SignificanceCoherent diffraction imaging (CDI), a lensless imaging technique, is mainly developed to reconstruct the complex amplitude of objects with simple optical paths. Combining CDI and Ptychography, Rodenburg proposed a Ptychographic Iterative Engine (PIE) in 2004, which scans the sample through a localized probe light to a raster of positions and records all diffraction intensities formed in a far field. The complex amplitude of the object under observation can be reconstructed accurately and promptly with proper overlapping between two neighboring illuminated regions. As a powerful phase retrieval method, the PIE algorithm has been widely applied in bioimaging, wavefront diagnosis, and optical elements measurement. Presently, the PIE has been successfully realized with a high energy electron beam, Xray, visible light, and terahertz wave.Applying the multislice theory of electron microscopy, threedimensional (3D) imaging can also be realized with the PIE by regarding a 3D object as a series of 2D infinitely thin layers. In comparison with traditional 3D imaging methods such as optical coherent and magnetic resonant tomography, which generates intensity images, the 3D PIE (3PIE) can provide a high quality 3D phase image for a transparent volume object rapidly. A singleshot 3PIE was also realized by recording a subdiffraction patterns array with one detector exposure, making 3D phase imaging for dynamic imaging possible.The coherence of synchrotron radiations is not as ideal as that of common laser beams, and a clear reconstruction was not available for the common GS algorithm, Fienup’s algorithms, standard PIE algorithm, and coherent modulation imaging (CMI) algorithm in most of cases. By treating the recorded incoherent intensity I(x, y) as the summation of several coherent diffraction patterns of different wavelengths as I0(x,y)=∑n I ( λ n, x, y ), the light field of each wavelength can be reconstructed separately using the synchronized constrain ∑n I '( λ n, x, y ) / I0 ( x, y ) · I '( λ n, x, y ). This is the principle of multimode PIE (mmPIE). It has been proved experimentally that this multimode PIE can achieve satisfying reconstruction with temporally or spatially incoherent illumination. Several studies on the influence of partial coherence on the reconstruction of CDI have been conducted, and various physical and numerical approaches have been proposed to improve the quality of reconstructed images with partially incoherent illumination.The original PIE algorithm requires a known illumination, and this makes the use of PIE much difficult because the distribution of illumination cannot be accurately measured in majority of cases. In parallel PIE and nonlinear optimization algorithms, both profiles of the target object and probe light can be reconstructed in the absence of prior accurate information on illumination, and a similar strategy based on an extended PIE (ePIE) was proposed by Andy Madden to remarkably improve the convergence speed and robustness to noise. Because of the ability to retrieve illumination and objects simultaneously, ePIE has become the mainstream CDI algorithm.CDI methods including PIE, CMI, and ePIE are all iterative phase retrieval algorithms, and no formula exists to compute the object under inspection directly from recorded intensity in previous decades. The underlying mathematical mechanisms and the existence of a unique solution PIE are problematic and controversial. This makes the error analysis on their reconstructed images impossible and hinders the application of CDI including PIEs optical measurement and metrology fields, where mathematical uniqueness and error analysis are crucial.ProgressTo investigate the underlying physics and mathematics of the CDI technique, a set of linear mathematical modes was set up and an efficient computing method to obtain analytical solutions was proposed. The diffraction intensities were written as a series of linear equations where the sample or illumination spectra were unknown, and the spatial components of the sample and illumination can be analytically determined by solving this linear equation set. The underlying mathematical mode and the existence of the unique solution for mmPIE, 3PIE, and ePIE algorithms were illustrated for the first time in this study. Furthermore, the influences of experimental factors such as detecting noise and positioning error were considered and the robustness to the noise of the proposed method was also testified. The studies have laid a physical and mathematical foundation for CDI as a measurement instrument and provided a guiding error analysis theory.Conclusions and ProspectsCDI technique as an important phase retrieval tool that can achieve high resolution imaging without imaging elements. The mathematical analytic solutions theory makes the diffraction imaging technique more universal and provides theoretical basis in the field of measurement and metrology. In addition, the CDI technique can be applied for beam measurement, wavefront diagnosis of highpower laser devices, thermal distortion measurement, stress measurement, and damage measurement of large aperture optical components, promoting the rapid development of various cuttingedge studies.

SignificancePulsed single-frequency fiber lasers can be applied in fields such as LIDAR, remote sensing, and spectroscopy. For example, with the rapid development of the air travel, meteorology, and clean wind energy sectors, it has become increasingly important to realize long-distance, real-time, and high-resolution detection of 3D wind fields. Therefore, the laser source carried on the LIDAR system should exhibit high power/energy, narrow linewidth, and good beam quality. Based on optical fiber waveguides, fiber laser sources have attracted attention owing to not only their high-performance laser output, but also their high compactness and robustness, which fulfill the requirements of the aforementioned applications.In recent years, significant progress has been made in single-frequency fiber laser techniques in terms of laser power, linewidth, noise, and operation wavelength. Although some reviews on single-frequency fiber lasers have been published, no specific work has focused on pulsed single-frequency fiber laser amplifiers. This motivated us to summarize the progress in pulsed single-frequency fiber amplifiers considering the application areas, critical techniques, and bottlenecks for further development.ProgressConsidering the narrow linewidth of single-frequency lasers, the main issue in the development of pulsed single-frequency fiber laser amplifiers is the severe stimulated Brillouin scattering (SBS) effect. Different strategies have been developed to suppress the SBS effect in pulsed single-frequency fiber amplifiers to improve the laser power and energy. The first is the development of a novel gain-fiber structure. Based on a polarization-maintaining (PM) Er-doped fiber with mode area up to 1100 μm2 (Fig. 1), pulsed single-frequency laser at 1572 nm has been demonstrated with an energy of 541 μJ, while good beam quality can be maintained with an M2 of 1.1 by virtue of 1480 nm core-pumping scheme. Microstructured optical fibers provide more space for achieving a large-mode-area (LMA) single-mode fiber. With 39 Er-doped cores stacked in size of 24 μm×32 μm, a multifilament-core fiber was developed to achieve pulsed single-frequency laser with energy up to 750 μJ, in which the M2 is 1.3 due to the low core numerical aperture (NA) of only 0.022. Moreover, tapered gain fibers with gradually increasing core diameters have also been used to improve the SBS threshold owing to the decreased laser power density. With a PM Yb-doped tapered fiber, whose core and cladding diameters at input and output ports are 17 μm/170 μm and 49 μm/490 μm, linear-polarized single-frequency laser with peak power of 2.2 kW has been demonstrated while the beam quality M2 is only 1.08. Compared with silica glass, multicomponent glass exhibits a much higher rare-earth-ion doping capability and can achieve more precise manipulation of the refractive index, which facilitates a low NA under a large fiber core diameter. mJ-level single-frequency pulsed laser energy was demonstrated based on rare-earth-doped silicate, phosphate, and germanate glass fibers.Temperature and strain gradients have also been employed to manipulate the gain spectrum of Stokes light along the fiber, which decreases the gain accumulation of the SBS effect. With the strain gradient along an Er/Yb co-doped fiber, pulse energy of 540 μJ was demonstrated for a 500-ns single-frequency laser at 1540 nm.Considering that the SBS effect originates from the interaction between the signal light and the acoustic phonons, whose lifetime is approximately 10 ns, the SBS effect can be suppressed using a laser pulse shorter than 10 ns. In addition to a 12-cm long Er/Yb co-doped phosphate fiber, a peak power of up to 128 kW was demonstrated for a 5-ns single-frequency laser at 1.5 μm.Up to now, high-power and high-energy pulsed single-frequency fiber amplifiers have seen significant performance improvement in the wavelength region from 1 μm to 2 μm. In 1 μm, around 913 W average power has been realized for a 3-ns pulsed laser with repetition rate of 10 MHz. A peak power of up to 91 kW was achieved from a 2.4 ns single-frequency pulsed laser based on a commercial Yb-doped LMA silica fiber. At the wavelength region of 1.5 μm, peak power up to 200 kW for a 0.9 ns laser has been demonstrated with a piece of tapered Er-doped fiber and further power improvement was only constrained by the self-phase modulation effect other than SBS effect. For 2 μm pulsed single-frequency laser, around 1 mJ has been demonstrated with 41-cm long Tm-doped germanate fiber.Conclusions and ProspectsOver 20 years of development, the performance of pulsed single-frequency fiber amplifiers in terms of laser power, energy, linewidth, and beam quality has greatly improved. Peak powers of up to hundred kW, and pulse energies at the millijoule level have been demonstrated. For further development, a proper balance between the laser gain and different nonlinear effects, as well as the laser beam quality, should be considered. New gain fiber designs, manipulation of pulse properties in both the time and frequency domains, and the combination of fiber and solid laser amplifiers can be further explored to achieve new milestones in the development of high-performance pulsed single-frequency fiber lasers.

SignificanceThe performance of high-power fiber lasers has rapidly improved over the years. Consequently, high-power fiber lasers have become instrumental in the development of industrial processes and national defense technologies. Researchers are attempting to further improve the output power of high-power fiber lasers, but inefficient transmission methods are limiting their widespread application. Free-space transmission introduces complex spatial optical paths, resulting in insufficient flexibility and stability. Therefore, the demand for high-power laser transmission based on flexible fibers is increasing. Traditional solid silica fibers are available for such applications. However the damage threshold and material nonlinearity eventually affect them and thereby limit the increases of transmission power and length. Moreover, the severe material absorption in the mid-infrared band and the large Rayleigh scattering loss at short wavelengths of silica materials limit the laser transmission range.Hollow-core anti-resonant fibers (HC-ARFs) provide novel solutions for flexible high-power laser transmission by guiding light through air, a vacuum, or a gas-filled hollow core. Compared with solid-core fibers, HC-ARFs inherently reduce the overlap between the optical field and glass structure by five orders of magnitude, which results in low optical nonlinearity and a higher damage threshold. Recently, numerous multilayer structures have emerged, based on the knowledge of light-guiding mechanisms, which rapidly decrease the losses of HC-ARFs. The unique properties of these innovatively structured HC-ARFs offer significant potential in terms of transmission distance, flexibility, and power-handling capacity. In terms of their transmission bands, HC-ARFs regulate the operational window by adjusting the silica wall thickness. HC-ARFs overcome the transmission bandwidth of silica materials and realize laser transmission from the UV band to the mid-infrared band.ProgressThe rapid development of HC-ARFs has attracted extensive attention from researchers in the field of high-power laser transmission, and a series of breakthroughs have been achieved. Owing to the high damage threshold and low nonlinearity of HC-ARFs, most research teams focus on the enhancement of transmission power. Currently, in terms of continuous-wave laser transmission, 3 kW high-power laser transmission over 110 m and 1 kW high-power laser transmission over 1 km have been achieved using HC-ARFs. In terms of nanosecond high-energy laser transmission, 30 mJ single pulse energy transmission has been achieved using HC-ARFs. In terms of ultra-short pulse laser transmission with peak power, 20 GW of peak power transmission has been achieved using HC-ARFs. The above achievements fully demonstrate the enormous potential of HC-ARFs in the field of high-power laser transmission.The implementation of high-power laser transmission in special wavelength bands based on HC-ARFs is also an important direction for development. In the mid-infrared band, HC-ARFs have good physicochemical properties and low absorption losses. Currently, an HC-ARF can achieve up to 6 µm wavelength guidance. In terms of high-power laser transmission, our team has exceeded 20 W average power in the 3.1 µm band using HC-ARFs. In the ultraviolet bands, HC-ARFs overcome the limit of Rayleigh scattering loss to achieve low loss transmission. Our team realized high power ultrashort pulse laser transmission in the UV band using HC-ARFs, achieving a peak power of 5.3 MW.Conclusions and ProspectsHC-ARFs have the characteristics of low nonlinearity, low dispersion, a high damage threshold, and a controllable number of transmission modes. High-power laser transmission based on HC-ARFs has become a research hotspot. With the improvement of the structural design and fabrication processes for HC-ARFs, the transmission losses of multiple important laser bands are significantly reduced. Research reports on high-power laser transmission based on HC-ARFs continue to emerge, and high-power continuous laser and pulse laser transmission from the ultraviolet to mid-infrared bands has been achieved. However, there is still significant space for improvement, such as further improving transmission power and efficiency and exploring full fiber integrated transmission methods under high power, among others. With the in-depth research and resolution of related technical difficulties, high-power laser transmission technology based on HC-ARFs will become a new generation of transmission solutions and thereby promote the rapid development of related application fields.

