirror (PM-FRM) was used to rotate the polarization of the two light components by 90°, ensuring that they would travel the same distance along each axis of the PM fiber during a complete round trip, thus canceling out the birefringence-induced walk-off. The 21 m PM fiber was divided into 10 segments and cross-fusion-spliced to enhance the laser’s stability. Figure 9b–d show the characteristics of the mode-locked pulses with a central wavelength of 1588.9 nm. The performance of the APM fiber lasers mode-locked with NPR is summarized in

Table 3. Performance of the APM fiber lasers mode-locked with NPR. NA: non-available.

Year	Gain Fiber	Compensation Method	Laser Configuration	Total Dispersion	Duration	Energy	Repetition Rate	Ref.
2016	Yb	Cross-splicing	Ring cavity	All-normal dispersion	11.7 ps	2.1 nJ	43.8 MHz	[40]
2018	Yb	Cross-splicing	Ring cavity	All-normal dispersion	192 fs	0.47 nJ	111 MHz	[41]
2017	Yb	Cross-splicing	Ring cavity	0.278 ps2	150 fs	0.85 nJ	20.54 MHz	[42]
2020	Yb	Cross-splicing	Ring cavity	All-normal dispersion	260 fs	0.2 nJ	24.5 MHz	[44]
2023	Yb	Cross-splicing	Ring cavity	0.316 ps2	5.99 ps	0.54 nJ	20.47 MHz	[45]
2019	Er	Cross-splicing	Ring cavity	0.0099 ps2	90 fs	77 pJ	90.5 MHz	[43]
2024	Tm	Cross-splicing	Ring cavity	−1.77 ps2	2.02 ns	21.64 nJ	10.34 MHz	[46]
2007	Yb	Faraday mirror	Linear cavity	NA	5.6 ps	1.34 nJ	5.96 MHz	[47]
2014	Yb	Faraday mirror	Linear cavity	NA	17.8 ps	68 pJ	948 kHz	[48]
2018	Yb	Faraday mirror	Ring cavity	All-normal dispersion	5.9 ps	2.9 nJ	6.7 MHz	[50]
2020	Yb	Faraday mirror	Linear cavity	NA	0.8 ns	26.9 nJ	4.54 MHz	[51]
2020	Yb	Faraday mirror	Linear cavity	0.039 ps2	161.37 fs	2.265 mW	6.17 MHz	[52]
2018	Er	Faraday mirror	Ring cavity	NA	700 fs	382 μW	15.4 MHz	[49]
2023	Er	Cross-splicing + Faraday mirror	Ring cavity	−1.03 ps2	1.25 ps	102 μW	3.9 MHz	[53]
2024	Er	Cross-splicing + Faraday mirror	Ring cavity	−1.03 ps2	1.69 ps	80 μW	3.9 MHz	[53]

.

2.3.2. APM Mode-Locked Fiber Laser Based on NPR with Phase Bias

In the APM mode-locked fiber lasers mentioned above, to achieve NPR mode locking, a long section of PM fiber is usually introduced to provide sufficient nonlinear phase shift for the laser to self-start. Incorporating an FR in NPR [55,56] to introduce a 90° phase shift between the two polarization states can facilitate self-starting and simplify the laser structure.

The NPR mode-locked laser with a phase shifter is compact and easy to start. Subsequently, extensive research has been carried out on this laser structure [38,57,58,59,60,61,62,63,64,65,66,67]. M. Edelmann et al. [65] used a large-mode-area fiber with a 25 µm core diameter to increase the single-pulse energy of the APM fiber laser. The laser setup is shown in Figure 10. Unlike other configurations, this laser uses two FRs. One FR, along with the mirror M2, swaps the pulses propagating along the fast and slow axes of the fiber to compensate for the group-velocity mismatch caused by birefringence. The other FR, together with a quarter-wave plate (QWP2) and a half-wave plate (HWP), forms a phase shifter that introduces a phase shift between two orthogonally polarized pulses. The mode-locking mechanism of nonlinear polarization rotation with phase bias is based on the nonlinear phase shift accumulated between the orthogonal polarization components in the polarization-maintaining fiber, while the linear phase shift is compensated by applying a Faraday mirror. The pulse transmitted from the polarization beam splitter (PBS) is separated into two orthogonal polarization components by the quarter-wave plate and the half-wave plate. These two components propagate along the fast and slow axes of the polarization-maintaining fiber. The subsequent Faraday rotator provides a 45-degree rotation. The Faraday mirror at the end causes a 90° rotation of the two polarization components and enables them to exchange their propagation paths, thus compensating for the walk-off effect caused by the group-velocity mismatch. After reflection, the two polarization components pass through the Faraday rotator again, thereby obtaining a non-reciprocal π/2 phase shift. Then, the two orthogonal polarization components are projected onto the p and s directions of the polarization beam splitter. The coherent interference between the two combined beams allows the pulse with a larger nonlinear phase difference to pass through the polarization beam splitter; otherwise, it will be reflected by the polarization beam splitter. This process forms a pulse discrimination mechanism, that is, an artificial saturable absorber [58]. The laser operates at a pulse repetition rate of 17.3 MHz, with an output power of 170 mW and a single-pulse energy of about 10 nJ. The authors also found that cross-phase modulation within the laser affects the saturable absorption process, thus influencing the characteristics of the output pulses. They showed that using an YVO₄ crystal to separate the pulses can effectively suppress cross-phase modulation [64].

