2 kW incoherent beam combining of four narrow-linewidth photonic crystal fiber amplifiers

Real-time synchronized monitoring for the overlap accuracy of the combining beam spot in a wavelength beam combination based on confocal planes

Beam overlap accuracy in a wavelength beam combination system determines the beam quality and efficiency, so systematic monitoring of overlap accuracy is essential. In this work, a method of performing real-time synchronized monitoring and recording overlap accuracy for a combining beam spot is proposed. Firstly, theoretical calculations for monitoring different wavelength sub-beam positions and angular errors are established. Then, an optical design and grayscale centroid algorithm are developed to analyze and simulate the combination spots. A monitoring device was designed and constructed to meet the requirements of combining system applications, which achieved an accuracy of 8.86 µrad. Finally, the method successfully monitored the system spot fluctuation range within ±22 µrad. This study resolves the issue of distinguishing the different wavelength sub-beams and their response delays in traditional combining beams. It offers precise error data for real-time synchronized calibration of the overlap accuracy in laser beam combining technology.

A 2 kW, 8 GHz-Linewidth Yb-Doped Polarization-Maintained Fiber Laser with Quasi-Flat-Top Pseudo Random Binary Sequence Phase Modulation for SBS Suppression

We demonstrated a narrow-linewidth high-power Yb-doped polarization-maintaining (PM) fiber laser with near-diffraction-limited beam. The laser system consisted of a phase-modulated single-frequency seed source and four-stage amplifiers in the master oscillator power amplifier configuration. A quasi-flat-top pseudo random binary sequence (PRBS) phase-modulated single-frequency laser with a linewidth of 8 GHz was injected into the amplifiers for suppressing stimulated Brillouin scattering. The quasi-flat-top PRBS signal was readily generated from the conventional PRBS signal. The maximum output power was 2.01 kW with polarization extinction ratio (PER) of ~15 dB. The beam quality (M2) was less than 1.3 over the power scaling range.

Simply structured polarization-independent high efficiency multilayer dielectric gratings

A polarization-independent multilayer dielectric diffraction grating with a low aspect ratio and high diffraction efficiency was designed and fabricated. The diffraction grating designed with a grating density of 1200 lines/mm had an aspect ratio of 0.59, mean polarization-independent diffraction efficiency in the Littrow angle of ±2.5^∘, and 1030-1080 nm wavelength range of 97.2%. The designed grating was fabricated using ion assisted deposition and reactive ion etching techniques. The mean polarization-independent diffraction efficiency of the fabricated grating was 96.1%, and its standard deviation was 0.68%. The fabricated diffraction grating was irradiated with a 1064 nm cw laser, with a power density of 30kW/cm², for 1 min to measure the temperature change before and after the laser application. It was verified that the temperature variation of the diffraction grating without heat treatment was 8.8°C, and the temperature variation after heat treatment at 400°C decreased to 2.3°C.

Numerical analyses of a spectral beam combining multiple Yb-doped fiber lasers for optimal beam quality and combining efficiency

Physical parameters of a spectral beam combining (SBC) system for multiple Yb-doped fiber lasers (YDFLs) were identified and numerically analyzed to obtain the optimal beam quality and the combining efficiency. We proposed an optimal range of the parameters that can be utilized in SBC systems. For a practical SBC system composed of a multi-layer dielectric grating and a transform mirror, we systematically varied input laser parameters such as the incident angle, beam diameter, laser linewidth, spectral spacing, number of beams, and their spatial separation. Characteristics of diffracted beams by the SBC system were numerically analyzed using a Fourier modal method (FMM). The beam quality M2 and the combining efficiency, η, were optimized by varying the laser beam parameters. We found that M2 and η were most affected by the angle of incidence and the laser linewidth, respectively. We presented the optimal parameters for three, five, and seven linear beam array SBCs along with a range of allowed parameters that could be used in the laser power scaling.

