Johan Nilsson

Coherent beam combination (CBC) is a promising technique to scale optical power far beyond the limits of a single fiber laser. In CBC, independent control over each channel allows the far-field intensity distribution to be reshaped, within the physical constraints of the system. Applications such as beam shaping, steering, and turbulence correction naturally benefit from this capability, as they require precisely the kind of wavefront adjustments that phase-only control provides. In practice, however, CBC systems are susceptible to phase noises, making real-time feedback control an essential requirement. Deep learning presents a promising approach, enabling single-step phase retrieval without iterative algorithms. In this work, we demonstrate a deep learning framework using a differential training strategy, which enables not only phase locking to maximize power-in-the-bucket, but also, flexible, programmable, and stable beam shaping and steering, including instantaneous intensity redistribution across target patterns. Additionally, we demonstrate its performance in the presence of turbulence. Altogether, this opens new possibilities for high-power laser systems requiring agile, real-time beam management.

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 64×64 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 λ /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.

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 report the performance of a narrow-linewidth, actively Q-switched erbium-doped fibre ring laser emitting at ~1.55μm. 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 33nm. Proper adjustment of the spectral overlap of the cascaded filters allowed the linewidth to be reduced to 65pm (8GHz). Q-switching was achieved using an acousto-optic modulator, providing stable pulses at repetition frequencies between 9kHz and 166kHz, with a minimum pulse length of 400ns. These results demonstrate the feasibility of such a system for use in a narrow linewidth, tuneable laser.

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 ~5-13W and frequencies of ~500Hz, 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.

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.

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.