Silica-clad neodymium-doped lanthanum phosphate fibers and fiber lasers

We have fabricated a neodymium-doped phosphate glass fiber with a silica cladding and used it to form a fiber laser. Phosphate and silicate glasses have considerably different glass transition temperatures and softening points making it hard to draw a fiber from these two glasses. A bulk phosphate glass of composition (Nd2O3)(0.011)(La2O3)(0.259)(P2O5)(0.725)(Al2O3)(0.005) was prepared and the resultant material was transparent, free from bubbles and visibly homogeneous. The bulk phosphate glass was drawn to a fiber while being jacketed with silica and the resultant structure was of good optical quality, free from air bubbles and major defects. The attenuation at a wavelength of 1.06 mu m was 0.05 dB/cm and the refractive index of the core and cladding at the pump wavelength of 488 nm was 1.56 and 1.46, respectively. The fibers were mechanically strong enough to allow for ease of handling and could be spliced to conventional silica fiber. The fibers were used to demonstrate lasing at the F-4(3/2) - I-4(11/2) (1.06 mu m) transition. Our work demonstrates the potential to form silica clad optical fibers with phosphate cores doped with very high levels of rare-earth ions (27-mol % rare-earth oxide).

A molecular dynamics model of the atomic structure of dysprosium alumino-phosphate glass

Molecular dynamics (MD) has been used to identify the relative distribution of dysprosium in the phosphate glass DyAl0.30P3.05O9.62. The MD model has been compared directly with experimental data obtained from neutron diffraction to enable a detailed comparison beyond the total structure factor level. The MD simulation gives Dy ... Dy correlations at 3.80(5) and 6.40(5) angstrom with relative coordination numbers of 0.8(1) and 7.3(5), thus providing evidence of minority rare-earth clustering within these glasses. The nearest neighbour Dy-O peak occurs at 2.30 angstrom with each Dy atom having on average 5.8 nearest neighbour oxygen atoms. The MD simulation is consistent with the phosphate network model based on interlinked PO4 tetrahedra where the addition of network modifiers Dy3+ depolymerizes the phosphate network through the breakage of P-(O)-P bonds whilst leaving the tetrahedral units intact. The role of aluminium within the network has been taken into explicit account, and A1 is found to be predominantly (78 tetrahedrally coordinated. In fact all four A1 bonds are found to be to P (via an oxygen atom) with negligible amounts of Al-O-Dy bonds present. This provides an important insight into the role of Al additives in improving the mechanical properties of these glasses.

Titanium phosphate glass microspheres for bone tissue engineering

We have demonstrated the successful production of titanium phosphate glass microspheres in the size range of ~10-200 µm using an inexpensive, efficient, easily scalable process and assessed their use in bone tissue engineering applications. Glasses of the following compositions were prepared by melt-quench techniques: 0.5P2O5-0.4CaO-(0.1 - x)Na2O-xTiO2, where x = 0.03, 0.05 and 0.07 mol fraction (denoted as Ti3, Ti5 and Ti7 respectively). Several characterization studies such as differential thermal analysis, degradation (performed using a novel time lapse imaging technique) and pH and ion release measurements revealed significant densification of the glass structure with increased incorporation of TiO2 in the glass from 3 to 5 mol.%, although further TiO2 incorporation up to 7 mol.% did not affect the glass structure to the same extent. Cell culture studies performed using MG63 cells over a 7-day period clearly showed the ability of the microspheres to provide a stable surface for cell attachment, growth and proliferation. Taken together, the results confirm that 5 mol.% TiO2 glass microspheres, on account of their relative ease of preparation and favourable biocompatibility, are worthy candidates for use as substrate materials in bone tissue engineering applications.

Numerical modeling of holmium-doped fluoride fiber lasers

We combine all the known experimental demonstrations and spectroscopic parameters into a numerical model of the Ho3+ -doped fluoride glass fiber laser system. Core-pumped and cladding-pumped arrangements were simulated for all the population-bottlenecking mitigation schemes that have been tested, and good agreement between the model and the previously reported experimental results was achieved in most but not in all cases. In a similar way to Er3+ -doped fluoride glass fiber lasers, we found that the best match with measurements required scaled-down rate parameters for the energy transfer processes that operate in moderate to highly concentrated systems. The model isolated the dominant processes affecting the performance of each of the bottlenecking mitigation schemes and pump arrangements. It was established that pump excited-state absorption is the main factor affecting the performance of the core-pumped demonstrations of the laser, while energy transfer between rare earth ions is the main factor controlling the performance in cladding-pumped systems.

