Thirty years ago, Coullet et al. proposed that a special optical field exists in laser cavities bearing some analogy with the superfluid vortex. Since then, optical vortices have been widely studied, inspired by the hydrodynamics sharing similar mathematics. Akin to a fluid vortex with a central flow singularity, an optical vortex beam has a phase singularity with a certain topological charge, giving rise to a hollow intensity distribution. Such a beam with helical phase fronts and orbital angular momentum reveals a subtle connection between macroscopic physical optics and microscopic quantum optics. These amazing properties provide a new understanding of a wide range of optical and physical phenomena, including twisting photons, spin-orbital interactions, Bose-Einstein condensates, etc., while the associated technologies for manipulating optical vortices have become increasingly tunable and flexible. Hitherto, owing to these salient properties and optical manipulation technologies, tunable vortex beams have engendered tremendous advanced applications such as optical tweezers, high-order quantum entanglement, and nonlinear optics. This article reviews the recent progress in tunable vortex technologies along with their advanced applications.

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Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities

In this paper, we summarize the fundamental properties and review the latest developments in high power fiber lasers. The review is focused primarily on the most common fiber laser configurations and the associated cladding pumping issues. Special attention is placed on pump combination techniques and the parameters that affect the brightness enhancement observed in single-mode and multimode high power fiber lasers. The review includes the major limitations imposed by fiber nonlinearities and other parasitic effects, such as optical damage, transverse modal instabilities and photodarkening. Finally, the paper summarizes the power evolution in continuous-wave and pulsed ytterbium-doped fiber lasers and their impact on industrial applications.

High Power Fiber Lasers: A Review

Fiber lasers have seen progressive developments in terms of spectral coverage and linewidth, output power, pulse energy, and ultrashort pulse width since the first demonstration of a glass fiber laser in 1964. Their applications have extended into a variety of fields accordingly. In this paper, the milestones of glass fiber laser development are briefly reviewed and recent advances of high-power continuous wave, Q-switched, mode-locked, and single-frequency fiber lasers in the 1, 1.5, 2, and 3 μm regions and their applications in such areas as industry, medicine, research, defense, and security are addressed in detail.

Fiber lasers and their applications [Invited]

2D Layered Materials: Synthesis, Nonlinear Optical Properties, and Device Applications

Commercial 0.5 kW Yb-doped fiber amplifiers have been characterized and found to be suitable for coherent beam combining. Eight such fiber amplifiers have been coherently combined in a tiled-aperture configuration with 78% combining efficiency and total output power of 4 kW. The power-in-the-bucket vertical beam quality of the combined output is 1.25 times diffraction limited at full power. The beam-combining performance is independent of output power.

Coherent combining of a 4 kW, eight-element fiber amplifier array.

We present theoretical and experimental research on coherent beam combining of fiber amplifiers using stochastic parallel gradient descent (SPGD) algorithm. The feasibility of coherent beam combining using SPGD algorithm is detailed analytically. Numerical simulation is accomplished to explore the scaling potential to higher number of laser beams. Experimental investigation on coherent beam combining of two and three W-level fiber amplifiers is demonstrated. Several application fields, i.e., atmosphere distortion compensating, beam steering, and beam shaping based on coherent beam combining using SPGD algorithm are proposed.

Coherent Beam Combining of Fiber Amplifiers Using Stochastic Parallel Gradient Descent Algorithm and Its Application

Coherent beam combination of a 1.08 kW fiber amplifier array has been demonstrated for the first time, to our knowledge. In the experiment, nine fiber amplifiers are tiled into a 3×3 array, and the output power of each amplifier is approximately 120 W. A single frequency dithering algorithm is used to compensate the phase noises between the elements, which runs on a signal processor based on field programmable gate array for phase control on the fiber amplifiers. When the phase control system goes into closed loop, the fringe contrast of the far-field intensity pattern is improved to more than 85%, and the residual phase error is less than λ/15.

Coherent beam combination of 1.08 kW fiber amplifier array using single frequency dithering technique.

Taking the improved SRS threshold formula into consideration, the power scaling of tandem-pumped Yb-doped silica fiber lasers and amplifiers is analyzed by new models. The results show that the power scaling of tandem-pumped Yb-doped fiber lasers and amplifiers is primarily limited by optical damage, SRS and thermal lens, while the pump brightness induced limitation is almost removed. It is also found that tandem-pumped Yb-doped fiber lasers and amplifiers, based upon state-of-art fiber technology, have the potential to achieve a power limit of 70.7 kW with a core diameter of 63.4 μm, and in the case of a strict single-mode fiber, the power limit is about 13.3 kW with a core numerical aperture of 0.03.

Power scaling analysis of tandem-pumped Yb-doped fiber lasers and amplifiers.

Coherent polarization beam combining (CPBC) of fiber lasers has the potential to scale the output total power while simultaneously maintaining good beam quality. In this paper, we will present the very recent technical advance in a single-channel coherently combinable linearly polarized narrow-linewidth fiber amplifier and high-power CPBC system. The noise property of a recently developed near 2 kW fiber amplifier and its feasibility in a CPBC system, CPBC of four 500-W-level fiber amplifiers and two kilowatt fiber amplifiers are demonstrated for the first time, which is also the first result for a 2 kW CPBC system. We have also performed numerical analysis on the performance scaling of CPBC, and deduced handy design guidelines for CPBC of a 64-element high-power system.

High-power coherent beam polarization combination of fiber lasers: progress and prospect [Invited]

Multitone radiation is a promising technique to mitigate stimulated Brillouin scattering effects in narrow-linewidth fiber amplifiers. We demonstrate coherent beam combination of three two-tone fiber amplifiers using a stochastic parallel gradient descent (SPGD) algorithm. Phase control on the fiber amplifiers are performed by running the SPGD algorithm on a digital signal processor. The contrast of far-field intensity pattern of a coherently combined beam is more than 85%. Experimental results validate that a single-frequency seed laser is not indispensable for coherent beam combination in master oscillator power amplifier configuration.

Yanxing Ma

Papers

Coherent Beam Combining of Fiber Amplifiers Using Stochastic Parallel Gradient Descent Algorithm and Its Application

Coherent beam combination of 1.08 kW fiber amplifier array using single frequency dithering technique.

Power scaling analysis of tandem-pumped Yb-doped fiber lasers and amplifiers.

High-power coherent beam polarization combination of fiber lasers: progress and prospect [Invited]

Coherent beam combination of three two-tone fiber amplifiers using stochastic parallel gradient descent algorithm.