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We report here the first hundred-watt continuous wave fiber gas laser in H2-filled hollow-core photonic crystal fiber (PCF) by stimulated Raman scattering. The pump source is a homemade narrow-linewidth fiber oscillator with a 3 dB linewidth of 0.15 nm at the maximum output power of 380 W. To efficiently and stably couple several-hundred-watt pump power into the hollow core and seal the gas, a hollow-core fiber end-cap is fabricated and used at the input end. A maximum power of 110 W at 1153 nm is obtained in a 5 m long hollow-core PCF filled with 36 bar H2, and the conversion efficiency of the first Stokes power is around 48.9%. This work paves the way for high-power fiber gas Raman lasers.

We report on the generation of single transverse mode output in large-mode-area fiber with a core diameter of 50 µm using a 3×1 photonic-lantern-based adaptive spatial mode control system. We have designed and fabricated the photonic lantern composed of a single mode fibers bundle taper region and a multi-segment multimode fiber splicing region. From simulation and experiments, we demonstrate that the quality of the output beam is significantly influenced by the size of the fibers bundle’s waist and the segmented splicing scheme of the multimode fiber. Stable single transverse mode output is achieved at 1064 nm with M2 ∼1.4, which will provide a possible technical solution to increase the mode instability threshold in high power large-mode-area fiber systems.

Thermal blooming effect is one of the significant factors affecting the propagation performance of high-power ytterbium-doped fiber lasers (YDFLs) in the atmosphere. In this paper, two 20 kW YDFL systems with typical wavelengths (1070 nm and 1080 nm) are fabricated for propagation comparison experiments, which are used to investigate the thermal blooming effect induced by high-power YDFL propagation through the atmosphere. Under approximately the same laser system parameters (except wavelength) and atmospheric environment, the 1070 nm laser has better propagation characteristics than the 1080 nm laser. Due to the combined effect between the different central wavelengths of the two fiber lasers and the spectral broadening caused by output power scaling, the thermal blooming caused by the different absorptivity of water vapor molecules to the two fiber lasers is the main factor for the variation of the propagation properties. Through theoretical analysis and numerical calculation of factors affecting the thermal blooming effect, and considering the industrial manufacturing difficulty of YDFLs, a reasonable selection of fiber laser parameters can effectively improve atmospheric propagation performance and reduce manufacturing costs.

Chirped and tilted fiber Bragg gratings (CTFBGs) are important all-fiber filtering components in high-power fiber lasers for stimulated Raman scattering (SRS) suppression. The fabrication of CTFBGs in large-mode-area double-cladding fibers (LMA-DCFs) by femtosecond (fs) laser is reported for the first time to the best of our knowledge. The chirped and tilted grating structure is obtained by scanning the fiber obliquely and moving the fs-laser beam relative to the chirped phase mask at the same time. By this method, the CTFBGs with different chirp rates, grating lengths, and tilted angles are fabricated, and the maximum rejection depth and bandwidth are ∼25 dB and ∼12 nm, respectively. To test the performance of the fabricated CTFBGs, one is inserted between the seed laser and the amplifier stage of a 2.7 kW fiber amplifier, and an SRS suppression ratio of ∼4 dB is achieved with no reduction in laser efficiency and degradation in beam quality. This work provides a highly fast and flexible method to fabricate large-core CTFBGs, which is of great significance to the development of high-power fiber laser systems.

In this manuscript, a narrow linewidth fiber amplifier based on confined-doped fiber is established, and the power scaling and beam quality maintaining capabilities of this amplifier are investigated. Benefitted from the large mode area of the confined-doped fiber and precisely controlling the Yb-doped region in the fiber core, the stimulated Brillouin scattering (SBS) and transverse mode instability (TMI) effects are effectively balanced. As a result, a 1007 W signal laser with just 1.28 GHz linewidth is obtained by combining the advantages of confined-doped fiber, near-rectangular spectral injection, and 915 nm pump manner. As far as we know, this result is the first beyond kilowatt-level demonstration of all-fiber lasers with GHz-level linewidth, which could provide a well reference for simultaneously controlling spectral linewidth, suppressing the SBS and TMI effects in high-power, narrow-linewidth fiber lasers.

