Self-imaging-based high-power all-fiber coherent combiners: fabrication and preliminary demonstration.

Self-imaging combiners can achieve near-perfect filled-aperture coherent beam combination in an all-fiber format with a high-power operation capability. In this Letter, the fabrication of proposed self-imaging combiners is presented, along with a demonstration of a 2 × 2 configuration that uses commercially available large-mode-area fibers, glass tube and square-core fiber. Two types of self-imaging combiners have been fabricated using polarization-maintaining fibers and non-polarization-maintaining fibers, respectively, and these have been tested in an all-fiber coherent beam combination system. Preliminary results reveal that non-polarization-maintaining fibers can achieve better positioning precision, and a maximal combining efficiency of 52.7% has been achieved. The deviation of the demonstrated combining efficiency from the theoretical prediction is mainly attributed to the distortion of the fiber bundle and square-core output fiber, which can be further improved by refining the fabrication process and employing specially developed square-core fiber with better geometrical precision. To the best of the authors' knowledge, this is the first validation of all-fiber coherent beam combining based on the self-imaging effect.

A 3.7-kW Oscillating-Amplifying Integrated Fiber Laser Featuring a Compact Oval-Shaped Cylinder Package.

Combining the advantages of high efficiency, environmental robustness, and anti-reflection behavior, oscillating-amplifying integrated fiber lasers have become popular for use in high-power laser structures in industrial applications, wherein the size of the laser source matters. Here, an oscillating-amplifying integrated fiber laser in an oval-shaped cylinder package has been proposed and demonstrated, the footprint for which only occupies an area of 0.024 m2 apart from the pump diode, which is much smaller than in traditional planar fiber laser packages. Numerical simulations have been carried out, which have revealed that an oval-shaped cylinder package can effectively suppress the high-order mode in large mode area fiber setups and thereby benefit the integration of fusion points and the unpackaged elements at the same time. Over 3.7 kW of transverse mode instability (TMI)-free output power has been obtained, with a slope efficiency higher than 80%. With a custom-made chirped and tilted fiber Bragg grating (CTFBG), the Raman suppression ratio is improved to reach 38 dB at peak output power. The oval-shaped design has been verified to assist with the realization of TMI suppression and improve the integration of high-power fiber lasers. To the best of our knowledge, this fiber laser has among the smallest footprints of the various fiber sources at such high-power operating levels.

Highly Integrated Cladding Mode Stripper Array for Compact High-Power Industrial Fiber Laser.

A design integrating multiple cladding mode strippers used in fiber laser architectures into a single device is proposed. This approach can increase the compactness of fiber lasers, thus contributing to industrial laser processing applications. By offset-placing the most intense light-stripping parts, for instance, by inversing the laser injection directions or by displacing the beginning of etched sections, multiple cladding mode strippers bundled together into a single housing can have the hottest regions separated and can operate at full power simultaneously, with no evident cross-influence on each other. Two and three cladding-mode-stripper arrays have been implemented, and validation tests have been conducted with ~500-W cladding power being injected into each input port. For both arrayed devices, compared to the scenario in which only a single cladding mode stripper is working, no greater than a 2.1 °C temperature increment is generated when all components are operating concurrently, which demonstrates the effectiveness of the integration method. In this way, one half and two thirds of space/weight reduction can be realized, respectively, for the two and three cladding-mode-stripper arrays, which is meaningful, since cladding mode strippers are among the most bulky and hottest components in fiber lasers. Moreover, this integration provides a valuable reference for the miniaturization of other components, and thus, could contribute to the development fiber lasers with higher power-to-volume ratios, which would be more economical for industrial applications.

High-power cylindrical vector beams generated from an all-fiber linearly polarized laser by metasurface extracavity conversion.

Five-hundred-watt cylindrical vector beams (CVBs) at 1030 nm with the 3 dB linewidth being less than 0.25 nm have been generated from a narrow linewidth all-fiber linearly polarized laser by metasurface extracavity conversion. At maximum output power, the transmission efficiency and polarization extinction ratio of radially polarized cylindrical vector beams (RP-CVBs) are beyond 98% and 95%, respectively. The average power is approximately an order higher than previously reported high-power narrow-linewidth CVBs generated from fiber lasers. The temperature rise of the metasurface is less than 10°C at 500 W output power, which means that the system can be further power-scaled in the near future. The high-power, high-purity, and high-efficiency RP-CVBs generated by the metasurface demonstrate potential application of a metasurface in high-power CVBs lasers.

SRS suppression in multi-kW fiber lasers with a multiplexed CTFBG.

