Molten Core Method Research Articles

Nanoparticle doping and molten-core methods towards highly thulium-doped silica fibers for 0.79 μm-pumped 2 μm fiber lasers – A fluorescence lifetime study

We study the fluorescence lifetime of Thulium-doped silica fibers for efficient laser operation in the 2 μm “eye-safe” wavelength range pumped by 0.79 μm radiation. A large set of optical fibers prepared by Modified Chemical Vapor Deposition combined with solution doping or nanoparticle doping, and Molten-core method is investigated in terms of fluorescence lifetimes of the 3F4 and 3H4 excited levels, which contain information about the Tm3+ ion environment, the rate of unwanted processes, such as concentration quenching or multiphonon relaxation, as well as the inter-ionic energy-transfer processes, including the cross-relaxation. Maximum evaluated lifetime values are 778 μs for the 3F4 level and 56 μs for the 3H4 level. From the lifetime study, we conclude that for Tm3+ concentrations above 10,000 ppm and enhanced aluminum concentrations above 5 mol. % Al2O3, the cross-relaxation efficiency is enhanced and multiphonon relaxation is remarkably reduced due to the lower phonon energy of the resulting alumino-silicate glass.

WITHDRAWN: Comprehensive phase analysis of GaAs:(Cu, Au) systems in molten core fibers

The recently developed “flux assisted molten core method,” used previously to fabricate GaAs core fiber with Sn as a solvent phase, is further extended here to Cu and Au fluxes. Unlike the GaAs:Sn system whose observed phases were same in the precursor, as-drawn, and laser annealed fiber, the GaAs:Cu and GaAs:Au systems exhibited differing phases at each stage of the process. These systems undergo reactions during the molten core fiber fabrication and CO2 laser post-processing. X-ray diffraction is employed to determine the phases resulting from As volatility during the pre-melt, fiber draw, and CO2 laser post-processing. The phases, while unanticipated, are determined to be thermodynamically stable. In some of the systems, remnant Ga is suggested as being located in the flux compounds. This is a break with the conventional molten core method, where the fiber consists of the same phases as the starting material, and cases where reactions are driven in the molten core. The volatility of GaAs brings about unexpected reactions.

High-efficiency single-frequency DBR fiber laser at 1091 nm utilizing a Yb:YAG crystal-derived silica fiber

A single-frequency distributed Bragg Reflector (DBR) fiber laser operating at 1091 nm was demonstrated by using a Yb:YAG crystal-derived silica fiber (YDSF). The YDSF was prepared via the molten core (MC) method, with a Yb2O3 doping concentration of 5.60 wt.% in the core, resulting in a gain coefficient of 1.45 dB/cm at 1091 nm. Employing 0.8 cm of the YDSF, we attained a single-frequency laser with a maximum output power of 145 mW and a slope efficiency of 31.8%. The laser exhibited an optical signal-to-noise ratio (OSNR) exceeding 71 dB, a linewidth of ∼34 kHz, and a stabilized relative intensity noise (RIN) at -132 dB/Hz for frequencies over 4.5 MHz. The fiber laser could serve as an outstanding seed source for high-power, narrow-linewidth fiber amplifiers operating at 1091 nm.

Comprehensive phase analysis of GaAs:(Cu, Au) systems in molten core fibers

The recently developed “flux assisted molten core method,” used previously to fabricate GaAs core fiber with Sn as a solvent phase, is further extended here to Cu and Au fluxes. Unlike the GaAs:Sn system whose observed phases were the same in the precursor, as-drawn, and laser annealed fiber, the GaAs:Cu and GaAs:Au systems exhibited differing phases at each stage of the process. These systems undergo reactions during the molten core fiber fabrication and CO2 laser post-processing. X-ray diffraction is employed to determine the phases resulting from As volatility during the pre-melt, fiber draw, and CO2 laser post-processing. The phases, while unanticipated, are determined to be thermodynamically stable. In some of the systems, remnant Ga is suggested as being located in the flux compounds. This is a break with the conventional molten core method, where the fiber consists of the same phases as the starting material, and cases where reactions are driven in the molten core. The volatility of GaAs brings about unexpected reactions.

Broadband 1.0 µm emission in Nd3+/Yb3+ co-doped phosphate glasses and fibers for photonic applications.