The stimulated Raman scattering (SRS) process in gas-filled hollow-core fiber is mostly used to realize the wavelength conversion, which has the potential to produce narrow-linewidth and high-power fiber laser output. However, limited by the coupling technology, the current research is still at a few watts power level. Here, through the fusion splicing between the end-cap and the hollow-core photonics crystal fiber, several hundred watts pump power can be coupled into the hollow core. Homemade narrow-linewidth continuous wave (CW) fiber oscillators with different 3 dB linewidths are used as the pump sources, then the influences of the pump linewidth and the hollow-core fiber length are studied experimentally and theoretically. As the hollow-core fiber length is 5 m the H2 pressure is 30 bar, 109 W 1st Raman power is obtained with a Raman conversion efficiency 48.5%. This study is significant for the development of high-power gas SRS in hollow-core fibers.

In this Letter, to the best of our knowledge, the first kilowatt monolithic Yb-doped fiber laser operating near 980 nm is demonstrated. The fiber laser is achieved with the help of an all-fiber amplifier fabricated with a 105/250- µ m core/cladding-diameter double-cladding Yb-doped fiber. With 11-W seed light, about 1.11-kW output power was obtained with 65.3% slope efficiency. The center wavelength was around 978 nm with a 10-dB bandwidth of about 0.6 nm. About 34-dB suppression of 1030-nm amplified spontaneous emission was realized, and the output beam quality ( M 2 factor) was about 16.2 at maximum output power. Better beam quality can be expected by optimizing the seed light and active fiber. This work can provide significant guidance for the study and designation of high-power fiber lasers operating near 980 nm and other three-level fiber sources.

Side-pumping combiner is used for pumping double-clad fiber in various fiber laser schemes. However, its coupling efficiency and temperature characteristics suffer when pumped via a large numerical aperture (NA) pump light. We investigated the method of optimizing the coupling efficiency of a (2 + 1) ×1 combiner under a large NA pump light injection. After optimization of taper ratio and length of the pump fiber and fusion area between pump and signal fiber, the coupling efficiency increased and the temperature characteristic improved, which could be useful for fabrication of a side-pumping combiner for high-power fiber laser applications.

Fiber tapering technology has been used to realize low-loss coupling from solid-core fibers to antiresonant hollow-core fibers (AR-HCFs) at low-power level, by inserting the tapered solid-core fibers into the hollow-core of AR-HCFs. Here, we have demonstrated the high-power coupling capacity of this way. For ice-cream type AR-HCF, a transmission efficiency of ~47% is achieved with injected 1080 nm laser power of ~90 W in a ~2 m HCF; for nodeless type AR-HCF, a transmission efficiency of ~40% is achieved with injected 1550 nm laser power of ~32 W in a ~2 m HCF. The output mode field with good beam quality shows the mode filtering characteristics of AR-HCFs.

A novel, to the best of our knowledge, cantilever construction design of an adaptive fiber-optics collimator (AFOC) based on piezoelectric bimorph actuators for tip/tilt control is introduced. With this new cantilever structure, an AFOC with a diameter of only 6 mm was developed, and the output laser beam deviation angle and resonance frequency of the device were measured. The experimental results show that this new AFOC can provide more than 1 mrad deflection angle at a 20 V driving voltage, and the first resonance frequency is about 500 Hz. Further, in order to verify whether the cantilever structure can be used in a high-power fiber collimator, a high-power X − Y positioner with an 8 mm diameter fiber end cap was developed. The experimental results show that the high-power X − Y positioner can output more than 2 kW laser power and provide about 330 µm displacement of the fiber end cap in the X direction and about 770 µm in the Y direction at a 150 V driving voltage.

Side pumping combiners are widely used in fiber laser schemes for their high coupling efficiency, low insertion loss, and multi-point pumping capability. However, side pumping combiners perform differently in coupling efficiency when pumping with a laser diode (LD) and a high-brightness 1018 nm Yb-doped fiber laser (YDFL). In this paper, for the first time, to the best of our knowledge, we investigated the different parameters to fabricate the (2+1)×1 combiner with high coupling efficiency when pumping with an LD and a YDFL, respectively. After optimization, the maximum coupled pump power from one single-pump port of the combiner was 1200 W and 2730 W when pumping with a LD and a YDFL, respectively.

We demonstrate two critical rules of designing photonic lanterns for applications in adaptive spatial mode control: (1) optimized input fiber arrangements to effectively control modes; (2) appropriate input fiber core-cladding ratio to expand the optional range of the output fiber. The 3×1 and 5×1 photonic lanterns according to above design requirements have been fabricated. Using stochastic parallel gradient descent algorithm, the phases of the inputs are actively modulated to stabilize the output of novel 5×1 photonic lantern with 30/125 µm output fiber. When the control target is the fundamental mode, the M2 factor of output beam is below 1.2 stably, which will provide a possible technical solution to increase the mode instability threshold in large mode area fiber laser systems. Furthermore, we obtain single orbital angular momentum mode (OAM01 or OAM02 mode) and high order linearly polarized mode (LP11 or LP21 mode) with the purity of the corresponding modes over 0.85 by altering evaluation function, which will be of benefit in optical communication and atomic optics.

