Laser Heated Pedestal Growth Technique Research Articles

Yb,Er,Tm:Sc2O3 single crystal fibers for multi-mode optical thermometry

Fluorescence intensity ratio (FIR) thermometers have obtained great attention in industrial production, medical diagnosis and scientific research. However, the sensitivity has a large decline at high temperatures, which seriously limits the practical applications. Here, the Yb,Er,Tm:Sc2O3 single crystal fibers were grown by laser-heated pedestal growth (LHPG) technique and its multi-mode FIR thermometry based on thermal coupled energy levels (TCLs) and non-thermal coupled energy levels (NTCLs) has been well demonstrated. The temperature measuring range of Yb,Er,Tm:Sc2O3 single crystal fibers covers 298–973 K, with an increase of 2 orders of magnitude in absolute sensitivity. The relative sensitivity is higher than 0.02 % K−1 in the whole T-range of 298∼973 K, with a maximum value of 1.02 % K−1. Especially at the high temperatures of 973 K, it can still maintain 0.31 % K−1. This work proposes an innovative fluorescence intensity ratio detection strategy to achieve complementary disadvantages between TCLs and NTCLs and provides an effective method for improving detection sensitivity.

Selective Laser Spectroscopy of the Bixbyite-Type Yttrium Scandate Doped by Rare-Earth Ions.

Yttrium scandate crystals doped by Nd3+ and Tm3+ ions have been successfully grown in the form of fibers using the laser-heated pedestal growth (LHPG) technique. The selective laser spectroscopy methods have identified and distinguished three distinct types of optically active centers associated with Nd3+ and Tm3+ ions. The substitution of Y3+ and Sc3+ for rare-earth ions in the C2 structural site leads to the formation of two distinct basic long-time centers. In Nd3+:YScO3, another type of center (a short-lifetime one) is formed known as the Nd3+-Nd3+ aggregate pair. This center arises from the substitution of Y3+ or Sc3+ for Nd3+ cation in the adjacent MO6 polyhedra that share an edge. In Tm3+:YScO3, the third optical center is formed as a result of the substitution of Y3+ or Sc3+ for Tm3+ in the MO6 octahedra with the C3i site symmetry. The fluorescence decay lifetimes of Nd3+ and Tm3+ ions in the YScO3 crystal structure have been accurately measured and estimated. A Stark level diagram illustrating the splitting of 4F3/2, 4I11/2, and 4I9/2 multiplets of Nd3+ ions has been constructed to show features of the active optical centers with the C2 site symmetry.

Anisotropic growth and characterizations of large length-diameter ratio sapphire fibers via laser-heated pedestal growth technique

Sapphire fiber is a promising candidate for harsh environment sensing due to its high melting point, compact structure, and corrosion resistance, where the fiber with a smaller diameter and a larger length-diameter ratio is being widely demanded. In this work, high quality sapphire crystal fibers with minimum diameter of 25 μm and maximum length of 24.5 m were successfully prepared by laser-heated pedestal growth technique, which is the dimension record for crystal fibers that ever reported. Sapphire fibers exhibit anisotropic growth behaviors and the c-oriented sapphire fibers were demonstrated to have higher tensile strength compared to a-oriented sapphire fibers, illustrating superior mechanical properties. Moreover, various essential properties such as flexibility, surface roughness, and crystallinity have also been systematically investigated.

Dopant segregation in YAG single crystal fibers grown by the laser heated pedestal growth technique

In this study, we report self-segregation of dopants in a crystal matrix within a single crystal (SC) fiber. Neodymium and holmium-doped yttrium aluminum garnet (YAG) fibers were grown using the Laser Heated Pedestal Growth (LHPG) technique and cross-sectional dopant concentration was measured using electron-probe micro-analysis. It was observed that the degree of auto-segregation of the rare-earth dopant depended on the difference in ionic size of the dopant ion and the Y3+ ion in the YAG matrix. While holmium showed little tendency to self-segregate, the concentration of neodymium ions varied as much as 25% across the cross-section of the fiber. Strong correlation between the dopant concentration profile and fiber draw speed was also demonstrated. Since the local refractive index depends on the concentration of dopants, a refractive index profile can be achieved by a dopant profile across the fiber cross-section. Engineered index profiles can be realized by varying growth conditions, dopants, crystal matrix, etc. Such an approach is promising in applications such as the development of monolithic SC fibers with graded-index profiles.