SignificanceBeam combining of fiber laser is an effective way to break the power limitation of single fiber laser and to achieve more powerful laser beams. In recent years, significant breakthrough has been achieved in power combining, spectral beam combining and coherent beam combining. In this paper, the recent progress of beam combining of fiber lasers is reviewed, the trend and the characteristics are summarized, and the challenges for further increasing the performance of laser beam combining systems are discussed.ProgressAll fiber signal combiner is widely used for power combining because of the advantage of compact structure. In 2013, IPG Photonics Corporation developed the world’s first 100 kW high-power fiber laser system, which was based on 90 fiber lasers with kW level output power. And such a 100 kW laser system was successfully applied to laser processing. In 2021, the 100 kW level fiber laser systems, which were power combined by using all fiber signal combiner, were reported by University of South China and Raycus Fiber Laser Technologies Corporation. In 2024, high-power fiber laser systems from 150 kW to 200 kW were reported by BWT Corporation, Han’s Laser Technology Industry Corporation, and Maxphotonics Corporation, respectively.For spectral beam combining, most of the high power outputs were achieved by using reflective diffraction gratings or dichroic mirrors as the beam combination components. In 2016, Lockheed Martin achieved a fiber laser system with output power of >30 kW by spectral beam combining of 96 individual fiber lasers based on reflective diffraction grating. Using the similar method, 10 kW level spectral beam combining systems were developed by Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences and China Academy of Engineering Physics in the same year. In recent years, the output power of the diffraction grating based spectral beam combining system was increased rapidly. By spectral beam combining of 60 fiber lasers, output power of 150 kW was obtained by Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. By using dichroic mirrors, several fiber lasers can also be cascaded combined into one beam. 10 kW level output power was generated by spectral beam combining of 2‒3 fiber lasers based on dichroic mirrors by Nanjing University of Science and Technology, National University of Defense Technology and the 11th Research Institute of China Electronics Technology Group Corporation.And then, the progress of coherent beam combining of nonlinear frequency conversion laser and ultrafast laser was reviewed in this paper. In some practical applications, lasers in ultraviolet, visible and mid-infrared bands are required. So, coherent beam combining was employed for power scaling of nonlinear frequency conversion lasers. For example, the average powers of 600 W at 520 nm (second harmonic) and 300 W at 347 nm (third harmonic) were generated from an eight-beam, sub-nanosecond fiber laser system by Osaka University. For another example, coherent combining of two mid-infrared difference frequency generators by active phase control was reported by the French Aerospace Lab. In recent years, great progress has also been made in coherent combining of ultrafast fiber lasers. In 2020, ultrafast pulses with 10.4 kW average output power based on coherent combining of 12 fiber amplifiers was reported by Friedrich Schiller University Jena, and coherent beam combining of 61 femtosecond fiber amplifiers in a tiled aperture configuration with combining efficiency of ∼50% was achieved by Institut Polytechnique de Paris. In 2023, by combining of 16 fiber amplifiers, the nearly Fourier transform limited pulses with an energy of 32 mJ and a duration of 158 fs were obtained by Friedrich Schiller University Jena. Besides, coherent beam combining technique has also been employed in versatile applications, such as laser guide star, laser interferometer gravitational-wave observatory (LIGO), laser communication and optical field manipulation.Besides, those three beam combining technologies can be used in a hybrid way, therefore more powerful laser and much narrower pulse width could be expected. For example, coherent spectral combining technique was proposed to generate pulses with shorter pulse durations.Conclusions and ProspectsBy using power, spectral and coherent combining technologies, the output power has been increased by a magnitude compared with that from a single fiber laser. And beam combining technologies have been moved from laboratory researches to practical applications, such as laser processing, laser communication and national defense. The developments of beam combining technologies have great significance in the history of high-power fiber laser, and demonstrate the possibility of obtaining higher output power and developing ultra-large scale fiber laser systems in the future. The current researches of beam combination of fiber lasers are basically concentrated in the near infrared band, and combining of lasers in ultraviolet, visible and mid-infrared bands will be an important direction of the next development. With the breakthrough of technologies such as deep learning in recent years, artificial intelligence has been used in the field of laser beam combining, and many outstanding research results have emerged. In the future, artificial intelligence can be further used in modulation signal optimization, combination element design, multi-parameter control, beam quality evaluation, and so on.

SignificanceMid-infrared lasers operating in the wavelength region of 2.5‒5.0 μm, which overlap both the atmospheric window and the so-called molecular fingerprint region, have found a growing number of applications in defense, medical treatment, and advanced scientific research. Compared with other laser sources, rare-earth-ion-doped fiber lasers exhibit the advantages of power scalability, wavelength tunability, thermal management, and beam quality, and are thus a promising approach for generating high-power mid-infrared laser emission. Of the different types of mid-infrared rare-earth-doped fiber lasers, Er-doped fluoride fiber lasers are the most studied because they can be conveniently pumped by laser diodes and have a mature fiber material base. Er ions provide two significant mid-infrared emission bands in fluoride glass hosts, namely, ~2.8 and ~3.5 μm, which correspond to 4I11/2→4I13/2 and 4F9/2→4I9/2 transitions, respectively. With the development of ZBLAN fibers and ZBLAN fiber-based devices, the performance of mid-infrared Er-doped ZBLAN fiber lasers has benefitted significantly. In this study, we review the recent research progress of 2.8 μm and 3.5 μm continuous-wave (CW) Er-doped fluoride fiber lasers, including improvements in output power scaling, efficiency, and wavelength extension.ProgressFor 2.8 μm Er-doped fluoride fiber lasers, output power scaling has been mostly limited by the self-termination of 4I11/2→4I13/2 transition induced by the long lifetime of the lower laser level. The most popular approach to depopulate the lower laser level is to use heavily Er-doped fluoride fibers (typically >7%) via the strong energy upconversion transfer (ETU) process (Fig. 2). In a previous study, a record output power of 41.6 W was obtained using this approach in a dual-end pumped Er-doped fluoride fiber laser; to date, this remains the highest mid-infrared laser output power obtained from rare-earth-doped fibers (Fig. 3). However, the high quantum defect-induced heat accumulation subjects the fiber tip to thermal damage, particularly in this type of heavily doped fiber. To reduce the heat load, researchers proposed a 2.8 μm/1.6 μm cascaded lasing scheme, depopulating the lower laser level via 4I13/2→4I15/2 transition instead of through phonon relaxation (Fig. 4). This method does not rely on the ETU process and thus allows for the use of lightly Er-doped fluoride fibers for output power scaling. Based on this approach, a 10 watt-level 2.8 μm Er-doped fluoride fiber laser with a high efficiency of 50% (as compared with absorbed 0.98 μm pump power) was demonstrated (Fig. 5). In addition, Er ions have shown strong excited state absorption (ESA, 4I13/2→4I9/2) at 1.6‒1.7 μm, and the overlap with the ground state absorption (GSA) spectrum enables the Er-doped fluoride fiber to be directly pumped by the 1.6‒1.7 μm fiber laser (Fig. 6). Through this new pumping scheme, the efficiency of 2.8 μm Er-doped fluoride fiber lasers has been increased to greater than 50%.For 3.5 μm Er-doped fluoride fiber lasers, the major milestone is the development of the 0.98 μm+2 μm dual-wavelength pumping scheme (Fig. 8), wherein the Er ions are first pumped at 0.98 μm via GSA to provide initial ion accumulation at the 4I11/2 level or virtual ground state (VGS) and are then pumped at 2 μm to populate the 4F9/2 level through VGS absorption (VGSA). This pumping scheme effectively addresses the bottleneck induced by ion accumulation in the longer-lived levels, based on which a 15 W record output power at 3.55 μm has been obtained from an all-fiber Er-doped fluoride fiber laser (Fig. 9). The broad emission band of 4F9/2→4I9/2 transition with the aid of a wavelength selector enables the operating wavelength of Er-doped fluoride fiber laser to be further extended. For example, a 2 W output at 3790 nm has been achieved with a fiber Bragg grating (FBG)-based all-fiber configuration. Continuous wavelength tuning over a 450 nm span (3.33‒3.78 μm) was also demonstrated using diffraction grating (Fig. 10). Recently, by optimizing the cavity arrangements, our group further extended the operating wavelength to 3810 nm, which is the longest wavelength achieved with Er-based lasers. In addition, single-frequency operation and pulsed laser emission have also been demonstrated in this long-wavelength region.Conclusions and ProspectsWith the development of laser diodes, soft glass fibers, and fiber devices, rare-earth-doped fiber-based mid-infrared laser sources have rapidly developed in recent years. Due to their advantages of pumping convenience and their use of fiber materials, Er-doped fluoride fiber lasers are some of the most widely researched lasers. Many studies have reported major breakthroughs in output power scaling and operating wavelength extension. Future development of Er-doped fluoride fiber lasers should focus on the following two directions. First, new fiber glass with a high damage threshold and broad transparent windows should be developed, and existing fiber fabrication techniques should be improved to provide a high-performance gain medium for mid-infrared lasers. Second, mid-infrared fiber-based devices should be further developed using FBG and signal/pump combiners. These devices are the means by which mid-infrared oscillators and amplifiers with all-fiber configurations can be produced. Through these approaches, we believe that mid-infrared fiber lasers will eventually progress from mere laboratory status to more practical uses, thereby promoting the development and progress of technology in the industrial, medical, defense, and related fields.

