Laser sources emitting in the infrared range at around 2 µm are attracting great interest for a variety of applications like processing of transparent thermoplastic polymers in industry as well as plenty of applications in medicine, spectroscopy, gas sensing, nonlinear frequency conversion to the mid-infrared, to mention a few. Of late, fiber lasers compared to other kinds of lasers benefit from their all-fiber design, leading to a compact, robust, and well thermally manageable device. Particularly, thulium- and holmium-doped fiber lasers are the first choice in fiber lasers emitting light around 2 µm. In this paper, we give an overview of our recent results in the research on thulium- and holmium-doped optical fibers, fiber lasers, and related research topics in the 2-µm spectral range. In particular, we present, to our knowledge, the first results of improvement of pump absorption in double-clad fibers thanks to the fiber twist frozen during drawing. Finally, a brief demonstration of material processing by thulium all-fiber laser operating at 2 µm is presented.

Efficient holmium fiber lasers have been studied as attractive laser sources operating around 2.1 μm. We report on holmium-doped silica fibers prepared by the modified chemical vapor deposition in combination with either a solution-doping method or a nanoparticle-doping method. A set of 15 fibers with various compositions was characterized and compared with respect to their fluorescence lifetime, laser slope efficiency and laser threshold. This set of fibers in wide concentration ranges allowed us to assess reliably the influence of material composition and the influence of doping method. The best-performance fibers exhibited slope efficiency 83.1%, laser threshold 155 mW and a record value of upper laser level lifetime of 1.35 ms. These results were achieved in fibers with holmium concentration lower than 800 molar ppm and Al/Ho molar ratio greater than 70. Significant differences between fibers prepared by solution doping and nanoparticle doping were not observed. The behavior of $\text{Al}_2 \text{O}_3$ nanoparticles during fiber preparation is discussed in details.

Energy transfer coefficients in thulium-doped silica fibers are key parameters for a reliable numerical modeling and optimization of high-power fiber lasers pumped at 790 nm and emitting around eye-safer wavelength of 2 µm. We report on determination of the energy transfer coefficients by comparing experimental values and rate-equation-based modeling of thulium fluorescence decays. The study was performed using two pump, 792 nm and 1620 nm, and two signal, 800 nm and 2 µm, wavelengths. The following values were obtained k₃₀₁₁ = (1.9 ± 0.2) × 10⁻²² m³s^-1, k₁₁₃₀ = (1.4 ± 0.2) × 10⁻²³ m³s^-1 and k₁₁₂₀ = (3.5 ± 0.1) × 10⁻²³ m³s^-1. To the best of our knowledge, this study involves so far the most comprehensive comparison of various fluorescence decays in terms of excitation and detection wavelengths as well as excitation powers.

We investigate the influence of various optical fiber fabrication processes on the fluorescence decay of RE ions commonly used in fiber lasers and amplifiers, i.e. Yb³⁺, Tm³⁺ and Ho³⁺. Optical fiber preforms were prepared using the MCVD method combined with Al₂O₃ nanoparticle doping and subjected to subsequent heat treatment processes such as preform elongation and fiber drawing. The fluorescence decay of RE ions was measured in multiple stages of optical fiber preparation: in an original preform, in an elongated preform (cane), in a standard fiber, and in an overcladded fiber. It was found that heat treatment processing of the preforms generally leads to a faster fluorescence decay, which can be explained by the diffusion of dopants and clustering of RE ions. The fiber drawing exhibited a greater effect compared to preform elongation, which was ascribed to a faster cooling rate of the process. In general, the heat treatment of RE-doped silica glass preforms leads to the decline of fluorescence decay.

We report on high-power thulium-doped fiber amplifiers built using a nanoparticle-doped double-clad, pedestal-style large mode area fiber. The applied nanoparticle doping technique allowed achieving uniform high doping concentrations of Al ₂ O ₃ and Tm ₂ O ₃ across the fiber core. The fiber outer layer was fluorine-doped providing a decrease in pump radiation interaction with the coating, thereby enhancing the reliability of the entire laser system under high-power operation. The laser system operating at a wavelength of 2034 nm delivered an output power of up to 441 W with a slope efficiency of 57.4%, and the electro-optical conversion efficiency of the amplifier was 25%. The 1940-nm-wavelength all-fiber amplifier seeded with a narrowband Tm-doped fiber laser provided an output power of up to 273 W with a slope efficiency as high as 61.8%. In both cases the output spectrum was characterized by a 3-dB spectral bandwidth below 160 pm. Moreover, a special splicing procedure for large mode area fibers transmitting high power radiation was proposed and successfully demonstrated.

We report on a flat and broadband thulium-doped amplified spontaneous emission (ASE) fiber source working around 1.85 µm in the eye-safe spectral region. Core-pumped thulium fibers were prepared in-house using the modified chemical vapor deposition method. The amplified spontaneous emission source in a backward direction with respect to the pump and in a single-ended configuration produces stable, non-polarized radiation with an output power of up to 280 mW, corresponding to a slope efficiency of about 36% with respect to the pump power. To the best of our knowledge, the device reported herein is the broadest ASE source based on Tm-doped fiber without internal spectral filtering with an output power exceeding 90 mW and full width at half-maximum of the spectrum greater than 155 nm.