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In recent years, mid-infrared fiber lasers based on gas-filled photonic crystal hollow-core fibers (HCFs) have attracted enormous attention. They provide a potential method for the generation of high-power mid-infrared emissions, particularly beyond 4 μm. However, there are high requirements of the pump for wavelength stability, tunability, laser linewidth, etc., due to the narrow absorption linewidth of gases. Here, we present the use of a narrow-linewidth, high-power fiber laser with a highly stable and precisely tunable wavelength at 2 μm for gas absorption. It was a master oscillator power-amplifier (MOPA) structure, consisting of a narrow-linewidth fiber seed and two stages of Thulium-doped fiber amplifiers (TDFAs). The seed wavelength was very stable and was precisely tuned from 1971.4 to 1971.8 nm by temperature. Both stages of the amplifiers were forward-pumping, and a maximum output power of 24.8 W was obtained, with a slope efficiency of about 50.5%. The measured laser linewidth was much narrower than the gas absorption linewidth and the wavelength stability was validated by HBr gas absorption in HCFs. If the seed is replaced, this MOPA laser can provide a versatile pump source for mid-infrared fiber gas lasers.

In recent years, photonic crystal fibers (PCFs) have attracted increasing attention. Compared with traditional optical fibers, PCFs exhibit many unique optical properties and superior performance due to their high degree of structural design freedom. Using large-mode area (LMA) fibers with single-mode operation is essential to overcoming emerging problems as the power of fiber lasers scales up, which can effectively reduce the power density and mitigate the influence of nonlinear effects. With a brief introduction of the concept, classification, light transmission mechanism, basic properties, and theoretical analysis methods of PCFs, this paper mainly compiles the worldwide development of large-mode area and polarization-maintaining (PM) PCFs, and finally proposes possible technical routes to realize the single-mode operation of LMA-PCFs and PM-LMA-PCFs. Finally, the future development prospects of the PCFs are discussed.

Stimulated Raman scattering (SRS) effect is considered to be one of the main obstacles for power scaling in general-type fiber lasers. Different from previous techniques that aim at suppressing SRS, nonlinear fiber amplifier (NFA), which manipulates and employs the SRS for power scaling in rare-earth-doped fiber, is under intensive research in recent years. In this paper, the authors will present an all-round study on this new kind of high-power fiber amplifier. A theoretical model is proposed based on the rate equation and amplified spontaneous emission (ASE), with random noise taken into account. By numerical solving of the theoretical model, the power scaling potential, heat analysis and advantages in suppressing the undesired backscattering light are quantificationally analyzed for the first time. Then two different types of high-power NFAs are demonstrated individually. Firstly, a laser diode pumped NFA has reached kilowatt output power, and the results agree well with theoretical predictions. Secondly, a tandem-pumped NFA is proposed for the first time and validated experimentally, in which 1.5 kW output power has been achieved. The authors also briefly discuss several new issues relating to the complex nonlinear dynamics that occur in high-power NFAs, which might be interesting topics for future endeavors.

As a novel design of large-mode-area fiber, the single trench fiber (STF) providing high higher-order-mode suppression with a large mode area for the fundamental mode shows potential for high-power and high-brightness applications. However, the output power of STFs has remained relatively low over the past decade. In this paper, we first conducted a design process for STFs and determined the optimal ratio of the fiber structural parameters. Following this ratio, we fabricated an ytterbium-doped STF and demonstrated an all-fiberized fiber amplifier. The system achieved an output power of 2.5 kW with an M2 factor of 1.396. To the best of our knowledge, the power of the STF in this study is approximately three times higher than the previous single-mode power record.

We present a thulium-doped single-oscillator monolithic fiber laser emitting successively at three wavelengths, especially at unusual long wavelengths as 2.09 µm and even at 2.12 µm. The 793 nm core absorption of 8.42 dB/m allows for achieving a slope efficiency higher than 43% both at 1.94 µm and 2.09 µm. The operation of the laser at 1.94 µm, 2.09 µm, and 2.12 µm is compared by using different fiber Bragg gratings to push the limit of thulium ions emission above 2.05 µm. This is the first demonstration of emission exceeding wavelengths of 2.1 µm of an only thulium-doped fiber laser, to the best of our knowledge.