SignificanceRandom fiber lasers (RFLs) utilize the distributed feedback in optical fibers to form resonant cavities of random lengths. They combine the low coherence of random lasers with the high brightness of fiber lasers. Therefore, RFLs have been widely used in fields such as environmental sensing and optical communications.Because all-fiber-integrated RFLs were proposed using Rayleigh backscattering as a feedback mechanism, RFLs have attracted significant attention in the power scaling and wavelength extension of fiber lasers. Rayleigh backscattering occurs in silicon fibers owing to disordered fluctuations in the refractive index, which provides randomly distributed feedback for RFLs. Because the output of an RFL is the sum of the resonant cavities with random lengths, the RFL is free of the self-pulsing effect, unlike traditional fiber laser generated in a fixed cavity. Amplifying the RFL through the master oscillator power amplification (MOPA) configuration can suppress spectral broadening because no peak power from the RFL seed is enhanced during this process. Control experiments were conducted in a 10 kW-level MOPA system by adopting an RFL and a fiber laser with a fixed cavity as the seed. The results demonstrate a suppression effect.Rayleigh backscattering provides broadband reflection in optical fibers, which can replace the reflection component in wavelength-tunable fiber-laser systems and supercontinuum (SC) sources. Therefore, RFLs can easily achieve wavelength tunability using a single-frequency selection component. By taking advantage of cascaded Raman scattering, the operating wavelength of the RFL can cover the transmission band of the silicon fiber. Research on 1.1‒2.0 μm RFLs proves the flexibility of their output wavelength. In addition, the modulation instability provides a wide band gain for light waves near the zero-dispersion wavelength of the optical fiber. This type of light wave can be generated by cascaded Raman scattering in the RFLs. In addition, the nonlinear effect is enhanced by Rayleigh backscattering because it increases the effective length. Thus, RFLs are an excellent choice for SC sources. The feedback provided by the optical fiber itself, without extra optical components, promises great capability for high-power handling; therefore, RFLs can be a good platform for high-power SC generation.ProgressIn the second section, generation methods for high-power RFLs are introduced and hundred-watt- to kilowatt-level RFL oscillators are summarized (Fig. 2). Furthermore, MOPA configuration seeding by RFLs is introduced to further scale their power (Fig. 4, Table 1). The suppression of stimulated Raman scattering (SRS) contributes to the 10 kW-level amplification of the RFL. In the third section, the flexibility of the operating wavelength enabled by the RFL configuration is reviewed and the wavelength-tunable RFLs gained by rare-earth ions are summarized (Fig. 6). The broadband feedback of Rayleigh backscattering simplifies the structure for wavelength tuning. Taking advantage of cascaded Raman scattering, random Raman fiber lasers (RRFLs) are introduced (Fig. 7). An amplification configuration adopting a hybrid gain of ytterbium ions and Raman scattering is used to achieve a high-power RRFL (Fig. 8). In the fourth section, SC generation in optical fibers using an RFL is reviewed. Both half-open-cavity (Fig. 9) and full-open-cavity RFLs (Fig. 10) can be utilized to realize an SC output whose spectral range can cover the transmission band of the silicon fiber. By combining Rayleigh backscattering and continuous-wave pumping, the average power of the SC generation in the RFL is scaled from the hundred-watt to 3 kW level (Fig. 11, Table 2). In the fifth section, practical applications of RFLs are discussed. Owing to their high brightness and low coherence, RFLs enable speckle-free imaging and are compatible with fiber-integrated imaging systems (Figs. 12 and 13).Conclusions and ProspectsRFLs pave the way for power scaling and wavelength extension of high-performance fiber lasers. Their temporal stability contributes to the suppression of spectral broadening during power amplification. The broadband feedback of Rayleigh backscattering and the gain of the cascaded Raman effect make them suitable for the wavelength extension of fiber lasers. Rayleigh backscattering not only simplifies the structure to achieve fiber lasers operating in a broad spectral range but also improves the power-handling ability of the reflection component. Because of their high brightness and low coherence, RFLs have been widely used in fields such as imaging through fibers and inertia-confinement fusion.

SignificanceCharacterized by ultrashort pulse and ultra-high peak power, high intense lasers provide unprecedented experimental possibilities and extreme physical conditions for human beings, thus enabling various applications in major scientific and technological frontier fields, such as advanced manufacturing, high-order harmonic generation, particle acceleration, and ultra-high-speed phenomenon research. However, owing to limitations by the thermo-optic effect, scaling the average power and repetition frequency of ultrafast intense lasers is challenging, and discrepancy by an order of magnitude exists between the current technologies and the relevant application requirements (Fig. 1). To achieve ultrafast intense lasers with higher average power, researchers have adopted multichannel fiber-laser coherent-beam combination (CBC) to generate ultrashort laser pulses with high peak and average power levels simultaneously. Specifically, owing to the merits of large surface-to-volume ratio and excellent thermal optical performance endowed by the unique waveguide structure of optical fiber, fiber lasers can realize a high conversion efficiency (electro-optical efficiency exceeding 30%) via a flexible and compact configuration, which is beneficial for implementing a multichannel CBC system. Nevertheless, demonstrating the CBC of ultrafast fiber lasers is extremely challenging owing to the requirement of high precision control of multidimensional parameters such as delay, phase, beam pointing, and polarization. Over the past 15 years, significant efforts have been devoted to address those issues, and significant progresses have been achieved regarding the average power and energy scaling of ultrafast fiber-laser CBC systems. Specifically, the development of time-domain CBC and multidimensional parameter control technologies for fiber lasers has further enhanced CBC, thus reflecting the integrated development of ultrafast intense lasers technology and fiber-laser CBC technology. Based on this perspective, the research progress and development status pertaining to the CBC of ultrafast fiber lasers domestically and internationally in recent years are comprehensively reviewed herein, and the development trend of domestic high-power ultrafast intense lasers based on CBC technology is prospected.ProgressAt the early stage of the CBC of ultrafast fiber lasers, the general scheme is similar to that of the continuous-wave counterparts, i.e., the spatial-domain CBC, which includes tiled- and filled-aperture configurations. Tiled-aperture CBC was first proposed for the International Coherent Amplification Network (ICAN) project (Fig. 2), which aims to realize a single-pulse energy of 10 J with a repetition rate of 10 kHz by combining ten thousands of fiber chirped pulse amplifiers (CPAs). Currently, researchers have realized 61 channels of tiled-aperture CBCs (Fig. 3), whereas the corresponding average power of a single channel is only 25 W. Additionally, the combing efficiency is intrinsically limited by the space duty cycle of the fiber-laser array (maximum of 67%). Meanwhile, filled-aperture CBC is based on the successive combination of two collimated beams achieved using spatial optical components, and the combining efficiency can reach 100% in ideal conditions (Fig. 5). Thus far, ultrafast lasers with an average power exceeding 10 kW has been realized through the filled-aperture CBC of 12 high-power CPAs (Fig. 6), and a higher power of 100 kW is anticipated.For the time-domain CBC, the main schemes are divided pulse amplification (DPA) and pulse stacking assisted by passive resonators. In principle, DPA leverages polarization beam splitting and delay control to segment a pulse into several discrete subpulses, which are subsequently amplified (most likely in a single channel CPA) and then combined into a single pulse via the reverse process of pulse dividing for power and energy scaling (Fig. 8). The pulse stacking includes two representative techniques, i.e., the stack-and-dump enhancement cavity (Fig. 10) and the Gires–Tournois interferometer (Fig. 11), which can stack a pulse train into a single pulse to increase the pulse energy, although at the expense of decreased repetition rate and average power. In recent years, to realize high-power and -energy ultrafast lasers, researchers have developed the spatiotemporal CBC technique, which associates the advantages of spatial- and time-domain CBC. Currently, this technique has afforded a maximum average power of 700 W and a pulse energy of 32 mJ. Moreover, ultrafast lasers with hundreds of kilowatts of average power and tens of joules of pulse energy are believed to be compatible with such a scheme (Fig. 13).Conclusions and ProspectsThe development of ultrafast fiber-laser CBC technologies in recent years has enabled the achievement of fiber lasers that exhibit high power, high energy, and narrow pulse widths simultaneously, thus enabling novel applications. Nonetheless, the discrepancy between related domestic and international research progresses is significant. However, ultrafast fiber-laser CBC is anticipated to develop rapidly in China, where solid research outcomes have been obtained concerning the relevant unit technologies.

SignificanceCompared with traditional random polarization fiber lasers, some special applications (such as coherent beam combining and nonlinear frequency conversion) require fiber lasers with high power, a narrow linewidth, linear polarization, and good beam quality output. To fulfill this requirement, high-power narrow-linewidth linearly polarized fiber lasers have recently attracted extensive attention, and their output power has increased from 1 kW to 5 kW in the last decade. The output power of narrow linewidth linearly polarized fiber lasers used for coherent beam combining and nonlinear frequency conversion has also significantly improved. This paper reviews the recent development of high-power narrow-linewidth linearly polarized fiber lasers. Representative application results for high-power narrow-linewidth linearly polarized fiber lasers in coherent beam combining and nonlinear frequency conversion are also presented.ProgressHigh-power narrow-linewidth linearly polarized fiber lasers are generally based on a master oscillator power amplifier (MOPA) structure. Several types of laser seeds have been used in these MOPAs, including single-frequency, phase-modulated single-frequency, and fiber-oscillator laser seeds. At present, a single-frequency linearly polarized laser has an output on approximately the kilowatt level, but the stimulated Brillouin scattering (SBS) effect seriously restricts further increases in output power. A linewidth on approximately the gigahertz scale can also meet the requirements for coherent beam combining and nonlinear frequency conversion. The SBS effect on the amplification process can be greatly suppressed by using the phase modulation technique to broaden the spectrum of a single-frequency laser, making it possible to achieve a higher output with a narrow-linewidth linearly polarized fiber laser. Currently, sine-wave, white noise signal (WNS), pseudo-random binary sequence (PRBS), and optimized signal phase modulation techniques are used in high-power narrow-linewidth linearly polarized fiber laser systems. At present, narrow-linewidth linearly polarized MOPAs based on these phase modulation methods have achieved laser outputs measured in kilowatts. Among them, the most representative is the realization of a 5 kW linearly polarized power output with a 10 GHz linewidth based on nonlinear optimized signal phase modulation, which fully verified its ability to obtain an output with higher power and a narrower linewidth. A MOPA based on a narrow linewidth fiber oscillator seed is also an effective method to achieve a high-power narrow-linewidth linearly polarized fiber laser output. Currently, this method can be used to achieve a narrow-linewidth linearly polarized power output with a maximum value of 4.6 kW and linewidth of 91 GHz. Narrow-linewidth linearly polarized MOPAs based on these different seed sources have different advantages and disadvantages. In general, a single-frequency or single-frequency phase-modulated seed source has a stable time domain, and the spectrum broadening is not obvious during amplification, which has a high stimulated Raman scattering (SRS) threshold. However, SBS is an important limiting factor. A narrow-linewidth linearly polarized MOPA based on a fiber-oscillator seed has a simple structure and low cost, along with a high SBS threshold. However, self-phase modulation (SPM) and four-wave mixing (FWM) cause a serious spectrum-broadening effect during the amplification process. At the same time, because of the time-domain instability of the multi-longitudinal mode seed, the SRS threshold during the amplification process is much lower than the theoretical expectation.This paper also presents representative results for the application of high-power narrow-linewidth linearly polarized fiber lasers in the fields of coherent beam combining and nonlinear frequency conversion. A 21.6 kW laser output has been achieved by using 19 narrow-linewidth linearly polarized fiber amplifiers for aperture-divided coherent beam combining. Recently, the maximum number of combining channels has also exceeded 1000. A 5.02 kW laser output has been achieved by using four narrow-linewidth linearly polarized fiber amplifiers for filled-aperture coherent beam combining. For nonlinear frequency conversion, a kilowatt continuous wave 532 nm green laser has been realized based on the frequency doubling technology of a high-power narrow-linewidth linearly polarized fiber laser.Conclusions and ProspectsAlthough the output power of linearly polarized fiber lasers has reached 5 kW, there is still a significant difference in the maximum output power values of randomly polarized fiber lasers. In the future, based on the subsequent development of polarization-maintaining fiber materials, artificial intelligence, and other technologies, further improvements in the output performances of narrow-linewidth linearly polarized fiber lasers are expected to support future applications such as higher power coherent synthesis and nonlinear frequency conversion.