Apart from the commonly used Er-doped and Yb-doped fiber lasers, Ho-doped and Tm-doped fiber lasers have also been reported. J. Wang et al. [38] proposed an APM Ho-doped mode-locked laser. The laser has a linear cavity structure with two output ports. One port generates stable soliton-like pulses with a pulse width of 439 fs, while the other port produces complex, low-quality pulses. The stable pulses have significantly better performance in relative intensity noise and power stability compared to the complex pulses. S. Peng et al. [61] studied harmonic mode locking and noise-like pulse phenomena in Tm-doped fiber lasers. In addition to stable single-pulse operation, Y. Zhou et al. [62] observed multi-soliton molecules. By reducing the pump power, the number of solitons in the bound state can be decreased from four to one. The soliton spacing and spectral shape can be adjusted by slightly rotating the angle of a half-wave plate or a λ/8 wave plate.

Table 4. Summary of the NPR mode-locked fibers with phase bias. NA: non-available.

Year	Gain Fiber	Laser Configuration	Total Dispersion	Duration	Energy	Repetition Rate	Ref.
2021	Er	Linear cavity	NA	1.20 ps	0.11 nJ	115 MHz	[56]
2021	Er	Linear cavity	−0.016 ps2	1.35 ps	0.78 nJ	133 MHz	[57]
2021	Yb	Linear cavity	−0.005 ps2	91 fs	263 mW	105 MHz	[58]
2021	Yb	Linear cavity	−0.104 ps2	0.83 ps	400 pJ	36.7 MHz	[64]
2023	Er	Linear cavity	−0.049 ps2	129 fs	0.17 nJ	105.8 MHz	[59]
2023	Er	Linear cavity	−0.0025 ps2	90 fs	67.1 pJ	116.76 MHz	[60]
2023	Er	Linear cavity	NA	1.35 ps	780 pJ	133 MHz	[63]
2023	Yb	Linear cavity	−0.121 ps2	1.2 ps	5.4 nJ	17.3 MHz	[65]
2024	Ho	Linear cavity	−0.32 ps2	439 fs	0.12 nJ	61.67 MHz	[38]
2025	Er	Linear cavity	−0.62 ps2	550 fs	429 fJ	7 MHz	[67]
2025	Tm	Linear cavity	−0.612 ps2	409 fs	259 pJ	229 MHz	[61]

summarizes the characteristics of APM NPR mode-locked fiber lasers with phase bias.

The NPR mode-locking mechanism features a fast response time and a large modulation depth, which are conducive to generating high-performance pulses with short durations and high energies. Furthermore, the parameters of NPR mode-locked lasers are flexibly controllable, and it is easy to generate various types of pulses, such as dissipative solitons and noise-like pulses. This serves as an excellent platform for studying nonlinear dynamics in optical fibers. However, in APM NPR mode-locked lasers, temperature changes can cause differences in thermal expansion between the stress-applying parts and the cladding of the polarization-maintaining fiber, leading to non-uniform deformation of the fast and slow axes of the fiber. Although the pulse quality can be improved by increasing the number of fiber segments, this also increases the complexity of laser fabrication. By combining NPR with other advanced technologies such as machine learning algorithms to assist in laser design and optimization, it is expected to achieve more precise control and higher performance.

It should be noted that the use of Faraday mirrors or cross-splicing methods can compensate for the group-velocity difference caused by birefringence, but the pulse evolution process is not completely consistent with the traditional NPR mechanism. In the traditional NPR mechanism, when different parts of a pulse travel in the fiber, they accumulate different nonlinear phase shifts, and the polarization states of each part evolve at different speeds. As a result, the polarization states of each part are inconsistent when passing through the polarizer (polarization-dependent isolator) at the end. In polarization-maintaining fibers, the two orthogonal polarization components along the fast and slow axes of the fiber have different group velocities. After traveling a certain distance, the two pulses may completely separate. The Faraday mirror or cross-splicing method can compensate for this separation. After compensation, the polarization states of each part of the pulse are inconsistent, achieving pulse narrowing. Therefore, in terms of the evolution process, NPR in polarization-maintaining fibers is not completely consistent with the traditional NPR mechanism.