In this work, the spectroscopic properties of 1.0 µm emission in Nd3+/Yb3+ co-doped phosphate glasses were systematically investigated under 808 nm excitation. Notably, broadband 1.0 µm emission with a full width at half maximum (FWHM) of 96 nm was obtained in the phosphate glass doped with 2 mol.% Nd2O3 and 1 mol.% Yb2O3. In addition, the energy transfer microscopic parameter and transfer efficiency were analyzed. What is more, multimaterial fibers with Nd3+/Yb3+ co-doped phosphate glass core and silicate cladding were successfully drawn by using the molten core method. An intense 1.0 µm amplified spontaneous emission (ASE) can be realized in a 3 cm long multimaterial fiber. More importantly, the FWHM of the ASE can reach as large as 60 nm when excited at 976 nm. These results demonstrate that the Nd3+/Yb3+ co-doped phosphate glasses and fibers are promising gain materials for amplifier and laser applications in photonics.

Dual-Core Ce: YAG-Derived Silicate Scintillation Fiber for Real-Time X-Ray Detection

A dual-core Ce:YAG-derived scintillation fiber (DCSF) was proposed conceptually, simulated numerically, and prepared experimentally by the modified molten core method (MCM) for the first time, of which, the core material is amorphous yttrium aluminosilicate glasses and the cladding material SiO2. Theoretically, it is found that such kind of DCSF exhibits very low inter-core crosstalk with the help of finite element simulation analysis, which may result from the high index difference of ∼0.097 between core and cladding. The average decay lifetime of the prepared DCSF are 31.4 ns and 36 ns at 350 nm and 500 nm, which are shorter than those of its Ce:YAG bulk crystal counterpart. Radiation monitoring was demonstrated by an all-fiber detection system. The radiation response of the DCSF bundle was also investigated, whose radio-luminescence response intensity is ∼1.66 and ∼1.12 times higher than that of the single-core fiber bundles with similar core-diameter and with similar core-area, respectively. In conclusion, with radiation detection performance improvement enabled by space division multiplexing capability, the DCSF not only provides a solution for the dilemma between low transmission loss and high optical yield, but also makes a substantial step towards the development of practical and reliable radiation detectors.

A novel route to fibers with volatile crystalline semiconductor cores part 2: Selenides and phosphides

The flux assisted molten core method, previously employed to fabricate glass-clad crystalline GaAs core fiber, is applied here to realize crystalline selenide and phosphide cores. These semiconductor families show great promise in photonics, optoelectronics, and thermoelectrics, although their volatility has previously prohibited successful fabrication utilizing the conventional molten core method. Through the introduction of a flux, the alloy melt temperature and associated vapor pressure can be sufficiently reduced to permit thermal drawing of the melt inside a glass cladding into fiber. Reported here are demonstrations of crystalline selenide and phosphide cores. Also reported is the CO2 laser post processing of a ZnSe core fluxed with Cu8GeSe6, which resulted in lateral segregation of the ZnSe phase from the flux. X-ray diffraction and electron microscopy were deployed to determine the resulting ZnSe, InP and flux alloy phases and elemental distributions in the fiber core. X-ray diffraction of the InP: flux and most of the ZnSe: flux systems showed intermetallic flux phases that were not present on the phase diagrams reported in the literature. Formation enthalpy and phase diagrams of the intermetallic phases at room temperature were used to determine which observed phases were thermodynamically stable or meta-stable. The results indicate that the influences at play are primarily thermodynamic, but in some cases, kinetics plays a role during the fabrication of these fibers, forming metastable phases. Lastly, a review and discussion of the new demands on cladding glasses is presented for laser post-processing such fibers. This work demonstrates an approach to incorporating volatile and incongruently melting phases, that are not compatible with the standard molten core method, to realize novel in-fiber optoelectronic compositions.

Enhanced broadband NIR emission in glass–ceramic fibers containing Nd3+/Yb3+ co‐doped YCa4O(BO3)3 disordered nanocrystal

AbstractDemands are increasing for ultrashort pulse laser in industrial applications, where the gain bandwidth of most optical fiber material is not wide enough, and developing a wide bandwidth gain medium is challenging. Glass–ceramic fibers containing Nd3+/Yb3+ co‐doped YCa4O(BO3)3 nanocrystals were fabricated by the molten core method and successive heat treatment. After a careful heat treatment, Nd3+/Yb3+ co‐doped YCa4O(BO3)3 nanocrystals were precipitated in the fiber core. Enhanced broadband near‐infrared (NIR) emission from 850 to 1150 nm (bandwidth: ∼252 nm) was obtained in the glass–ceramic fiber compared to that of precursor fiber. These results suggest that the Nd3+/Yb3+ glass–ceramic fibers are promising for broadband NIR optical amplifications and lasers.

