Ho-doped Soft Glass Optical Fibers for Coherent Wavelength Sources Above 2 Micron

摘要

This report results from a contract tasking Politecnico de Torino as follows: B. TECHNICAL PROPOSAL/DESCRIPTION OF WORK: Aim of the project is to fabricate and characterize rare earth doped soft glass optical fibers for fiber lasers emitting at wavelengths longer than 2 micron to allow efficient narrow linewidth emission in the atmospheric window useful for coherent detection and sensing. The research activity will focus on the design of the laser host material in terms of suitable glass composition and rare earth doping (quality and quantity of the dopants), then fiber preform fabrication and fiber drawing will follow with the aim of maximizing dopant concentration (which induces a reduction of the laser cavity), lifetime of the upper laser level and emission cross section. Proof of concept of fibers suitable for single frequency operation will be demonstrated. Understanding of the active material suitable for optimum laser design is expected as final result. The investigated host material for the incorporation of rare earth ions will be tellurite glass because it is a better candidate than silica for emission above 2 micron: it shows high transparency (up to 6 micron), lower phonon energy (700 cm-1 vs. 1100 cm-1), higher rare earth solubility (up to 10 mol% of Ln2O3). These properties should allow the fabrication of short cavity narrow linewidth (also called single frequency) optical fiber lasers and in general of compact active devices. The main rare earth dopant will be Ho3+ ion, which features emission at wavelengths longer than 2 micron. Since Ho does not allow efficient direct pumping with laser diodes, Tm and Tm/Yb co-doping will be carried out and pumping at 790 and 980 nm will be investigated, respectively.