In this paper, a 500-W-level all-fiber amplifier operating near 980 nm is demonstrated firstly, to the best of our knowledge. The amplifier is fabricated with a 105/250-μm core/cladding-diameter double-cladding Yb-doped fiber (DCYF). By optimizing the length of DCYF, the record 556-W output power and 50.5% slope efficiency are obtained with a 1.85-W seed light, and about 30.9 dB peak-to-peak suppression of 1030-nm amplified spontaneous emission (ASE) is realized. Furthermore, the strong in-band ASE is also observed in experiment, which limits the power up-scaling of the fiber amplifier. By analyzing the origin of the in-band ASE, it is revealed that optimizing the seed light should be beneficial to the suppression of in-band ASE. Besides, the beam quality is measured at the maximum output power and the M2 factor is about 23.6. The experimental study should be able to provide significant guidance for understanding and designing the high-power three-level fiber lasers and amplifiers.

Among many types of side-pumping combiner, which have been reported recently, a fused tapered method, which is based on direct fusion of one or several tapered pump fibers to the surface of a signal fiber, seems to be a potential method because of its high coupling efficiency and the capability of handling high pump power up to kW-class. However, the fused depth is seldom taken into consideration in the numerical analysis, which is not in agreement with the real situation. We present the numerical and experimental investigation about the influence of fused depth on the performance of the side-pumping combiners for all-fiber lasers and amplifiers. The experimental results agree well with the theoretical analysis. The fused depth shows great importance and should be paid much attention when designing and developing a side-pumping combiner by the fused tapered method. We develop a fused tapered side-pumping combiner, which is successfully applied in a backward pumped fiber amplifier with a 1080 nm output power of 1.458 kW. Our results may also offer detailed suggestion and instruction of fabricating side-pumping combiner for high-power fiber laser applications.

Supercontinuum (SC) generation directly from an Ytterbium-doped fiber amplifier (YDFA) has many advantages such as high output power, low splicing loss, and high optical to optical conversion efficiency. However, the output spectrum can only extend to longer wavelength. We report a method to further extend the spectral range of SC generated in an YDFA at short wavelength direction by using cascaded double-clad passive fiber tapers. A 750 ps seed pulse at 1060 nm is amplified to 15.9 W and spectrally broadened from 1000 to 1600 nm in an YDFA system. Using this broadband source to pump cascaded double-clad passive fiber tapers, a 14.1 W SC source is obtained, covering 630 to 2000 nm, which is in an all-fiber and low-cost structure. It is found that, in fiber tapers, especially at the taper waist, the frequency shift rate of solitons increases significantly, and those red-shifted solitons are group-velocity matched to dispersive waves even in the red wavelength region. To our best knowledge, this is the first time there has been generation in tapered double-clad passive fiber.

In addition to the pump coupling efficiency and power handling capability of a signal/pump coupler, it is of great importance for high power fiber lasers and amplifiers to maintain the optical properties of the signal light propagating through the component. We report on the comprehensive theoretical and experimental investigations on the beam quality degradation of the signal light in a side pumping coupler with large-mode-area (LMA) signal fiber, fabricated by tapered-fused technique. For the purpose of further understanding the mechanism of beam quality degradation of a side pumping coupler, the impact of micro deformation and thermal dopant diffusion in the signal core is analyzed and discussed theoretically through Beam Propagation Method (BPM). The beam quality and signal insertion loss of several home-made couplers with different fabrication parameters are measured experimentally. We show theoretically and experimentally (using a hydrogen/oxygen flame) that the beam quality degradation and the signal insertion loss are dependent on the temperature and duration of heating process, as well as on the size of the heating region. Accordingly, several solutions and suggestions about the component design and fabrication with the consideration of beam quality are proposed.

We investigate a high-beam-quality (6+1)×1 signal and pump combiner with 50 μm core diameter of the output fiber based on the tapered-fused fiber bundle technique for the first time. The influence of taper ratio of the taper bundle for different mode proportions of the output fiber and the transition efficiency of the signal light are analyzed numerically. In the experiment, a signal fiber core diameter matching from 10 to 50 μm in the combiner is achieved. We experimentally demonstrate that the total 1018 nm pump efficiency of the combiner is >98% and the signal light insertion loss is less than 2%, with a beam quality factor M2 of 1.59. Also, we have set up an amplifier based on the combiner, whose output laser power can achieve 2340 W with the slope efficiency of 73.76%. The beam quality factor M2 of the output laser is 2.83 at the maximum power output, and a high beam quality is maintained when a large-mode-area fiber is employed in the laser system.