The structure of the rare-earth phosphate glass(Sm2O3)0.205(P2O5)0.795 studied by anomalous dispersion neutron diffraction

The role of the Sm3+ ions in the structure of vitreous Sm2O3•4P2O5 has been investigated using the neutron diffraction anomalous dispersion technique, which employs the wavelength dependence of the real and imaginary parts of the neutron scattering length close to an absorption resonance. The data described here represent the first successful complete neutron anomalous dispersion study on an amorphous material. This experimental methodology permits one to determine exclusively the closest Sm• •• Sm separation. Knowledge of the R•••R (R = rare-earth) pairwise correlation is key to understanding the optical and magnetic properties of rare-earth phosphate glasses. The anomalous difference correlation function, ΔT''(r), shows a dominant feature pertaining to a Sm•••Sm separation, centred at 4.8 Å. The substantial width and marked asymmetry of this peak indicates that the minimum approach of Sm3+ ions could be as close as 4 Å. Information on other pairwise correlations is also revealed via analysis of T (r) and ΔT (r) correlation functions: Sm3+ ions display an average co-ordination number, n Sm(O), of 7, with a mean Sm–O bond length of 2.375(5) Å whilst the PO4 tetrahedra have a mean P–O bond length of 1.538(2) Å. Second- and third-neighbour correlations are also identified. These results corroborate previous findings. Such consistency lends support to the application of the anomalous dispersion technique to determine separations.

The effect of zinc and titanium on the structure of calcium-sodium phosphate based glass

An array of different structural probes has been used to define the effect of adding Zn and Ti to a sodium-calcium phosphate glass. X-ray absorption spectroscopy at the Zn K-edge suggests that the Zn atoms occupy mixed (4- and 6-fold) sites within the glass matrix. X-ray diffraction reveals a feature at 2.03 angstrom that develops with the addition of Zn and Ti and is consistent with Zn-O and Ti-O near-neighbour distances. Neutron diffraction is used to resolve two distinct P-O distances and highlights the decrease in P center dot center dot center dot P coordination number from 2.0 to 1.7 as the Ti metal concentration rises, which is attributed to the O/P fraction moving away from the metaphosphate value of 3.0 to 3.1 with the addition of Ti. Other correlations, such as those associated with CaO(x) and NaO(x) polyhedra, remain largely unaffected. These results suggest that the network forming P center dot center dot center dot P correlation is most disrupted, with the disorder parameter rising from 0.07 to 0.10 angstrom with the additional modifiers. Zn appears to be introduced into the network as a direct replacement for Ca and causes no structural variation over the composition range studied.

Investigation of ultrafast laser-photonic material interactions: Challenges for directly written glass photonics

Currently, direct-write waveguide fabrication is probably the most widely studied application of femtosecond laser micromachining in transparent dielectrics. Devices such as buried waveguides, power splitters, couplers, gratings, and optical amplifiers have all been demonstrated. Waveguide properties depend critically on the sample material properties and writing laser characteristics. In this paper, we discuss the challenges facing researchers using the femtosecond laser direct-write technique with specific emphasis being placed on the suitability of fused silica and phosphate glass as device hosts for different applications.

Waveguide fabrication in lithium-niobo-phosphate glasses by high repetition rate femtosecond laser:route to non-equilibrium material’s states

We study waveguide fabrication in lithium-niobo-phosphate glass, aiming at a practical method of single-stage fabrication of nonlinear integrated-optics devices. We observed chemical transformations or material redistribution during the course of high repetition rate femtosecond laser inscription. We believe that the laser-induced ultrafast heating and cooling followed by elements diffusion on a microscopic scale opens the way toward the engineering non-equilibrium sates of matter and thus can further enhance Refractive Index (RI) contrasts by virtue of changing glass composition in and around the fs tracks. © 2014 Optical Society of America.

Direct observation of R...R distances in rare-earth (R) phosphate glasses by magnetic difference neutron diffraction

A magnetic difference neutron diffraction study of a rare-earth (Tb) phosphate glass has revealed exclusively the Tb...Tb distances. The difference between data taken with and without an applied magnetic field of 4 T shows Tb...Tb pairwise atomic correlations at 3.9 and 6.4 A, respectively, with relative coordination numbers of 1:14. The first distance arises when two Tb3+ ions share a common oxygen neighbor, and indicates a clustering of rare-earth ions. The second distance arises when two Tb3+ ions are coordinated to different oxygens in the same PO4 group, in a near-linear arrangement.

Structure of dysprosium and holmium phosphate glasses by the method of isomorphic substitution in neutron diffraction

The relative distribution of rare-earth ions R3+ (Dy3+ or Ho3+) in the phosphate glass RAl0.30P3.05O9.62 was measured by employing the method of isomorphic substitution in neutron diffraction and, by taking the role of Al into explicit account, a self-consistent model of the glass structure was developed. The glass network is found to be made from corner sharing PO4 tetrahedra in which there are, on average, 2.32(9) terminal oxygen atoms, OT, at 1.50(1) Å and 1.68(9) bridging oxygen atoms, OB, at 1.60(1) Å. The network modifying R3+ ions bind to an average of 6.7(1) OT and are distributed such that 7.9(7) R–R nearest neighbours reside at 5.62(6) Å. The Al3+ ion also has a network modifying role in which it helps to strengthen the glass through the formation of OT–Al–OT linkages. The connectivity of the R-centred coordination polyhedra in (M2O3)x(P2O5)1−x glasses, where M3+ denotes a network modifying cation (R3+ or Al3+), is quantified in terms of a parameter fs. Methods for reducing the clustering of rare-earth ions in these materials are then discussed, based on a reduction of fs via the replacement of R3+ by Al3+ at fixed total modifier content or via a change of x to increase the number of OT available per network modifying M3+ cation.

