Fibre lasers in the mid-infrared regime are useful for a diverse range of fields, including chemical and biomedical sensing, military applications and materials processing. This Review summarizes the different rare-earth cations and host materials used in mid-infrared fibre laser technology, and discusses the future applications and challenges for the field.

Towards high-power mid-infrared emission from a fibre laser

Material-dependent properties influencing the performance of fiber amplifiers are reviewed together with the available data for Er/sup 3+/. The major glass types potentially useful in this application are considered and compared to silica. The topics addressed include quenching processes and the solubility of rare-earth ions, transition strengths and bandwidths at the 1500-nm gain transition, and the characteristics at the 800-, 980-, and 1480-nm pump bands. Aluminum is shown to be an extremely useful codopant for silica, improving its ability to dissolve rare-earth ions and providing desirable spectroscopic properties for Er/sup 3+/. For some of the attributes considered, other glasses have advantages over Al silica, but only with respect to gain bandwidth and pumping performance at 800 nm is significantly better than expected from other glass compositions. >

Erbium-doped glasses for fiber amplifiers at 1500 nm

To utilize the excellent properties of silica (SiO2) glass for a glass laser host, neodymium‐aluminum (Nd–Al) and neodymium‐phosphorous (Nd–P) co‐doped SiO2 glasses were studied. They were prepared by plasma‐torch chemical vapor deposition (CVD). It was found that a doping level less than ten times the number of Nd for the Al co‐dopant and less than about fifteen times for the P co‐dopant was enough to remove undesirable fluorescence properties of Nd‐doped SiO2 glasses and make them suitable for laser application. The clustering Nd ions disperse well in a glass matrix and lasing fluorescence increases. The effects of the Al dopant on the density and Raman spectra were also studied to obtain structural information. On the basis of glass science and solution chemistry, the marked effects of both dopants were explained by the following model. Nd ions can be well incorporated into a SiO2 glass network through co‐dopant oxide forming a solvation shell around the Nd ions. This model leads to an expansible metho...

Aluminum or phosphorus co-doping effects on the fluorescence and structural properties of neodymium-doped silica glass

A new infrared luminescence from bismuth-doped silica glass is discovered. Spectroscopic properties of this glass are different from those of previously reported Bi2+- or Bi3+-doped glasses or crystals. Its luminescence spectrum is wide (full-width at half maximum; about 200 nm), with the peak at 1140 nm with 500 nm excitation. Absorption bands exist between the visible and near-infrared region. The lifetime of bismuth-doped silica glass is 630 µs at room temperature, which is longer than the lifetimes of Bi3+ luminescence reported previously.

https://iopscience.iop.org/article/10.1143/JJAP.40.L279/pdf

Infrared Luminescence from Bismuth-Doped Silica Glass

We take familiar inorganic oxide glasses and non-oxide glasses and the liquids from which they derive to review the current understanding of their atomic structure, ranging from the local environments of individual atoms to the long-range order which can cover many interatomic distances. The structural characteristics of important glasses and melts, like silicates, borates, alumino-silicates, halides and chalcogenides, are drawn from the results of recent spectroscopy and scattering experiments. The techniques include Nuclear Magnetic Resonance (NMR) and X-ray Absorption Fine Structure (XAFS), Neutron Scattering (NS) and Small- and Wide-angle X-ray Scattering measurements (SAXS/WAXS), and are often combined with computer simulation experiments in order to obtain detailed images of structure and diffusion in the glassy as well as in the molten state. We then review the current understanding of relaxation in glasses, liquids and polyamorphic states. This includes phenomenological models and theories of rela...

Inorganic glasses, glass-forming liquids and amorphizing solids

An optical waveguide for transmission of light in the middle infrared band is produced by having its clad part formed of glass consisting preponderantly of germanium dioxide and its core part left unfilled.

Optical waveguide for middle infrared band

Silica glass has not been fully characterized as a host material for glass lasers. To evaluate the intrinsic fluorescent characteristics of Nd-doped SiO2 glass, a plasma torch CVD system was specially designed for the preparation of clear high purity glasses. Both of the observed fluorescences of Nd3+ (4F3/2–4I9/2 and 4F3/2–4I11/2) consist of a fast (τ0.35 µs) and a slow (τ450 µs) decay component. The spectral features and intensity of the former remarkably depend on Nd concentration and preparation conditions, while those of the latter are similar in every sample and its fluorescence peak positions distinctly shift to a wavelength longer than those observed in ordinary multicomponent glasses.

Preparation of Nd-Doped SiO 2 Glass by Plasma Torch CVD

The time resolved fluorescence spectra and their Nd3+ concentration CNd dependence were observed in Nd-doped SiO2 glasses prepared by plasma torch CVD. The 4F3/2 radiative emissions observed consist of a fast (FDC, τm0.35 µs) and a slow (SDC, τm470 µs) decay component. The FDC are characterized by the intensity ratio (4F3/2–4I11/2/4F3/2–4I9/2>1), but the SDC by the inverse ratio and the spectral shift to the longer wavelength. The SDC increases and reaches almost constant values in higher CNd. The FDC can not be observed in very low CNd region, but increases abraptly in higher CNd. The FDC is ascribed to the Nd3+ in the clustering microstructure and the SDC is ascribed to the isolated Nd3+ in crystal field unique to unitary SiO2 glass.

Fluorescence and its Nd 3+ Concentration Dependence of Nd-Doped SiO 2 Glasses Prepared by Plasma Torch CVD

To prepare Nd-doped SiO2 glasses by axial injection plasma torch CVD, a special type of NdCl3 gas generator for axial injection was developed and its optimum operating conditions were examined. The technical advantages and disadvantages of this method were discussed in comparison with previously reported tail injection plasma torch CVD. Although a large difference exists between these two methods, little difference in the fluorescence properties of the resultant glasses was observed.

Ken Kumata

Papers

Aluminum or phosphorus co-doping effects on the fluorescence and structural properties of neodymium-doped silica glass

Optical waveguide for middle infrared band

Preparation of Nd-Doped SiO 2 Glass by Plasma Torch CVD

Fluorescence and its Nd 3+ Concentration Dependence of Nd-Doped SiO 2 Glasses Prepared by Plasma Torch CVD

Preparation of Nd-Doped SiO2 Glasses by Axial Injection Plasma Torch CVD and Their Fluorescence Properties