Silica Optical Fiber with a Ring-Shaped Active Layer Doped with Thulium Ions

Currently, there is a dynamic development of optical radiation sources (e.g., fiber lasers and amplified spontaneous emission (ASE) sources) based on fiber optic structures. Particular attention is given to designs operating in the eye-safe spectral range (above 1.4 μm) [1]. One of the most well-known and widely used materials for their production is silica. It is characterized by thermal stability and a high damage threshold, which are crucial for achieving high-quality optical beam propagation [2]. Achieving stable single-mode propagation is essential for many applications. However, the typical core diameter of a single-mode fiber (approximately 8 μm) results in high optical density and the occurrence of nonlinear effects [3]. For this reason, LMA (Large Mode Area, NA < 0.1) fiber designs are currently being investigated. These designs employ a special refractive index profile (e.g., ring-core, multi-ring core) and enable the shaping of the mode field by optimizing the refractive index and dopant distribution in the core area [4]. Advanced refractive index profile fibers can be fabricated using the well-known modified chemical vapor deposition (MCVD) method.