Coherent beam combining of 107 beams has been demonstrated for the first time to the best of our knowledge. When the system was in closed loop, the pattern in far-field was stable and the fringe contrast was >96%. The impact of the dynamic tilt error, the piston error, and power inconsistency was theoretically analyzed. Meanwhile, the distribution law of dynamic tilt error was estimated and the correlation of the tilt dithering of different axis was analyzed statistically. The ratio of power in the central lobe was ∼22.5%. The phase residue error in the closed loop was ∼λ/22, which was evaluated by the root-mean-square error of the signal generated from the photoelectric detector.

/pdf/first-experimental-demonstration-of-coherent-beam-combining-4yzraz0dpt.pdf

First experimental demonstration of coherent beam combining of more than 100 beams

Coherently combining of seven-channel narrow-linewidth, linear-polarized, kW-level power all-fiber amplifiers has been successfully demonstrated by using the stochastic parallel gradient descent (SPGD) algorithm. The seven polarization-maintained fiber amplifiers are tiled by a collimator array with a high lenslet filling factor (~95%). Total emitting power is ~8 kW, and ~7.1 kW combined output power is achieved and the contrast of the far-field intensity pattern is measured to be ~86% when the system is in the closed loop. This work could provide practical reference on scaling the output power of the tiled aperture coherent beam combining system.

7.1 kW coherent beam combining system based on a seven-channel fiber amplifier array

Ultra-short pulse lasers, generating coherent light pulses with pulse durations in the picosecond and femtosecond range, are becoming popular in precision laser microfabrication. They are benefiting not only from well-predicted laser ablation with the suppressed heat-affected zone but also by opening new processing opportunities, especially in transparent materials, due to enhanced non-linear interaction with the material. In this review, the evolution of various kinds of mode-locked lasers from a scientific toy to a robust industrial tool is reviewed. The utilization benefits of high-average-power, high-pulse-repetition-rate ultra-short pulse laser are closely related to beam shaping and manipulation techniques. Fast beam scanning with galvanometric and polygon scanners as well as multi-beam parallel processing methods were developed. Some hot applications areas are briefly described. The efficient use of photons from ultra-short pulse lasers pushed the development of optimization methods in the ablation process. Efforts toward upscaling the process by increasing the average power of mode-locked lasers made feedback to laser developers, and burst regime became a necessity as well as high-speed beam scanning devices. Unique opportunities of ultra-short pulses are successfully exploited in machining transparent materials, with glass separation being the leading application.

/pdf/ultra-short-pulse-lasers-for-microfabrication-a-review-48dars3u1m.pdf

Ultra-Short Pulse Lasers for Microfabrication: A Review

We report on the use of a 61 beamlets coherent beam combination femtosecond fiber amplifiers as a digital laser source to generate high-power orbital angular momentum beams. Such an approach opens the path for higher-order non-symmetrical user-defined far field distributions.

/pdf/orbital-angular-momentum-beams-generation-from-61-channels-33hjmb0xth.pdf

Orbital angular momentum beams generation from 61 channels coherent beam combining femtosecond digital laser

OAM beam generation in space and its applications: A review

We report on the coherent beam combining of 61 femtosecond fiber chirped-pulse amplifiers in a tiled-aperture configuration along with an interferometric phase measurement technique. Relying on coherent beam recombination in the far field, this technique appears suitable for the combination of a large number of fiber amplifiers. The 61 output beams are stacked in a hexagonal arrangement and collimated through a high fill factor hexagonal micro-lens array. The residual phase error between two fibers is as low as λ/90 RMS, while a combining efficiency of ∼50% is achieved.

https://hal.archives-ouvertes.fr/hal-02995221/document

Coherent beam combining of 61 femtosecond fiber amplifiers.

Coherent beam combining (CBC) opens the way to a new paradigm in laser architecture [1] . The quest to high peak and average power indeed faces several fundamental limitations (transverse amplified spontaneous emission in wide amplifiers, thermal management issues) which can be addressed when relying on numerous smaller scale reliable lasers coherently combined. Achieving CBC for a large number of amplifiers will not only rely on the use of an efficient phase-locking scheme but also on fiber management inside the laser head (such as accurate angular and lateral positioning, at µm or mrad levels), optics, and amplifiers management as well as laboratory environment control. An accurate diagnosis of the laser array is mandatory in order to get significant feedback on its design, and potentially improve or stabilize its behavior, either in phase-locked loop or in free run.

PISTIL interferometry diagnosis on a 61 channels coherent beam combining digital laser

Tiled-aperture Coherent Beam Combination architecture opens the way to digital laser operating in high peak and
average power regimes.

/pdf/61-channels-coherent-beam-combining-femtosecond-digital-zrlg47kqid.pdf

61 channels coherent beam combining femtosecond digital laser

We report on the first coherent beam combining of 60 fiber chirped-pulse amplifiers in a tiled-aperture configuration along with an interferometric phase measurement technique. Relying on coherent beams recombination in the far field, this technique appears well suited for the combination of a large number of fiber amplifiers. The 60 output beams are stacked in a hexagonal arrangement and collimated through a high fill factor hexagonal microlens array. The measured residual errors within the fiber array yields standard deviations of 4.2 μm for the fiber pitch and 3.1 mrad for the beam-to-beam pointing, allowing a combining efficiency of 50 %. The phasing of 60 fiber amplifiers demonstrates both pulse synchronization and phase stabilization with a residual phase error as low as λ/100 RMS.

/pdf/coherent-beam-combining-of-60-femtosecond-fiber-amplifiers-3v9xo5xxsx.pdf

Matthieu Veinhard

Papers

Coherent beam combining of 61 femtosecond fiber amplifiers.

Orbital angular momentum beams generation from 61 channels coherent beam combining femtosecond digital laser

PISTIL interferometry diagnosis on a 61 channels coherent beam combining digital laser

61 channels coherent beam combining femtosecond digital laser

Coherent beam combining of 60 femtosecond fiber amplifiers