On the other hand, after compensation by cross-splicing or a Faraday mirror, the two pulses propagating along the fast and slow axes recombine into one pulse. The polarization states of each part of the recombined pulse are no longer the same, so that when passing through the polarizer, it can play a role in narrowing the pulse. From the perspective of the final effect, the principle of pulse shaping is still based on the inconsistency of the polarization states of each part of the pulse. From this perspective, the mode locking after compensation by cross-splicing or a Faraday mirror can still be considered to be consistent with the essence of NPR. In terms of the principle of final pulse shaping, the two are consistent.

2.4. APM Mode-Locked Fiber Laser Based on the Figure-8 Cavity Using NOLMs/NALMs

NOLMs and NALMs are two types of equivalent saturable absorbers based on nonlinear fiber effects. They were proposed as mode-locking components in 1988 [68] and 1990 [69], respectively. Due to their 8-shaped structure, they are commonly called figure-8 cavity mode-locked lasers. NOLMs rely on the interference between two counter-propagating light beams, which makes it easier to build an APM laser [70,71,72,73,74,75,76].

Figure 11 [70] shows the typical structure of an NOLM mode-locked fiber laser. The laser has two loop sections. The nonlinear optical fiber loop, including the gain fiber (YDF), allows unidirectional light propagation. The secondary loop is formed by a 20/80 coupler. The 20/80 coupler divides the incident light into two parts, which propagate in different directions within the nonlinear optical fiber loop. Due to the asymmetric placement of the optical fiber that provides the amplification gain, the two parts of the light accumulate different nonlinear phase shifts during their propagation in the optical fiber. When they pass through the coupler again, interference occurs, resulting in different transmission rates for different parts of the pulse, thus achieving NOLM mode locking.

In 2021, A. Borodkin et al. [13] built an all-normal-dispersion NOLM mode-locked Yb-doped fiber laser, achieving a single-pulse energy of 5.9 nJ. After external compression, the pulse width was reduced to 265 fs. K. Watanabe et al. [71] reported a similar Yb-doped mode-locked fiber laser with an average pulse power of 8.95 mW and a repetition rate of 10.7 MHz. Thanks to the APM design, the root-mean-square (RMS) of the output power fluctuation over 3 h was only 0.4%. By using large-mode-area PM fibers, the single-pulse energy of the mode-locked laser could be increased to 12 nJ [72]. Moreover, J. Shi et al. [73] pointed out that traditional NOLM mode-locked fiber lasers are hard to self-start. They showed that using a bidirectional output coupler with unbalanced losses can enhance the modulation depth of the equivalent saturable absorber and improve self-starting [73].

Figure 12 [77] depicts the typical structure of an NALM mode-locked fiber laser. The laser consists of a left main loop and a right NALM loop. The main loop provides the cavity for laser oscillation, while the NALM loop enables mode locking. An amplifier is added to the NALM based on the NOLM, and the amplifier is asymmetrically placed on one side of the coupler. This structural difference enables the NALM to more easily accumulate a sufficient nonlinear phase-shift difference to initiate mode locking compared to the NOLM. Figure 12 also shows the pulse characteristics at a repetition rate of 6 MHz, with a single-pulse energy of 10 nJ and a spectral width of 31 nm. After external compression, the pulse width is reduced to 93 fs. The laser has excellent stability, with an RMS power stability as low as 0.04% over 10 h.

Nearly half of the reported studies on APM NALM mode-locked fiber lasers focus on Yb-doped lasers [78,79,80,81,82,83,84,85,86,87,88,89,90]. The research aims to generate higher-performance pulses, such as those with higher pulse energy, higher average power, lower repetition rates, and narrower pulse widths. One way to increase pulse energy is to extend the laser’s cavity length. J. Zhou et al. [81] showed that high pulse energy could be achieved by increasing the cavity length, even in the presence of Raman scattering. They constructed an Yb-doped dissipative soliton fiber laser with a single-pulse energy of 32 nJ. Also, the position of the gain fiber affects the output pulse power [87]. By properly placing the Yb-doped fiber, a single-pulse energy of 22 nJ was obtained. In terms of repetition rates, Y. Liu et al. [89] reported a 140 kHz ultralow-repetition-rate Yb-doped fiber laser. After amplification, the single-pulse energy reached 27 µJ, with a compressible pulse width of 1.16 ps. For Yb-doped NALM mode-locked fiber lasers, the narrowest achievable pulse width is around 100 fs [80,90]. In addition to conventional pulses, dissipative soliton resonance pulses in Yb-doped fiber lasers have also been investigated and reported [88]. For Tm-doped APM NALM mode-locked fiber lasers [91,92,93,94,95,96], most studies have reported noise-like pulses [91,92,93] or dissipative soliton resonance pulses [94,95]. Compared to solitons and dissipative solitons, dissipative soliton resonance and noise-like pulses offer higher single-pulse energies. M. Michalska et al. [91] studied the pulse characteristics of Tm-doped noise-like lasers at different repetition rates and achieved a maximum single-pulse energy of 43.4 nJ at a repetition rate of 4.055 MHz.

