Abstract
In this work we report on the thulium-doped silica-based optical fibers with increased fluorescence lifetime of the 3F4 level thanks to the modification of the local environment of thulium ions by high content of alumina. The determination of the cross-relaxation energy-transfer coefficients from the measurements of the fluorescence lifetimes of the 3F4 and 3H4 energy levels of Tm3+ ions in the experimentally prepared optical fiber is provided as well. Preforms of optical fibers were prepared either by conventional solution-doping of Tm3+ and Al3+ ions or by dispersion-doping of Tm3+ ions with alumina nanoparticles. Optical fibers were characterized by means of Tm, Al, and Ge concentrations, refractive index profiles, optical spectral absorption and luminescence, and by time-resolved fluorescence spectroscopy. Highly aluminium-codoped thulium silicate optical fibers exhibited fluorescence lifetimes of over ~500 μs with maximum value of 756 μs, which means a fluorescence lifetime enhancement when compared to the thulium-doped fibers reported elsewhere. We show an application of the thulium-doped fiber in a compact all-fiber ring laser that is passively mode-locked by using graphene-based saturable absorber. The output pulsewidth and repetition rate were 905 fs and 32.67 MHz, respectively.
Highlights
Thulium-doped fiber lasers operate in the region of wavelengths around 2 μm and can be used as high-power laser sources
Various compositions of optical fiber preforms were analyzed in order to choose appropriate properties for further evaluation of cross-relaxation coefficients of Tm ions as well as for the fabrication of all-fiber ring laser
The properties of prepared optical fibers and preforms are summarized in Table 1 and include glass matrix compositions, refractive index differences, numerical aperture (NA), cutoff, concentrations of selected dopants, and measured values of the 3 F4 level fluorescence lifetime
Summary
Thulium-doped fiber lasers operate in the region of wavelengths around 2 μm and can be used as high-power laser sources. Devices operating in the region of wavelengths around 2 μm receive growing attention nowadays because they can be used for so-called “eye-safe“ applications, e.g., laser surgery, biological imaging, materials processing, atmospheric transmission, defense, etc. Thulium-doped fibers are attractive 2 μm sources despite the low quantum conversion efficiency (QE). Low QE usually causes shortening of energy storage time and increase of laser threshold. High-power, continuous-wave (cw) Tm-doped fiber lasers can be built even with active medium of impaired QE efficiency. High QE is required especially in photonic devices such as ASE (Amplified Spontaneous Emission) sources, low- and moderate-average-power fiber lasers and amplifiers, or Q-switched pulsed sources
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