Yttrium Aluminum Garnet Fibers Research Articles

Supercontinuum generation in single-crystal YAG fibers pumped around the zero-dispersion wavelength.

Yttrium aluminum garnet (YAG) is a common host material for both bulk and single-crystal fiber lasers. With increasing interest in developing optical technologies in the short-wave infrared and mid-infrared wavelength range, YAG may be a potential supercontinuum medium for these applications. Here, we characterize femtosecond laser pumped supercontinuum generation with 1200-2000 nm pump wavelengths (λp) for undoped, single-crystal YAG fibers, which are representative of the normal, zero, and anomalous-dispersion regimes. Supercontinuum was observed over the spectral region of about 0.2 to 1.6λp. Z-scan measurements were also performed of bulk YAG, which revealed little dispersion of the nonlinear index of refraction (n2) in the region of interest. The measured values of n2 (∼1×10-6cm2/GW) indicate a regime in which the nonlinear length, LNL, is less than the dispersion length, LD, (LNL≪LD). We report intensity clamping of the generated filament in the normal group velocity dispersion (GVD) regime and an isolated anti-Stokes peak in the anomalous GVD regime, suggesting further consideration is needed to optimize supercontinuum generation in this fiber medium.

Effects of sintering atmosphere on the densification and microstructure of yttrium aluminum garnet fibers prepared by sol-gel process

Yttrium aluminum garnet (YAG) fibers were prepared by a sol-gel method, and then sintered in air or nitrogen atmosphere, respectively. The effects of sintering atmosphere on the densification process and microstructure of YAG fibers were investigated. No obvious difference can be found in the fibers sintered below 800 °C. At 1100 °C, the grain size of YAG fibers sintered in nitrogen is much smaller than in air. This difference is much clearer at the higher temperature of 1200 °C. The fine grains are explained by the existence of residual carbon in YAG fibers, which can be trapped at the grain boundaries to hinder the movement of grain boundary. Meanwhile, the densification degree of fibers sintered in nitrogen is higher than in air at 1200 °C, due to the smaller grain size and higher oxygen vacancy concentration generated in the nitrogen atmosphere, which leads to a higher fiber densification rate.

Preparation, mechanical properties, and diffuse reflectance of YAG continuous fibers and nanofibers

Yttrium-aluminum-garnet (YAG) ceramic fibers not only have good flexibility, a large aspect ratio, good continuity, light weight, and high strength but also a high melting point, stable physical and chemical properties, high-temperature creep resistance, and excellent optical properties of YAG itself. These properties allow YAG fibers to be used as structural reinforcement and as basic optical materials. In this study, a YAG precursor was selected by comprehensive analysis of the spinning properties of solutions synthesized from aluminum powder, AlCl3·6H2O, and Y(CH3COO)3·4H2O. YAG precursor continuous fibers and nanofibers were spun by extrusion-forming and electrospinning, respectively. The decomposition, crystallization, and microstructure of the fibers were analyzed by thermogravimetry-differential thermal analysis (TG-TDA), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The mechanical properties and diffuse reflectance of the YAG ceramic fibers were also determined. The results showed that the YAG precursor synthesized with an AlCl3·6H2O/Al powder molar ratio of 1:4 and Al/Y ratio of 3:5 had the best spinnability. The bundle of continuous fibers was 50 × 40 μm, and the length of the individual fibers was greater than 50 cm. The diameter of as-obtained YAG continuous fibers was approximately 23 μm. Both YAG continuous fibers and nanofibers showed good flexibility, uniform diameters, and high crystallinity. During the heat treatment process, loss of free water and crystalline water, decomposition and carbonization of organic matter, and crystallization and densification of the fibers occurred. YAG was directly crystallized from an amorphous material at 750 °C with a ceramic yield of 32.5%. The tensile stress and Young's modulus of the as-obtained YAG continuous ceramic fibers reached 851 MPa and 150 GPa, respectively. The average diffuse reflectance of the YAG nanofibers in the wavelength range of 700–2500 nm reached 92.7%.

