Raman suppression in high-power fiber oscillators by femtosecond-written chirped and tilted fiber Bragg gratings.

Realizing stimulated Raman scattering (SRS) suppression is a key topic for high-power fiber lasers. Here, we report an effective and simple strategy for SRS suppression using chirped and tilted fiber Bragg gratings (CTFBGs) in high-power fiber oscillators while maintaining the compactness and stability of the system. The CTFBG is inserted on the side of a cavity mirror FBG without cutting the gain fiber. To improve power handling capability, the CTFBG and cavity mirror FBGs are inscribed by femtosecond (fs) lasers. The optimal SRS suppression effect can be realized when the CTFBG is inserted into the resonant cavity and on the side of the output coupler FBG. The SRS threshold is increased by approximately 11% with an SRS suppression ratio of nearly 14 dB. Moreover, the output power of the fiber oscillator is improved to 3.5 kW, which is the maximum power achieved in fiber oscillators with SRS suppression using CTFBGs, to the best of our knowledge. The temperature of the air-cooled CTFBG is 50.2 °C, which has the potential to handle higher power. This work provides new insights for suppressing SRS in fiber oscillators, promoting the application of CTFBGs in high-power lasers.

10 kW tandem pumping fiber amplifier with good beam quality based on tapered ytterbium-doped fiber.

In this paper, we established a high power tandem pumped fiber amplifier based on tapered ytterbium-doped fiber (TYDF). The TYDF is developed in-house with a core/inner cladding diameter of 30/250 µm at the small-core region and 48/400 µm at the large-core region. The key parameters of the amplifier in a co-pumped and counter-pumped configuration are experimentally investigated, such as slope efficiency, stimulated Raman scattering (SRS) threshold, and beam quality evolution. Up to 10.28 kW laser free of SRS or transverse mode instability is obtained from the counter-pumped amplifier, and the beam quality factor M2 is 2.29, which is significantly improved compared with the 48/400 µm uniform YDF. To the best of our knowledge, this is the highest average output power achieved so far based on the TYDF. This work could provide a solution for balancing the SRS suppression and high order modes control in high power tandem pumped YDF lasers.

All-fiberized linearly polarized superfluorescent fiber source with 5 kW power output.

A superfluorescent fiber source (SFS) is a special fiber source that commonly possesses high temporal stability and a wide spectral linewidth. In this work, an all-fiberized linearly polarized SFS with, to our knowledge, record output power and near-diffraction-limited beam quality is presented. Up to 5.03 kW SFS is achieved at a pump power of 6.18 kW with a corresponding conversion efficiency of ∼81.1%. At maximum output power, the signal-to-noise ratio to background spectral noise is over 50 dB, the polarization extinction ratio is ∼17d B, and the beam quality factor is M x 2=1.49, M y 2=1.44. Further comparisons confirm the power scalability of fiber amplifiers employing SFSs as seed lasers. Overall, this work could provide a good reference for potential exploration of high-power fiber laser systems.

Robust femtosecond-written chirped and tilted fiber Bragg gratings for Raman filtering in multi-kW fiber lasers.

We present a robust chirped and tilted fiber Bragg grating (CTFBG) in a large-mode-area double-cladding fiber (LMA-DCF) written by a femtosecond (fs) laser. By implementing the fs-CTFBG into the output end of a high-power fiber laser for Raman filtering, a power handling capability of 4 kW is achieved with a Raman filtering ratio of ∼13 dB. To the best of our knowledge, this is the maximum handling power of a CTFBG for Raman filtering. The signal loss of the fs-CTFBG is 0.03 dB, which has little effect on the output laser beam quality. The air-cooled fs-CTFBG has a minimum temperature slope of 7.8°C/kW due to a self-annealing effect. This work proves the excellent performance of the fs-CTFBG, promoting the development of high-power CTFBGs.

Analytical study on the steady-state thermal blooming effect of high-power ytterbium-doped fiber lasers propagating through the atmosphere.

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.

Femtosecond laser fabrication of chirped and tilted fiber Bragg gratings for stimulated Raman scattering suppression in kilowatt-level fiber lasers.

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.

Stimulated Raman scattering of H2 in hollow-core photonics crystal fibers pumped by high-power narrow-linewidth fiber oscillators.

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.

Confined-doped fiber enabled kilowatt-level all-fiber laser with 1.28†GHz linewidth.

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.

Stable single transverse mode excitation in 50 µm core fiber using a photonic-lantern-based adaptive control system.

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.

Cantilevered adaptive fiber-optics collimator based on piezoelectric bimorph actuators.

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.

Demonstration of kilowatt monolithic Yb-doped fiber laser operation near 980 nm.

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-µm 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 (M2 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.

Investigation of the pump coupling efficiency of a side-pumping combiner based on tapered-fused method.

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.