SignificanceShort-cavity single-frequency fiber lasers and high-repetition-rate passively mode-locked fiber lasers based on high-gain rare-earth (RE)-doped fibers have significant applications in the manufacturing, healthcare, and military fields. Although RE-doped silica fibers have advantages such as low propagation loss and easy splicing, the low fiber unit gain coefficient caused by the low doping concentration of RE ions severely limits the performance of short-cavity lasers. In recent years, with the expansion of fundamental research on RE-doped silica glasses and significant improvements in fiber fabrication techniques, RE-doped silica fibers have been developed and successfully applied for short-cavity lasers. The performance of short-cavity lasers at various wavelengths has been significantly improved, unleashing their potential for application. Considering fiber fabrication techniques and short-cavity laser applications using different RE-doped silica fibers, this review systematically analyzes the latest developments and application progress of highly RE-doped silica fibers and predicts their future development.ProgressOwing to the continuous improvement of fiber fabrication techniques and the further development of basic research on RE-doped silica glasses, the doping concentration of RE ions and the pump absorption coefficient of silica fibers have been significantly improved in recent years. Table 4 lists the high absorption coefficient Nd3+, Yb3+, Er3+, and Tm3+-doped silica fiber products developed by special fiber manufacturers such as Nufern, Coractive, and Liekki. The most typical is the Coractive Yb406 Yb3+-doped silica fiber, which has a core absorption coefficient of 2400 dB/m at 976 nm. According to estimation, the Yb doping content (mass fraction) in the core can reach 5% (or even higher), which is significantly higher than the Yb doping content (mass fraction) of conventional Yb3+-doped silica optical fibers (~1%). Taking these high-absorption RE-doped silica fibers as gain media, many research groups have successfully demonstrated their applications in short-cavity single-frequency and high-repetition-rate mode-locked fiber lasers with wavelengths ranging from the 0.9 to 2.0 μm bands. For instance, in the 0.9 μm wavelength band, the research team from the Shanghai Institute of Optics and Fine Mechanics successfully developed a highly Nd3+-doped silica single-mode fiber with a 4.1 dB/cm pump absorption coefficient. The research team demonstrated DBR single-frequency lasers at 890‒910 nm based on this fiber, extending the single-frequency laser of RE-doped silica fibers to below 900 nm (Fig. 11). Additionally, they also demonstrated an over 200 MHz high-repetition-rate passively mode-locked laser at 920 nm with an F‒P cavity structure (Fig. 12).In the 1.0 μm wavelength band, in 2023, a research team from Tianjin University realized a high-efficiency 1064 nm single-frequency DBR fiber laser using the above mentioned Coractive Yb406 fiber. The slope efficiency reached 66.4% with only a 1.2 cm Yb3+-doped fiber (Fig. 13), which is the highest efficiency recorded for short-cavity single-frequency lasers using Yb3+-doped silica fibers. This efficiency is comparable to that of high-gain Yb3+-doped phosphate fiber, indicating that the fabrication technique for high-doping-concentration RE-doped silica fibers has been significantly improved.In the 1.5 μm wavelength band, in 2021, the research team from Shandong University conducted studies on a high-repetition-rate passively mode-locked laser using a commercial highly Er3+-doped silica fiber (Liekki-Er110-4/125). Through in-depth analysis of SESAM (semiconductor saturable absorber mirror) parameters and experiments, they noted that an SESAM with a smaller modulation depth was better for high-repetition-rate passively mode-locked lasers. After optimization, they realized a 5 GHz fundamental-repetition-rate passively mode-locked laser at 1.5 μm using only a 2.0 cm Liekki-Er110 fiber as the gain medium (Fig. 20).In the 2.0 μm wavelength band, in 2024, a research team from the University of Adelaide in Australia realized single DBR fiber lasers at 1908 nm, 1950 nm, 1984 nm, and 2050 nm, using a custom high-absorption 5/125 Tm3+-doped silica fiber from Coherent. The numerical aperture of the fiber core is 0.21, and the absorption coefficient at 1540 nm is 195 dB/m±6 dB/m. The length of the Tm3+-doped silica fiber is 25 mm. The slope efficiencies corresponding to the 1908 nm, 1950 nm, 1984 nm, and 2050 nm bands are 33%, 37%, 48%, and 26%, respectively (Fig. 22). Among them, the direct output power from the cavity at 1984 nm exceeds 1 W. The efficiency and power at 1984 nm are currently the highest recorded for a DBR single-frequency laser at a wavelength of 2.0 μm.Conclusions and ProspectsRE-doped specialty optical fibers are crucial components of fiber lasers. For short-cavity fiber lasers, the performance of RE-doped fibers directly determines the laser parameters, including laser efficiency, wavelength, and repetition rate. As indicated above, highly RE-doped silica fibers are being increasingly applied in short-cavity lasers in the 0.9‒2.0 μm bands. Although some laser parameters obtained from RE-doped silica fibers are still worse than those from RE-doped soft glass fibers, the performance gap between RE-doped silica fibers and RE-doped soft glass fibers is constantly narrowing, indicating the application feasibility of RE-doped silica fibers in short-cavity lasers. However, we note that the fabrication techniques of highly RE-doped silica fibers are mainly mastered by international manufacturers such as Liekki, Nufern, and Coractive, and in comparison, domestic research and fiber product development are lagging behind and require further breakthroughs. For the application of single-frequency narrow-linewidth and high-repetition-rate mode-locked fiber lasers, further research on highly RE-doped silica fibers can be conducted considering two aspects: further improvement of the performance of highly RE-doped silica fibers and short cavity lasers and the regulation of the fiber gain spectrum.

SignificanceThe mid-infrared (MIR) spectral band, spanning the 2.5‒5.0 µm wavelength range, is included within the atmospheric transmission window and features medium molecular absorption and concentrated thermal radiation energy. These unique characteristics render MIR lasers crucial for applications in atmospheric remote sensing, medical diagnostics and therapy, gas sensing, precision detection, and material processing. Among the various laser technologies, fiber lasers are particularly favored because of their excellent beam quality, compact structure, effective heat dissipation, and high optical-to-optical conversion efficiency. With advancements in fiber laser technology, the output wavelength range has gradually expanded into the MIR band. Fluoride glasses, which are known for their low phonon energy, broad MIR transmittance, and high solubility for rare-earth ions, have been identified as ideal gain media for MIR lasers. Among the various types of fluoride glass, fluoroaluminate glass stands out because of its moisture resistance and mechanical strength, which significantly promote its application in high-power laser systems. This study traces the evolution of the composition, structure, MIR emission characteristics, and fiber fabrication techniques of fluoroaluminate glasses since the 1950s, focusing on the latest representative research findings on MIR fluoroaluminate fiber lasers. Hence, this study provides a strong basis for the future research on MIR fluoroaluminate fiber lasers and the applications of the technology.ProgressThe development of fluoroaluminate glass, with its unique properties (including its low refractive index, high glass transition temperature, and enhanced mechanical strength), has attracted significant interest since it was patented by Sun in 1949. The glass-forming regions of the AlF3-RF2, AlF3-RF2-YF3 (AYF), and AlF3-YF3-PbF2 (AYP) systems have been extensively studied, leading to improved stability and reduced crystallization tendencies, as shown in Figs. 1‒4. The structure of fluoroaluminate glass, which is primarily composed of [AlF6]3- octahedra, has been investigated using techniques such as X-ray diffraction, neutron diffraction, Raman spectroscopy, and molecular dynamics simulation, revealing insights into its network structure. Furthermore, the MIR luminescent properties of fluoroaluminate glass doped with rare-earth ions have been investigated, with significant achievements in realizing emissions at 2.7 μm, 3.5 μm, and beyond, thereby demonstrating the potential applications of the material in the advancement of laser technology.There have been several innovations in the techniques used for the fabrication of fluoroaluminate glass fibers. The development of AlF3-BaF2-CaF2-YF3-SrF2-MgF2-ZrF4-NaF(AZF) glass fibers with low losses is a particularly significant milestone. The subsequent successful fabrication of glass fibers from both A1F3-BaF2-YF3-PbF2-MgF2 (ABYPM) and AYF glass systems underscores the progressive refinement of fluoroaluminate fiber technology.Fluoroaluminate glass fibers with higher mechanical strength and stronger resistance to moisture can theoretically facilitate the output and stability of high-power MIR lasers. Continuous research on fluoroaluminate fibers doped with rare-earth ions, such as Er3+, Ho3+, and Dy3+, has resulted in the active development of MIR fiber laser technology. In 1990, Yanagita et al. reported a 2.715 μm laser that employed Er3+-doped AZF glass fiber with a maximum output power of 2.1 mW. Over nearly 25 years of development, the outputs of MIR fluoroaluminate fiber lasers have been scaled up to approximately 10 W, as shown in Table 3. In 2018, Jia et al. used a 1120 nm fiber laser to pump Ho3+-doped ABYPM glass fiber, achieving laser output at 2.868 μm. The maximum unsaturated output power was 57 mW with a slope efficiency of 5.1% (Figs. 7 and 8). In the following two years, He et al., Wang et al., and Zhang et al. optimized the doping of rare-earth ions, using co-doped Ho3+/Pr3+ to reduce the population of the laser lower energy levels, and achieved higher power and higher efficiency MIR laser output at approximately 3 μm. Significantly, Liu et al. achieved MIR laser emission with a Watt level of approximately 2.87 μm in 2021 (Figs. 12‒14). In 2022, Xu et al. first reported the use of 800 nm femtosecond laser line-by-line direct writing technology to write fiber Bragg gratings into co-doped Ho3+/Pr3+ AZF glass fibers (Figs. 16 and 17). The scheme to enhance the output performance of approximately 3 μm lasers via cascaded laser output has also been successfully implemented. In 2024, Liu et al. used a 1150 nm Raman fiber laser to pump Ho3+-doped AYF glass fiber and achieved cascaded laser output at approximately 3 μm and approximately 2 μm under normal atmospheric conditions at room temperature and humidity of 40%. The maximum unsaturated output power was 11.6 W (5.82 W @~3 μm and 5.76 W@~2 μm) with a slope efficiency of 29% (Fig. 15). This comprehensive overview of the evolution of fluoroaluminate glass and its pivotal role in the advancement of MIR laser technology, highlights the significant strides made in this field, and establishes a robust foundation for future innovations.ProgressConclusions and Prospects In this study, we review the progress in the research on fluoroaluminate glass and optical fibers in the development of MIR lasers and analyze the significance of the material with regard to the advancement of laser technology. Recent studies have demonstrated that MIR fluoroaluminate fiber lasers achieve remarkable output levels (approaching approximately 10 W) when a 1150 nm fiber laser is employed as the pump source. In the future, by developing new types of aluminum fluoroaluminate glass systems, exploring the preparation of double-cladding fluoroaluminate fibers, researching fiber grating engraving technology, and studying heterojunction fiber fusion technology, an all-fiber-structure MIR fluoroaluminate fiber laser can be developed. Such lasers are expected to deliver higher power outputs, improved efficiency, and extended wavelengths in the MIR spectrum. These advancements aim to satisfy the growing demands of practical applications and are expected to drive new breakthroughs in both the scientific research on MIR lasers and the industrial applications of the technology.

SignificanceStimulated Raman scattering offers both advantages and disadvantages to high-power fiber lasers. Although it limits the power scaling of rare-earth-doped fiber lasers, it is currently the only mechanism that can generate high-power lasers at new wavelengths, thus rendering it a “promising asset.” Owing to the rapid development of fiber and semiconductor technologies, Raman fiber lasers based on stimulated Raman scattering (SRS) have enabled new achievements. They have the output power of Raman fiber lasers at a single wavelength exceeded 10 kW, and the power across various new bands has increased significantly, thus fully demonstrating their significant potential for wavelength extension. This study summarizes the research progress of continuous-wave near-infrared Raman fiber lasers from two aspects: power scaling and wavelength extension. It outlines the unique gain, mode, and nonlinear integrated control technology pathways, as well as provides references and insights for the further development of high-power fiber lasers.ProgressIn recent years, owing to the rapid development of rare-earth-doped fiber lasers, the power level of Raman fiber lasers using high-power, high-brightness ytterbium-doped fiber lasers as pump sources has exponentially increased annually. Combined with methods such as cladding pumping and hybrid gain, Raman fiber lasers have achieved a power breakthrough at the 10-kW level, i.e., approaching the highest power level of rare-earth-doped fiber lasers. Raman fiber lasers based on pure Raman gain only, despite issues such as spectral broadening and insufficient pump conversion, have achieved the highest output powers of 4 kW and 1.8 kW for amplifiers and oscillators, respectively. Meanwhile, wavelength extension based on Raman fiber lasers has progressed significantly, with the output wavelength extending from the conventional 1130 nm and 1120 nm to the shortest visible light and the longest to the mid-infrared wavelengths; the spectral range with output powers of several hundred watts encompasses more than 700 nm.Currently, domestic and international review literatures pertaining to Raman fiber lasers, including the main research topics, are abundant. Owing to space limitations, this study focuses on summarizing the representative research achievements of near-infrared Raman fiber continuous lasers in terms of power enhancement and wavelength expansion, based on the aforementioned results. The types of fibers are primarily silicon-based fibers, and the pump sources include both fiber and semiconductor lasers, as well as different laser structures such as oscillators, amplifiers, and random cavities. Among these, power enhancement primarily includes two types of gain: ytterbium-Raman hybrid gain and single-stage pure Raman gain, whose main results are summarized in Table 1/Fig. 2 and Table 2/Fig. 4, respectively. Wavelength expansion primarily involves two implementation approaches: cascaded Raman and specific wavelength pumping (primarily direct pumping by semiconductor lasers), whose main results are summarized in Table 3/Fig. 11 and Table 4, respectively.Conclusions and ProspectsThe rapid development of Raman fiber lasers clearly indicate significant achievements in terms of power enhancement and wavelength expansion. Currently, single-stage Raman fiber lasers pumped by fiber lasers and laser diodes (LDs) have reached power levels of 10 kW and the kilowatt range, respectively, thus rendering them an effective method for expanding the high-power laser spectrum in the vicinity of the 1-μm spectral band. Additionally, cascaded Raman fiber lasers are an important scheme for achieving high-power laser outputs over a larger spectral range, although the power level remains at approximately hundreds of watts. Although the mode and target-wavelength laser gain must be further regulated, one can assume that Raman fiber-laser technology primarily entails the suppression of higher-order Raman scattering. Therefore, the development of Raman fiber lasers must account for the characteristics of different Raman conversion processes. Additionally, the design of Raman gain fibers, the pumping methods, and the temporal, spatial, and spectral domain characteristics of the pump and seed lasers must be optimized comprehensively to achieve comprehensive control of the laser gain, nonlinear effects, and beam quality.