Table 5. Summary of the NOLM/NALM mode-locked fiber lasers. NA: non-available.

Year	Gain Fiber	NOLM /NALM	Total Dispersion	Duration	Energy/Power	Repetition Rate	Ref.
2015	Yb	NOLM	All-normal dispersion	220 fs	3.46 nJ	15 MHz	[70]
2020	Yb	NOLM	Normal dispersion	250 fs	12 nJ	7.56 MHz	[72]
2021	Yb	NOLM	0.52 ps2	265 fs	5.9 nJ	9.45 MHz	[13]
2022	Yb	NOLM	0.024 ps2 −0.098 ps2 −0.051 ps2	161 fs 98 fs 2.77 ps	0.7 nJ 2.3 nJ 0.5 nJ	18.207 MHz 34.083 MHz 25.537 MHz	[74]
2023	Yb	NOLM	0.44 ps2	220 fs	0.84 nJ	10.7 MHz	[71]
2025	Yb	NOLM	NA	12.6 ns	4.1 nJ	1.085 MHz	[75]
2018	Er	NOLM	−1.24 ps2	530.6 fs	2.78 nJ	3.63 MHz	[73]
2025	Er	NOLM	NA	3.13 ps	17.28 μW	679.56 kHz	[76]
2012	Yb	NALM	0.53 ps2	344 fs	0.3 nJ	10 MHz	[78]
2012	Yb	NALM	NA	<400 fs	16 nJ 10 nJ 2.3 nJ	1.7 MHz 3.7 MHz 10 MHz	[79]
2013	Yb	NALM	All-normal dispersion	120 fs	4.2 nJ	10 MHz	[80]
2015	Yb	NALM	NA	615 fs	32 nJ	2.47 MHz	[81]
2016	Yb	NALM	All-normal dispersion	500 fs	6.9 nJ	506 KHz	[82]
2016	Yb	NALM	NA	360 fs	0.34 nJ	11.6 MHz	[83]
2017	Yb	NALM	NA	NA	11 nJ	7.26 MHz	[84]
2018	Yb	NALM	10.6 ps2	870 fs	134 nJ	448 kHz	[85]
2018	Yb	NALM	All-normal dispersion	200 fs	60 mW	8.04 MHz	[86]
2018	Yb	NALM	0.759 ps2	93 fs	10 nJ	6 MHz	[77]
2020	Yb	NALM	All-normal dispersion	195 fs	22 nJ	8.7 MHz	[87]
2020	Yb	NALM	NA	288 ps 1.38 ns	2.4 nJ 20 nJ	2.59 MHz	[88]
2021	Yb	NALM	NA	1.16 ps	27 μJ	140 kHz	[89]
2022	Yb	NALM	All-normal dispersion	114 fs	27 nJ	7.81 MHz	[90]
2018	Er	NALM	0	500 fs	10 nJ	12 MHz	[97]
2018	Er	NALM	0.82 ps2	500 fs	110 pJ	7.9 MHz	[98]
2019	Er	NALM	NA	660 fs	4 nJ	1.96 MHz	[99]
2024	Er	NALM	NA	20.6 ns	NA	413.8 kHz	[100]
2019	Tm	NALM	−3.632 ps2 −0.877 ps2 −0.711 ps2	>7 ns 388 ps 232 fs	43.4 nJ 11 nJ 11.6 nJ	4.06 MHz 16.59 MHz 20.39 MHz	[91]
2020	Tm	NALM	−4.8 ps2	13.5 ns	29.1 nJ	1.62 MHz	[94]
2021	Tm	NALM	−4.88 ps2	13.5 ns	27.5 nJ	1.59 MHz	[95]
2022	Tm	NALM	−2.1 ps2	357 fs	20 mW	7.267 MHz	[96]
2022	Tm	NALM	−4.8 ps2	303 fs	13.6 mW	3.25 MHz	[93]
2017	Er:Yb	NALM	−0.9274 ps2 −4.3084 ps2	4.35 ns 92 ns	199 nJ 1.01 μJ	4.6 MHz 0.994 MHz	[103]
2019	Nd	NALM	2.9 ps2	6 ns	20 nJ	2.19 MHz	[101]
2024	Ho	NALM	1.006 ps2	5.8 ns	2.1 μJ	7.081 MHz	[104]
2019	Er	NALM 3 × 3	−0.133 ps2	194 fs	7 mW	36.56 MHz	[105]
2020	Yb	NALM 3 × 3	All-normal dispersion	178 fs	1.2 nJ	52 MHz	[106]
2022	Yb Tm	NALM 3 × 3	0.01 ps2 −0.58 ps2	125 fs 215 fs	1.2 nJ 580 pJ	NA 30.3 MHz	[107]
2023	Yb	NALM 3 × 3	0.171 ps2	162 fs	1.15 nJ	11.62 MHz	[108]

