Erbium Host Offers High Doping Levels, Flat Gain

Anne L. Fischer

Researchers at the Universities of Nottingham and Sheffield in the UK and at Universidad de La Laguna on the island of Tenerife in Spain are investigating a new class of material as an erbium host for telecommunications applications. The material, which they call “nano-glass-ceramic,” accepts doping levels of erbium on the order of 45,000 parts per million without concentration quenching, exhibits fairly flat emission in the range of 1530 to 1560 nm and displays the broadest half-height-width emission for erbium in any host to date.

Internet traffic travels along fiber optic cables as streams of light with wavelengths close to 1.55 µm, for which high-silica glass fiber has low optical loss. The erbium-doped fiber amplifiers that are spliced into the network have an emission that coincides with this window, enabling them to be used to boost the strength of these signals and extend transmission distances. However, the high-silica glass can take only so much erbium before concentration quenching of 1.55-µm emission occurs, and the gain spectrum of current erbium-doped fiber amplifiers is rather narrow and is not flat over the window.

The new material is produced by heating the parent oxyfluoride glass above the transformation temperature of approximately 400 °C. This induces homogeneous nucleation and the growth of an erbium-doped nanocrystalline fluoride phase that is dispersed in and, hence, protected by the glass matrix (Figure 1). The researchers have found little extra scattering loss incurred by the nanocrystals and have noted good wavelength divergence of the absorption and emission bands in the nano-glass-ceramic.

Figure 1. High-resolution transmission electron micrographs show nano-glass-ceramics with nanocrystal diameters of ~12 nm (a) and ~ 3 nm (b).

Because this material offers broader, flatter gain than current erbium-doped fiber amplifiers and can accept levels of erbium more than two orders of magnitude higher than silica fiber, they think that nano-glass-ceramic has promise in the development of erbium-doped waveguide amplifiers. Given the doping levels, they suggest that it should be possible to produce amplifiers with active regions only a few millimeters in length, in contrast to the meters that are required in erbium-doped fiber amplifiers.

To that end, the scientists recently demonstrated direct laser writing of erbium-doped nanocrystalline buried channels within the bulk parent glass using focused femtosecond or CW laser radiation (Figure 2). This may pave the way for the development of novel telecommunications components such as 3-D integrated erbium-doped waveguide amplifiers and other integrated 3-D photonic devices.

Figure 2. An optical reflection micrograph shows 1-μm-diameter buried channel features written in nano-glass-ceramic using a Ti:sapphire laser. The image is perpendicular to the write laser beam. These potential waveguides are written first at a depth of 200 μm (parallel bright lines, top to bottom) and then at 100 μm (parallel dark lines, left to right) in a 2.5-mm-thick sample of the parent glass.

Angela B. Seddon, head of the novel photonic glasses research group at the University of Nottingham’s School of Mechanical, Materials and Manufacturing Engineering, noted further that nano-glass-ceramics may enable the development of amplifiers covering the wider transmission window of fiber such as AllWave from Lucent Technologies of Murray Hill, N.J. The high-silica glass host of current erbium-doped fiber amplifiers does not work with other dopants to provide gain across this wider window. The investigators are working to demonstrate that nanoglass-ceramic works with other dopants.