SignificanceThe 2 μm laser operates in the human eye-safe wavelength range, with excellent atmospheric transmittance and non-metallic material absorption characteristics. Combined with the high power, high beam quality, high conversion efficiency, and high integratin capacity of the fiber lasers, the 2 μm laser is widely used in LIDAR, industrial processing, national defense and security, and biomedical applications. Recently, 2 μm high-power thulium-doped fiber lasers (TDFLs) are being developed rapidly with output power exceeding the order of kilowatts, for applications in medical care, military security, space communications, atmospheric pollution detection, and materials processing. However, the thermal effect, mode instability, and nonlinear effect severely limit the improvement in the output power of thulium-doped fiber lasers.ProgressIn 1988, Hanna et al. from the University of Southampton, UK, reported thulium-doped quartz fiber lasers for the first time. Limited by laser pumping technology and the preparation process of thulium-doped fibers, the average output power was 100 μW, and the slope efficiency was only 13%. The technological breakthroughs in the field of high-brightness 793‒795 nm semiconductor lasers, have also ushered in a golden stage of rapid development for TDFLs. By improving the absorption coefficient, changing the fiber structure, optimizing the pumping mode, improving the laser conversion efficiency, suppressing parasitic lasing, and optimizing the mode instability, researchers have increased the output power of TDFL from milliwatts to kilowatts (Fig. 1). In recent years, kilowatt-class narrow linewidth laser and mJ-class high pulse energy output have been achieved and used for combining laser beams through optimization, laying a solid foundation for the 10 kW-class high-power TDFL laser output.In 2010, Ehrenreich et al. from Nufern, USA, reported the utilization of a two-stage master oscillator power amplifier (MOPA) to achieve a laser output power of >1 kW with a conversion efficiency of 53.2% in 20/400 μm TDFs. In 2018, Ramírez-Martínez et al. at the University of Southampton, UK, prepared TDFs with a high concentration of Tm/Al co-doping by improved modified chemical vapor deposition (MCVD). This was followed by a combination of solution methods, with a doping concentration of 5.6% (mass fraction) for thulium, and a maximum laser conversion efficiency of 72.4%, which is the highest conversion efficiency of TDFs utilizing the cross-relaxation process. In 2022, Heuermann et al. at the University of Jena, Germany, achieved a single-pulse energy output of 1.65 mJ with an average power of 167 W and a repetition frequency of 101 kHz in a 2 μm region, using a low-pass filter (LPF) with a core diameter of 80 μm or a rod-shaped thulium-doped fiber (rod-type fiber) in conjunction with a four-channel laser coherent combining beam (Fig. 12). This result not only demonstrates the importance of TDF in the field of ultrashort pulsed lasers but also verifies the potential of coherent beam combining technology in expanding the laser output of the 2 μm band. In 2023, the research group optimized the optical path to increase the single-pulse energy output to 1.86 mJ at the 2 μm band.Conclusions and ProspectsImproving the laser conversion efficiency and reducing the effect of thermally induced mode instability of the laser output performance are still the focus of future research on 2 μm high-power laser output. As the 2 μm laser output breaks through the kilowatt barrier, nonlinear effects such as stimulated Brillouin source scattering will limit the laser output power, especially for the narrow linewidth high-power lasers. In conclusion, with aluminum or germanosilicate glass material as the main line, the fiber laser conversion efficiency is being continuously improved from the perspectives of rare-earth ion-doped glass spectral properties and glass acoustic/optical field coordination, combined with the fiber structure design. The principles of mode selection in step-type fibers, numerical aperture matching in step-type fibers, higher-order mode gain suppression in ring-type fibers, and mode filtering in photonic crystal fibers, are used along with mode instability and excited Brillouin scattering threshold, essentially breaking through the TDF laser output power limit. With the development of kW-class narrow linewidth lasers and laser beam combining technology at 2 μm region, 10 kW-class TDFL will also emerge and become the focus of research and development in high-energy lasers.

We present a high-power, high-stability all-polarization-maintaining (all-PM) fiber femtosecond pulse laser source at 780 nm with full engineering integration that comprises an all-PM fiber amplified chirped pulse laser at 1560 nm, a grating pair compressor and a second-harmonic generation (SHG) module. The all-PM fiber amplified chirped pulse laser at 1560 nm operates with a repetition rate of 111.5 MHz and an average power of 5.37 W, which is compressed with a pulse width of about 204 fs. The 780-nm pulses are subsequently generated by SHG in a frequency-doubling crystal. We achieve an average power of 2.22 W at 780 nm with a pulse width of 288 fs, a SHG efficiency of 52%, and a root-mean-square power fluctuation of 0.23%, which is the best performance merit as far as we know. The whole laser system is finally integrated for practical applications with reliable laser operation. We anticipate that this 780-nm all-PM fiber femtosecond pulse laser with full engineering integration is promising for frontier applications such as two-photon imaging, nanofabrication, terahertz generation, etc.

ObjectiveFiber lasers have advantages, such as high conversion efficiency, good beam quality, convenient thermal management, and flexible transmission. They are widely used in industrial processing, national defense research, and other fields. In recent years, with the development of fiber technology, fiber devices, and pump source technology, the output power of high-power fiber lasers has been rapidly improved. Currently, the output power of the fiber oscillator exceeds 9 kW. For fiber amplifiers, solutions with an output power exceeding 10 kW have become more mature, and multiple research teams have reported on fiber amplifiers with an output power exceeding 20 kW. With the increasing demand for laser size, weight, cost, and reliability in industries and other fields, the traditional concept of size, weight, and power (SWaP) is gradually becoming unsuitable for a wide range of application scenarios. Moreover, it is difficult to simultaneously consider multiple application scenarios using traditional fiber oscillators and amplifiers. Now, bidirectional output fiber laser (BOFL) and oscillating-amplifying integrated fiber lasers (OAIFL) are quickly receiving widespread attention as new structure fiber lasers, with obvious advantages in efficiency, nonlinear effect suppression, transverse mode instability (TMI) suppression, system size, cost, and weight. Currently, a BOFL can achieve a 2×4 kW laser output, and an OAIFL can achieve a 6 kW high beam quality laser output. In this study, we introduce new indicators with which to evaluating the performance of fiber lasers: cost‒size‒weight per unit power (CSWpP) and cost‒size‒weight per unit brightness (CSWpB). In addition, we analyze the application potential of the two new structure fiber lasers in different demand scenarios via theoretical and experimental methods.MethodsFirst, based on the rate equation system of fiber lasers, the theoretical models of BOFL and OAIFL are established. Based on theoretical models, the advantages of the two new structure fiber lasers are analyzed in terms of efficiency, temperature control, and nonlinear effect suppression. In an experiment, a BOFL is designed, based on a double cladding ytterbium-doped fiber with a core/cladding diameter of 30/600 µm. In addition, an OAIFL is designed, based on a self-designed low NA (numerical aperture) double cladding ytterbium-doped fiber with a core cladding diameter of 30/600 µm. Finally, by combining the bidirectional output structure with the oscillating amplifying integrated structure, a scheme of using an oscillating‒amplifying integrated bidirectional output fiber laser (OAI-BOFL) to simultaneously reduce the CSWpP and CSWpB of the laser system is proposed, and experimental verification is conducted.Results and DiscussionsTheoretical simulation results show that, compared with UOFL, BOFL has a higher efficiency, lower fiber temperature, and better stimulated Raman scattering (SRS) suppression ability under the same conditions. If the strength of the SRS is controlled similarly, the total output power of the BOFL is more than twice that of the UOFL. The OAIFL also has a higher efficiency, better temperature control ability, and better SRS suppression ability, compared with UOFL. In an experiment, SRS mutual feedback is suppressed by shortening the length of the passive fiber. A 2×5 kW laser output is achieved with a total efficiency of 81.0%, and the beam quality values at both ends are 2.59 and 2.74, respectively. Based on a self-designed low NA double cladding ytterbium-doped fiber with a core cladding diameter of 30/600 µm, an OAIFL with an output power of over 10 kW with an efficiency of 70.6% is achieved. Finally, based on the OAI-BOFL, a high beam quality 2×4 kW laser output is achieved, thereby demonstrating the potential to simultaneously balance CSWpP and CSWpB.ConclusionsIn this study, the concepts of CSWpP and CSWpB in fiber lasers are proposed, based on SWaP. Low CSWpP fiber lasers focus on improving output power while reducing the size, weight, and cost of the laser, which confers obvious advantages in industrial applications. Moreover, low CSWpB fiber lasers focus on improving the beam quality of the output laser while reducing the size, weight, and cost of the laser, which confers obvious advantages in the field of national defense. In response to application requirements in different fields, the concepts of BOFL that are beneficial for reducing CSWpP and those of OAIFL that are beneficial for reducing CSWpB are proposed. In an experiment, a double cladding ytterbium-doped fiber with a core/cladding diameter of 30/600 µm is used to achieve a total power of over 10 kW using a BOFL. Based on a self-designed low NA double cladding ytterbium-doped fiber with a core/cladding diameter of 30/600 µm, an OAIFL with an output power of over 10 kW is achieved. An OAI-BOFL is developed based on the above two structures, which can simultaneously balance the CSWpP and CSWpB of laser systems and is an important development direction for future high-power fiber lasers.

