Efficient phase-locking of 37 fiber amplifiers by phase-intensity mapping in an optimization loop

Coherent beam combining of cylindrical vector beams for power scaling

Coherent beam combining (CBC) of cylindrical vector beams (CVBs) based on an active phase controller is proposed and demonstrated. Experimentally, two pieces of spatially variant wave plates (S-wave plate) were employed as vector mode convertors for two individual 1064-nm fiber amplifier channels. When the system was in a closed loop, a combined output power of 10 W and a CBC efficiency of higher than 94% were achieved, for both TM01 (radially polarized) mode and TE01 (azimuthally polarized) mode cases. Moreover, the laser system showed a high vector mode purity, which was measured to be ∼97.4% (TM01) and ∼97.3% (TE01). To the best of knowledge, this could possibly be the first demonstration of CBC of CVBs, which paves the way for power scaling of CVBs.

Optical coherent combining of high-power optical amplifiers for free-space optical communications

Highly efficient coherent beam combining (CBC) of two very-high-power optical amplifiers (VHPOA) with applications to long-range FSO communications such as ground-to-space feeder links is presented. The CBC setup is designed to minimize the telecom signal degradation, with a polarization beam splitter used to minimize the power fluctuations and to control the output polarization state of the beam. The system delivers 80 W output power and is proven to be compatible with 25 Gb/s telecom signals with a less than 1 dB power penalty.

System design for coherent combined massive fiber laser array based on cascaded internal phase control

Coherent beam combining (CBC) of a fiber laser can scale the output power while maintaining high beam quality. However, phase detection and control remain a challenge for a high-power CBC system with a massive laser array. This paper provides a novel, to the best of our knowledge, cascaded phase-control technique based on internal phase detection and control, called the cascaded internal phase-control technique. The principle of the technique was introduced in detail, and the numerical simulations were carried out based on the stochastic parallel gradient descent (SPGD) algorithm. The results indicated that the cascaded internal phase-control technique was compatible with the massive laser array. Compared with the traditional CBC based on the SPGD algorithm, the control bandwidth could be improved effectively about seven times (120 steps) than the traditional SPGD algorithm (830 steps). Furthermore, the average root mean square of residual phase error was decreased to 0.03 rad (∼λ/209) with a laser array of 259 channels (7∗37), which was 0.36 rad (∼λ/17) in the traditional SPGD algorithm. In addition, the element expanding capacity was analyzed. Since there is no large-aperture optical device in the phase-detection system, this technique has the advantage of freely designing the caliber of the laser emitting system. This paper could offer a reference for the high-power massive laser array system design and phase control.

Phase noise measurements and diagnoses of a large array of fiber lasers by PISTIL

One of the most promising solutions to access high power laser chains is to achieve a coherent combination of a large number of elementary lasers. To interfere constructively, these laser sources should be identical and operate under the same conditions. However, despite these efforts, differential delays appear in the course of time, which must be compensated for. While designing the required correction system, knowing the behavior of a laser as a function of the environmental conditions is not crucial, whereas having access to the differences in the behaviors of identical lasers is, leading to difficulties in modeling. The purpose of this paper is to illustrate how a large set of lasers can be simultaneously analyzed to estimate their variations and optimize a correction system. The X-Coherent Amplified Network laser relies on 61 fiber amplifiers, which are as identical as possible. This state of the art femtosecond digital laser therefore appears as an ideal candidate to study a large number of fiber lasers working under controlled conditions.

Scalable all-fiber coherent beam combination using digital control

This paper describes a filled-aperture coherent beam combining (CBC) system based on locking of optical coherence via single-detector electronic-frequency tagging (LOCSET). The sensing and control architecture is implemented using a field-programmable gate array and high-bandwidth electro-optic phase modulators. The all-fiber optical configuration consists of a narrow linewidth 1560 nm seed laser separated into three channels, each containing 7 W erbium-doped fiber amplifiers. The system was demonstrated experimentally, achieving a total stabilized output power of 20 W, a combination efficiency greater than 95%, and an output RMS phase stability of λ/493. As this architecture employs an entirely digital sensing and control scheme based on LOCSET, it presents a highly scalable and cost-effective solution for CBC that is wavelength agnostic and can support an arbitrarily large number of channels.

