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.

Supercontinuum generation of 314.7 W ranging from 390 to 2400 nm by tapered photonic crystal fiber.

To consider both high-power handling and blue-extended supercontinuum (SC) generation, a long-tapered photonic crystal fiber is pumped by a high-power laser source. An SC ranging from 390 to 2400 nm with 314.7 W output power is obtained. A spectral component below 960 nm accounts for 36.1% of the total output power, exceeding 113.5 W, with a spectral flatness within 16 dB. To the best of our knowledge, this is the first time an SC coverage of all visible wavelengths with more than 300 W output power has been achieved. This result increases the output power of the SC covering the visible range by a factor of three.

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.

High brightness laser based on Yb:YAG MOPA chain and adaptive optics system at room temperature.

We demonstrate a master oscillator power amplifier (MOPA) architecture based on Yb:YAG amplifiers and adaptive optics (AO) systems with a high power and high beam quality laser output. With two conduction cooled, dual-end-pumped Yb:YAG zigzag-slab amplifiers at room temperature, the fiber laser of 300 W was scaled to 11.9 kW. Moreover, AO system positioned downstream was utilized to correct wavefront of amplified laser. The beam quality ß at maximum output power was 2.8 times diffraction limited with closed-loop AO system.

kW-level 1030 nm polarization-maintained fiber laser with narrow linewidth and near-diffraction-limited beam quality.

kW-level 1030 nm polarization-maintained fiber laser with narrow linewidth and near-diffraction-limited beam quality is demonstrated. Theoretical simulations based on the power balance equation are first performed to optimize the system parameters of the 1030 nm ytterbium-doped fiber laser for the maximum suppression of amplified spontaneous emission (ASE). With the optimized parameters, both the copumped and counterpumped MOPA lasers are implemented to obtain an output power of >1 kW. In both cases, the ASE suppression ratio reaches 40 dB with a 3 dB linewidth of about 0.14 nm, and the polarization extinction ratio is about 12 dB at 1 kW of output power. The beam quality starts degrading at 900 W of output power in the copumped structure, but maintains nearly single mode (Mx2,My2)=(1.07,1.12) until power is over 1 kW in the counterpumped structure.

5kW GTWave fiber amplifier directly pumped by commercial 976nm laser diodes.

With home-made fiber perform and special fiber drawing & coating technique, a new-type of (3 + 1) GTWave fiber theoretically designed for bi-directional pump method, was successfully fabricated and justified of integrating multi-kW pump energy from commercial 976nm laser diodes. This (3 + 1) GTWave fiber amplifier demonstrated uniform absorption of pump light and easy thermal management characteristics along the whole fiber length. This amplifier is capable of simultaneously aggregating 5.19kW pump power at 976nm and finally generating 5.07kW laser output at 1066.5nm with an optical-to-optical efficiency of 74.5%, the first publically-reported multi-kW GTWave fiber directly pumped with commercial 976nm laser diodes to the best of our knowledge. No power roll-over was found at 5kW level and further power scaling can be expected with more pump power. The results indicate that GTWave fiber is a competitive integrated fiber device to collect enough pump energy from low-cost commercial laser diodes for multi-kW fiber laser development.

Spectral broadening in high-power Yb-doped fiber lasers employing narrow-linewidth multilongitudinal-mode oscillators.

High-power continuous wave fiber lasers with narrow linewidths are required for spectral beam combining systems. A 2.9 kW Yb-doped fiber laser with an output linewidth as narrow as 0.31 nm employing a narrow-linewidth multilongitudinal-mode oscillator is experimentally demonstrated. The numerical simulations reveal that the spectral broadening of the bandwidth grows almost linearly with injected mode number, which provides a potential way to further suppress the nonlinear spectral broadening in high-power fiber lasers.

Parameter space for the collective laser coupling in the laser fusion driver based on the concept of fiber amplification network.

Collective laser coupling of the fiber array in the inertial confinement fusion (ICF) laser driver based on the concept of fiber amplification network (FAN) is researched. The feasible parameter space is given for laser coupling of the fundamental, second and third harmonic waves by neglecting the influence of the frequency conversion on the beam quality under the assumption of beam quality factor conservation. Third harmonic laser coupling is preferred due to its lower output energy requirement from a single fiber amplifier. For coplanar fiber array, the energy requirement is around 0.4 J with an effective mode field diameter of around 500 µm while maintaining the fundamental mode operation which is more than one order of magnitude higher than what can be achieved with state-of-the-art technology. Novel waveguide structure needs to be developed to enlarge the fundamental mode size while mitigating the catastrophic self-focusing effect.

Coherent and incoherent combination of Gaussian beams employing lens array distributed on the spherical chamber.

Coherent and incoherent combination of Gaussian beams employing a lens array distributed on the spherical chamber is theoretically analyzed. The output field of each source in the array is coupled through an individual optical system whose local optical axis coincides with the radial direction of the chamber. The resulting intensity profile near the origin is derived. The intensity profile and power in the bucket on the target for rectangular and hexagonal arrangement are numerically calculated. The influences of the center-to-center separation and the ring number of the focusing lens array are given. The synthetic intensity profile of incoherent combination changes little for a lens array scale much smaller than the chamber size. In contrast, the synthetic intensity profile of coherent combination shows an interference pattern with a sharp central peak and sidelobes.

Intermodal interference induced significant frequency modulation to amplitude modulation conversion in a broadband large-mode-area fiber laser.

The conversion of the FM-to-AM effect induced by intermodal interference in the broadband large-mode-area (LMA) fiber laser was first investigated theoretically and experimentally. The numerical simulation results show that the spectrum transfer functions are different at different positions of the LMA fiber end face owing to the intermodal interference, so the output broadband pulses are different. We attain the similar results in the experiment when measuring the output pulse with the single mode fiber sampling oscilloscope. Whereas there is no amplitude modulation for the output pulse when measured by the bulk detector owing to the orthogonal characteristic of the eigenmodes.