Growth and lasing of single crystal YAG fibers with different Ho3+ concentrations

A method to grow single crystal (SC) yttrium aluminum garnet (YAG) fibers with varied rare-earth ion dopant concentration has been proposed. Crystalline holmium aluminum garnet (HoAG), prepared via sol-gel process, was dip-coated on to previously grown SC YAG fibers. The HoAG coated SC YAG fiber preforms were re-grown to a smaller diameter using the laser heated pedestal growth (LHPG) technique. The final dopant concentration of the re-grown SC fiber was varied by changing the number of HoAG coatings on the preform. 120 μm diameter SC Ho:YAG fibers with four different dopant concentrations were grown. Lasing was demonstrated at 2.09 μm for these fibers. A maximum of 58.5% optical-to-optical slope efficiency was obtained.

Preparation and grain-growth of chromia-yttrium aluminum garnet composites fibers by sol–gel method

The chromia-yttrium aluminum garnet long fibers were prepared by the sol–gel method using aluminum chloride, aluminum powder, yttrium oxide, chromium trioxide, and acetic acid as raw materials. The spinnability of precursor sol increased while the addition content of polyvinylpyrrolidone increased. The precursor sol has excellent spinnability at 40–60 °C. The chromium (VI) was completely reduced to chromium (III) in the preparation process of chromia-yttrium aluminum garnet fibers. A 5 wt% chromium trioxide completely dissolved in solid solution (Y3(Al1-xCrx)5O12). The chromium can hinder grain growth in yttrium aluminum garnet fibers. The grain growth exponent of the fibers was about 2.88 at 1600 °C. The chromia-yttrium aluminum garnet (Cr-YAG) long fibers were prepared by the sol–gel method using aluminum chloride, aluminum powder, yttrium oxide, chromium trioxide (CrO3), and acetic acid as raw material. A 5 wt% CrO3 completely dissolved in solid solution (Y3(Al1-xCrx)5O12). The Cr6+ can change to Cr3+ in the preparation process of Cr-YAG fibers.

Preparation and crystal activation energy of long yttrium aluminum garnet gel fibers

A spinning yttrium aluminum garnet (YAG) precursor sol was obtained by using aluminum chloride, aluminum and yttria powder as raw materials. The spinnability and stability of precursor sol were improved by adding acetic acid. Long gel fiber of 40 cm was prepared by using PVP as spinning additive. The YAG fiber with fine grain was obtained by sintering gel fiber at 1000 °C for 2 h. However, a rough surface with an average grain size of 0.8 μm can be observed when gel fibers were fired at 1500 °C for 2 h. Calculated from Kissinger equation, the crystallization activation energy value (E a) of YAG fibers was 582.6 kJ/mol. A spinning yttrium aluminum garnet (YAG) precursor sol was obtained by using aluminum chloride, aluminum and yttria powder as raw materials. The spinnability and stability of precursor sol were improved by adding acetic acid. Long gel fiber with 40 cm was prepared by using PVP as spinning additive. The YAG fiber with smooth surface was obtained by sintering gel fiber at 1000 °C for 2h. However, a rough surface with an average grain size of 0.8 μm can be observed when gel fibers were sintered at 1500 °C for 2 h. Calculated from Kissinger equation, the crystallization activation energy value (E a ) of YAG fibers was 582.6 KJ/mol.

Development of polycrystalline yttrium aluminum garnet (YAG) fibers

Polycrystalline yttrium–aluminum garnet (YAG) fibers are attractive for high-power lasers and for high-temperature structural materials. Processing methods for <30μm diameter polycrystalline YAG fibers suitable for single-mode laser operation are presented. The methods use extrusion of classified YAG powders with binders. Extrusion rheologies and rates, and heat-treatment temperatures, times, and environments that yield the most dense, defect-free fibers were explored. Fiber tensile testing, followed by fractography, was used to identify defects and to guide determination of optimal processing conditions. The effects of processing variables on fiber microstructures and properties are discussed. Advantages and disadvantages of processing methods are compared.