Single crystal Er3+ : YAG fibers with tailored refractive index profiles.

Erbium-doped yttrium aluminum garnet (Er3+:YAG) rods were inserted inside undoped tubes and grown into single-crystal fibers of a diameter of 300μm using the laser-heated pedestal growth technique. Growth at various rates resulted in radially graded distributions of Er3+ dopant ions, as observed using laser-induced fluorescence imaging. Profiles of the refractive index were measured using cross-sectional reflectometry in a microscope. Dopant distributions and the corresponding index profiles were compared with thermal diffusion theory to determine the inter-diffusion coefficient of Y3+ and Er3+ ions at 2000°C, yielding an estimated value of D=(9.10±0.8)×10-11 m2/s. This work constitutes a step toward controlled growth of fibers with high thermal conductivities, low Brillouin gain, and waveguiding properties required for high-power optical amplifier and laser applications.

Optimizing alignment and growth of low-loss YAG single crystal fibers using laser heated pedestal growth technique.

The effect of misalignments of different optical components in the laser heated pedestal growth apparatus have been modeled using Zemax optical design software. By isolating the misalignments causing the non-uniformity in the melt zone, the alignment of the components was fine-tuned. Using this optimized alignment, low-loss YAG single crystal fibers of 120μm diameter were grown, with total attenuation loss as low as 0.5dB/m at 1064nm.

Higher Gain of Single-Mode Cr-Doped Fibers Employing Optimized Molten-Zone Growth

Higher gain of single-mode Cr-doped crystalline core fibers (SMCDCCFs) with longer fiber length employing an optimized molten-zone in laser-heated pedestal growth (LHPG) technique is demonstrated. The optimized molten zone in the LHPG is determined by an empirical formula. The SMCDCCFs exhibit a length of 24-cm, a core diameter of 25-μm, and a V-value of 2.40 which confirms the LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> single-mode operation. A 6.44-dB gross gain and a 4.44-dB net gain with 24-cm fiber length of the SMCDCCF at wavelength of 1.4-μm were obtained. Further studies on higher gain of more than 10-dB gross gain of the SMCDCCFs including a longer length of 30-cm, a lower loss fiber, and a suitable annealing technique are essential to develop the SMCDCCFs as broadband fiber amplifiers for practical use in the next-generation fiber transmission systems and are currently under investigation.

Single crystal fiber growth of cerium doped strontium yttrate, SrY2O4:Ce3+

First single crystal fibers of cerium doped strontium yttrate were fabricated using the laser-heated pedestal growth technique. Through thermodynamic equilibrium calculations and by high-temperature mass spectrometry suitable growth conditions could be determined. The atmosphere played an important role during crystallization. It affected the composition shift, on the one hand, and the valence state of cerium, on the other hand. These dependencies can be explained by combining X-ray diffraction, elemental analysis, and optical spectroscopy. Crystallization in slightly reducing nitrogen atmosphere proved to be a reasonable choice, because evaporation is suppressed and trivalent cerium is stabilized. Strong green emission that depends on the oxygen fugacity during crystallization could be excited using UV light. Optical properties of SrY2O4:Ce3+ were measured for the first time.

Gain Enhancement of Single-Mode Cr-Doped Core Fibers by Online Growth System

Gain enhancement of single-mode Cr-doped crystalline core fiber (SMCDCCF) with longer fiber length fabricated by online laser-heated pedestal growth (LHPG) technique is demonstrated. In comparison with the SMCDCCF fabricated without online growth, the online technique enables real-time monitoring and controlling small molten zone in the LHPG to achieve longer length with better uniformity and smaller core diameter of the SMCDCCFs. The SMCDCCF exhibits a length of 10.6 cm, a core diameter of 25 mu m, and a V-value of 2.40, which confirms the LP01 single-mode operation by the far-field pattern measurement. A 3.9-dB gross gain and a 1.9-dB net gain of the SMCDCCF at the wavelength of 1400 nm were obtained. The gross and net gains are the highest yet reported of the SMCDCCFs. Further development on higher gain of the SMCDCCF may be functioned the SMCDCCF as a broadband fiber amplifier for use in the next-generation fiber transmission systems.

Growth of single-crystal YAG fiber optics.

