Thulium-doped fiber lasers (TDFL) emitting around 2 µm receive growing attention. The Tm3+ ions may be pumped around 0.8 µm into the 3H4 level, but the maximum achievable efficiency in theory reaches only 40 %. Much higher efficiencies are achievable in practice thanks to the cross-relaxation (CR) effect, also called „2-for-1“ process. To trigger the CR effect, very high contents of Tm3+ ions are required, which places significant requirements on the material design; high concentrations of Al2O3 are typically needed to prevent concentration quenching in highly-doped fibers.
MCVD method combined with nanoparticle doping is one of the most perspective methods for the preparation of highly doped alumino-silicate fibers. In this contribution, a large set of thulium-doped fibers was prepared by various methods. The fibers were analyzed with emphasis on the fluorescence lifetime and laser performance, and the nanoparticle doping method was evaluated in comparison with conventional fabrication methods.
Nanostructured or “pixelated” core fibers have attracted great attention thanks to possible design of optical fibers with almost arbitrary refractive index profile, including gradient index nanostructured core, large mode area fibers for high power applications or fiberized free-form optical components. A short review of applications of (nano)structured core active fibers in fiber lasers will be given followed by detailed study of the effect of heat treatment and fiber drawing on the luminescence properties important for fiber laser performance; and application of the erbium- and ytterbium structured-core active fibers in fiber lasers that operate simultaneously at 1 and 1.55 micrometer wavelengths.
Nanocrystalline holmium-doped titanates have been widely investigated for their luminescence properties. Low-phonon pyrochlore lattice supports the radiative energy transfers improving the efficiency of high-power lasers and amplifiers operating around 2 m. However, the chemical reactivity toward the silica prevents the incorporation of ceramic nanoparticles in common optical fibers. Deposition of active ceramic layer on the inner wall of capillary or hollow core fiber represents a promising alternative. We present a versatile sol-gel route to active capillary fibers doped by nanocrystalline (Ho0.05La0.95)2Ti2O7. Nanocrystalline films with tailored properties were prepared by sol-gel method. The sols were coated on silica glass slides to receive a set of reference samples and soaked into silica capillary fibers making the coatings on the inner capillary wall. The presented approach led to the formation of homogenous nanocrystalline (Ho0.05La0.95)2Ti2O7 films with tailored nanocrystal size up to 87 nm and refractive index of about 2.2. All prepared samples showed an intensive emission at 2.0 m under an excitation at 450 nm and the luminescence decay time of about 7.3 ms. The presented method enables the preparation of homogenous and highly transparent thin films with tailored properties. These films are suitable for preparation of bulk luminophores and planar active optical components operating at 2 μm.
Recent evolution in nanoscience and nanotechnologies has brought novel possibilities in the development of optical fibers. Dual-wavelength fiber lasers have attracted scientific attention due to their prospective applications in fields including next-generation optical fiber communication, ranging systems, and spectroscopy. Nanostructurization has shown itself as a suitable method for preparing fiber lasers operating simultaneously at dual wavelengths. We report on the design of nanostructured or “pixelated” core fabricated by assembling erbium- and ytterbium elements, as well as on the optimization of the average concentration of rare earth elements using numerical modeling. Preliminary experimental results of erbium- and ytterbium-doped nanostructured-core fiber will be presented.
For applications in fiber lasers and amplifiers, silica glass remains a perspective host for rare-earth ions thanks to favorable material properties. However, the luminescence of RE ions is hindered by the high phonon energy of silica lattice and low solubility of RE ions, which cause luminescence quenching. Pure silica thus needs to be co-doped with suitable additives such as Al2O3, which form a beneficial low-phonon environment and increase the solubility of RE ions. Luminescence lifetime is one of the most important parameters to determine the suitability of RE-doped silica fibers for laser operation. Optical fibers with higher luminescence lifetime typically exhibit higher values of slope efficiency and lower laser threshold. It was previously shown that the environment of Tm3+, Ho3+ or Yb3+ ions and luminescence lifetime may be significantly affected by fabrication processing at high temperatures, probably due to the chemical changes occurring in the matrix. However, the effect of fabrication processing on the spectroscopic properties of another important RE ion, Er3+, is different to the other ions and remains unclear. In this contribution, we present a study on the fluorescence lifetime of a highly-doped optical fiber prepared by the MCVD method combined with nanoparticle-doping. The fluorescence lifetime of Er3+ was studied in several stages of fabrication processing. The influence of fabrication processing on the fluorescence lifetime of Er3+ ions was analyzed and discussed.