Predicted performance limits of yttrium aluminum garnet fiber lasers

The effect of various parameters that limit the performance of fiber lasers are calculated for yttrium aluminum garnet (YAG) as the fiber core. The optimum core diameter, the optimum fiber length, and the corresponding output power capability are determined for a variety of optical and structural parameters of YAG, based on available data in the literature to date. It is shown that a conservative limit for YAG fiber laser is 6 kW per fiber in multimode operation, compared to 1.8 kW for a silica-based fiber laser. Furthermore, it is shown that if stimulated brillouin scattering gain, a key limiting parameter, is <10−12 m/W in YAG, as a recent publication suggests, the maximum output power is 30 kW per fiber without a significant increase in fiber core diameter. Similar gains are predicted for single-mode operation, although with much lower output power limits. A parametric study shows the relative merits of improving on various optical parameters.

Nanoindentation of melt-extracted amorphous YAG and YAG:Eu, Nd micrometric fibers synthesized by the citrate precursor method

We report the structural and mechanical characterization of yttrium aluminum garnet fibers obtained with Eu and Nd additions by the melt-extraction method (ME). Powders were synthesized by the modified citrate precursor method. This method produces in a single step of heat treatment the pure YAG phase avoiding the formation of YAlO 3, Y 4Al 2O 9 or Y 2O 3 phases. Rare earth additions delay proportionally to the added quantity of dopant the crystallization temperature of the YAG phase. Using crystalline powders, cylindrical fibers (20–30 μm) in an amorphous state irrespective of doping type and amount were obtained by ME. Nanoindentation tests were carried out showing hardness and reduced elastic modulus of 5.8 ± 0.3 and 79.3 ± 4.9 GPa for pure YAG fibers, respectively. 2 at.% additions of Nd to the YAG phase lead to an increment in hardness and reduced elastic modulus (9.1 and 128.6 GPa, respectively). However, further increments of Eu or Nd (5 at.%) cause a clear degradation of the mechanical properties respective to the values reached with 2 at.% of rare earth additions. This degradation effect seems to be correlated to the atomic interaction due to difference between radii of Eu and Nd, respective to Y.

Preparation, microstructure and properties of yttrium aluminum garnet fibers prepared by sol–gel method

Yttrium aluminum garnet (YAG) fiber was prepared by sol–gel method using water as the solvent. The spinnable YAG sol was synthesized using Al powder, Y(CH3COOH)3·4H2O and HCl as precursors, polyethylene oxide as viscosity adjusting agent. Gel fibers with diameter of 5–10 μm were prepared from the YAG sol by using centrifugal spinning technique. YAG crystalline fibers were obtained by drying gel fibers and heat-treating at selected temperature. TG/DTA analysis showed an exotherm at 906 °C attributed to formation of YAG phase and weight loss of 45% at 1000 °C. XRD and FT-IR analysis showed that phase-pure YAG can be formed at 900 °C, and no other intermediate was observed. The grain size of YAG fibers increased from 25 to 220 nm and tensile strength decreased rapidly from 970 to 380 MPa when the sintering temperature increased from 900 to 1550 °C.

Effect of yttria on crystallization and microstructure of an alumina–YAG fiber prepared by aqueous sol–gel process

Continuous fiber development is needed for high performance and high temperature composites. Various methods have been used to make ceramic fibers. In this research, composite fibers (yttrium aluminum garnet (YAG)/Al 2O 3) were prepared by a sol–gel method using aqueous solution. They were synthesized from aluminum salt, aluminum metal, yttrium oxide and water used as solvent. Transparent gel fibers were obtained by immersing a thin wire into the viscous sol, then pulling it out by hand. The obtained fibers contained very fine grains with diameter ranging from 10 to 80 μm after heat treatment. When yttria content was increased, the crystallization of YAG shifted to a lower temperature, whereas the transformation temperature to α-Al 2O 3 shifted to a higher temperature. Nevertheless, the fibers with different amounts of yttria contained alumina and YAG after heat treatment at 1400 °C. The composite fibers had vermicular structure and were denser than alumina fibers. The yttria percent concerning the limits of this study (≤10 wt%) effected on fiber diameter. As the yttria content was increased, the fiber diameter increased, whereas grain size and densification of the composite fibers decreased.

