New construction of a silica-based multi-ring profile of Tm ³⁺ doped fiber laser is presented. Optimisation of both multi-ring refractive index profile and active ions distribution towards high beam quality mode propagation is a new approach in developing active fiber constructions. Using a system of layers forming the core allows controlling the dopant profile and, consequently, the emission properties of fiber lasers (power density, beam profile shape etc.). The preform fabricated using MCVD technology is presented. The refractive index profile (Δn = 9·10 ⁻³ ), EPMA-EDS analysis (Tm ₂ O ₃ and Al ₂ O ₃ concentration 0.56 and 5.2 wt% respectively), and excited state ( ³ F ₄ ) decay time are shown. The double-clad fiber laser based on the Fabry-Perot resonator construction (17/240 μm core/cladding) was used for single-mode laser beam generation at a wavelength of 1938.8 nm (exc. @ 790 nm) with an FWHM bandwidth of 0.17 nm (1.68 W output power, M ² _x = 1.10, M ² _y = 1.23). For the optimized gain fiber (25/330 μm core/cladding diameters) length, the laser provided an output power of >5 W with a slope efficiency of ∼26%, determined to the absorbed pump power.

LMA (Large Mode Area) optical fibers are presently under active investigation to explore their potential for generating laser action or broadband emission directly within the optical fiber structure. Additionally, a wide mode profile significantly reduces the power distribution density in the fiber cross-section, minimizing the power density, photodegradation, or thermal damage. Multi-stage deposition in the MCVD-CDT system was used to obtain the structural doping profile of the LMA fiber multi-ring core doped with Tm³⁺ and Tm³⁺/Ho³⁺ layer profiles. The low alumina content (Al₂O₃: 0.03wt%) results in low refractive index modification. The maximum concentrations of the lanthanide oxides were Tm₂O₃: 0.18wt % and Ho₂O₃: 0.15wt%. The double-clad construction of optical fiber with emission spectra in the eye-safe spectral range of (1.55–2.10 µm). The calculated LP₀₁ Mode Field Diameter (MFD) was 69.7 µm (@ 2000 nm, and 1/e of maximum intensity), which confirms LMA fundamental mode guiding conditions. The FWHM and λmax vs. fiber length are presented and analyzed as a luminescence profile modification. The proposed structured optical fiber with a ring core can be used in new broadband optical radiation source designs.

Introduction: Mothers play a key role in ensuring the oral health of their children, They should have wide knowledge of both the risk factors for dental caries and methods of its prevention. Research conducted in Poland to-date indicates that pregnant women have inadequate knowledge of dental...

A novel optical fiber design featuring a multi-ring core refractive index profile co-doped with Tm³⁺ is presented. The fiber was fabricated using the MCVD-CDT technique, enabling precise control of the spatial distribution of dopants and refractive index. Unlike conventional fibers with homogeneous cores, the proposed multi-ring architecture allows intentional engineering of modal and emission properties, representing a key novelty of this work. Numerical analysis confirmed weakly guiding conditions and few-mode emission with a low numerical aperture (NA = 0.132) for a 48/250 µm core/cladding geometry. The diffusion process in the multi-ring structure during fiber drawing is also shown. The measured maximum concentrations of active and modifier oxides in the optical fiber preform are: 0.2mol% Tm₂O₃ and, 3.22 mol% Al2O3 and 0.75 mol% for the P2O5,. The ASE generation process in the main emission band can be observed by a decrease in FWHM from 543 nm to 114 nm as the pump power increases from 3.2 to 18.6 W. The proposed fiber architecture demonstrates new opportunities for spectral (1.7-2.1 µm) and modal engineering of fiber-based light sources and amplifiers operating in the eye-safe wavelength region.

The Large Mode Area (LMA) fibers are attractive in the field of new components in optical fiber technology in the field of sensing and novel radiation sources. The numerical analysis of multi-ring (multi trench) fiber construction is investigated for eye-safe transmission spectral range (1400-2100 nm). The optical fiber refractive profile construction of ∆n (0.001- 0.004) for different radial step ∆r (1.0-1.6 µm) is used for the investigation of linearly polarized modes (LP01 and LP11) propagation. The maximum mode field diameter (MFD) of LP01 was calculated. The results confirm the LMA propagation possibility in the presented fiber construction. The single-mode guiding was confirmed for different ∆n and ∆r parameters in the investigated spectral range. The cutoff wavelength <1400 nm and single-mode operation are possible for ∆n=0.001 and ∆r=1 µm.