Beam quality degradation of signal light in a side pumping coupler with a large-mode-area signal 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.

High-beam-quality signal and pump combiner with large-mode-area fiber for high-power fiber laser and amplifier.

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.

Highly efficient coherent conformal projection system based on adaptive fiber optics collimator array.

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.

Incoherent beam combining of fiber lasers by an all-fiber 7 × 1 signal combiner at a power level of 14 kW.

We demonstrate an all-fiber 7 × 1 signal combiner with an output core diameter of 50 µm for high power incoherent beam combining of seven self-made Yb-doped single-mode fiber lasers around a wavelength of 1080 nm and output power of 2 kW. 14.1 kW combined output power is achieved with a total transmission efficiency of higher than 98.5% and a beam quality of M2 = 5.37, which is close to the theoretical results based on finite-difference beam propagation technique. To the best of our knowledge, this is the highest output power ever reported for all-fiber structure beam combining generation, which indicates the feasibility and potential of >10 kW high brightness incoherent beam combining based on an all-fiber signal combiner.

Realization of large energy proportion in the central lobe by coherent beam combination based on conformal projection system.

In this paper, we experimentally validate a tiled-aperture conformal projection system with the largest array filling factor and element beam truncation factor to the best of our knowledge. The conformal projection system, which is made up of a hexagonal adaptive fiber-optics collimator (AFOC) array with the proximate ideal intensity distributions, is fabricated and the performance of output beam is tested and evaluated properly and carefully. Both of the active phase-locking control and precise tip-tilt control are implemented successfully in the CBC of the hexagonal seven-beam-array. Experimental results show a large energy proportion (47%, which increases by over 10% comparing with the previously demonstrated largest value) in the central lobe is achieved and the residual phase error is lower than λ/27. When the AFOC array performs, the precise tilt control makes the combining beams overlap well and the average normalized metric value is improved from 0.336 without control to 0.947 with both of active piston and tip-tilt phase controls while the fringe contrast increases from 19% to more than 91% correspondingly. This work presents a promising structure for the achievement of large energy proportion in the central lobe in high power fiber laser CBC systems.

Monolithic fiber end cap collimator for high-power free-space fiber-fiber coupling.

In this paper, we present the design, construction, and testing of a monolithic fiber end cap collimator for high-power free-space fiber-fiber coupling applications. The collimator is based on a large-sized fiber end cap and a spherical lens design on the output facet. Values of the spot size and working distance are theoretically analyzed based on Gaussian approximation and ABCD transmission matrix. The free-space fiber-fiber coupling process is also analyzed for different lens curvature radii and coupling distances. In the experiment, a collimated laser beam is obtained with Rayleigh length of about 400 mm. A high-power laser with 1.1 kW output is tested on the end cap collimator, which only heats up by 7°C at the output facet without active cooling. Free-space fiber-fiber coupling between two 20/400 µm fibers is achieved based on these collimators, with measured coupling loss lower than 0.3 dB.

Large deflection angle, high-power adaptive fiber optics collimator with preserved near-diffraction-limited beam quality.

We report on the development of a monolithic adaptive fiber optics collimator, with a large deflection angle and preserved near-diffraction-limited beam quality, that has been tested at a maximal output power at the 300 W level. Additionally, a new measurement method of beam quality (M2 factor) is developed. Experimental results show that the deflection angle of the collimated beam is in the range of 0-0.27 mrad in the X direction and 0-0.19 mrad in the Y direction. The effective working frequency of the device is about 710 Hz. By employing the new measurement method of the M2 factor, we calculate that the beam quality is Mx2=1.35 and My2=1.24, which is in agreement with the result from the beam propagation analyzer and is preserved well with the increasing output power.

Theoretical and experimental research of supercontinuum generation in an ytterbium-doped fiber amplifier.

The theoretical research of supercontinuum (SC) generation in a fiber amplifier system has been seldom reported. For the purpose of further understanding the mechanism of SC generation in fiber amplifiers, we propose a combined numerical model of the laser rate equations and the generalized non-linear Schrödinger equation to simulate the amplification of 1060 nm picosecond pulses and their spectral broadening in an ytterbium-doped fiber amplifier. The calculation results of this model are compared with the experimental results under the same conditions and a good agreement is achieved. We find that the pulse is gain amplified initially, and then dominated by stimulated Raman scattering in the normal dispersion region. In anomalous dispersion region, modulation instability, higher-order soliton fission and soliton self-frequency shift dominates the spectral broadening. It is found numerically and experimentally that the length of the gain fiber and the 976 nm pump power are the most imperative parameters to control the output power, spectral range and flatness of the SC. The pulse width of signal pulse also plays a part in influencing SC generation. The results verify that our model is promising for analyzing the physical processes of pulse evolution and SC generation in a fiber amplifier system.