Active fiber optics doped with lanthanides – the current directions

Optical fibres doped with lanthanide ions have enabled incredible developments in lasers, optical amplifiers, and broadband radiation sources. The construction of an active optical fibre requires high optical quality of the core glass, but also its good technological parameters for fiber drawing. In results, the advantages and disadvantages of oxide, fluoride and chalcogenide optical fibres are known, particularly in terms of their phonon limitations. This leads to the search for new fiber designs in both silica glass and multicomponent glasses. In addition, due to the limitations of fiber glasses, systems are being proposed in which active nanoparticles are introduced whose emission properties depend on their crystalline environment. In this talk, three approaches involving silica, germanium and glass-crystal optical fibers will be presented.

Firstly, silica Large Mode Area (LMA) double-clad multi-ring core optical fiber for the emission in the eye-safe range 1.71 µm to 2.1 µm will be presented. Examples of Tm³⁺/Ho³⁺ ion doping profiles will be discussed. During the research, various optical fiber structures with Tm³⁺/Ho³⁺ and SiO₂ dopants were manufactured, as well as configurations of Tm³⁺ and Tm³⁺/Ho³⁺ layers in order to modify the emission band. The fabricated optical fibers (with double cladding) were used in the construction of broadband optical fiber sources and laser source operated around λ= 1939 nm. The applied doping profiles provide a low numerical aperture range (NA = 0.054–0.118) and a mode field diameter (MFD) of up to approx. 70 μm (at 2000 nm).The measured distributions of rare earth dopant concentrations (Ho₂O₃ and Tm₂O₃) in the optical fiber preform in the range of 0.2–0.6 wt.% will be shown. The results of luminescence decay time measurements at a wavelength of 1800 nm (Tm³⁺: ³F₄ → ³H₆) and 2050 nm (Ho³⁺: ⁵I₇ → ⁵I₈) will also be presented. The experiments showed a significant modification of the luminescence profile depending on the fiber length, resulting from the reabsorption process and the formation of spontaneous stimulated emission (ASE).

The second approach is broadband emission in the NIR (1.4-2.1 µm) range which was explored in multicomponent barium gallo-germanate double-clad fibers doped with Yb³⁺/Er³⁺/Tm³⁺/Ho³⁺ ions. A comparative study of single-core and multi-core fiber design (up to 11 active cores) on luminescence properties and bandwidths of 3 dB and 10 dB has been carried out. The observed luminescence spectra can be described as a superposition of the emission bands of Er³⁺: ⁴I_13/2→⁴I_15/2, Tm³⁺: ³H₄→³F₄, Tm³⁺: ³F₄→³H₆ and Ho³⁺: ⁵I_­7→⁵I₈ ions. The presented results show the potential of using a multi-core design due to the increase of active material per meter of fiber and the possibility of profiling the emission spectrum by a number of cores and dopant concentrations in individual cores. Moreover, increased dopant concentration per meter of fiber, which is proportional to the number of active cores, allows for a decrease in the required optical fiber length. This leads to reduced total fiber attenuation, which is higher than silica fibers fabricated by the MCVD method. Presented multicore double-clad fibers offer new possibilities for constructing eye-safe, ultra-broadband ASE sources.

The third new method, called “glass powder doping” was used to produce the first glass-ceramic optical fibre doped with YPO₄:Yb³⁺ active nanocrystals (ANCs). The advantage of this construction is the use of core glass with refractive index (n=1.78, l=633nm) matched to the ANCs. It was obtained due to the separate preparation of ANCs and glass matrix. The YPO₄:Yb³⁺ nanocrystals were synthesized by the solvothermal method and optimized for the size (~50 nm), shape, dispersion and Yb³⁺ concentration (3 mol.%) to obtain the required optical properties. Optical fibre was fabricated using powder-in-tube method, where the mixture of glass powder and ANCs (5% wt.) and Duran® glass tube were used as a core and cladding respectively. The presence of the crystals was confirmed by luminescence measurements and FIB/TEM-EDX analysis. Optical and luminescence properties of the nanocomposite optical fibre allow to conclude that it may be a new class of active materials for lasing applications.