Design considerations and performance analysis of a fiber laser array system for structuring orbital angular momentum beams: a simulation study

Since the advent of optical orbital angular momentum (OAM), advances in the generation and manipulation of OAM beams have continuously impacted on intriguing applications including optical communication, optical tweezers, and remote sensing. To realize the generation of high-power and fast switchable OAM beams, coherent combining of fiber lasers offers a promising way. Here in this contribution, we comprehensively investigate the coherent fiber laser array system for structuring OAM beams in terms of the design considerations and performance analysis. The performance metric and evaluation method of the laser array system are presented and introduced. Accordingly, the effect of the main sections of the laser array system, namely the high-power laser sources, emitting array configuration, and dynamic control system, on the performance of the output coherently combined OAM beams is evaluated, which reveals the system tolerance of perturbative factors and provides the guidance on system design and optimization. This work could provide beneficial reference on the practical implementation of spatially structuring high-power, fast switchable OAM beams with fiber laser arrays.

On-Demand Phase Control of a 7-Fiber Amplifiers Array with Neural Network and Quasi-Reinforcement Learning

We report a coherent beam combining technique using a specific quasi-reinforcement learning scheme. A neural network learned by this method enables the tailoring and locking of a tiled beam array on any phase map. We present the experimental implementation of on-demand phase control by a neural network in a seven-fiber laser array. This servo loop needs only six phase corrections to converge to the desired phase set at any profile, with a bandwidth higher than 1 kHz. Moreover, we demonstrate the dynamical feature of adaptive phase control, performing sequences of controlled phase sets. It is the first time, to the best of our knowledge, that an actual array of seven-fiber amplifiers has been successfully phase-locked and controlled by machine learning.

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.

Beam-quality improvement with a bio-inspired sunflower array for coherent beam combining

The beam quality of coherent beam combing (CBC) is significantly affected by the beam array schemes, which are regular hexagon arrays in most research. Here, we propose a bio-inspired Fermat spiral array (FSA) for large-array CBC, for the first time to our knowledge. The far-field distribution and beam quality of CBC with various designed FSAs was investigated numerically and experimentally; the simulated and experimental results agreed with each other. The power in the bucket (PIB) increased with the central space density of the FSA, accompanying by the weakening of the far-field sidelobes. In addition, for the FSA with constant space density, the PIB increased, and sidelobes weakened with the increase of the array filling factor. The FSA could effectively improve the PIB and weaken the sidelobe of the CBC far field by the aperiodic and nonuniform space density arrangement, compared to the regular arrays. These results provide a new approach for the beam arrangement of large-array CBC.

Experimental phase control of a 100 laser beam array with quasi-reinforcement learning of a neural network in an error reduction loop

An innovative scheme is proposed for the dynamic phase control of a laser beam array. It is based on a simple neural network included in a phase correction loop that predicts the complex field array from the intensity of the induced scattered pattern through a phase intensity transformer made of a diffuser. A crucial feature is the use of a kind of reinforcement learning approach for the neural network training which takes account of the iterated corrections. Experiments on a proof-of-concept system demonstrated the high performance and scalability of the scheme with an array of up to 100 laser beams and a phase setting at λ/30.

Small footprint phase locking system for a large tiled aperture laser array

A phase sensing system fitted to the control of coherent laser beam array of large cross section is experimented. It is based on the use of a fiber bundle that collects a weak part of the synthetic wavefront, that scales it down (1/40) and that reshapes it in a more compact arrangement (2D to 1D array). Then, the reconfigured beam array can be analyzed by a small footprint system making the large laser beam array easier to phase-lock. The discrete laser array wavefront transmitted by the meter long fiber bundle was stabilized thanks to a multiple arm servo loop. Laser array phase locking was further ensured by random scattering through a diffuser, associated to an alternating projection algorithm. Six fiber laser beams constituting a 110 mm diameter synthetic aperture, were phase-locked with λ/16 accuracy.

Coherent Beam Combining Using an Internally Sensed Optical Phased Array of Frequency-Offset Phase Locked Lasers

Coherent beam combining can be used to scale optical power and enable mechanism-free beam steering using an optical phased array. Coherently combining multiple free-running lasers in a leader-follower laser configuration is challenging due to the need to measure and stabilize large and highly dynamic phase differences between them. We present a scalable technique based on frequency-offset phase locking and digitally enhanced interferometry to clone the coherence of multiple lasers without the use of external sampling optics, which has the potential to support both coherent and spectral beam combining, and alleviates issues of voltage wrapping associated with actuating feedback control using electro-optic modulators. This technique was demonstrated experimentally using a tiled-aperture optical phased array in which the relative output phase of three free-running lasers was stabilized with an RMS output phase stability of λ/104.