An experiment that failed
The serependious discovery of heavy metal fluoride glasses was the starting point of an extraordinary adventure. It took place in 1974 at Rennes University. The two brothers Michel and Marcel Poulain, researchers in the solid state chemistry department, were investigating new fluoride complexes based on zirconium fluoride. The synthesis process encompassed heating metallic sealed tubes and quenching to stabilize high temperature phases.
When opening the nickel tube, they first observed large crystals that later turned to be amorphous. An amorphous crystal… that was a glass !
This happy accident would have hudge consequences on glass science and technological developments. A first paper was published in 1975, and gradually research effort moved from crystalline materials to these new glasses. Encouraged by Jacques Lucas, the head of the Laboratory, Marcel Poulain developed a new synthesis route using open platinum crucible and in situ fluorination by ammonium fluoride. This reduced synthesis time from 15 hours to less than 30 minutes, while increasing batch size. With PhD students, chemical compositions were systematicly investigated to define optimum glass forming ranges. The stabilizing effect of aluminum fluoride emerged as a key feature, and rapidly enough the ZBLAN glass composition was identified.
On the scientific side, fluoride glasses perturbed the scientific community as they did not fit the classical model of glass structure : they made an exception to the famous « Zachariasen rules ». The potential of fluoride glasses for ultralow optical losses aroused immense interest and intense research effort in USA, Japan and Europe.
The first fluoride glass fibers were drawn in 1976 and a patent was issued one year later. While keeping the management of a University research team, Marcel Poulain decided to make a company to develop these new glasses. Le Verre Fluoré was established with Gwenael Mazé as manager in 1977.
Since then, intense and continuous R & D work was conducted in relation with the University team. 50 cm long tubes were manufactured by rotationnal casting for the french CEA. New glasses based on indium fluoride, scandium and other heavy metal fluorides, were discovered by Michel Poulain and patented in 1980. The financial support of France Telecom allowed large improvements in optical transmission. In 1985 large numerical aperture otical fibers were produced for the NASA Aviris project. Stimulated by the prospects for ultralow optical losses, numerous research groups investigated this field that encompassed material characterization and low loss optical fibers.
The high transparency of fluoride glass fibers in the mid-IR attracted the attention of astronomers. The first coupling of optical telescopes was achieved with LVF fibers. This « FLUOR » project was followed by other astronomical programs that were made possible only by LVF fibers.
President, Emeritus Professor at Rennes University