Towards Ultimate High-Power Scaling: Coherent Beam Combining of Fiber Lasers

Fiber laser technology has been demonstrated as a versatile and reliable approach to laser source manufacturing with a wide range of applicability in various fields ranging from science to industry. The power/energy scaling of single-fiber laser systems has faced several fundamental limitations. To overcome them and to boost the power/energy level even further, combining the output powers of multiple lasers has become the primary approach. Among various combining techniques, the coherent beam combining of fiber amplification channels is the most promising approach, instrumenting ultra-high-power/energy lasers with near-diffraction-limited beam quality. This paper provides a comprehensive review of the progress of coherent beam combining for both continuous-wave and ultrafast fiber lasers. The concept of coherent beam combining from basic notions to specific details of methods, requirements, and challenges is discussed, along with reporting some practical architectures for both continuous and ultrafast fiber lasers.

Optimization and visualization of phase modulation with filtered and amplified maximal-length sequence for SBS suppression in a short fiber system: a theoretical treatment

We use a model to investigate both the temporal and spectral characteristics of a signal lightwave which has been spectrally broadened through phase modulation with a maximal-length sequence (MLS), which is a common type of pseudo-random bit sequence. The enhancement of the stimulated Brillouin scattering (SBS) threshold of the modulated lightwave in a fiber system is evaluated by numerically simulating the coupled three-wave SBS interaction equations. We find that SBS can build up on a nanosecond-level time scale in a short fiber, which can reduce the SBS suppressing capability of MLS modulation waveforms with GHz-level clock rate, if the sub-sequence ("run") lengths with the same symbol (zero or one) of the MLS extend over several nanoseconds. To ensure the SBS buildup is perturbed and thus suppressed also during these long sub-sequences, we introduce a low-pass filter to average the signal over several bits so that the modulation waveform changes gradually even during long runs and amplify the RF modulation waveforms to the level required for sufficient spectral broadening and carrier suppression of the optical signal. We find that the SBS suppression depends non-monotonically on the parameters of the filtered and amplified MLS waveform such as pattern length, modulation depth, and the ratio of low-pass filter cutoff frequency to clock rate for maximum SBS mitigation. We optimize the SBS suppression through numerical simulations and discuss it in terms of the temporal and spectral characteristics of the lightwave and modulation waveform using derived analytical expressions and numerical simulations. The simulations indicate that the normalized SBS threshold reaches a maximum for a RMS modulation depth of 0.56π and a ratio of filter cutoff frequency to clock rate of 0.54 and that MLS9 is superior to other investigated patterns.

Photonic integrated circuits based hybrid integration for wavelength beam combining

In this Letter, we have demonstrated wavelength beam combining (WBC) through hybrid integration of photonic integrated circuits (PICs) to significantly reduce the size, weight, and operation power of the laser combining system. The hybrid integration WBC includes III/V semiconductor optical amplifiers (SOAs), which provide gain, and the silicon nitride PICs, which perform as the external cavity. We first show that the arrayed waveguide grating (AWG) -based hybrid laser defines the lasing wavelength through the AWG passband. We then demonstrate that the AWG successfully forms multiple channel lasers by combining SOAs in the hybrid platform.

High-power, high-beam-quality spectral beam combination of six narrow-linewidth fiber amplifiers with two transmission diffraction gratings

We demonstrate 10 kW spectral beam combination of six narrow-linewidth fiber amplifiers by two transmission gratings with a combining efficiency about 90%. The wavelengths of incident beams range from 1056 to 1088 nm. Each fiber amplifier delivers 2 kW output power laser with 0.25 nm root-mean-square (RMS) linewidth. A dual-grating beam-combining system is constructed to combine the six beams into a 10 kW level high-power beam and control the beam quality factor ${{\rm M}^2} \lt {2}$M²<2 by dispersion compensation. To the best of our knowledge, this is the highest output power combined by transmission gratings, which verifies the feasibility of transmission grating under high-power density laser radiation and provides an alternative approach for high-power beam combining.

2.05 kW all-fiber high-beam-quality fiber amplifier with stimulated Brillouin scattering suppression incorporating a narrow-linewidth fiber-Bragg-grating-stabilized laser diode seed source

We experimentally demonstrate an all-fiber high-power fiber amplifier with high beam quality and a slope efficiency of 81.8%, using a fiber-Bragg-grating-stabilized laser diode as a narrow spectral linewidth (0.08 nm) seed source. During amplification, the spectral linewidth of the laser output is broadened from 0.08 to 0.24 nm due to nonlinear phenomena. To the best of our knowledge, we report the first experimental observation of the suppression of stimulated Brillouin scattering (SBS), with increased output power. In addition, we investigated the SBS suppression by simultaneously measuring the optical backscattered power, backscattered spectrum, and output spectrum at different values of output power. The beam quality, M², was measured to be ∼1.28 at the maximum output power of 2.05 kW, and modal instability was not observed.