summarizes the characteristics of APM NALM mode-locked fiber lasers. In addition to Yb- and Tm-doped fiber lasers, Er-doped [97,98,99,100], Nd-doped [101,102], Er/Yb-co-doped [103], and Ho-doped [104] fiber lasers have also been reported.

A conventional NALM mode-locked fiber laser is composed of one 2 × 2 coupler. In 2019, D. Kim et al. [105] demonstrated an APM NALM mode-locked fiber laser based on a 3 × 3 coupler. The schematic of the proposed fiber laser is shown in Figure 13. They compared the mode-locking performance of the 3 × 3 laser to that of the conventional 2 × 2 fiber laser, and they confirmed that the mode-locking threshold of the 3 × 3 laser was significantly reduced. Since the 3 × 3 coupler generates 120-degree phase bias, the 3 × 3 lasers can easily acquire a mode-locked state even without any additional phase-biasing components [105,106,107,108].

Lasers based on NOLMs and NALMs can achieve an APM structure, showing high stability against environmental factors such as mechanical vibration and temperature changes. NOLM/NALM lasers facilitate the realization of mode-locked output with different mechanisms by adjusting the parameters and components in the cavity. However, the startup of such lasers requires the accumulation of a sufficient difference in nonlinear phase shift in the cavity, which is difficult. In addition, in the 1.55 μm and 2 μm wavelength bands, due to the relatively low nonlinear Kerr coefficient, the nonlinear effects are less obvious than those in the 1 μm wavelength band. It is necessary to continuously explore new structural and component configurations to optimize the nonlinear phase shift in the cavity and improve the performance of the lasers. Moreover, it is essential to miniaturize and integrate the lasers to enhance the stability and compactness of the system.

Both NPR and NOLMs/NALMs are passive mode-locking techniques that utilize the nonlinear effects in optical fibers. For NPR, when a pulse propagates in the polarization-maintaining fiber, two orthogonal polarization states in different parts of the pulse accumulate different nonlinear phase shifts, resulting in inconsistent polarization states between the central and edge parts of the pulse. When the pulse passes through a polarizer (such as a polarization-dependent isolator), the transmittance of each part is different, thereby achieving pulse narrowing. The NOLM/NALM is usually formed by an asymmetrical couple whose output ports are connected to form a loop. When light enters the NOLM loop, it is split into two counter-propagating beams. Due to nonlinear effects such as self-phase modulation, these two beams with power imbalance acquire different nonlinear phase shifts after one round trip in the loop. This phase difference makes the interference result intensity-dependent, and then it generates an intensity-dependent transmission function similar to that of a saturable absorber to achieve mode locking.

The pulse evolution process of an APM NPR mode-locked laser is slightly different from that of a non-APM NPR mode-locked laser. The working principle of an APM NOLM/NALM mode-locked laser is exactly the same as that of a non-APM NOLM/NALM mode-locked laser. However, in an APM NOLM/NALM mode-locked laser, fast-axis cutoff fiber devices are generally used to confine the light to the slow axis for transmission. The all-polarization-maintaining NPR mode locking is relatively sensitive to environmental factors. Temperature changes may cause differences in thermal expansion between the stress-applying parts and the cladding in the polarization-maintaining fiber, affecting the output pulse characteristics and even disrupting the mode locking. The all-polarization-maintaining NOLM and NALM adopt an all-fiber structure, which has a certain resistance to environmental interference.

As mentioned above, NPR relies on two orthogonal polarization components. NOLMs/NALMs usually use an asymmetric coupler to generate two counter-propagating beams with different intensities, realizing the function of a saturable absorber through the interference of these two beams. By combining the two, mode-locked pulses can also be obtained by using two counter-propagating beams with different polarization states [109,110,111,112]. Ref. [109] reports a mode-locked laser based on a symmetric NOLM loop. A quarter-wave plate and a twisted low-birefringence fiber are added to the laser to generate two counter-propagating beams with different polarization states. The laser can self-start and obtain stable mode-locked pulses. In this work, the twisted fiber was similar to a polarization-maintaining fiber, which can resist environmental disturbances to a certain extent and has a certain degree of stability. Compared with APM fiber lasers, this hybrid configuration combining single-mode fiber and polarization-maintaining fiber adds some design flexibility.