In this paper, we present experimental studies on newly developed multiplexed chirped tilted fiber Bragg gratings (MCTFBGs) for stimulated Raman scattering (SRS) suppression for high-power fiber laser systems. The MCTFBG device is composed of five continuous segments of chirped tilted fiber Bragg gratings (CTFBGs), which are inscribed into the large-mode-area (LMA, 25/400µm) fibers. The SRS suppression capability of the MCTFBG device has been successfully demonstrated with a master oscillator power amplification (MOPA) fiber laser system at the output level of 3.4 KW. The experimental observation thus indicates the MCTFBG's excellent SRS suppression capability at a very high power level (∼15 dB under >3 kW) and high thermal handling capacity (∼1.48℃/kw). Our work thus provides a key development of essential fiber grating components that can effectively suppress the SRS suppression at a very high power level.

Experimental investigation of quasi-static mode degradation in a high power large mode area fiber amplifier.

In this work, quasi-static mode degradation in high power fiber amplifiers has been investigated experimentally. An increase of M2 from 1.3 to 2.6 with distortion of the beam profile is observed, which results in the signal spectra and backward light characterization departing from the traditional phenomena. The amplifier has been operated at the same input pump power of 705 W for nearly 2.2 hours to investigate the relationship between quasi-static mode degradation and photodarkening. The evolution of M2 factor/beam profile, mode correlation coefficient and output laser power at different working times indicate that the quasi-static mode degradation in the high power fiber amplifiers is dependent on photodarkening and evolves on the scale of tens of minutes. A visible green light has been injected to photobleach the gain fiber for 19 hours, which reveals that the quasi-static mode degradation has been suppressed simultaneously. To the best of our knowledge, this is the first detail report of photodarkening-induced quasi-static degradation in high power fiber amplifiers.

3 kW high OSNR 1030 nm single-mode monolithic fiber amplifier with a 180 pm linewidth.

In this Letter, the amplified spontaneous emission (ASE) effect of a 1030 nm fiber laser is studied theoretically and, based on the theoretical results, a 3 kW high optical signal-to-noise ratio (OSNR) 1030 nm fiber amplifier with a 180 pm linewidth and near-diffraction-limited beam quality is achieved. A theoretical model, which takes simulate ASE light falling in the range of Raman light as the Raman seed, has been used to optimize the power scaling capability of 1030 nm fiber amplifiers. It shows that the SRS effect seeded by the ASE is the main limiting factor for the fiber amplifiers operating at 1030 nm, and >3kW output power with a high OSNR can be achieved by proper parameter designing of the fiber laser system. A 1030 nm monolithic narrow linewidth fiber amplifier, which delivers 3 kW output power with the OSNR being 37 dB and a 0.18 nm spectrum linewidth, has been demonstrated. At the maximum 3 kW output power, the SRS light peak is obviously higher than ASE light, which agrees with the theoretical predictions. Neither a stimulated Brillouin scattering effect nor a thermal-induced mode instability effect has been observed at ultimate power level, and the beam quality factor M2 is measured to be less than 1.2. To the best of our knowledge, this is the highest average power for a narrow linewidth single-channel fiber laser system reported so far operating at 1030 nm.

Experimental study of the influence of mode excitation on mode instability in high power fiber amplifier.

Mode instability with different mode excitation has been investigated by off-splicing the fusion point in a 4 kW-level monolithic fiber laser system, which reveals that the fiber systems exciting more high order mode content exhibits lower beam quality but higher mode instability threshold. The static-to-dynamic mode degradation and dynamic-only mode degradation have also been observed in the same high power fiber amplifier by varying the mode excitation, which implicates that the mode excitation plays an important role in mode characteristics in high power fiber lasers. By employing a seed with near fundamental mode beam quality, only dynamic mode degradation-mode instability sets in with negligible static beam quality degradation. Then the fusion point in the seed laser is offset spliced to excite high order mode. As the output power of the main amplifier scales, the beam quality degrades with the beam profile being static, and then the dynamic mode instability sets in, the power threshold of which is higher than that with good beam quality seed. We consider that the static mode degradation is caused by the presence of incoherent supposition of fundamental and high order mode, which leads to that the measured dynamic mode instability threshold is higher.

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.

Dynamic characteristics of stimulated Raman scattering in high power fiber amplifiers in the presence of mode instabilities.

Impact of mode instability on dynamic characteristics of stimulated Raman scattering in high power fiber amplifiers has been studied for the first time, which reveals another characterization of mode instability from the aspect of optical spectrum. It shows that, after the onset of mode instability, the measured light spectrums, especially the Raman light spectrums, are different from those without mode instability, which become burr-like. As mode instability evolves into different stages, the intensity of stimulated Raman scattering effects as laser power increasing also behaves differently. During the transition region, the stimulated Raman scattering effect becomes stronger as the lasing power increases until the mode instability evolves into chaotic regions, where the stimulated Raman scattering effect weakens. The effect of stimulated Raman scattering on mode instability has also been studied. Due to that the stimulated Raman scattering effect is weak and that the fraction of Raman light is only a few percent, the stimulated-Raman-scattering-induced mode instability has not been observed in the experiment, and the observed mode instability is induced by ytterbium ion gain of signal laser. It also revealed that the stimulated Raman scattering has negligible influence on the mode instability induced by ytterbium ion gain.