Modeling and Analysis of the Influence of an Edge Filter on the Combining Efficiency and Beam Quality of a 10-kW-Class Spectral Beam-Combining System

Filter-based spectral beam combining (FSBC) is a promising power-scaling concept for high-power, broad-linewidth fiber lasers, as it relaxes the requirements for linewidth control and also the sizes of the individual beams. As the combining element in the FSBC system, the steep-edge filter plays a major role in achievement of the combining efficiency and the beam quality. In this case, we combine the uncorrelated surface roughness model and the combining efficiency model, and we conduct a comprehensive analysis of the effects of surface roughness, thickness error, and incident angle on the filter’s optical properties and the combining efficiency, in order to determine the optimal configuration for the laser beam-combining system. The simulation results show a good agreement with the measured ones. Meanwhile, through the adoption of the angular spectrum theory, this paper has also conducted a preliminary analysis of the influence of the combining elements on the quality of the combined beam, and some theoretical instructions on the future design of the spectral beam-combining system are provided.

High-efficiency polarization-independent wideband multilayer dielectric reflective bullet-alike cross-section fused-silica beam combining grating

A high-efficiency polarization-independent wideband multilayer dielectric reflective bullet-alike cross-section (combination of trapezoidal-rectangular grating profile) fused-silica beam combining grating (BCG) used in the -1st order for spectral beam combining (SBC) is designed and fabricated. Exact grating profile parameters are optimized by using the rigorous coupled-wave analysis and simulated annealing algorithm. As a comparison, traditional pure trapezoidal and pure rectangular gratings are also designed. Numerical results show that such a bullet-alike cross-section BCG exhibits wide bandwidth with the lowest maximum electric field enhancement in the grating material, which is greatly beneficial for the promotion of the power scaling level of the grating-based SBC system. A two-step dry-etching procedure is developed to fabricate such a grating. The averaged diffraction efficiency of greater than 91% was experimentally demonstrated.

Stimulated Brillouin scattering suppression of thulium-doped fiber amplifier with fiber superfluorescent seed source

We investigate the stimulated Brillouin scattering (SBS) effect in high-power thulium-doped fiber amplifier seeded with a narrow-linewidth fiber superfluorescent source or a conventional narrow-linewidth fiber laser. No random backward SBS pulses are observed when using a narrow-linewidth fiber superfluorescent source as the seed. The corresponding average power and peak power reach 153 W and 3.4 kW, respectively, only limited by the available pump power. This gives the average power and peak power extraction from the thulium-doped fiber amplifier with 17 fold enhancement, in comparison with the situation using the conventional narrow-linewidth fiber laser with similar central wavelength and spectral linewidth as the seed. This work indicates that using low coherent fiber superfluorescent sources is a good solution for power scaling in narrow-linewidth fiber amplifier system in order to overcome the limitation of SBS effect.

Coupling efficiency model for spectral beam combining of high-power fiber lasers calculated from spectrum

We utilize the spectral broadening of Yb-doped fiber lasers in the spectral beam combining scheme to develop an analytical model of the coupling efficiency, which forms a critical factor in evaluating the practicality of the beam combination system. The simulation results predict a trend similar to the measured ones. Via increasing the number of simulating lasers, the model can be extended to calculate the combining efficiency of the resulting multiple-beam-combination system and estimate the optimal output power and combining efficiency. Moreover, the analytical model is suitable to investigate key parameters of Yb-doped lasers and filters, which is beneficial in enhancing the combining efficiency.