Objective2 μm high-power narrow-linewidth fiber lasers have a wide range of application demands in frontier applications and scientific research fields, such as coherent optical communications, Lidar, remote sensing, and next-generation gravitational wave detection, owing to their favorable spatiotemporal coherence. Generally, the power scaling of narrow-linewidth fiber lasers is limited by the stimulated Brillouin scattering effect. Using phase modulation to broaden the linewidth of a single-frequency seed source to reduce the SBS gain peak, researchers achieved >1 kW narrow-linewidth laser output for high-power fiber amplifiers in the 1.0?1.5 μm band, thus proving that phase modulation is the most promising technical path currently to achieve kilowatt-level narrow-linewidth fiber-laser operations. Compared with 1.0?1.5 μm Yb- or Er-doped fiber lasers, 2 μm-band Tm-doped fibers with a larger mode field area can effectively increase the SBS threshold, thus demonstrating significant potential in achieving high-power single-frequency laser operations. Thus far, 2 μm Tm-doped fiber lasers have achieved 316 W single-frequency outputs and 1.1 kW laser powers with a narrow linewidth of ~5 GHz. However, the effect of the laser linewidth on the SBS suppression capability of kilowatt-level narrow-linewidth thulium-doped fiber amplifiers has not been investigated comprehensively. In this study, we constructed a thulium-doped fiber laser system with >1 kW power and a linewidth of <500 MHZ based on pseudo-random binary sequence phase modulation and a multihotspot pumping scheme. Additionally, we investigated the effect of the seed-source linewidth on the SBS threshold in thulium doped fiber amplifier (TDFA), both theoretically and experimentally.MethodsFirst, based on the theoretical model of the SBS threshold enhancement factor after filtered pseudo-random binary sequence (PRBS) signal modulation, the effects of the PRBS7 signal modulation frequency and seed linewidth on the SBS threshold enhancement factor of a thulium-doped fiber amplifier were analyzed. In the experiment, a kilowatt-class all-fiber narrow-linewidth thulium-doped fiber amplifier system was constructed based on pseudo-random binary phase-modulation technology and a multihotspot pumping scheme design, and the laser performance was characterized. Finally, the experimental and theoretical results of the effect of laser linewidth on the SBS threshold were compared and discussed.Results and DiscussionsThe theoretical simulation results show that the SBS threshold enhancement factor first increases and then saturates as the PRBS7 modulation frequency increases under different filter bandwidths. Additionally, the saturated SBS threshold enhancement factor is positively correlated with the filter bandwidth, as shown in Fig. 4 (a). In the experiment, the SBS threshold is defined as the forward laser power when the reverse optical power reaches 1000 times the forward output power. The measured SBS threshold of the thulium-doped fiber amplifier is approximately 257 W under an unmodulated single-frequency seed-source injection with a linewidth of 1 MHz. When the seed linewidth is modulated using a PRBS signal with a modulation frequency of 1 GHz and broadened to 116, 232, and 480 MHz, the SBS threshold increases approximately linearly to 547, 683, and 1020 W, respectively, with a slope efficiency of 1.36 W/MHz. In other words, the thulium-doped fiber amplifier can achieve SBS threshold enhancement factors of 2.13, 2.66, and 3.97, respectively, while the linewidth of the seed source is broadened 116, 232, and 480 times, respectively. The experimental results agree well with the theoretical SBS threshold enhancement factor curve at a modulation frequency of 1 GHz, as shown in Fig. 4 (b). The output power of the thulium-doped fiber amplifier increases linearly to 1025 W at a pump power of 1980 W while the seed linewidth is fixed at 480 MHz, thus resulting in a slope efficiency of 52.6%. The central wavelength is 1998 nm, with a signal-to-noise ratio of > 46 dB. This shows that the linewidth broadening of the seed source through phase modulation can linearly enhance the SBS threshold, thereby effectively suppressing the SBS effect in thulium-doped fiber amplifiers and realizing a narrow linewidth kilowatt-level laser operation.ConclusionsBy performing PRBS phase modulation to broaden the linewidth of the single-frequency seed source, the relationship between the SBS threshold and laser linewidth in a kilowatt-level narrow-linewidth thulium-doped fiber amplifier was investigated theoretically and experimentally. The thermal effect of the thulium-doped gain fiber under high-power laser operation was alleviated by adopting a multihotspot pumping design. An output power of 1025 W with a slope efficiency of 52.6% is achieved under a seed-source linewidth of 480 MHz. The central wavelength of the laser output is 1998 nm and the signal-to-noise ratio is >46 dB. The SBS threshold of the thulium-doped fiber amplifier increases linearly with the seed linewidth broadening, thus resulting in an SBS enhancement factor of 4 at a seed linewidth of 480 MHz. The results of this study can provide a technical reference for the optimization of 2 μm kilowatt-level single-frequency thulium-doped fiber laser systems.

ObjectiveThe objective of this study is to enhance the power of high-power fiber lasers by designing high-performance fibers and to overcome the challenges associated with the digital design of complex fiber structures. Currently, the optical characteristic parameters of fibers are primarily obtained through experimental measurements or numerical calculations, which are expensive and difficult to accomplish. Therefore, the development of fiber modeling and simulation software for aiding the design is particularly urgent. Existing commercial fiber photonics simulation software typically present issues such as complex operation and non-specialization. In this study, SeeNano, which is a fiber-waveguide structure-design software, was developed. We analyze the cross-sectional structure of multilayer refractive-index fibers from the lateral dimension, establishe various material library models, and optimize the characteristic parameters of the fibers by analyzing their mode, loss, and dispersion characteristics to provide new solutions for enhancing the power of high-performance fibers and high-power fiber lasers.MethodsFirst, the optical scale of fiber waveguides was considered via mathematical modeling and software development. Initially, a numerical model of the characteristic equation transmission matrix for fibers and other dedicated algorithm models were established to provide a theoretical basis for calculating the fiber mode-field, loss, and dispersion characteristics. Subsequently, based on software engineering, a fiber-waveguide design and simulation software named SeeNano was developed, which features a simple and intuitive graphical user interface with a guided operation flow. It was designed to help users understand the usage of the software promptly and reduce the learning difficulty. Subsequently, the design processes of two cases, i.e., step- and graded-index trench-assisted cases, were introduced, and key characteristic parameters, such as the effective mode-field area, effective mode-field diameter, material dispersion, and waveguide dispersion, were calculated and compared with the results yielded by the commercial software OptiFiber to validate the results. However, the functionality of the software is not yet completed, and the developed features target primarily concentric multilayer refractive-index fibers. Hence, the software functionality must be continuously supplemented and gradually enhanced.Results and DiscussionsThe design of different layers, such as rings (higher refractive index) or grooves (lower refractive index), in the design of novel step- or graded-index multilayer refractive-index fibers has been adopted increasingly in various scenarios to satisfy specific application requirements. This paper introduces the design cases of step- and graded-index trench-assisted fibers. Under different evaluation parameters, the simulation results were compared with those obtained using commercial software OptiFiber. This paper presents comparisons of the mode-field intensity distributions for different fiber structures (Figs. 4 and 7). The effective refractive indices of various modes with eight significant figures match (Tables 2 and 4), and the variations in the mode-field diameter and effective mode area with wavelength show consistent results (Fig. 5). The dispersion curves of material dispersion, waveguide dispersion, and total dispersion as a function of wavelength are similar in most cases (Fig. 8).ConclusionsThis paper introduces the basic functions and case demonstrations of preliminarily developed fiber-waveguide structure design and simulation software, SeeNano. The software provides assistance and guidance in the optimal design of fibers and the selection of fiber parameters, thus enhancing the efficiency of fiber design. The accuracy of the software calculations was verified by comparing simulation results with those obtained using the commercial software OptiFiber. This reduces the difficulty in investigating and designing multilayer refractive-index fibers, reduces the dependence on foreign softwares, and promotes the development of domestic fiber-design softwares. Notably, our research and development team has developed a fiber-laser simulation software named SeeFiberLaser, which begins from the fiber-structure scale and considers the processes of various nonlinear effects, the mode evolution, and the coupling of various physical effects under different time-domain regimes. The software analyzes the effects of fiber device parameters on the laser power, spectrum, and pulse evolution in the longitudinal dimension. The next step is to conduct laser-fiber performance modeling and analysis on the full parameter dimension and multiphysical scale. By continuously improving the basic theory and performing collaborations to develop fibers and fiber-laser software, the associated application requirements shall be satisfied.

ObjectiveHigh average and peak powers are key technical indicators for the development of ultrafast laser technology, enabling a wide range of applications. With the rapid growth of ultrafast laser technologies, high-average-power lasers with ultrahigh repetition rates in the order of MHz have gained attention as fascinating secondary sources, such as the laser-like desktop source of high-order harmonic generation (HHG) in the extreme ultraviolet (XUV) regime with ultrahigh repetition rate, which has great potential in coherent diffraction imaging, coincidence detection, photoelectron spectroscopy, XUV optics metrology, and ultrafast atomic and molecular physics and chemistry. This is because of its excellent coherence, short wavelength, and femtosecond-to-attosecond pulse duration, making it an excellent supplementary source for synchrotron and free-electron lasers. However, high-average-power laser technologies with ultrahigh repetition rates are limited to pulse duration of hundreds of femtoseconds to a few picoseconds , which obstructs gas HHG. In this study, two different post-compression modules are developed to optimize HHG driving pulses. Sufficient driving conditions related to phase matching are comprehensively studied.MethodsHigh-photon-flux coherent XUV sources are developed using a fiber laser with an ultrahigh repetition rate. In the experiment, two nonlinear post-compression systems are investigated: an argon-filled hollow-core fiber (HCF, Fig. 1) and a multi-pass cell (MPC, Fig. 2) filled with argon. Both approaches successfully broaden the laser spectrum and further reduce the pulse duration from 230 fs to below 40 fs after proper dispersion compensation with chirped mirrors. The ytterbium-doped fiber laser with a center wavelength of 1030 nm provides a maximum average power of 80 W with a pulse duration of 230 fs and supports flexible pulse repetition rates from 50 kHz to 19 MHz. The maximum pulse energy (400 μJ) is assigned at 200 kHz. As depicted in Fig. 4, the post-compressed pulses are subsequently focused by a gold-coated spherical mirror and interacted with the noble gases spurted out from a tapered gas jet. Because of the divergence difference between the infrared (IR) laser and the generated XUV beam, an iris is installed immediately after the gas jet to preliminarily reduce the residual driving pulse. Owing to the high average power driving conditions, a grazing incident plate is used to further diminish the residual IR, which allows reflectivity of 70% for the spectral range from 25 nm to 40 nm at a 7o grazing incident angle. A relay imaging system is installed using an ellipsoidal mirror to set up a high-resolution XUV spectrometer and an application-friendly beamline. The tangential and sagittal radius of curvature of this relay imaging mirror are -12308.3 mm and -182.8 mm, respectively. To prevent heating due to ultrahigh-repetition-rate IR pulses, an aluminum filter (209.6 nm thick) is placed behind the ellipsoidal mirror, which ultimately stops the residual IR and any scattering light along the beamline. Pure XUV spectra are then detected using an in-house-built XUV flat-field spectrometer, which consists of a holographic grating and an X-ray charge-coupled device (CCD) camera. A removable silicon plate is used to switch between the spectrometer and the photon flux detector.Results and DiscussionsThe two post-compression modules excellently reduce the laser pulse duration for subsequent HHG. The energy transmission efficiencies of HCF and MPC are over 60% and 90%, respectively. As shown in Fig. 3, the former module enables pulse compression from 230 fs to 27 fs, while the latter approach reaches 36 fs under optimal conditions. The improved pulses are then sent to the vacuum chamber to interact with Ar and Kr from the gas jet. Simulations and experiments on gas-based HHG are systematically performed. A variety of parameters related to important phase-matching conditions are investigated, such as driving pulse energy, pulse duration, gas type, gas pressure, and light-field coupling in the Z direction. By increasing the driving-pulse energy, the HHG signal becomes stronger. However, when the peak intensity is sufficiently high to induce overionization, the HHG exhibits a phase mismatch and the HHG signal decreases (Fig. 9). Because the gas density determines the neutral atom dispersion term and plasma dispersion term in phase matching, the gas pressure is one of the major tuning parameters for optimizing the HHG signal. Spatial splitting is observed in the HHG spectra when the gas jet is scanned along the beam path. This is mainly attributed to the contribution of electron trajectories in the HHG process. The long orbit tends to achieve phase matching at off-axis positions, whereas the short orbit favors phase matching at on-axis positions. Both experiments and simulations using a three-dimensional strong-field approximation code show good agreement on the above impacts on HHG optimization. A very bright harmonic signal between 21 nm and 40 nm is obtained at a 500 kHz repetition rate with a krypton gas jet backing pressure of 5.3 bar. The photon flux of each harmonic order exceeds 1.6×1010 photon/s, and the strongest harmonic order (35th, the wavelength of 29.4 nm) is 1.8×1012 photon/s ( the power of 12.2 μW), with an energy conversion efficiency of approximately 5×10-7 (Fig. 7).ConclusionsHigh-photon-flux HHG signals in the XUV regime (21?40 nm) are obtained with an ultra-high repetition rate femtosecond fiber laser associated with in-house developed HCF and MPC pulse post-compression systems. Under a loosely focusing configuration, the main impacts of the HHG process are investigated experimentally and through simulations, including the driving pulse energy and pulse width, gas type, gas pressure, and light field coupling conditions in the Z direction. Optimized photon flux of the 35th harmonic order (29.4 nm) reaches 1.8×1012 photon/s (12.2 μW), with a single harmonic order conversion efficiency of approximately 5×10-7. Under the current experimental platform, further harmonic signals with higher brightness and shorter wavelengths (~10 nm) can be achieved by using a cascaded pulse post-compression system with MPC and HCF. Furthermore, fiber laser technology has successfully exceeded the average output power of 200 W and is an important driving laser for high-brightness XUV light source. For even shorter wavelengths, such as 6.7 nm and the water window, ultra-high-repetition-rate optical parametric chirped pulse amplification (OPCPA) mid-IR lasers (wavelength of 2.0?2.5 μm) are a viable option. The rapid development of disk laser technology has successfully surpassed the average output power by over kilowatts. When combined with a high-power MPC post-compression module, disk lasers are expected to be effective driving tools for milliwatt-level, short-wavelength (<10 nm) XUV light sources, with broad applications in nanostructure imaging, XUV metrology, ultrafast dynamics of atomic molecular physics, and component damage and system tests related to the lithography.