Coherent beam combination (CBC) of fiber laser array, which is a promising approach to overcome the power scaling limitations of the single fiber laser and to achieve high-brightness laser with good beam quality, can be employed in coherent conformal projection system (CCPS) for beam projection and free space laser communication system with enormous potential. To date, all the demonstrated CBC system through several kilometers atmospheric turbulence have employed collimated beams, which significantly decrease the final CBC effect because of the inefficient wavefront overlap. On the other hand, focused CCPS has lots of advantages over collimated projection system since the wave front of the projected beam is closer to that of a convergent spherical wave. In this paper, we design and manufacture focused CCPS based on an adaptive fiber optics collimator array for the first time, which is achieved by introducing controllable spherical aberration through the adaptive fiber optics collimator. A CBC system is setup to evaluate and validate its performance. Results show that compared with the collimated CCPS, the energy portion of the central lobe could be increased by a factor of 44%, which agrees well with the theoretical analysis.

High-peak-power transform-limited narrow-linewidth nanosecond all-fiber lasers are desired in a range of applications. However, their linewidths will be broadened by self-phase modulation (SPM). We propose a novel concept that generates transform-limited laser pulses by temporally shaping the pulse seed. The impact of the pulse shape on SPM-induced spectral broadening was studied numerically and experimentally. It was found theoretically that the square-shape pulsed laser is immune to SPM-induced spectral broadening. Based on this principle, we built a high-peak-power, linearly polarized, square-shape nanosecond all-fiber laser in a master oscillator power amplifier (MOPA) configuration. Stimulated Brillouin scattering (SBS) limited peak powers of 4.02 kW, 5.06 kW, 6.52 kW and 9.30 kW were obtained at pulse widths of 8 ns, 7 ns, 6 ns and 5 ns. Thanks to the square-shape pulsed seed, the linewidths at maximum peak power remained at 129.5 MHz, 137.6 MHz, 156.2 MHz and 200.1 MHz, respectively, close to the transform-limited values of 110.8 MHz, 126.6 MHz, 147.7 MHz and 177.3 MHz.

In this paper, we investigate a 19 × 1 tapered fiber bundle (TFB) pump combiner for high power fiber lasers theoretically and experimentally, based on the twist method. Theoretically, we analyze the influence of the taper length, taper ratio and twist angle on the performance of a 19 × 1 TFB combiner by using the beam propagation method. Then, a 19 × 1 all-fiber combiner is fabricated experimentally, with input fibers and output fiber having core/clad diameter of 220/242 μm (numerical aperture (NA) = 0.22) and 30/600 μm (NA = 0.06/0.46) respectively. The transmission efficiency of each port is higher than 98.6% and the average efficiency of all 19 ports is 99.3%. The combiner achieved an output power of 1.5 kW with transmission efficiency of 99.3% when tested with four ports of 976 nm laser diode. To our best knowledge, this is the first time that 220/242 μm (NA = 0.22) multimode fibers act as the pump fibers of a 19 × 1 end-pumping combiner.

In this paper, we demonstrate a 3 × 1 signal combiner using hydrofluoric acid etched optical fibers and a low index capillary tube to fabricate tapered fused bundle(TFB). The diameter of the output fiber is chosen to be 50 μm to control the beam quality. A high output power of 6015 W is obtained with a power of 6140 W launched into the combiner by three self-made Yb-doped fiber lasers(YDFLs) operating at wavelength of 1064 nm. The total transmission efficiency reaches 98 %. The beam quality factor, M2, is measured to be about 3.6, which is the best beam quality compared with the presented results to the best of our knowledge.

Beam combining is one of the most potential methods to break through the output power limitations of a single fiber laser. In this letter, we report a high-power experimental result achieved by incoherent beam combining based on a $7\times 1$ all-fiber signal combiner. The final output power is 6.26 kW with high beam quality ( $M^{2}\approx 4.3$ ). Beam quality of the incoherent combination laser is analyzed both by theory and experiment in details. Experimental measured values fit well with the simulation results. These results can demonstrate the feasibility of high-power high-brightness incoherent beam combining based on all-fiber signal combiners.