Holmium-doped silica-based optical fibers belong to intensively studied materials for fiber laser sources operating around 2.1 μm. In this contribution, we deal with silica-based optical fibers doped with holmium and aluminum oxide. The fibers were prepared by the modified chemical vapor deposition method combined either with a solution doping or a nanoparticle doping. A large set of fibers with various dopant concentrations was characterized related to their fluorescence lifetime, laser threshold and slope efficiency. The best-performance fibers exhibited a fluorescence lifetime longer than 1 ms, a laser threshold under 200 mW and a slope efficiency around 80%. These characteristics are discussed regarding the doping method and dopant concentrations.
The high-power fiber lasers rely on the use of double-clad active fibers with noncircular symmetry of their inner-cladding cross-section. Therefore, the optical fiber preforms had to be shaped before fiber drawing. A new technique of preform-shaping by a CO2 laser is now available along with the conventional mechanical-based grinding. This innovative technique retains the advantages of enabling to produce complex inner-cladding shapes that not easily achievable by a conventional grinding technique. However, one of the drawbacks of the CO2 laser-based preform-shaping is weak of hydroxyl OH-groups reduction during the ablation process. The water is often penetrating into the preform surface via the oxyhydrogen flame during preform manufacturing. The thermophysical nature of the CO2 laser ablation process causes further diffusion of the OH-ions deeper towards the preform center during shaping. The diffused OH-groups in the glass material cause high attenuation at some wavelengths which are associated with the overtones of the fundamental OH absorption peaks. Unfortunately, some of these peaks lay rather close to the commonly used laser pumping wavelengths. This should be considered when designing a double-clad fiber laser as well as when selecting the preform-shaping technique. In this work, we will present a new method of mitigation of the water penetration into the optical fiber preform when a CO2 laser preform-shaping technique is applied. This method includes an optical fiber preform etching procedures prior to the preform laser shaping and to the fiber drawing. The acquired data helps also to predict the thickness of the layer that should be removed from the preform surface. The knowledge of the thickness of the optimal layers is of great benefit for the advanced estimation of the inner-cladding attenuation, an important parameter of double-clad fibers intended for high-power fiber lasers.
We present the preparation and characterization of holmium-doped silica-based optical fibers for fiber lasers operating around two micrometers. The fibers were prepared by a modified chemical vapor deposition process and co-doped with aluminum oxide. Alumina was doped with the use of two different methods – solution doping and nanoparticle doping. Prepared optical preforms and fibers were characterized according to their optical, spectral and laser properties. It was observed that the doping by alumina nanoparticles improve a fluorescence lifetime and laser characteristics such as laser threshold and slope efficiency. The comparison of both doping methods is presented and results are discussed.
Since an extension of spectral region from near-infrared to mid-infrared covered by fiber lasers belongs to ambitions of today’s research, germanate-based glasses have been proposed, fabricated and their optical properties studied. Glass samples were prepared by Modified Chemical Vapor Deposition method and by conventional glass melting processes. Fabricated preforms suitable for fiber drawing were optically characterized by refractive index and absorption spectra measurements. A limit of rare earths doping of GeO2-based glasses without phase separation was found comparable to SiO2-based glasses (thousands of ppm). Preforms prepared by Modified Chemical Vapor Deposition contained only up to 29 mol. % of GeO2 in core, although pure GeO2 glass layers were deposited. This effect caused transparency limitation of such materials to 2-2.5 μm. Implementing of effective drying processes during GeO2-based glass conventional melting lead to decrease of residual OHcontent to the level comparable with optical silica glass HOQ 310. The shift of infrared transmission edge of bulk GeO2-based conventionally prepared glasses to longer wavelengths towards to 5 μm was observed. These facts support the idea of potential use of GeO2-based glass matrices for fiber lasers operating in mid-infrared region
In this work we report on 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 fibers.
Optical fiber preforms were prepared by solution-doping of Tm3+ ions with either Al3+ ions or dispersed alumina
nanoparticles. Optical fibers were characterized by means of Tm, Al and Ge concentrations, refractive index profiles,
optical spectral attenuations, luminescence spectra and fluorescence lifetimes. Highly aluminium-codoped optical fibers
exhibited fluorescence lifetimes of up to 756 μs.
In this contribution we report and discuss the results of laser characterizations of experimental thulium-doped optical
fibers. These active fibers were fabricated in house and were tested in two laser systems to verify their characteristics.
The first one, a monolithic fiber laser, was of great interest to us due to its potentially lower overall resonator losses,
improved laser lifetime and better robustness. The compact laser cavities with a Bragg gratings inscribed directly into the
active optical fiber differs to the second laser system where the Bragg gratings were inscribed into a passive fiber which
had to be spliced to the active fiber. The tested fibers were manufactured by the modified chemical vapor deposition
method and a solution-doping of thulium ions with Al2O3 or alumina nanoparticles, respectively. We focused on
comparison of laser output powers, slope efficiencies, and laser thresholds for particular thulium-doped fiber in different
We present preparation and characterization of thulium-doped silica-based optical fibers for fiber lasers. The fibers were
prepared by modified chemical vapor deposition process and doped with alumina and thulium ions. Alumina co-doping
was achieved through two different methods – solution doping and nanoparticle doping method. Prepared preforms were
characterized in terms of refractive index profiles and dopants distribution. For the drawn fibers, their spectral
attenuation, fluorescence lifetime and laser performance were measured. In the case of nanoparticle doping, better laser
characteristics were observed. Discussion and explanation of the trends for laser efficiency improvement is given.
Silica optical fibers doped with rare-earth elements are key components of high-power fiber lasers operating in near-infrared region up to 2.1 μm. In this contribution we deal with preparation and optical characterization of silica-based optical preforms and fibers doped with thulium for fiber lasers operating around 2 μm. A set of fibers with thulium concentration ranges 1000-5000 ppm was prepared by the MCVD solution doping method and characterized. A decrease of fluorescence lifetime of thulium from 487 μs to 378 μs was observed with increasing rare-earth concentration in fiber core. This phenomenon can be explained by energy transfer between ions and ion clustering. Fabricated fibers were suitable for use in fiber lasers.
Spontaneous laser-line sweeping refers to fiber laser instabilities with regular laser wavelength drift within a broad range that may exceed 10 nm; other characteristics of the laser output are sustained relaxation self-pulsing and narrow spectral linewidth. The laser wavelength drift is caused by standing-wave in the cavity; it can be regarded as a special case of mode instability of longitudinal modes of the laser resonator. Self-sweeping was observed so far in Yb, Er, Tm and Bidoped fiber lasers. We report on Ho-doped fiber laser self-sweeping in interval of 3-5 nm near 2100 nm. The sweeping rate was typically 0.7-0.9 nm/s. The thulium-doped fiber lasers at around 2030 nm and 1950 nm were tested as pump sources. The self-sweeping was registered by FTIR spectrometer and the data processing is discussed.