Holmium-doped aluminosilicate fibers are frequently used in holmium-doped fiber lasers (HDFL) thanks to their strong emission at 2 μm. Fluorescence lifetime is one of the most important parameters to determine the suitability of holmium doped optical fibers for use in fiber lasers. One of the potential mechanisms for the shortening of fluorescence lifetime is the diffusion of RE ions and Al2O3 at high temperatures during the fiber preparation process. We have prepared a Ho-doped aluminosilicate optical fiber preform using MCVD combined with nanoparticle doping. The prepared preform was subjected to various fabrication processes such as preform elongation, fiber drawing or additional heat treatment, the fluorescence lifetime was measured in all stages of the experiment and its dependency on the fabrication process was discussed. The original preform exhibited a long fluorescence lifetime of 1.433 ms. Gradual application of fiber fabrication processes such as preform elongation or fiber drawing resulted in a decline of fluorescence lifetime down to 1.174 ms in the case of overcladded optical fiber. The decrease of fluorescence lifetime was ascribed to the diffusion of dopants and the changes in the Ho3+ ion environment, which increased the rate of multiphonon relaxation, as well as clustering of holmium ions, which increased concentration quenching.
In this paper, we investigate the influence of various nanostructured-core fiber fabrication processes, such as preform elongation or fiber drawing, on the fluorescence lifetime of Yb3+ ions. The optical fiber preform was prepared using Modified Chemical Vapor Deposition (MCVD) method combined with Al2O3 nanoparticle doping. The optical fiber preform was subjected to various processing treatments involving heat and mechanical stresses, i.e. preform elongation and fiber drawing, and the fluorescence lifetime was measured in all stages of fiber fabrication, i.e. original preform, elongated preform (cane), fiber and overcladded fiber. It was found that the time-resolved photoluminescence properties of Yb3+ ions in silica glass are strongly dependent on the processing of the material. The fluorescence lifetime of the 2F5/2 level of Yb3+ ions decreased with the heat and mechanical treatment, which was explained by the break-up of Al2O3 nanoparticles, diffusion of dopants and changes in the Yb3+ phonon environment as well as clustering of the Yb3+ ions. The fiber drawing exhibited a stronger effect compared to preform elongation which was ascribed to the high rate of cooling and mechanical stresses during the drawing process. In general, the heat and mechanical processing of Yb-doped optical fiber preforms leads to a deterioration of time-resolved photoluminescence properties.
Recently optical capillaries modified by Bragg reflection mirrors applied on the inner walls have been investigated for transmitting radiation of MIR lasers. Such capillaries include Bragg and omniguide fibers, holley fibers, or silica Kagome like fibers. Although OmniGuide fibers are commercially available and have been used for delivery of radiation of CO2 fibers at 10.6 μm, novel types of hollow-core fibers are still investigated for MIR applications. In this paper a novel approach for the preparation of capillary optical fibers for MIR region is presented. This approach employs the application of thin layers of arsenic sulfide glass and acrylate polymer from their solutions onto the inner wall of silica capillary. Arsenic sulfide forms high-index and polymers the low-index parts of reflection mirrors. By controlling optical thicknesses of such layers, Bragg mirrors can be obtained. In experiments, input solutions of arsenic sulfide in n-propylamine and UV-curable acrylate in acetone were prepared. Such solutions were applied by dip-coating method on glass slides in order to obtain samples of single layers and multilayer coatings for the determination of thicknesses and refractive indices. Acrylate layers were UV cured and arsenic sulfide layer were thermally treated at 80°C. By passing columns of the input solutions through a silica capillary with a hole diameter of 80 μm and a length of 50 cm multilayer coatings on the inner capillary wall were prepared. The column velocity for each solution was controlled as a main factor influencing the layer thickness. Applied layers were UV cured or thermally treated under a nitrogen flow through the capillary. Coatings of three pairs of the high- and low-index layers were fabricated. Single layers and multilayers applied on planar substrates were characterized by transmission spectroscopy and by optical microscopy. Attenuation coefficients of internally coated capillary fibers of 10-20 dB/m were determined at a wavelength of 1940 nm.
Chalcogenide materials due to high refractive indices, transparency in the mid-IR spectral region, nonlinear refractive
indices, etc, have been employed as fibers and films in different photonic devices such as light amplifiers, optical
regenerators, broadband radiation sources. Chalcogenide films can be prepared by physical methods as well as by
solution-based techniques in which solutions of chalcogenides in amines are used. This paper presents results on the
solution-based fabrication and optical characterization of single arsenic sulfide layers and multilayer stacks containing
As2S3 layers together with porous silica layers coated on planar and fiber-optic substrates.