2.5. Figure-9 Cavity APM Mode-Locked Fiber Laser

The figure-9 cavity mode-locked fiber laser improves upon the figure-8 cavity fiber laser. By decoupling the secondary loop of the figure-8 cavity laser and employing a mirror to retro-reflect the optical beam, the figure-9 cavity laser is constructed. To enable easier self-starting, non-reciprocal optical elements are usually integrated into the figure-9 cavity mode-locked fiber lasers [113].

An optical frequency comb is essentially a precisely controlled femtosecond laser. A PM fiber frequency comb based on an NALM has lower inherent noise than one based on NPR [114]. The figure-9 cavity mode-locked fiber laser was first used to build frequency combs [114]. During the same period, T. Jiang et al. [115] developed a compact reflective non-reciprocal phase shifter and achieved an APM, all-fiber, Yb-doped mode-locked laser. W. Hänsel et al. [116] demonstrated femtosecond pulse generation at three different wavelengths (1030 nm, 1565 nm, and 2050 nm), using Yb, Er, and Tm/Ho-co-doped fibers as the gain media, respectively. Stable mode-locked pulses were obtained across a wide range of parameters. Figure 14 [116] shows the structure of the laser and the output pulse characteristics of the Er-doped laser.

There has been extensive research on the figure-9 cavity mode-locked fiber laser. In Er-doped figure-9 cavity fiber lasers [116,117,118,119,120,121,122,123,124,125,126,127,128,129], traditional solitons are easily obtained due to the negative dispersion of standard single-mode fibers in the 1.5 µm wavelength band [118,125]. By adding dispersion-compensating fibers, dispersion-managed solitons and dissipative solitons can be achieved [121,122,123]. F. Chen et al. [117] amplified and compressed the pulses from a figure-9 cavity laser. By optimizing the lengths of the gain fiber and compression fiber, they obtained stable pulses with a duration of 28 fs and an energy of 3 nJ.

For Yb-doped lasers [130,131,132,133,134,135,136], in all-normal-dispersion mode-locked lasers, dissipative solitons can be generated by adding a filter. Common dispersion compensation methods include chirped fiber Bragg gratings or separate diffraction gratings. After dispersion compensation, soliton-like or stretched-pulse solitons can be obtained. Using standard single-mode fibers or double-clad fibers, the single-pulse energy of dissipative solitons is usually in the range of several nJ [131,134]. With large-mode-area photonic crystal fibers [130], a single-pulse energy of 28 nJ has been achieved.

In the 2.0 µm wavelength band, Tm-doped fiber lasers [137,138,139] can produce traditional solitons [138] and dispersion-managed solitons [137]. Additionally, in Er/Yb-co-doped lasers [140], the use of large-mode-area fibers has enabled the generation of dissipative solitons with an average power of 690 mW and a single-pulse energy of 13.8 nJ. For Ho-doped figure-9 cavity mode-locked lasers [141], a single-pulse energy of 140 pJ and a pulse width of 1.3 ps have been reported.

In addition to pursuing high-performance pulses, researchers have also explored various aspects of figure-9 cavity mode-locked lasers, such as their self-starting characteristics [119,138], dispersion management properties [121,123,135], wavelength tuning capabilities [128,141], and period-multiplication bifurcation behaviors [132]. In some figure-9 cavity mode-locked fiber lasers, the laser starts in a multi-pulse state and needs a reduction in pump power to switch to a single-pulse state. Z. Łaszczych et al. [138] studied the transition dynamics from multi-pulse to single-pulse operation in Tm-doped lasers. In 2021 [123]; they also investigated the output characteristics of Er-doped lasers when the net cavity dispersion changed from −0.034 ps² to −0.006 ps². In 2022, Y. Shi et al. [135] used chirped fiber Bragg gratings for dispersion compensation and studied the output characteristics of Yb-doped lasers under large positive dispersion and near-zero dispersion conditions. Regarding wavelength tuning, H. Zhang et al. [128] demonstrated through numerical simulations and experiments that increasing the pump power in Er-doped lasers causes a significant redshift of the operating wavelength, while rotating a wave plate in different directions can lead to either a redshift or a blueshift. In Ho-doped fiber lasers [141], the operating wavelength could be tuned between 2035 nm and 2075 nm by rotating a bandpass filter. Additionally, J. Zhou et al. [132] analyzed the period-multiplication phenomenon in figure-9 cavity mode-locked lasers, which shows a typical path from periodic steady-state to chaotic behavior.