Spatially-distributed orbital angular momentum beam array generation based on greedy algorithms and coherent combining technology.

A novel approach to generate a spatially-distributed orbital angular momentum (OAM) beam array based on coherent combining technology is presented. The arrangement of the multiple fundamental Gaussian beams at the initial plane, as well as the intensity weights and the phase distributions of the array beams, is determined by the reversal of Huygens Fresnel diffraction and the greedy algorithm. This method ensures that a vortex beam array is formed at a specified distance, and the distance can be adjusted by phase modulation. The evolution properties of the synthesized beam array near the receiver plane are studied as well to estimate the robustness of the method. The experimental limitations of this technique are discussed, including the maximum number of beams, the relative separation of each beam and the maximum topological charges. The results illustrate that a spatially-distributed OAM beam array can be effectively generated within a finite distance interval, and the distance is adjustable. This new method enables further applications of a structured optical field, such as optical communication and spatial light structuring.

Mode instability dynamics in high-power low-numerical-aperture step-index fiber amplifier.

The study on mode instability (MI) in the large-mode-area fiber is generating great interest regarding the high-power applications of fiber lasers. To the best of our knowledge, we have investigated for the first time the dynamics of the output beam from a kilowatt-level all-fiber amplifier based on the low-numerical-aperture (<0.04) step-index (SI) fiber before and after the onset of the MI, including the temporal dynamics and mode evolution. The temporal power fluctuations indicate three evolution stages apart from the onset threshold of the MI, defined as stable, transition, and chaotic regions. In addition, the mode decomposition technique is utilized to accurately observe and investigate the mode evolution and relevant modal content corresponding to the transition and chaotic regions in the SI fiber laser for the first time. According to the mode decomposition results, the reduction of the extracted power can be explained by the high bending loss of the high-order mode excited in the MI process. Finally, the difference of MI dynamics between the fiber lasers based on the SI fiber and rod-type photonic crystal fiber is discussed.

1.89 kW all-fiberized and polarization-maintained amplifiers with narrow linewidth and near-diffraction-limited beam quality.

In this manuscript, we demonstrate high power, all-fiberized and polarization-maintained amplifiers with narrow linewidth and near-diffraction-limited beam quality by simultaneously suppressing detrimental stimulated Brillouin scattering (SBS) and mode instability (MI) effects. Compared with strictly single frequency amplification, the SBS threshold is scaled up to 12 dB, 15.4 dB, and higher than 18 dB by subsequently using three-stage cascaded phase modulation systems. Output powers of 477 W, 1040 W, and 1890 W are achieved with full widths at half maximums (FWHMs) of within 6 GHz, ~18.5 GHz, and ~45 GHz, respectively. The MI threshold is increased from ~738 W to 1890 W by coiling the active fiber in the main amplifier. Both the polarization extinction ratio (PER) and beam quality (M2 factor) are maintained well during the power scaling process. To the best of our knowledge, this is the first demonstration of all-fiberized amplifiers with narrow linewidth, near linear polarization, and near-diffraction-limited beam quality at 2 kW power-level.

560 W all fiber and polarization-maintaining amplifier with narrow linewidth and near-diffraction-limited beam quality.

We present a narrow linewidth, all-fiber polarization-maintained amplifier chain seeded by a phase-modulated single-frequency laser, which is a narrow linewidth. Different from previous phase-modulation techniques, the phase-modulation signal is generated by simply imposing an excited signal to an acoustic-optical driven source. Theoretical simulation results show that this method can suppress stimulated Brillouin scattering (SBS) to a better degree, and the output power can be boosted to about 1.2 kW in terms of the SBS threshold. By amplifying the phase-modulated seed based on master-oscillator power-amplification configuration in experiments, a 560 W output laser is achieved with slope efficiency of 87.2% and linewidth of <5 GHz. Further power scaling is limited by mode instability instead of an SBS effect. At maximal output power, the beam quality (M2 factor) and polarization extinction ratio is measured to be within 1.3 and 14 dB, respectively.

1.5 kW ytterbium-doped single-transverse-mode, linearly polarized monolithic fiber master oscillator power amplifier.