Identification of the relative distribution of rare-earth ions in phosphate glasses

The relative distribution of rare-earth ions R3+ (Dy3+ or Ho3+) in the phosphate glass RAl0.30P3.05O9.62 was measured by employing the method of isomorphic substitution in neutron diffraction. It is found that 7.9(7) R-R nearest neighbors reside at 5.62(6) Angstrom in a network made from interlinked PO4 tetrahedra. Provided that the role of Al is explicitly considered, a self-consistent account of the local matrix atom correlations can be developed in which there are 1.68(9) bridging and 2.32(9) terminal oxygen atoms per phosphorus.

High-power diode-pumped fiber laser operating at 3 μm

A high-power diode-cladding-pumped Ho-doped fluoride glass fiber laser operating in cascade mode is demonstrated. The 5|6 -> 5|7 and 5|7 -> 5|8 laser transitions produced 0:77W at a measured slope efficiency of 12.4% and 0:24Wat a measured slope efficiency of 5.2%, respectively. Using a long fiber length, which forced a large threshold for the 5|7 -> 5|8 transition, a wavelength of 3:002 µm was measured at maximum output power, making this system the first watt-level fiber laser operating in the mid-IR.

High-power diode-pumped fiber laser operating at 3 μm

A high-power diode-cladding-pumped Ho-doped fluoride glass fiber laser operating in cascade mode is demonstrated. The 5|6 -> 5|7 and 5|7 -> 5|8 laser transitions produced 0:77W at a measured slope efficiency of 12.4% and 0:24Wat a measured slope efficiency of 5.2%, respectively. Using a long fiber length, which forced a large threshold for the 5|7 -> 5|8 transition, a wavelength of 3:002 µm was measured at maximum output power, making this system the first watt-level fiber laser operating in the mid-IR.

Femtosecond laser writing of buried waveguides in erbium III-Doped oxyfluoride glasses and nano-glass-ceramics

Femtosecond-pulsed laser writing of waveguides, a few mm long, is demonstrated; waveguides were written orthogonally to the writing beam inside the bulk of ErIII-doped oxyfluoride glasses at a depth of 160 mum. The writing beam was 795 nm wavelength, 54 fs pulse duration and 11 MHz repetition rate. Tracks were written at pulse energies of 13.1 nJ to 26.1 nJ and sample translational velocity of 10 mmmiddot.s-1 to 28 mmmiddots-1. The influence of translational velocity and pulse energy on the cross-sectional shape and integrity of the written tracks is reported. Tracks tend to be narrower as the pulse energy is lowered or translational velocity decreased. Above 22.9 nJ, pulse energy, tracks tend to crack. The estimated refractive index profile of one track has a maximum increase of refractive index of 0.003 at the centre. These glasses normally form nano-glass-ceramics on heat treatment just above the glass transformation temperature (Tg). Here, a post-fs-writing heat-treatment just above Tg causes nano-ceramming of the glass sample and removes a light-guiding peripheral region of the fs-written tracks suggesting that this region may have been fs-modified by stress alone. Waveguiding at 651 nm and 973 nm wavelengths, and upconversion, are demonstrated in optimally written tracks.

Femtosecond laser writing of buried waveguides in erbium III-Doped oxyfluoride glasses and nano-glass-ceramics

Femtosecond-pulsed laser writing of waveguides, a few mm long, is demonstrated; waveguides were written orthogonally to the writing beam inside the bulk of ErIII-doped oxyfluoride glasses at a depth of 160 mum. The writing beam was 795 nm wavelength, 54 fs pulse duration and 11 MHz repetition rate. Tracks were written at pulse energies of 13.1 nJ to 26.1 nJ and sample translational velocity of 10 mmmiddot.s-1 to 28 mmmiddots-1. The influence of translational velocity and pulse energy on the cross-sectional shape and integrity of the written tracks is reported. Tracks tend to be narrower as the pulse energy is lowered or translational velocity decreased. Above 22.9 nJ, pulse energy, tracks tend to crack. The estimated refractive index profile of one track has a maximum increase of refractive index of 0.003 at the centre. These glasses normally form nano-glass-ceramics on heat treatment just above the glass transformation temperature (Tg). Here, a post-fs-writing heat-treatment just above Tg causes nano-ceramming of the glass sample and removes a light-guiding peripheral region of the fs-written tracks suggesting that this region may have been fs-modified by stress alone. Waveguiding at 651 nm and 973 nm wavelengths, and upconversion, are demonstrated in optimally written tracks.