Co-drawing of technical and high-performance thermoplastics with glasses via the molten core method

Among the different fundamental aspects that govern the design and development of elongated multimaterial structures via the preform-to-fiber technique, material association methodologies hold a crucial role. They greatly impact the number, complexity and possible combinations of functions that can be integrated within single fibers, thus defining their applicability. In this work, a co-drawing strategy to produce monofilament microfibers from unique glass-polymer associations is investigated. In particular, the molten core-method (MCM) is applied to several amorphous and semi-crystalline thermoplastics for their integration within larger glass architectures. General conditions in which the MCM can be employed are established. It is demonstrated that the classical glass transition temperature compatibility requirements for glass-polymer associations can be overcome, and that other glass compositions than chalcogenides can be thermally stretched with thermoplastics, here oxide glasses are considered. Composite fibers with various geometries and compositional profiles are then presented to illustrate the versatility of the proposed methodology. Finally, investigations are focused on fibers produced from the association of poly ether ether ketone (PEEK) with tellurite and phosphate glasses. It is demonstrated that upon appropriate elongation conditions, the crystallization kinetics of PEEK can be controlled during the thermal stretching and crystallinities of the polymer as low as 9 mass. % are reached in the final fiber. It is believed such novel material associations as well as the ability to tailor material properties within fibers could inspire the development of a new class of hybrid elongated objects with unprecedented functionalities.

(INVITED)Investigation of intense visible defect luminescence from visible and infrared pumped barium fluorosilicate glass-core fiber

Optically active defects in silica have been studied for decades and are often indicators of network irregularities such as those that might result from optical or mechanical damage. They are well-known to be weak emitters and are usually present in relatively low concentration, thus precluding their use in a wide range of applications, including sensing and laser gain. Here, a new paradigm in intense defect emission in the visible wavelength range from a nominally passive optical fiber is presented. Optical fiber starting with 100 mol % BaF2 precursor core material and a pure silica cladding was successfully drawn utilizing the molten core method. These fibers demonstrate an intense, yet unexpected, green photoluminescence peaking near 537 nm (in addition to a second, weaker band near 704 nm) arising from relatively low-power CW pumping in the near-infrared at 976 nm. To understand the origins of this emission, absorption across the optical spectrum is analyzed and photoluminescence via excitation in both the visible and near-infrared wavelength ranges is studied. In addition, Raman spectra, decay lifetimes, magnetization curves, and temperature dependence measurements were collected. The emission spectra maintained a Pekarian-like spectral shape, suggesting an optically active defect as the mechanism behind the green emission. The results presented herein point towards the most likely origin being silanone or dioxasilyrane groups usually associated with surface defects. Importantly, such fibers, fabricated through less conventional methods and possessing novel compositions, may prove to be key in further extending the range of possibilities in defect engineering to well beyond what was previously thought possible.

3D Laser Engineering of Molten Core Optical Fibers: Toward a New Generation of Harsh Environment Sensing Devices

AbstractAluminosilicate glasses offer wide‐ranging potential as enabling materials for a new generation of optical devices operating in harsh environments. In this work, a nonconventional manufacturing process, the molten core method, is employed to fabricate and study sapphire (Al2O3) and YAG (yttrium aluminum garnet) derived all‐glass silicate optical fibers in which a femtosecond (fs) laser is used to imprint oriented nanostructures inside the fiber cores. Both writing kinetics and thermal stability of the laser‐modified regions are investigated over a wide temperature range (20–1200 °C). The laser‐imprinted modifications in these high alumina‐content fibers exhibit improved thermal stability with respect to commercial pure silica and GeO2‐doped silica analogs. Furthermore, optical devices in the form of Rayleigh backscattering and fiber Bragg grating sensors are fabricated to demonstrate the high‐temperature sensitivity and stability of these nonconventional fibers. This functionalization of aluminosilicate fibers by fs‐laser direct writing opens the door to a new generation of optical devices suitable for high‐temperature operation.