A linearly polarized monolithic fiber laser based on a master oscillator power amplifier structure with a master oscillator and a one-stage power amplifier is reported. We design a homemade oscillator based on the theory that, in the coiled gain fiber, the higher modes and the polarized mode of the fundamental mode along the fast axis are suppressed effectively because of their obviously higher bend loss than that of the polarized mode of the fundamental mode along the slow axis. The oscillator operates at 1080 nm, launching a 30 W seed laser with a high polarization extinction ratio of 19 dB into the power amplifier via a mode field adapter. The power amplifier utilizes Yb-doped polarization-maintaining fiber of 20/400 µm, which produces nearly diffraction-limited output power of about 1.5 kW with an optical-optical efficiency of 81.5% and a polarization extinction ratio of 13.8 dB. Both the M(x)² factor and the M(y)² factor of the collimated beam are measured to be about 1.2. The spectral width of the output power is broadened approximately linearly, and the full width at half maximum of the spectrum at the maximum output power is about 5.8 nm. It is known as the highest linearly polarized output power to the best of our knowledge.

Experimental investigation of thermal effects and PCT on FBGs-based linearly polarized fiber laser performance.

We experimentally study the impacts of thermal effects and polarization crosstalk (PCT) on the performance of FBGs-based linearly polarized all-fiber laser. The mechanism that the thermal effects and PCT influence the performance of the laser is analyzed. Thermally-dependent reflection peaks of polarization maintaining (PM) fiber Bragg gratings are revealed to be the prime reason why temperature influences spectrum, output power, and polarization property of the laser. The PCT would also influence the performance of the laser seriously in the case of mismatched angle even with effectively overlapped spectrum. It is revealed experimentally that stable linearly polarized output can be obtained if a certain pair of aligned principal axes of PM FBGs is not only spectrally overlapped but also strictly angle matched. Further, we point out that accurate temperature control and careful angle match are essential for stable linearly polarized output and even possible power scaling further.

240 W high-average-power square-shaped nanosecond all-fiber-integrated laser with near diffraction-limited beam quality.

We report an all-fiber-integrated high-average-power square-shaped nanosecond pulse laser operating at 1068 nm based on the master oscillator power amplifier configuration. The seed source is a passively mode-locked Yb-doped fiber laser with fundamental cavity repetition rate of 1.86 MHz. Output pulses with a square shape can be tuned in pulse width from 271 ps to the nanosecond level. The average output power reaches to 9.21 W after three preamplifiers. Finally, a main amplifier is developed to boost the average output power to 240 W, and the corresponding pulse energy and peak power are ∼ 129.3 µJ and 36 kW, respectively. The efficiency of the main amplifier is ∼ 61.3%, and the beam quality represented by M(2) factors is below 1.3 and 1.2 in the X and Y directions.

Pulsed Yb³âº-doped fiber laser operating at 1011 nm by intra-cavity phase modulation.

A 1011 nm pulsed Yb³âº-doped fiber laser is experimentally demonstrated by employing a commercially available LiNbO3 phase modulator (PM) in the linear cavity. The resonator is built up with a section of normal single-cladding Yb³âº-doped fiber, a PM, and a pair of fiber Bragg gratings. Active mode-locked stable trains of pulses with 2 and 1.4 ns are generated at repetition rates of 30.2478 and 60.4956 MHz, respectively. The maximum average output power is 10.6 mW at pump power of 200 mW, with the slope efficiency of 13.3%. Relaxation-oscillation-modulated pulses with width of 2 µs are obtained at a repetition rate of 27.778 kHz.

Coherent polarization beam combination of four mode-locked fiber MOPAs in picosecond regime.

In this manuscript, we report on coherent polarization beam combination (CPBC) of a four-channel pulsed amplifier array in the picosecond regime by using single frequency dithering technique. By employing a photo-detector with low-pass bandwidth (8.5 MHz at 10 dB gain) to filter the intensity fluctuation and obtain phase errors for feedback, a combined laser pulse with~480 ps pulse width at~60 MHz repetition rate is obtained with an average power of 88 W. By adjusting the optical path differences (OPDs) and controlling the pump power to ensure the synchronizations and alleviate the influence of nonlinear phases among each channel, more than 90% combining efficiency is achieved with excellent beam quality (M(2)~1.1). Finally, the efficiency loss of the system along with the power scaling process is discussed.

1.5 kW, near-diffraction-limited, high-efficiency, single-end-pumped all-fiber-integrated laser oscillator.

We report a monolithic single-end-pumped all-fiber laser oscillator with 1.5 kW power output operating at 1070 nm. The laser scheme design and fiber parameter selection are based on our theoretical analysis through one rate equation model consisting of changing pump wavelengths. The laser oscillator is pumped by 49 fiber-pigtailed 915 nm laser diodes with power of 50 W each through two stage combiners. The measured laser power is 1520 W at the pump power of 2054 W, and the corresponding optical-to-optical efficiency is 74%, in agreement with numerical simulation. Stimulated Raman scattering is observed when the laser power reaches 1030 W and the power ratio of Raman power is 4% at maximum output power. The experimental results show that the beam qualities represented by the M(2) factor are less than 1.2 at all pump power levels.