ObjectiveLithium niobate (LiNbO3, LN) is currently the preferred electro-optic (EO) Q-switch material for military lasers owing to its ease of growth and processing, low cost, and nonhygroscopic nature. Major national projects such as aerospace projects, lunar exploration, and Beidou satellite navigation demand lasers that can operate over a wide temperature range. However, the performance of LN EO Q switches deteriorates considerably at subfreezing temperatures. Currently, researchers attribute the unsatisfactory performance of cold Q-switching to three reasons: stress birefringence, the pyroelectric effect, and the thermo-optic effect. Most related studies focus on improving cold performance, whereas studies that investigate the mechanism underlying degraded cold Q-switching performance are few. Consequently, the reasons for unsatisfactory cold Q-switching performance are contradictory and confusing, which limits the development of LN EO Q switches with high-temperature stability. In this study, we perform comprehensive investigations to determine the primary factors affecting cold Q-switching performance and fabricate LN Q switches with improved temperature stability.MethodsFirst, the cold Q-switching performance of several LN Q switches is measured using a flash-lamp pumped Nd∶YAG laser, which is placed in a high?low temperature test chamber. The optical homogeneity of these Q switches at cold temperatures is characterized using high?low-temperature conoscopic interference technology, from which the variation in the birefringence at cold temperatures is clearly observed. Subsequently, we analyze the various factors that may affect the birefringence at cold temperatures and their influence characteristics. The distribution of the electric field created by pyroelectric charges inside the LN crystal is simulated using finite-element-analysis software. Combining the above with the EO effect of the LN crystals, we analyze the spatial-distribution characteristics of birefringence induced by the pyroelectric field and compare them with the experimental results of conoscopic interference. The pyroelectric coefficient of the LN crystals is measured using the dynamic current method, and the conductivity is measured using a high-resistance meter. For further experimental verification, we reduce LN thermochemically in the Li2CO3 powder atmosphere at 550 ℃ for 10 h. The Q-switching performance and optical homogeneity of the reduced LN crystals are measured at cold temperatures.Results and DiscussionsThe conoscopic interference patterns at cold temperatures change significantly compared with those at room temperature. Notably, the interference patterns in different areas of the light cross-section of LN differ significantly but show a clear distribution regularity (Figs. 4?6). In the X-axis direction, the interference patterns are symmetrically distributed relative to the crystal center, whereas the interference patterns are the same in the Y-axis direction. The interference ring becomes a distorted ellipse, with the major axis oriented at an angle of ±45° from the X-axis, and the extinction areas transform into two point-like areas separated along the major axis. The greater the distance from the center of the crystal, the greater is the variation in the interference pattern. The interference patterns of multiple LN crystals show the same variation characteristics but with different degrees, which is consistent with the Q-switched output at cold temperatures (Fig. 2 and Table 1). The spatial-distribution characteristics of birefringence at cold temperatures imply that stress and thermo-optic effects are not the primary factors affecting cold Q-switching performance. The results of finite-element simulations indicate that the pyroelectric charges create an uneven electric field inside the LN crystals (Fig. 7). The electric-field components are in the X- and Z-axis directions. The electric-field distribution is independent of the Y-axis and symmetrical within the XZ plane. Combining the above with the EO effect of LN, we conclude that the spatial distribution of birefringence induced by the pyroelectric field is consistent with the experimental results of conoscopic interference. Therefore, the electric field created by the pyroelectric charges is regarded as the dominant factor contributing to the unsatisfactory cold Q-switching performance. The differences in the cold performance of the LN Q switches are attributed to their different pyroelectric coefficients and conductivities (Table 2). The reduced LN crystal (Fig. 9) exhibits a significantly higher conductivity and its cold Q-switching performance and optical homogeneity improve considerably (Table 4, Fig. 10, and Fig. 11), which further confirms the leading role of the pyroelectric field on the cold Q-switching performance.ConclusionsWe conduct a comprehensive investigation to determine the primary factors affecting the cold Q-switching performance of LN EO Q switches. The variation in the birefringence at cold temperatures and its spatial-distribution characteristics are intuitively observed via cold-temperature conoscopic interference experiments. The effects of stress and thermo-optic effects are excluded because their influence on birefringence are inconsistent with the experimental results. The results of finite-element simulations reveal that pyroelectric charges create a spatially variable electric field inside the LN crystal. Furthermore, the spatial distribution of birefringence induced by the electric field via the EO effect is consistent with the experimental results. Therefore, the pyroelectric field is regarded as the primary factor affecting the cold Q-switching performance. LN crystals with significantly higher conductivities are achieved via thermochemical reduction. The cold Q-switching performance and optical homogeneity of the reduced LN improve significantly. However, the reduced LN crystals exhibit significant light absorption, which is undesirable for practical applications. This study is beneficial for guiding the development of LN EO Q switches with high-temperature stability.

ObjectiveThe primary objective of this study is to develop a high-power, all-fiber oscillator than can deliver exceptional output power and beam quality, which is crucial for advanced applications in material processing, laser manufacturing, and defense technology. The oscillator is intended to address the sustained demand for more robust and simpler laser systems that can operate at higher power levels without compromising stability and reliability. Nonlinear effects (NLEs) and transverse-mode instability (TMI) in fiber lasers, which are significant barriers to achieving high-power, high-beam-quality laser outputs, are addressed in this study.MethodsIn this study, a simple design of an all-fiber oscillator [Fig. 1(a)] using 30 μm /600 μm large-mode-area (LMA) ytterbium-doped fibers and matched fiber gratings inscribed by femtosecond laser phase-mask techniques is employed to form a resonant cavity. The system utilizes a low numerical aperture fiber to increase the loss of higher-order modes via a dual-wavelength pump source (969 nm and 982 nm) to reduce the thermal load and increase the gain saturation. TMI and NLEs are suppressed by appropriately designing the fiber grating bandwidth and optimizing the fiber coiling to ensure that multiple modes resonate adequately within the cavity while minimizing the intermodal coupling. Additionally, the angled cutting of the pigtail of a cladding light stripper can prevent backward Raman light from oscillating within the cavity.Results and DiscussionsThe oscillator achieves an unprecedented output power of 10.07 kW. The system demonstrates a slope efficiency of 72.3% and an optical-to-optical efficiency of approximately 72% [Fig.1 (b)]. The laser output spectrum features a 3-dB bandwidth of approximately 6.6 nm, with a Raman suppression ratio of approximately 16 dB [Fig. 1 (c)]. Notably, the laser maintains a stable operation with no significant TMI in the frequency domain [Fig. 1 (e)]. However, the beam quality degrades slightly with increasing power [Fig. 1 (f)]. This is attributed to the increasing contribution of higher-order modes, which is typically encountered in high-power fiber lasers. The results underscore the effectiveness of the implemented strategies in managing nonlinearities and TMI, thus providing a basis for further improving fiber-laser performance.ConclusionsIn this study, the development of a high-power, all-fiber oscillator that affords an unprecedented laser output exceeding 10 kW is successfully demonstrated. The innovative use of low numerical aperture LMA fibers and optimized fiber gratings as well as the effective management of nonlinear effects and TMI are pivotal in achieving these results. The findings not only validate the feasibility of scaling fiber laser power while maintaining operational stability but also highlight the potential for further improving beam quality by continuously refining the manufacturing technologies of optical fibers and fiber gratings. The advancements resulting from this study are critical for the further development of high-power laser applications, particularly in the demanding industrial and defense sectors.

ObjectiveKilowatt-level high-repetition-frequency picosecond lasers are widely used in fields such as free-electron lasers, attosecond science, and high-precision material processing. Currently, the average output power of commercial ultrafast lasers is on the order of hundreds of watts, and the pulse energy is on the order of tens of millijoules. However, some scientific and industrial applications require a higher average output power with high pulse energy. Thin-disk lasers have centimeter-level laser spots on the gain medium and low-thermal-distortion characteristics, thus rendering them ideal for achieving average power levels on the order of kilowatts and pulse energies on the order of millijoules.MethodsBased on the 72-pass self-developed thin-disk pump module, a thin-disk regenerative amplifier with a large stable range was designed, thus allowing the regenerative amplification cavity to achieve favorable mode matching within the operation range. To avoid damage to key optical components, the thin-disk regenerative amplifier was designed based on chirped pulse-amplification technology. The experimental setup is shown in Fig. 1. After injecting the seed laser, the high-voltage loading time of the Pockels cell or the number of round trips in the regeneration cavity was controlled, and a high-power picosecond laser output was obtained.Results and DiscussionsAs shown in Fig. 2(a), under a maximum continuous pump power of 650 W, a regenerative-amplifier laser output with an average power of 312 W and a repetition frequency of 50 kHz is obtained, which corresponds to an optical efficiency of 48.1% and a slope efficiency of 53.8%. The output pulse sequences at repetition rates of 50/100/200 kHz are shown in Fig. 2(d); the RMS of the single-pulse energy is 0.5% at 50 kHz. The beam quality factor M2 at a maximum output power of 312 W is 1.05/1.09 and the near-field output laser spot roundness is 91%. Owing to the gain-narrowing effect, the spectral width of the amplified laser is 1.80 nm, which theoretically supports the compression of the pulse width to 867 fs.ConclusionsBased on the self-packaged thin-disk and 72-pass thin-disk pump module, an amplified laser output with an average power of 312 W, a pulse width of 720 ps, and an optical efficiency of 48.1% is achieved at a repetition frequency of 50 kHz. To the best of our knowledge, this is the highest output power and optical?optical efficiency reported for thin-disk regenerative amplifiers in China. In the future, our group shall further optimize the thin-disk packaging technology, increase the pump power to increase the output power to the kilowatt level, and compress the output pulse width to less than 1 ps.