A Novel Route to Fibers with Incongruent and Volatile Crystalline Semiconductor Cores: GaAs

A novel extension to the molten core method was employed to realize a glass-clad, crystalline gallium arsenide (GaAs) core fiber for the first time to the author’s knowledge. GaAs is of considerable interest for its optoelectronic and nonlinear properties, but decomposition and volatility, usually leading to incongruent solidification, normally preclude melt-processing under ambient conditions into an optical fiber. Presented here are results on tin as a flux to melt GaAs at a temperature where volatility is negligible, permitting the thermal drawing of long lengths (100 m) of glass-clad, crystalline GaAs-containing fibers. Laser annealing is employed to segregate the Sn and GaAs phases, both across and along the fiber. This work presents an important new tool for scalable fabrication of glass-clad crystalline core materials previously thought incompatible with molten core processes and opens the door to a wide range of novel and useful in-fiber optoelectronic devices.

Nd3+ doped multi-component phosphate glass multi-material fiber for a 1.05 μm laser

Nd3+ doped multi-component phosphate glass was prepared by the conventional melt-quenching method. The absorption spectrum, emission spectrum, and fluorescence lifetime of the prepared glass were measured. Based on the absorption spectrum, Judd-Ofelt (J-O) intensity parameters, spectroscopic quality factor, absorption and emission cross-sections, and figure of merit were calculated and analyzed. What is more, multi-material fibers with Nd3+ doped multi-component phosphate glass core and silicate glass cladding were successfully drawn by using the molten core method. The Nd3+ doping concentration reaches as high as 3.82 × 1020 ions/cm3 (3.86 wt.%). More importantly, 1.05 μm amplified spontaneous emission (ASE) was realized in a 4-cm-long as-drawn multi-material fiber when pumped by an 808 nm laser diode (LD). These results suggest that the Nd3+ doped multi-component phosphate glass multi-material fibers are promising gain material for 1.05 μm fiber laser applications.

Concentration quenching and clustering effects in Er:YAG-derived all-glass optical fiber

A detailed investigation of concentration quenching and ion clustering effects in Er:YAG-derived all-glass optical fibers fabricated using the molten core method (MCM) is presented. Fibers are drawn from four precursor Er:YAG crystals, each possessing a different Er3+ concentration. The resulting fibers exhibited active ion densities ranging from 2.58 × 1025 m-3 to 19.5 × 1025 m-3. Compositional and refractive index profiles (RIPs) are shown to be uniformly graded across the fibers, for a given core diameter, facilitating the study of the impact of draw and host composition on rare earth spectroscopy, a first to the best of the Author’s knowledge. Measurements of the fluorescence lifetimes indicate some degree of clustering persists in all fibers; however, its reduction can clearly be correlated to an increase in sesquioxide (Al2O3 and Y2O3) concentration. Similarly, the critical quenching concentration is also revealed to increase with increasing sesquioxide concentration and ranged from 23.9 × 1025 m-3 to 40.4 × 1025 m-3 in the present fibers. Finally, emission and absorption spectra were found to be practically indistinguishable between the various fibers, with a zero-concentration radiative lifetime determined to be around 8.3 ms. Compared with other silica-based hosts, this lifetime is slightly lower, giving rise to proportionately higher cross-sections.

Preparation of Er:YAG Crystal-Derived All-Glass Silica Fibers For a 1550-nm Single-Frequency Laser

YAG crystal-derived all-glass silica fibers (YDSFs) with different doping concentrations of Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> were fabricated using the molten-core method. A maximum gain coefficient of 1.46 dB/cm was obtained when an Er:YAG rod at a doping concentration of 5 at.% was used as the precursor material. To the best of our knowledge, this is the highest gain coefficient at the 1.5-μm band in similar YDSFs. The diffusion between the Er:YAG core and silica cladding was investigated during the whole preparation process. Based on the homemade Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped YDSF, a 1.55-μm pulsed single-frequency YDSF laser was demonstrated for the first time, delivering an output power of 24.2 mW when the absorbed pump power was 174 mW. The corresponding slope efficiency was 15.1%. A single-pulse energy of ~32.7 nJ was obtained with a pulse duration of 78 ns and a repetition rate of 739 kHz. The signal-to-noise ratio was higher than 75 dB.

Silicate-clad heavily Yb3+ doped phosphate core multimaterial fiber with a high gain per unit length for mode-locked fiber laser applications.

