Quick note to John and Simon. A cladding grating with palladium and rhodium catalysts can give optical fibre feed back of cars catalytic converters. As part of a total auto-photonic solution with distributed sensors it could make sense. Gadolinium is an interesting rare earth dopant due to its magnetic properties - just checked there are now lots of papers on it. MP4 recording     MP3 recording -- -- Your browser does not support the audio element.