Therapeutic applications of lasers in urology: an update

There has been renewed interest in the use of lasers for minimally invasive treatment of urologic diseases in recent years. The introduction of more compact, higher power, less expensive and more user-friendly solid-state lasers, such as the holmium:yttrium–aluminum–garnet (YAG), frequency-doubled neodymium:YAG and diode lasers has made the technology more attractive for clinical use. The availability of small, flexible, biocompatible, inexpensive and disposable silica optical fiber delivery systems for use in flexible endoscopes has also promoted the development of new laser procedures. The holmium:YAG laser is currently the workhorse laser in urology since it can be used for multiple soft- and hard-tissue applications, including laser lithotripsy, benign prostate hyperplasia, bladder tumors and strictures. More recently, higher power potassium–titanyl–phosphate lasers have been introduced and show promise for the treatment of benign prostatic hyperplasia. On the horizon, newer and more effective photosensitizing drugs are being tested for potential use in photodynamic therapy of bladder and prostate cancer. Additionally, new experimental lasers such as the erbium:YAG, Thulium and Thulium fiber lasers, may provide more precise incision of soft tissues, more efficient laser lithotripsy and more rapid prostate ablation. This review provides an update on the most important new clinical and experimental therapeutic applications of lasers in urology over the past 5 years.

High performance oxide fibers for metal and ceramic composites

A family of oxide fibers, Nextel™ 610 Ceramic Oxide Fiber, Nextel™ 720 Ceramic Oxide Fiber and a new fiber, Nextel™ 650 Ceramic Oxide Fiber, has been developed specifically for the reinforcement of metal and ceramic matrix composites. This paper summarizes room and high temperature properties for these fibers. The strength of both single filaments and multi-filament rovings of Nextel 610, 650 and 720 fibers was determined between 25 and 250 mm gauge length. Weibull analysis was used to compare the statistical fracture distribution and gauge length dependence of strength. Fiber fracture statistics were in accord with Weibull theory; the effect of diameter variability on the statistical analysis was found to be small. Fractographic analysis on Nextel 610 fiber was used to identify primary fracture-causing defects; defect size was correlated with Griffith fracture predictions. High temperature single filament strength measurements were performed on Nextel 610, 650 and 720 fibers between 800 and 1400°C. High temperature strength varied inversely with strain rate. In combination with tensile creep tests at 1100 and 1200°C, these were used to compare the elevated temperature capability of each fiber and determine maximum use temperatures. The development of crystalline yttrium aluminum garnet fibers that demonstrate further improvements in creep performance relative to Nextel 720 fibers is also discussed.

Polycrystalline Yttrium Aluminum Garnet Fibers from Colloidal Sols

Polycrystalline yttrium aluminum garnet (YAG) fibers were prepared from commercially available colloidal sols of Y2O3 and AlOOH and water‐soluble polymers. The fibers were dry spun and all processing was performed in air. Transformation to YAG was complete by ≅1300°C, and the fibers were mostly dense by 1600°C with a final fired diameter of 120 μm. A bend test was used to characterize mechanical strength, and an average of 522 ± 186 MPa with a Weibull modulus of 3.5 was determined. The bend stress relaxation (BSR) test was used to characterize creep properties. The creep resistance was better than that of all commercially available oxide fibers with the exception of Saphikon single‐crystal alumina (c‐axis oriented). The creep strain of the YAG fibers compared well with that calculated for YAG monoliths with roughly the same grain size.