Coherent beam combination (CBC) is a route to breaking through the power limit of single fiber laser amplifiers, furthermore, tiled phased arrays enable rapid, flexible beam shaping for optimizing laser-based processes. In this work, we present PCANN, a principal component analysis (PCA) and neural network (NN) algorithm to simultaneously phase lock and beam shape CBC arrays, with no knowledge of phase, which we experimentally demonstrate with a three element tiled array from ytterbium-doped fiber amplifiers. The PCANN uses a differential mapping scheme through applying arbitrarily scaled phase controller inputs and recording pre- and post-step beam intensity images, allowing labeled training pairs to be generated without requiring direct phase measurement. The PCA reduces 64 & times; 64 pixel images to 13 features, enabling a lightweight mixture density NN to provide phase-actuator voltage corrections at 1.1 kHz with 1.4 ms latency. It achieved lambda/150 (41 mrad) RMS residual phase error, single-step beam shaping in 1 ms, and robustness over days. Scalability to higher channel counts is shown with a seven-channel fiber CBC experiment, and simulations up to 61 fibers indicate linear scalability in training data requirements.

Abstract Topologically structured optical fields exhibit rich physical properties and offer new opportunities for information encoding and light-matter interaction. In this work, we propose a general and scalable framework for generating and reconfiguring topological light using coherent beam combination (CBC). By treating the CBC system as an electronically addressable optical phased array, we demonstrate that the topology of the optical field becomes a programmable parameter. As a representative example, we focus on optical skyrmions and present an experimental demonstration of deterministic and dynamic switching between Bloch-type, Néel-type, and antiskyrmion textures using a 7-channel Yb-doped fiber master oscillator power amplifier CBC system. This approach establishes a scalable and programmable route toward high-power topological vector fields and reconfigurable structured light.

We report a record-high 5.5-mJ amplified pulse energy at 980 nm, achieving 0.55 kW peak power using a 1-meter long ultra-large-mode-area ( 290 mu m core) ytterbium fiber in a co-directional, amplifier configuration. Pumped at 915 nm and seeded by a modulated 980-nm laser diode. We provide a detailed fiber spectroscopy analysis, optimizing key parameters-such as seed and pump repetition rates to mitigate thermal load and other detrimental effects (e.g., re-absorption), enabling high-energy output. The amplifier's performance highlights its applications for high-brightness ultraviolet and visible light generation, and as an efficient pump for erbium-doped fiber systems, advancing their performances in fluorescence imaging, marine engineering and biophotonics. To the best of our knowledge, the achieved output energy of 5.5 mJ is the highest reported to date in an ytterbium-doped-fiber amplifier at 980-nm, while offering simpler fabrication, configuration and enhanced compatibility compared to other fiber systems at similar wavelengths.

We derive and assess an explicit analytic expression for the power conversion efficiency (PCE) of high-power continuous-wave optical fiber amplifiers with counter-propagating pump and signal in the presence of quenching, excited-state absorption, and background loss. The expression is uniquely simple to evaluate. A crucial assumption is that the level populations and thus the gain do not depend on the signal and pump powers separately, but rather on their ratio. In the ideal, “balanced”, case, this ratio remains constant throughout the amplifier, which is possible when the signal gain is equal to the operating pump depletion. This is achieved for certain (balanced) combinations of fiber length and input signal and pump power. With these assumptions, the PCE depends only on the spectroscopy and cross-sectional geometry of the gain fiber, but not depend on the absolute power.We use the equations to calculate and optimize the balanced PCE of homogeneously broadened cladding-pumped Er3+-doped fiber amplifiers based on phosphorus-rich silica fibers. Cases which fulfill as well as deviate from the ideal balanced assumptions are considered. The resulting PCE agrees well with that of well-established numerical simulations in most investigated cases, but agreement gets worse at large deviations from the ideal assumptions. The calculations are sufficiently fast for optimized curves to be updated real-time when parameters (e.g., describing quenching) are changed.We believe that our approach is valid for a range of realistic systems, including, for example, Yb-doped and Tm-doped fiber amplifiers as well as inhomogeneously broadened systems. We also discuss criteria for the expression’s validity and provide tests which are straightforward to evaluate in the balanced case. Validation in more general, “unbalanced” cases, is more difficult and may in many cases require comparisons to iterative numerical simulations.

Three Yb-fiber amplifier outputs were phase-locked and beam-shaped in a tiled aperture CBC array using a neural network algorithm executed at 1100Hz, achieving low differential phase noise (40mrad, lambda/150) with beam shaping in a single step (1ms).

A simple cladding-pumped single-stage neodymium-doped fiber amplifier reaches 40 dB of gain and 24 dBm of output power at 1.06 µm. The output remains temporally stable and free of pulsing in the absence of seeding.