Single-crystal YAG (Y3Al5O12) fibers have been grown by the laser heated pedestal growth technique with losses as low as 0.3 dB/m at 1.06 μm. These YAG fibers are as long as about 60 cm with diameters around 330 μm. The early fibers were grown from unoriented YAG seed fibers and these fibers exhibited facet steps or ridges on the surface of the fiber. However, recently we have grown fibers using an oriented seed to grow step-free fibers. Scattering losses made on the fibers indicate that the scattering losses are equal to about 30% of the total loss.

Growth of single-crystalline strontium titanate fibers using LHPG

Strontium titanate is an incipient ferroelectric material of the perovskite structure and high melting point (2080 °C). It is a material which has a remarkable chemical and thermal stability, and has an interesting potential for the development of applications, particularly in manufacturing laser diodes, sensors oxygen at elevated temperatures, UV sensors, and optical sensors, among other possibilities of development new optical devices. We present growth results SrTiO3 crystal fiber by the Laser Heated Pedestal Growth technique .

Resonance Raman spectroscopy of NdAlO3 single-crystal fibers grown by the laser-heated pedestal growth technique

NdAlO3 is a lanthanide monoaluminate compound presenting a trigonal structure with D3d6 space group. Single crystal fibers of NdAlO3 were grown by the laser-heated pedestal growth (LHPG) technique. The cylindrical fibers grew parallel to [100]h direction without cracks or striations. At room temperature, the Raman spectra presented an enlarged number of the measured bands. Raman bands associated to the D3d6 structure were compared with the prediction of lattice dynamic calculations performed using Wilson's GF-matrix method. By using group-theory calculations, it was possible to suggest a reduction in crystal symmetry induced by the Nd/Al non-stoichiometric ratio. The C3v6 and S62 phases are the best choices to describe the unit cell of our Al-deficient crystal fiber in place of D3d6 phase.

Cerâmica transparente de Y2O3 obtida por sinterização rápida em forno de radio frequência sob atmosfera de vácuo dinâmico

Monocristais de ítria (Y2O3) são importantes materiais hospedeiros para lasers. Entretanto, seu elevado ponto de fusão (2450 ºC) dificulta o processo de crescimento de monocristais. Uma alternativa para a obtenção de ítria policristalina com propriedades similares às dos monocristais é a fabricação de cerâmicas transparentes. Desta forma, este trabalho teve o propósito de desenvolver uma metodologia para fabricar cerâmicas transparentes de ítria, utilizando-se um forno de indução eletromagnética que permite o uso controlado de atmosferas inertes ou redutoras e pode atingir temperaturas próximas a 2000 ºC, com taxas de aquecimento maiores que 100 ºC/min. Os pós-precursores foram preparados por meio de reação do estado sólido usando 3% em mol de ZrO2 como aditivo. As amostras foram conformadas em discos com 12 mm de diâmetro e 1 mm de espessura sob uma pressão uniaxial de 50 MPa e isostática de 350 MPa. Em seguida, foram sinterizadas a 1800 e 1900 ºC com tempos de sinterização de 1 e 5 h. Difração de raios X e microscopia eletrônica de varredura foram usadas na caracterização estrutural e microestrutural das amostras e sua transmissividade ótica foi medida por meio de espectrofotometria. Através da sinterização em forno de indução foi possível obter cerâmicas com até 60 % de transmissão em linha na região de 800 nm em tempos de sinterização menores que os descritos na literatura.

Crucibleless crystal growth and Radioluminescence study of calcium tungstate single crystal fiber

In this article, single phase and high optical quality scheelite calcium tungstate single crystal fibers were grown by using the crucibleless laser heated pedestal growth technique. The as-synthesized calcium tungstate powders used for shaping seed and feed rods were investigated by X-ray diffraction technique. As-grown crystals were studied by Raman spectroscopy and Radioluminescence measurements. The results indicate that in both two cases, calcined powder and single crystal fiber, only the expected scheelite CaWO4 phase was observed. It was verified large homogeneity in the crystal composition, without the presence of secondary phases. The Radioluminescence spectra of the as-grown single crystal fibers are in agreement with that present in Literature for bulk single crystals, presented a single emission band centered at 420nm when irradiated with β-rays.