Traditional ytterbium-doped high-power fiber lasers generally use a unidirectional output structure. To reduce the cost and improve the efficiency of the fiber laser, we propose a bidirectional output fiber laser (BOFL). The BOFL has many advantages over that of the traditional unidirectional output fiber laser (UOFL) and has a wide application in the industrial field. In theory, the model of the BOFL is established, and a comparison of the nonlinear effect in the traditional UOFL and the BOFL is studied. Experimentally, high-power continuous wave (CW) and quasi-continuous wave (QCW) BOFLs are demonstrated. In the continuous laser, we first combine the BOFL with the oscillating amplifying integrated structure, and a near-single-mode bidirectional 2 × 4 kW output with a total power of above 8 kW is demonstrated. Then, with the simple BOFL, a CW bidirectional 2 × 5 kW output with a total power of above 10 kW is demonstrated. By means of pump source modulation, a QCW BOFL is developed, and the output of a near-single mode QCW laser with a peak output of 2 × 4.5 kW with a total peak power of more than 9 kW is realized. Both CW and QCW output BOFL are the highest powers reported at present.

Thanks to the advantage of balancing nonlinear effects and transverse mode instability, vary core diameter active fiber (VCAF) has been widely used in high power ytterbium-doped fiber lasers in recent years. Up to now, VCAF has developed from the basic form of the original tapered fiber to the spindle-shaped and saddle-shaped fiber with different characteristics and has been applied in conventional fiber lasers, oscillating–amplifying integrated fiber lasers, and quasi-continuous wave fiber lasers and successfully improved the performance of these lasers. In the present study, a 6110 W fiber laser amplifier is realized based on a tapered fiber. The maximum output power of a fiber laser amplifier based on spindle-shaped fibers is 6020 W with a beam quality of M2~1.86. In this paper, we first introduce the basic concept of VCAF and summarize its main fabrication methods and advantages in high-power fiber laser applications. Then, we will present the recent research results of high-power fiber laser employing VCAF in our group and clarify the outstanding advantages of VCAF compared with the constant core diameter active fiber (CCAF).

An all-silica-fiber thulium-doped fiber laser emitting at 0.82 μm on the transition from 3H4 to the ground state 3H6 outputs 105 W continuous-wave power and 555 W quasi-continuous-wave instantaneous power with 0.96% duty cycle in 240 μs rectangular pulses. The system comprises a double-clad thulium-doped fiber designed and fabricated in-house, incorporated into an all-fiber cavity and cladding-pumped by diode lasers at 0.79 μm. Co-lasing at 1.9 μm counteracts population trapping in 3F4. The slope efficiency reaches 64% and 77.5% under quasi-continuous-wave and continuous-wave operations, respectively. Under quasi-continuous-wave conditions, the beam quality M 2 becomes 2.2 (beam parameter product: 0.57 mm mrad) and 2.45 (0.64 mm mrad) in orthogonal directions at approximately 250 W of instantaneous output power. In addition, a modified quasi-continuous-wave setup is continuously wavelength-tunable from 812 to 835 nm. We believe this is the first reported demonstration of high-power laser operation of the 3H4 → 3H6 transition in a thulium-doped fiber.

In this paper, the fabrication method of a pump/signal (6 + 1) × 1 combiner based on a large-core (48 μm) multimode signal fiber is introduced. Since the signal fiber is not tapered in the production, and an effective feedback alignment method is adopted during the splice process, the degradation ratio of the M2 value of the signal light is only about 5% after passing through the beam combiner. In addition, with the help of a home-made beam combiner, a counter-directional tandem-pumping amplifier is built. The maximum output power of the amplifier is 15.31 kW with the slope efficiency of 83.2%. The temperature rise coefficient of the home-made combiner is 3.2 °C/kW and the backward isolation degree is more than 36 dB from each pump pigtail. Both test results prove the outstanding potential of the pump-signal combiner in high-power laser applications.

Transverse mode instability (TMI) is a primary limitation for power scaling in high-brightness fiber lasers. This paper reports observation of the TMI effect in the process of supercontinuum generation, and demonstrates a 993 W linearly polarized supercontinuum ranging from 800 to 2000 nm through an amplifier-based structure. To mitigate TMI, three strategies are implemented: repetition rate doubling to reduce peak power and mitigate the thermal loading; reducing the bending radius of the gain fiber to suppress higher-order modes; optimizing the polarization extinction ratio of the pre-amplifier. TMI-induced beam quality degradation and power-scaling limitation resemble those of conventional fiber lasers. In particular, burr-like spectral fluctuations in supercontinuum sources are observed upon the onset of TMI. This work deepens the understanding of TMI mechanisms in broadband laser systems and offers critical guidance for power scaling of high-power supercontinuum sources.