An all-silica-fiber thulium-doped fiber laser emitting at 0.82 μm on the transition from 3H4 to the ground state 3H6 outputs 105 W continuous-wave power and 555 W quasi-continuous-wave instantaneous power with 0.96％duty cycle in 240 μs rectangular pulses.The system comprises a double-clad thulium-doped fiber designed and fabricated in-house,incorporated into an all-fiber cavity and cladding-pumped by diode lasers at 0.79 μm.Co-lasing at 1.9 μm counteracts population trapping in 3F4.The slope efficiency reaches 64％and 77.5％under quasi-continuous-wave and continuous-wave operations,respectively.Under quasi-continuous-wave conditions,the beam quality M2 becomes 2.2(beam parameter product:0.57 mm mrad)and 2.45(0.64 mm mrad)in orthogonal directions at approximately 250 W of instantaneous output power.In addition,a modified quasi-continuous-wave setup is continuously wavelength-tunable from 812 to 835 nm.We believe this is the first reported demonstration of high-power laser operation of the 3H4 → 3H6 transition in a thulium-doped fiber.

We demonstrate a Q-switched, cladding-pumped thulium-doped silica fiber laser operating at 0.82 mu m, delivering energies up to 18 mu J in 530-ns pulses at a repetition rate of approximately 675 Hz. The system employs in-band pumping of an in-house-fabricated double-clad thulium-doped fiber (TDF). Q-switching was achieved by a mechanical optical chopper inside the laser cavity. To the best of our knowledge, this is the first reported demonstration of a Q-switched, cladding-pumped silica-based TDF laser at this wavelength, offering the potential for a power-scalable, compact, robust, and cost-effective source for high pulse energies in this wavelength range.

A new scheme of a highly efficient hybrid laser cavity is proposed and experimentally demonstrated utilizing a hot cesium (Cs) vapor cell as an optical gain medium. The laser cavity consists of a macroscopic concave reflector (> 99% reflectivity) and a 4% Fresnel-reflecting facet of a single mode fiber (SMF). The cesium gain cell is located between these two reflectors. The SMF serves multiple roles: (1) a passive mode-matching component to approximate the pump beam diameter to that of the laser cavity mode within the cesium cell, (2) an output coupler with a low reflectivity, and (3) a low loss laser delivery with a high beam-quality. Optimizing the pump beam waist diameter and the cesium vapor cell temperature, a high slope efficiency of 86% and an optical-to-optical conversion efficiency of 71% were achieved in the pump power range of 400–600 mW. The unique multi-functional role of the SMF in the hybrid cavity is fully described, which can also be applied to other high optical gain media.

This study examines the influence of quenching dynamics on the efficiency of erbium-doped fiber amplifiers (EDFAs) with high erbium-ion (E3+-ions) doping concentrations, comparing pulsed and continuous wave core-pumping methods. Our findings indicate that quenching, driven by energy transfer upconversion, substantially impacts signal gain in these fibers. The core-pumping configuration demonstrated significantly higher gain per unit length than both low-doping EDFAs and cladding-pumping systems, with effective energy storage achievable through short pump pulses to reduce concentration quenching effects. The highly doped homemade fiber (fiber under test - FUT No. 3) achieved a gain per unit length exceeding 8.4 dB/m, outperforming the lower doped commercial fiber (FUT No. 4) by a factor of 6.4, although FUT No. 4 displayed better compatibility with the core-pumping system. Additionally, the highest gain per unit length for a counter-directional, cladding-pumped amplifier configuration with a high-concentration fiber was recorded at 5.9 dB/m at a 1560 nm signal wavelength with a 2 μs pulse duration, positioning FUT No. 3 as a highly efficient option for high-gain applications despite its high doping concentration. Our experimental analysis of quenching dynamics not only highlights an approach for scaling pulse energy using shorter fiber lengths to mitigate nonlinear effects but also provides valuable insights into quenching-influenced gain behavior in pulsed fiber amplifier systems.

We report the performance of a narrow-linewidth, actively Q-switched erbium-doped fibre ring laser emitting at similar to 1.55 mu m. An investigation into the behaviour of a cascaded comb and tuneable filter system was carried out using a length of polarisation-maintaining fibre in a Lyot filter configuration. The resulting comb filter was used to suppress competing modes, and the addition of a tuneable filter allowed wavelength tuning over 33 nm. Proper adjustment of the spectral overlap of the cascaded filters allowed the linewidth to be reduced to 65 pm (8 GHz). Q-switching was achieved using an acousto-optic modulator, providing stable pulses at repetition frequencies between 9 kHz and 166 kHz, with a minimum pulse length of 400 ns. These results demonstrate the feasibility of such a system for use in a narrow linewidth, tuneable laser.

Controlling the optical phase in a fiber is a common task, but coherent beam combination (CBC) brings a unique set of requirement and challenges [1]. A common implementation is CBC of tree-like optical fiber amplifiers to take advantage of the kW-level channel powers available to obtain 10s or 100s kW laser sources, requiring sub-wavelength path length stabilisation of each fiber chain. The stabilisation algorithm used determines the bandwidth required of the phase controller, with iterative schemes such as stochastic parallel gradient descent requiring 10s kHz bandwidth, or LOCSET at MHz. Another approach is to use neural networks for single-step phase correction [2], where the total phase error is detected and can be corrected in one step, which reduces the phase controller bandwidth requirements to 100s Hz with accurate phase changes.