Silica cladded Nd^3+:YAG single crystal core optical fiber and its submicron residual stress detection

A Nd3+:YAG single crystal core optical fiber waveguide was successfully fabricated by the sapphire tube-assisted CO2 laser-heated pedestal growth (LHPG) technique with decent quality. Furthermore, a stress detection (mapping) technique with submicron spatial resolution was demonstrated with Nd3+ as distributive stress detection probe and scanning near field optical microscope (SNOM) as detection tool.

Simple and efficient defect-tailored fiber-based UV-VIS broadband white light generation

We propose and demonstrate a facile approach for ultraviolet-visible broadband generation from a sapphire crystal core-borosilicate glass cladding hybrid fiber using a laser-heated pedestal growth technique. Considerable formation of F- and F(2)-type color emitters is effectively facilitated by Ti(4+) ions and Al(3+) vacancies, retaining efficient luminescence and high crystallinity of the sapphire core. These color centers intensify the ultraviolet, blue, and green emissions at 370, 450, and 540 nm, whereas the 650-nm red emission is contributed by Cr(3+) in the octahedral sites of the corundum structure. Over 1-mW white light with an optical-to-optical efficiency of up to nearly 5% and 1931 Commission International de l'Eclairage chromaticity coordinate of (0.287, 0.333) is achieved under 325-nm excitation.

ITO electrode-embedded double-cladding single-crystal LiNbO_3 optical fiber

In this Letter, we demonstrate a way to make single-crystal LiNbO(3) optical fibers with silica glass cladding and In(2)O(3)-SnO (ITO) electrodes embedded between core and cladding with a one-step drawing process with the CO(2) laser-heated pedestal growth technique. A half-wave voltage (Vπ) of 6.6 V, effective electro-optic coefficient of 23.6 pm/V, and transmission loss of 0.89 dB/cm was achieved in the single-crystal LiNbO(3) optical fiber with core diameter of 9 μm and electrode distance of 50 μm. Furthermore, the technique can be applicable to make other single-crystal core optical fibers with ITO-embedded electrodes, which could potentially enable many electrically tunable fiber devices along with the unique property of the single-crystal core.

Few-Mode Cr-Doped Crystalline Core Fibers for Fiber Amplifier

A few-mode Cr-doped crystalline core fiber (FMCDCCF) is demonstrated. The fiber fabrication is a modified laser-heated pedestal growth (LHPG) technique that consists of over cladding a Cr:YAG crystal rod with silica and then fine tuning its core size with multiple core reduction through the LHPG system. The FMCDCCF exhibits a core diameter of 2 μm and a <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> -value around 3.77 to provide the few-mode characteristics. The optical properties of the FMCDCCF are similar to that of a pristine Cr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> :YAG crystalline. A gross gain of the FMCDCCF is measured around 2.3 dB at a wavelength of 1.4 μm.

High-resolution transmission electron microscopy analysis of the microstructures of Cr4+:Y3Al5O12 double-clad crystal fibers prepared by LHPG method

In order to unravel the relation of the nanostructure of Cr4+:Y3Al5O12 (Cr4+:YAG) crystal fibers controlled by the laser-heated pedestal growth (LHPG) technique with different growth conditions, the nanostructural analysis of Cr:YAG crystal fiber was investigated by high-resolution transmission electron microscopy (HRTEM), which offers nano-scale information in the core region and inner/outer cladding interface and the mechanism of inter-diffusion process during growth. HRTEM analysis indicates that the core has high crystallinity and a sharp core/inner-cladding interface. γ-Al2O3 nanocrystals were formed during YAG/SiO2 inter-diffusion. To further enhance the Cr4+ concentration in YAG crystal fiber, the Cr4+ ions were replenished using an electron gun to deposit Cr2O3 and divalent-ion oxide (CaO or MgO) on the source rod circumference before LHPG. It was observed that Ca2+ diffuses into the source rod more efficiently than Mg2+, while generating fewer defects and stacking faults.

Intracavity and resonant Raman crystal fiber laser

We demonstrated an efficient, compact, and continuous-wave Raman crystal fiber laser (RCFL) using an intracavity and resonant χ(3) approach. The gain and nonlinear medium was Cr4+:Y3Al5O12 double-clad crystal fiber grown using the codrawing laser-heated pedestal growth technique. The RCFL threshold was only 50 mW, and the slope efficiency reached 14.3% above a pump power of 350 mW. The result is in good agreement with theory, which indicates a near-100% quantum efficiency of resonant stimulated Raman scattering.