Input As2S3 solutions for the layer fabrications were prepared by dissolving As2S3 powder in n-propylamine in a
concentration of 0.50 mol/l. These solutions were applied on glass slides by dip-coating method and obtained layers were
thermally treated in vacuum at temperatures up to 180 °C. Similar procedure was used for As2S3 layers in multilayer
stacks. Such stacks were fabricated by repeating the application of one porous silica layer prepared by the sol-gel method
and one As2S3 layer onto glass slides or silica fibers (a diameter of 0.3 mm) by using the dip-coating method. It has been
found that the curing process of the applied layers has to be carefully controlled in order to obtain stacks with three pairs
of such layers.
Single arsenic and porous silica layers were characterized by optical microscopy, and by measuring their transmission
spectra in a range of 200-2500 nm. Thicknesses and refractive indices were estimated from the spectra. Transmission
spectra of planar multilayer stacks were measured, too. Interference bands have been determined from optical
measurements on the multilayer stacks with a minimum transmittance of about 50% which indicates the possibility of using such stacks as reflecting mirrors.
Layers based on TiO2-SiO2 systems fabricated by sol-gel method have been investigated for the preparation of planar
waveguides, antireflective coatings, Bragg mirrors, etc. However, at high titania contents such materials exhibit high
viscosities and tendency to phase separation. In this paper we present optical properties of films containing TiO2 which
are prepared via a novel approach sol-gel on the basis of ternary Na2O-TiO2-SiO2 glasses and which can exhibit lower
viscosities.
Films of Na2O-TiO2-SiO2 systems were prepared from input sols mixed of silica, titania and sodium oxide sols. The
silica sol was prepared from tetraethyl orthosilicate (TEOS), ethanol, hydrochloric acid and water, with a TEOS c= 2
mol/l and water/alkoxide ratio 1.75. The titania sol was mixed from titanium tetraisopropoxide (TiPr), propan-2-ol, nitric
acid and water, c= 0.5 mol/l, RW= 0.42. The sodium oxide sols with c= 0.474 mol/l were prepared from sodium ethoxide
and ethanol. Input sols were prepared by mixing the silica and titania sols first and then the sodium sol was added. The
input sols were aged for one hour. Stable input sols were obtained. The input sols were deposited on glass and silica
slides by dip-coating technique at a withdrawing speeds of 200 mm/min. Applied gel layers were thermally treated at
temperatures of 450 and 900°C. Layers containing sodium oxide and titania in concentration ranges of 0-20 mol.% and
0-30 mol.% respectively have been fabricated.
Optical properties of layers were determined by UV-VIS-NIR transmission and reflection spectrophotometry. Refractive
indices of layers were determined by spectral ellipsometry and from transmission spectra. Optical properties were
correlated with results of XRD spectroscopy, optical microscopy, and atomic force microscopy. Transparent
homogenous films with a maximum refractive index of 1.61 at a wavelength of 600 nm have been obtained.
The paper presents results on preparation and characterization of highly reflective coatings on planar substrates and
inside silica tubes. Coatings are designed for a maximum reflectivity at a wavelength of 550 nm and consist of several
pairs of oxide layers. Each pair is composed of one layer with a high refractive index and one layer with a low refractive
index with a refractive-index contrast of about 1.1.
The layers were prepared by the sol-gel method. High-index layers were applied from a sol based on titanium butoxide
while a sol of tetramethoxysilane was use for low-index layers. The sols were deposited onto silica slides or onto walls
of silica tubes by using the dip-coating technique. Applied gel layers were thermally treated at temperatures up to 450 °C
in order to obtain densified layers with thicknesses 50-100 nm. Coatings with one to five pairs of layers were fabricated.
Prepared coatings were characterized by transmission and reflection spectrometry in a wavelength range from 190 to-
1100 nm, by contact profilometry, and by spectral ellipsometry. Thicknesses and refractive indices of coatings were
determined from these measurements. For normal light incidence a reflectivity higher than 99% in a wavelength range of
500-650 nm was measured by transmission spectrometry on coatings prepared from four or five pairs of layers. Similar
reflectivity values were determined for angles of incidence of 15, 30, 45 and 60 degrees by using reflection
spectrometry. Transmission spectra measured on the coated tubes which show interference bands are also presented in
the paper.
This paper deals with the preparation of spherical silica whispering-gallery-mode (WGM) microresonators and with their resonance spectra measured in air and in acetone vapors. Spherical microresonators with a diameter ranging from 320 to 360 micrometers have been prepared by heating the tip of a silica fiber by a hydrogen-oxygen burner. Details of this preparation are shown on spherical and spheroidal microresonators. The prepared microspheres were excited by a fiber taper and their resonance spectra were measured and Q factors estimated. Changes in the resonance spectra of the microspheres due to their contact with acetone vapor heated to 55 °C or with liquid acetone have been observed. These changes are explained by interaction of acetone with silica and by temperature changes of the microspheres.
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