Table 6. Summary of the figure-9 cavity APM mode-locked fiber lasers. NA: non-available.

Year	Gain Fiber	Location of the Phase Shifter	Total Dispersion	Duration	Energy/Power	Repetition Rate	All-Fiber	Ref.
2019	Er	Ring cavity	−0.0027 ps2	50 fs	0.16 nJ	85 MHz	Yes	[118]
2019	Er	Ring cavity	−0.015 ps2	477 fs	NA	121 MHz	Yes	[119]
2020	Er	Linear cavity	+0.0005 ps2	70 fs	1.4 nJ	112 MHz	No	[120]
2019	Er	Ring cavity	+0.0036 ps2	132 fs	18 mW	44.9 MHz	No	[121]
2020	Er	Ring cavity	NA	95 fs	8.2 mW	41 MHz	Yes	[122]
2021	Er	Linear cavity	−0.034 ps2 −0.001 ps2	250 fs 79 fs	3.5 mW 6.6 mW	51.55 MHz 80.12 MHz	No	[123]
2021	Er	Ring cavity	0.041 ps2	354 fs	0.29 nJ	55.6 MHz	Yes	[124]
2021	Er	Ring cavity	−0.0124 ps2	510 fs	0.66 mW	201.14 MHz	Yes	[125]
2022	Er	Linear cavity	−1246 fs2	77 fs	37.72 mW	199.6 MHz	No	[126]
2023	Er	Ring cavity	0.03 ps2	96.3 fs	NA	59.3 MHz	No	[127]
2023	Er	Linear cavity	−0.09 ps2	142 fs	NA	52 MHz	No	[128]
2023	Er	Ring cavity	−0.02 ps2	48.2 fs	1.92 nJ	103.4 MHz	Yes	[129]
2016	Yb	Reflection type phase shifter	143,000 fs2	538 fs	4.1 mW	31.35 MHz	Yes	[115]
2018	Yb	Linear cavity	−0.04 ps2 −0.004 ps2 0.03 ps2	199 fs 68 fs 152 fs	8 nJ 13 nJ 28 nJ	26.4 MHz 30 MHz 72 MHz	No	[130]
2020	Yb	Ring cavity	−39.8 ps2	12.9 ps	3.5 nJ	26.4 MHz	Yes	[131]
2021	Yb	Ring cavity	−0.01 ps2	88 fs	51 mW	54.17 MHz	No	[133]
2022	Yb	Ring cavity	All-normal dispersion	378 fs	4.5 nJ	12.5 MHz	Yes	[134]
2022	Yb	Ring cavity	−0.004 ps2 0.019 ps2	175 fs 228 fs	0.26 nJ 0.52 nJ	47.3 MHz 39.1 MHz	No	[135]
2023	Yb	Ring cavity	NA	8.1 ps 5.7 ps	0.32 nJ 0.37 nJ	45.438 MHz	Yes	[136]
2022	Tm	Ring cavity	−0.0008 ps2	128 fs	0.30 nJ	121.3 MHz	Yes	[137]
2023	Tm	Linear cavity	−0.091 ps2	650 fs	177 pJ	52.4 MHz	No	[138]
2023	Tm	Ring cavity	−0.06 ps2	123 fs	2.61 pJ	30.6 MHz	Yes	[139]
2022	Er/Yb	Ring cavity	−0.704 ps2	1.7 ps	13.8 nJ	49.86 MHz	No	[140]
2022	Ho	Ring cavity	−0.43 ps2	1.3 ps	140 pJ	41.6 MHz	No	[141]

summarizes the characteristics of the figure-9 cavity all-polarization-maintaining mode-locked fiber lasers.

Through means such as dispersion management and spectral filtering, the figure-9 APM mode-locked laser can operate under multiple mode-locking mechanisms, such as conventional solitons, dispersion-managed solitons, dissipative solitons, and self-similar pulses. Meanwhile, the figure-9 laser has a high degree of integration, with a compact and stable structure. With the development of technology, the performance of the laser is expected to be continuously optimized, further increasing the pulse energy, shortening the pulse wavelength, expanding the wavelength range, and increasing the repetition rate. In combination with other advanced technologies such as novel material technologies, micro/nano processing technologies, and artificial intelligence algorithms, and through means such as structural optimization and intelligent control, it is expected that the overall performance and functionality of the laser will be further enhanced.