A silica-based thulium-doped fiber (TDF) laser emitting on the 3H4 -> 3F4 transition is continuously tunable in the range 1.45-1.49 mu m. We designed and fabricated a double-clad TDF and incorporated it into a cavity formed between a perpendicular fiber facet and a Littrow-configured diffraction grating for tunable wavelength-selective feedback. The TDF was cladding-pumped quasi-continuous-wave with up to 644 W of instantaneous launched power at 0.79 mu m from three pigtailed diode lasers, operating with a pump duty cycle of 0.95 % in 190-mu s rectangular pulses. This pump power readily overcomes the fast decay of 3H4 in silica. The highest instantaneous output power reached approximately 58 W at 1464 nm, exhibiting a slope efficiency of around 11.6 % relative to the launched pump power. The optical-to-optical conversion efficiency at 1464 nm was 9.0 % at full power. Concurrent emission at 1.9 mu m, on the 3F4 -> 3H6 transition, was required to deplete the 3F4 level and thus enhance the population inversion. Our work represents the first demonstration of a cladding-pumped silica-based TDF laser in this wavelength range.

Cladding-pumped thulium-doped fiber lasers (TDFLs) are attractive for their power scalability and the availability of high-power, high-brightness 0.79-μm diode-lasers for pumping. Their output power has reached over 1 kW at wavelengths around 2 μm [1]. A high concentration is generally used to overcome the large quantum defect and thus improve the efficiency through a “two-for-one” cross-relaxation process [2]. This however leads to high pump absorption and short device lengths. The thermal load per unit length becomes high and leads to coating failure in the absence of highly capable thermal management solutions. Measures such as off-peak pumping can reduce the pump absorption and thus distribute the heat over a longer fiber, even if this reduces the efficiency due to a relatively high background loss at 2 μm [3]. An alternative approach is to operate the TDFL quasi-continuous-wave (QCW), which we investigate in this work.

We have shown that fibre amplifiers based on the few-mode Yb-doped fiber and operating under low saturation and narrow-linewidth conditions have resulted in TMI thresholds of similar to 5-13W and frequencies of similar to 500 Hz, in large deviation from results obtained under strong saturation and broad-linewidth conditions. For the first time, we have carried out a frequency detuned, two-wave mixing experiment and shown that population inversion effects related with the two-wave mixing process are responsible for the observed low-power TMI thresholds and frequencies. This study shows that the TMI frequency is a very good indicator of the underlying fundamental physics of the instability.

We investigate and use the beam propagation method with equivalent input noise for the simulation of narrow-band amplified spontaneous emission (ASE) and signal amplification in continuous-wave Cr 2+ :ZnSe non-waveguiding “bulk” amplifiers with non-saturating signal and ASE in different configurations with weak reabsorption. Both the incident pump at 1901 nm and the signal at 2410 nm were diffraction-limited gaussian beams. We implemented the equivalent input noise as random realizations of one photon per gridpoint, and showed that this leads to one noise photon per mode. Simulation results of between 100 and 6000 realizations were ensemble-averaged to determine the power spectral density of the ASE in a Monte Carlo approach. We validated the approach by comparing results for single-mode and multimode fiber amplifiers to those obtained with well-established fiber amplifier models. We also calculated the beam quality of the ASE, $$\:{M}_{ASE}^{2}$$ , from its spatial distribution. We found that under some conditions, but not all, $$\:{{M}_{ASE}^{2}}^{2}$$ can serve as an estimate of an effective number of ASE modes and, together with the ASE PSD, predict the achievable signal gain. It is also possible to evaluate the PSD per unit solid angle due to spontaneous emission from the input noise seeding, and we found agreement down to the single-photon level.

Parallel two channel Yb-fiber amplifiers with low differential phase noise above 100 Hz, up to 200 W, were phase-locked for coherent beam combination using neural networks and principal component analysis, with lambda/120 stability at 900 cycles/s control.

We demonstrate that counterintuitively, silicon wafer stealth dicing can be performed with femtosecond laser pulses at 1030-nm, where linear absorption predominates. A 1.3-NA oil-immersion objective mitigated plasma defocusing and delocalization before the focal point.

A two-channel ytterbium-doped fiber amplifier system with active phase-locking reaches a differential phase noise of only 40 mrad (X/160) at 200-W channel power. Frequencies above 30 Hz did not require noise suppression, thus simplifying advanced beam-shaping through coherent beam combination.

In this paper, we demonstrate a single-pass Cr2+:ZnSe amplifier system with nearly 20 dB of net gain and 38 dB of on-off gain at 2304 nm under single-mode QCW pumping condition. In the experiment, the achromatic focusing technique and increased pump power allow for a gain enhancement over what we were able to obtain using an aspheric lens. Moreover, we will present the simulation results of this system.