10.8 kW spectral beam combination of eight all-fiber superfluorescent sources and their dispersion compensation

We report an 8-element spectral beam combination of Yb-doped all fiber superfluorescent sources around 1070 nm wavelength. Each source consists of a 60 mW front-end and a 1.5 kW three-stage fiber amplifier chain. The eight output beamlets are spectrally combined using a home-made polarization-independent multilayer dielectric reflective diffraction grating. 10.8 kW output power is achieved with an efficiency of 94%. Besides, both theoretical and experimental studies of dual grating dispersion compensation scheme have been performed, which is proved to be a prospective way for high brightness spectral beam combination.

kW-level narrow linewidth fiber amplifier seeded by a fiber Bragg grating based oscillator

This paper demonstrates an all-fiber narrow linewidth amplifier with a seed based on narrow linewidth fiber Bragg gratings (FBGs). The fiber amplifier achieves a narrow bandwidth output of 823 W, with an opto-optic efficiency of 84.5%. The pair of FBGs in the seed configuration helps to assure a narrow linewidth of the laser as 0.08 nm. In the laser profile, we introduce a cladding stripper with a sectional structure, which realizes high pump power leakage with high efficiency. The paper also discusses the impact of seed linewidth and fiber length on the SBS threshold in a narrow bandwidth laser. Based on this analysis, we discovered ways to inhibit SBS onset and scale power output.

Highly polarized all-fiber thulium laser with femtosecond-laser-written fiber Bragg gratings

We demonstrate and characterize a highly linearly polarized (18.8 dB) narrow spectral emission (<80 pm) from an all-fiber Tm laser utilizing femtosecond-laser-written fiber Bragg gratings. Thermally-dependent anisotropic birefringence is observed in the FBG transmission, the effects of which enable both the generation and elimination of highly linearly polarized output. To our knowledge, this is the first detailed study of such thermal anisotropic birefringence in femtosecond-written FBGs.

Stimulated Brillouin scattering thresholds in optical fibers for lasers linewidth broadened with noise

Phase and/or intensity modulation techniques to broaden the Linewidth of an optical source are well known methods to suppress stimulated Brillouin scattering (SBS) in optical fibers. A common technique used to achieve significant bandwidth enhancement in a simple fashion is to phase modulate with a filtered noise source. We will demonstrate here that, in this case the stochastic nature of noise requires an inclusion of length dependent corrections to the SBS threshold enhancement. This effect becomes particularly significant for short fiber lengths common to most high power fiber amplifiers.

Sequence combining of pulsed lasers using refraction-beam-displacement

We present an approach for effectively combining high-power pulsed lasers based on a refraction displacement pulse combining technique. This approach allows for lasers combining with various repetition rates, pulse duration, polarization, output power, and wavelength. The maximum number of lasers that can be combined mainly depends on their repetition rate and pulse duration. This approach is a feasible way to combine a large number of high average power lasers while maintaining good beam quality.

High density spectral beam combination with spatial chirp precompensation

A method for spectral combination of lasers with extremely high spectral density is introduced, enabling greater than 80% and theoretically approaching 100% spectral density utilization with no degradation in beam quality. Experiments demonstrating the utility of our method are described, cumulating in a demonstration of a compact, packaged laser with photonic-crystal-fiber-rod amplifiers at 0.5-MW peak power and 0.15-nm wavelength spacing. Our method is potentially scalable to many 100's of channels within the gain bandwidth of high average power or peak power rare earth doped fiber lasers at any wavelength in a compact footprint and uses only reflective optics and gratings.

Thermal effects in kilowatt all-fiber MOPA

Thermal effects and output power characteristics of kilowatt all-fiber master-oscillator power amplifier (MOPA) are investigated. Proper designs for cooling apparatus are proposed and demonstrated experimentally, for the purpose of minimizing splice heating which is critical for the reliability of high power operation. By using these optimized methods, a thermal damage-free, highly efficient ytterbium-doped double-clad fiber MOPA operating at 1080 nm with 1.17 kW output was obtained. The maximum surface temperature at the pump light launching end splice of the booster amplifier was 345 K, and the temperature rise for this key splice was 0.052 K/W.