3. Challenges and Future Directions

APM mode-locked fiber lasers have found extensive applications in micro-machining, imaging, and frequency comb metrology, playing a crucial role in facilitating the advancement of modern science and engineering technologies. In order to further enhance the performance of these lasers and broaden their application scopes, it is expected that developments and breakthroughs need to be achieved in several aspects of the existing challenges.

3.1. Existing Challenges

Currently, all-polarization-maintaining mode-locked fiber lasers mainly focus on the 1.0 μm and 1.55 μm wavelength bands, and there has been relatively less research in the visible-light and mid-infrared wavelength bands. For the visible-light band, the main reasons are attributed to the following three aspects [142]: Firstly, the fiber devices and gain fibers in the visible-light band are not mature. Secondly, there is a shortage of ultrafast optical modulators suitable for the visible-light band, such as visible-light saturable absorbers. Thirdly, the fiber resonator in the visible-light band has an extremely large positive dispersion value, making it very difficult to establish stable mode locking in the visible-light range. For the 2 μm mid-infrared band, there are problems such as high material costs and difficulties in nonlinear effects and thermal management [143].

The currently used saturable absorbers have many problems. Saturable absorbers based on materials generally have a relatively low damage threshold. Low-dimensional materials such as CNTs and graphene, although they have good nonlinear optical properties, are likely to be limited in their applications in high-power laser systems due to their low damage threshold. In addition, the preparation process of the materials is complex. Preparation methods such as liquid-phase exfoliation or post-transfer make it difficult to control the crystallization quality of the materials, and their repeatability is poor.

Despite the continuous development of integrated optics technology, there are still technical challenges in achieving a high degree of integration of APM mode-locked fiber lasers. When integrating various functional modules, such as the pump source, gain medium, and mode-locking element, it is necessary to solve the compatibility problem between different components and figure out how to ensure that the performance of each module is not affected while reducing the volume. For example, during the integration process, the differences in the thermal expansion coefficients of different components may lead to structural stress, affecting the stability of the laser, and the heat dissipation problem after integration is also quite prominent. If the heat dissipation is improper, it will cause the performance of the laser to decline or even be damaged.

3.2. Future Trends

The development of integrated optics is actively driving APM fiber lasers towards integration and miniaturization. Through the integration of various functional components within the laser, such as the pump source, gain medium, and mode-locking element, not only can the size and weight of the laser be substantially reduced, its stability and reliability can also be remarkably improved. Furthermore, miniaturized APM mode-locked fiber lasers are more convenient to carry and are highly suitable for applications in fields where equipment’s size and weight are strictly restricted, such as in vivo biomedical detection and miniaturized optical communication devices. In these fields, their application value is extremely high.

Different application fields have distinct wavelength requirements for lasers. Therefore, it is essential to develop APM mode-locked fiber lasers with more diverse wavelength outputs. Currently, these lasers are mainly concentrated on the 1.0 µm and 1.55 µm bands, while relatively less research has been conducted in the visible and mid-infrared spectral regions. The development of visible and mid-infrared mode-locked fiber lasers is of great significance for the development of spectroscopy, precision industrial machining, and biomedical applications. Additionally, in terms of wavelength-tunable output, the insertion of special wavelength-selective elements, such as fiber Bragg gratings and arrayed waveguide gratings, into the laser cavity can enable the generation of multiple specific wavelengths. These multi-wavelength lasers can be applied in wavelength division multiplexing communication systems to boost communication capacity, as well as in spectroscopic analysis to detect the characteristic spectra of different materials.

In addition, with the continuous breakthroughs in materials science, new materials hold broad application prospects in APM mode-locked fiber lasers. The utilization of new materials to develop saturable absorbers with a high modulation depth, wide bandwidth, and fast saturation recovery time may enable the generation of laser pulses with broader spectra, narrower pulse widths, and lower noise, representing a higher performance level.

4. Conclusions

APM mode-locked fiber lasers take advantage of the unique properties of PM fibers to effectively tackle the problem of mode-locking instability induced by polarization state fluctuations in conventional mode-locked fiber lasers. By integrating diverse mode-locking mechanisms and saturable absorber materials, including SESAMs, low-dimensional materials, nonlinear polarization rotation, NOLMs/NALMs, and figure-9 cavity structures, remarkable progress has been made in aspects such as pulse width, pulse energy, repetition rate, and stability. These lasers are most prevalently applied in the 1.0 µm and 1.55 µm wavelength bands, while research in the visible and mid-infrared regions is still relatively scarce. In the future, with the development of integrated optics technology, APM mode-locked fiber lasers are likely to become more compact and exhibit higher performance. They are also anticipated to develop towards multi-wavelength output and the utilization of novel materials, further broadening their potential in fields like optical communication, spectroscopy, and biomedicine.