The laser damage resistance of a crystal is closely related to its UV absorption performance. The 50% damage probability of the crystal with a translational speed of 50 mm/s is higher than those at 20 mm/s and 80 mm/s. The laser damage resistance of the crystal with the edge facing flow is higher than that with the face facing flow. The laser damage resistance of the crystals with an acceleration of 25 mm/s2 is better than that of 80 mm/s2. This is consistent with the ultraviolet absorption of the crystal; the absorption coefficient is high, and the resistance to laser damage is poor.ObjectiveKDP-type crystals are excellent nonlinear optical crystal materials that have a high laser threshold, wide transmittance band, high electro-optical coefficient, optical uniformity, and large size. Inertial confinement fusion (ICF) driven by high-power lasers has several requirements for crystal materials, such as a large aperture, a high laser damage threshold, large nonlinear optics, a wide transmission band, and low refractive index inhomogeneity. Thus far, only KDP-type crystals have met these requirements. Therefore, large KDP-type crystals are the only crystalline materials that can be used as electro-optical switches and frequency-conversion devices in ICF. The rapid growth method of KDP-type crystals eliminates the 0.5 mm/d growth rate limit of the traditional growth method and shortens the growth cycle from 2?3 years to 4?5 months. However, with the continuous power increase in ICF experiments worldwide, the requirements for KDP/DKDP crystals (particularly laser damage resistance) are increasing. Therefore, research on crystal growth remains significant. The key factor affecting the quality of crystal growth is the distribution of the solute concentration, which depends on the convective mode on the crystal surface. The two-dimensional growth mode allows the crystal surface to achieve a periodic reversible shear flow and improves the distribution uniformity of the solute concentration on the crystal surface.MethodsTo meet the requirements of the KDP crystal growth device, a set of experimental devices for two-dimensional crystal growth was designed and built. The device includes a growth tank, two-dimensional moving platform, and continuous filtration system. A temperature test was performed on the device to ensure that the temperature of the growth tank met the design requirements. To address the sealing problem of a crystal growth vessel (growth tank), a set of water-sealing devices that can satisfy the sealing demand of the tank was designed. A continuous filtration system can effectively reduce the particle concentration of a solution without affecting its stability. To reduce heterocrystal generation during the placement of long-seed crystals, a new spiral seed crystal method was proposed. The seed crystal was stably fixed on the crystal plate through a thread, which reduced the shaking amplitude and growth solution disturbance during the translation movement.Results and DiscussionsThe two-dimensional KDP crystals have stronger absorption in the ultraviolet region. With an increase in translation velocity, the absorption first decreases and then increases. The translation velocity of 50 mm/s is better than those of 20 mm/s and 80 mm/s. The absorption of the crystal in the ultraviolet region is lower than that in the plane. The absorption of the crystal in the ultraviolet band can be reduced by decreasing its acceleration during the translational motion.ConclusionsBased on the rapid growth of long-seed crystals, the two-dimensional translational growth technology of KDP crystals is explored. Equipment for two-dimensional translational growth was built, including a growth tank, two-dimensional moving platform, and continuous filtration system. A series of crystal growth processes were explored using this equipment. The grown crystals exhibit good transparency. The properties of the crystal were tested, including its optical properties, laser damage resistance, growth parameters, and mechanism. The absorption of the two-dimensional KDP crystals in the ultraviolet region is stronger. With an increase in translation velocity, the absorption first decreases and then increases. The absorption of the crystal in the ultraviolet region is lower than that in the plane. The absorption of the crystal in the ultraviolet band can be reduced by decreasing its acceleration during the translational motion. The laser damage resistance of the crystal is closely related to its UV absorption performance. The 50% damage probability of the crystal with a translational speed of 50 mm/s is higher than those at 20 mm/s and 80 mm/s. The laser damage resistance of the crystal with the edge facing flow is higher than that with the face facing flow. The laser damage resistance of the crystals with an acceleration of 25 mm/s2 is better than that of 80 mm/s2.

ObjectiveCoherent lidar uses coherent detection technology to probe target information. The echo signal is mixed coherently with a local oscillator signal to detect the target using the heterodyne detection method. The beat frequency signal is then analyzed to obtain the velocity and distance information of the target. However, a coherent detection system is affected by various factors, such as lasers, phase modulation, transmission distance, and atmospheric turbulence, during the detection process. This leads to the introduction of phase disturbances into the echo signal, consequently compromising the coherence of the coherent detection system, which significantly affects the detection capability of the coherent lidar. Maintaining system coherence is crucial to ensure high detection sensitivity and precision in coherent detection systems. Therefore, research on the coherence of coherent detection systems is crucial. By studying the coherence of a coherent detection system, we can determine the coherence length or time of the coherent lidar, thereby obtaining the maximum detection range and maximum coherent integration time of the coherent detection system. Additionally, the coherence of a coherent detection system should be restored by compensating for the effects of phase disturbances. Through phase compensation, the coherence of a coherent detection system can be restored, thereby enhancing its detection performance of coherent detection system.MethodsThis paper evaluates the overall coherence of a coherent detection system using the Strehl ratio and calculates the coherence time of the system based on the Strehl ratio. By designing a coherent detection system with two lasers as separate local oscillators and transmitting laser sources, velocity detection is simulated under the condition of the super-coherence length in actual detection. An acousto-optic frequency shifter and attenuator are employed to simulate the Doppler frequency and signal attenuation after transmission over distances exceeding the coherence length. Moreover, phase compensation for both internal and external phase disturbances within the coherent detection system is performed using phase measurements and an iterative phase-estimation algorithm. Phase measurement compensation utilizes a part of the local oscillator laser and the transmitting laser as the reference signal for coherent heterodyning to monitor the phase disturbances in the echo signal. Subsequently, in the digital domain, the phase disturbances measured from the reference signal are used to compensate for the phase of the echo signal, thereby eliminating the influence of the phase disturbances caused by the laser frequency drift. The iterative phase estimation algorithm performs phase estimation through phase perturbations of time delay and atmospheric turbulence and compensates for them in the digital domain using an iterative method to obtain the optimal algorithmic compensation results. In the experimental system used in this study, the iterative phase estimation algorithm compensated only for the effect of the time-delay phase.Results and DiscussionsThis paper systematically analyzes the uniform influence of various components of a coherent laser detection system (laser, modulator, distance, etc.) on the coherence of the system. The Strehl ratio is used to characterize the coherence degradation and coherence time (coherence length) of the coherent detection system. The simulation results verify that the coherence and detection accuracy of the coherent detection system are affected by the laser phase, phase modulation, and distance (Fig. 3, 4). This study simulates a coherent detection system with a super-coherence length using two lasers (Fig. 6) and evaluates the coherence time of the coherent detection system with a super-coherence length using the Strehl ratio (Fig. 8). The detection probability of the coherent detection system is 52%, the signal-to-noise ratio is 10.1 dB, and the velocity accuracy is 11.4 cm/s with a coherence time of 0.7 ms and echo signal power of 11 fW. After phase compensation (Fig. 7), the detection probability increases to 74%, the signal-to-noise ratio increases to 19.8 dB, and the velocity accuracy increases to 0.16 cm/s. Better detection performance is achieved with longer integration times (Fig. 10,11,12).ConclusionsDuring the detection process, coherent detection systems are affected by various factors, such as laser sources, phase modulation, transmission distance, and atmospheric turbulence. These factors can introduce phase disturbances into the echo signal, leading to the degradation of coherence in the coherent detection system. This severely affects the detection capability of the coherent lidar. To simulate velocity detection in a coherent detection system with a super-coherence length, this study designs a structure with two lasers as separate local oscillators and transmitting laser sources. Acousto-optic frequency shifters and attenuators are used to simulate echo signal conditions in an actual detection environment. Utilizing two compensation methods, phase measurement and a phase-estimation iterative algorithm, can effectively solve problems such as decreased coherence and detection capability caused by phase disturbances and enable coherent detection systems to detect target signals beyond the coherence length. The synthesis shows that coherent lidar still has the characteristics of high sensitivity and high detection accuracy with a super-coherence length, which is significant to providing a feasible experimental basis for the target detection of long distances and weak signals with super-coherence lengths.

ObjectiveDue to its many advantages, including low detection limits, high resolution, and minimal interference from water, enabling the real-time detection of trace substances, surface-enhanced Raman scattering (SERS) has garnered considerable attention and has been applied to various fields, including physical chemistry, biomedical science, and environmental monitoring. However, challenges remain for achieving quantitative analysis using SERS, primarily due to the uncertainties in the number of target molecules adsorbed on the enhancement substrate, uneven electromagnetic enhancement caused by the inherent nonuniformity of metal nanostructures, and molecular orientation fluctuations induced by chemical interactions. Currently, internal standard methods are primarily used for quantitative analysis. Internal standard methods involve the addition of internal standards or markers to achieve quantitative detection of analytes, partly addressing signal fluctuation issues. However, limitations such as the choice of internal standards, substrate materials, and preparation processes constrain their widespread practical applications. Carbon nanotubes (CNTs) exhibit distinctive 2D Raman characteristic peaks around 2696 cm-1, far from the characteristic peaks of common probe molecules, making them natural internal standards for SERS calibration detection. In addition, when the strong electromagnetic enhancement effect of silver nanoparticles (AgNPs) is leveraged, the combination of CNTs and AgNPs can synergistically enhance performance. Therefore, in this study, a chemical self-assembly method was employed to prepare AgNPs/CNTs composite structures. Rhodamine 6G and malachite green were used as probe molecules in Raman self-calibration experiments to investigate the self-calibrated Raman enhancement characteristics of AgNPs/CNTs composite structures.MethodsFirst, an electromagnetic simulation using finite-difference time-domain (FDTD) solutions was employed to analyze the structures of AgNPs/CNTs composite structures and AgNPs structures. SERS substrates based on AgNPs/CNTs composite structures were fabricated using a chemical self-assembly method. The prepared SERS substrates were characterized using SEM to assess their structural properties. Following characterization, a series of Raman spectroscopy measurements were conducted on the SERS substrates with AgNPs/CNTs composite structures using different concentrations of the target molecules R6G and MG. The obtained Raman spectral data were used to calculate the enhancement factors (k) for R6G and MG at various concentrations using the formula ki=Ii(analyte)/Ii(2D). A relationship curve (k-C curve) was established by plotting these values, and linear fitting was performed using Origin software to evaluate the quantitative detection capabilities of the AgNPs/CNTs composite structures.Results and DiscussionsElectromagnetic simulation results show that the electric field intensity around the AgNPs/CNTs composite structure is stronger than that around the AgNPs structure, with a maximum electric field intensity of Emax=162.975 V/m (Fig. 2). The calculated electromagnetic enhancement factor (FEM) for this structure is approximately 7.05×108. By contrast, for the AgNP structure, the maximum electric field intensity is Emax=138.481 V/m, resulting in an FEM of approximately 3.68×108. In addition, the AgNPs/CNTs composite structure demonstrates detection limits for R6G and MG of as low as 10-12 and 10-9 mol/L, respectively. Linear fitting of the obtained k-C relationship curves shows high goodness of fit at the R6G characteristic peaks around 613 cm-1 [R2=95.48%, Fig. 7(a)] and 773 cm-1 [R2=96.15%, Fig. 7(b)]. Similarly, for MG, the linear fitting results show R2 values of 95.11% at the characteristic peak 806 cm-1 [Fig. 9(a)] and 95.77% at 915 cm-1 [Fig. 9(b)].ConclusionsThe study utilized CNTs as natural internal standards for achieving quantitative SERS detection. The synergistic effect of these two components was investigated based on the strong electromagnetic enhancement of AgNPs. Simulation analysis of the electric field distribution around the AgNPs/CNTs composite structures and AgNPs structures revealed that the electromagnetic enhancement factor of the composite structure is approximately twice that of the AgNPs structure. The AgNPs/CNTs composite structures were prepared using chemical reduction and self-assembly methods. Self-calibration experiments were conducted using R6G and MG as probe molecules. Based on the normalized k-value method, experimental results demonstrate high linear fitting coefficients (R2) of 95.48% and 96.15% at the characteristic peaks of R6G at 613 cm-1 and 773 cm-1, respectively. Similarly, for MG, linear fitting coefficients are 95.11% at 806 cm-1 and 95.77% at 915 cm-1. This initial study achieves self-calibrated SERS detection through the use of AgNPs/CNTs composite structures.