Rare-earth-doped active films: basic problems to solve and hints for solutions

Because of the present success of the Erbium doped fibre amplifiers in telecommunication systems, there is now a trend to look for more compact amplifiers, also based on rare earth doped hosts in wave guide structures, but in the much more compact geometry of short planar wave guides. The shortening of the active length shall be obtain only if one of the following conditions are fuffihled: the active ion concentration-cross-sections product can be increased by three order of magrntude. From basic principle, it is shown that because of the rather weak interaction with the surrounthng, the ion crosssections does not vary a lot when changing the glass host arid that the ion-ion interactions shall fundamentally limit the active ion concentration. Tentative solutions to this problem are presented.