Simultaneous spectrum and coherent combining by active phasing dual two-tone all-fiber MOPA chains

We present a new approach for simultaneous spectral and coherent combining in a master oscillator power amplifier (MOPA) configuration with active phasing. Spectrally combined single-frequency seed lasers are employed as a master oscillator for the all-fiber power amplifier chain, which provides robust performance suppressing of the stimulated Brillouin scattering effect, while coherent combining of spectrally combined amplifiers with active phase control provides stable in-phase combining, despite the strong phase fluctuation. In experiments, two spectrally combined seed MOPA chains are coherently combined with a total output power of 390 W. The power of the main lobe in the closed loop is two times of that value in the open loop, and visibility of more than 75% of the long-exposure interference pattern at the receiving plane is obtained.

High power narrow-band fiber-based ASE source

In this paper we describe a high power narrow-band amplified spontaneous emission (ASE) light source at 1030 nm center wavelength generated in an Yb-doped fiber-based experimental setup. By cutting a small region out of a broadband ASE spectrum using two fiber Bragg gratings a strongly constrained bandwidth of 12±2 pm (3.5±0.6 GHz) is formed. A two-stage high power fiber amplifier system is used to boost the output power up to 697 W with a measured beam quality of M2≤1.34. In an additional experiment we demonstrate a stimulated Brillouin scattering (SBS) suppression of at least 17 dB (theoretically predicted ~20 dB), which is only limited by the dynamic range of the measurement and not by the onset of SBS when using the described light source. The presented narrow-band ASE source could be of great interest for brightness scaling applications by beam combination, where SBS is known as a limiting factor.

16 W output power by high-efficient spectral beam combining of DBR-tapered diode lasers

Up to 16 W output power has been obtained using spectral beam combining of two 1063 nm DBR-tapered diode lasers. Using a reflecting volume Bragg grating, a combining efficiency as high as 93.7% is achieved, resulting in a single beam with high spatial coherence. The result represents the highest output power achieved by spectral beam combining of two single element tapered diode lasers. Since spectral beam combining does not affect beam propagation parameters, M2-values of 1.8 (fast axis) and 3.3 (slow axis) match the M2-values of the laser with lowest spatial coherence. The principle of spectral beam combining used in our experiments can be expanded to combine more than two tapered diode lasers and hence it is expected that the output power may be increased even further in the future.

Coherent addition of fiber-amplified ultrashort laser pulses

We report on a novel approach of performance scaling of ultra-fast lasers by means of coherent combination. Pulses from a single mode-locked laser are distributed to a number of spatially separated fiber amplifiers and coherently combined after amplification. Splitting and combination is achieved by polarization cubes, i.e. the approach bases on polarization combining. A Hänsch-Couillaud detector measures the polarization state at the output. The error signal (deviation from linear polarization) is used to stabilize the synchronization of different channels. In a proof-of-principle experiment the combination of two femtosecond fiber-based CPA systems is presented. A combining efficiency as high as 97% has been achieved. The technique offers a unique scaling potential and can be applied to all ultrafast amplification schemes independent of the architecture of the gain medium.

Monolithic all-glass pump combiner scheme for high-power fiber laser systems

We report on an integrated all-glass pump combiner for ytterbium-doped high power fiber lasers. The combining of multiple pump fibers with an active double clad fiber for high power amplification was successfully achieved by splicing them to a dichromatically coated planar convex lens. The measured coupling efficiency of such a combining scheme was typically in excess of 80%, with 86.5% achieved in maximum. Theoretical analysis is discussed in order to get optimized parameters and to consider the scaling of this type of coupler to higher average powers.

Spectral beam combination of fiber amplified ns-pulses by means of interference filters

In this paper we introduce a simple scheme to spectrally combine four single beams using three low-cost dielectric interference filters as combining elements. 25 ns pulses from four independent and actively Q-switched fiber seed-sources are amplified in a single stage fiber-amplifier. Temporally and spatially combined 208 W of average power and 6.3 mJ of pulse energy are obtained at two different repetition frequencies. A detailed analysis of beam quality as well as the thermal behavior of the combining elements is carried out and reveals mutual dependency.

Average power of 1.1 kW from spectrally combined, fiber-amplified, nanosecond-pulsed sources

We report on spectral combination of four high repetition rate 5 ns pulsed Yb-doped fiber amplifiers at 1 microm wavelength. The output beam is spatially and temporally superposed by use of an all-reflective diffraction grating. 1.1 kW average power and 220 microJ pulse energy were extracted with a combining efficiency of 99%.