Silicate-clad heavily Yb3+ doped phosphate core multimaterial fiber (MF) was successfully drawn by using a molten core method, which has a high gain per unit length of 5.44 dB/cm at 1.06 µm. What is more, an all-fiber-integrated passively mode-locked fiber laser based on a 5 cm long MF was built. The mode-locked pulses operate at 1055 nm with a period of ∼555ps, and the fundamental repetition rate is 1.787 GHz. For the first time, to the best of our knowledge, we demonstrate the realization of a mode-locked fiber laser with a gigahertz fundamental repetition rate based on a silicate-clad heavily Yb3+ doped phosphate core MF.

Single crystal tellurium semiconductor core optical fibers

Tellurium (Te) semiconductor core optical fibers with silicate glass cladding were drawn by the molten core method. The as-drawn precursor fiber has a large core diameter of about 123 µm, which was found to be polycrystalline. What is more, a Bridgman-type fiber postprocessing technique was constructed and used for the first time to anneal the polycrystalline Te semiconductor core optical fibers. The Te core in precursor fiber was melted and recrystallized to single crystal Te with c-axis orientation parallel to fiber axis, which was confirmed by X-ray diffraction, single crystal X-ray diffraction, micro-Raman spectra, and transmission electron microscope measurement results. Enhanced conductivities were observed in single crystal Te semiconductor core optical fibers under illuminated and stress states, respectively. This work demonstrates that the Bridgman-type fiber postprocessing technique could be an effective way to fabricate single crystal semiconductor core optical fibers with large core diameters (∼ 100 µm) and long lengths (a few centimeters).

CO2 laser annealed SiGe core optical fibers with radial Ge concentration gradients

CO2 laser annealing of SiGe core, glass-clad optical fibers is a powerful technique for the production of single-crystal cores with spatially varying Ge concentrations. Laser power, laser scan speed and cooling air flow alter the Ge distribution during annealing. In this work, near-single crystal fibers exhibiting a central axial feature with peak Ge concentration ∼15 at% higher than the exterior of the semiconductor core have been prepared. Preferential transmission of near infrared radiation through the Ge-rich region, and spectral data confirm its role as a waveguide within the semiconductor core. This proof-of-concept step toward crystalline double-clad structures is an important advancement in semiconductor core optical fibers made using the scalable molten core method.

Tm:YAG ceramic derived multimaterial fiber with high gain per unit length for 2 µm laser applications.

In this work, Tm:YAG (Tm:${{\rm Y}_3}{{\rm Al}_5}{{\rm O}_{12}}$Y3Al5O12) ceramic-derived multimaterial fiber was fabricated by using the molten core method, which has a high gain per unit length of 2.7 dB/cm at 1950 nm. To our knowledge, this is the highest gain per unit length at 2 µm band in similar Tm:YAG-derived multimaterial fibers. A distributed Bragg reflector (DBR) fiber laser was built based on a 10-cm-long as-drawn fiber. The achieved 1950 nm laser, which has a maximum output power of $\sim{240}\;{\rm mW}$∼240mW and a slope efficiency of 16.5%, was pumped by a self-developed 1610 nm fiber laser. What is more, an all-fiber-integrated passively mode-locked fiber laser based on the 10-cm-long as-drawn fiber was realized. The mode-locked pulses operate at 1950 nm with duration of $\sim{380}\;{\rm ps}$∼380ps, and the repetition rate is 26.45 MHz. The results described here indicate that the Tm:YAG ceramic-derived multimaterial fiber with high gain per unit length has promising applications in 2 µm all-fiber fiber lasers.

Powder-in-Tube Reactive Molten-Core Fabrication of Glass-Clad BaO-TiO2-SiO2 Glass-Ceramic Fibers.

In this paper we report the fabrication of glass-clad BaO-TiO2-SiO2 (BTS) glass–ceramic fibers by powder-in-tube reactive molten-core drawing and successive isothermal heat treatment. Upon drawing, the inserted raw powder materials in the fused silica tubing melt and react with the fused silica tubing (housing tubing) via dissolution and diffusion interactions. During the drawing process, the fused silica tubing not only serves as a reactive crucible, but also as a fiber cladding layer. The formation of the BTS glass–ceramic structure in the core was verified by micro-Raman spectroscopy after the successive isothermal heat treatment. Second-harmonic generation and blue-white photoluminescence were observed in the fiber using 1064 nm and 266 nm picosecond laser irradiation, respectively. Therefore, the BTS glass–ceramic fiber is a promising candidate for all fiber based second-order nonlinear and photoluminescence applications. Moreover, the powder-in-tube reactive molten core method offers a more efficient and intrinsic contamination-free approach to fabricate glass–ceramic fibers.