In-line singlemode optical amplifiers have been constructed using a re
sonant coupling of light power from polished singlemode fibre to plana
r multimode waveguide with a laser-active LiF:F-2(+) or LiF:F-2(-) col
our centre crystal superstrate. The F-2(+) (or LiF:F-2(-)) luminescenc
e was produced in the superstrate by excitation with an evanescent wav
e from the waveguide. A relative gain greater than or equal to 2 was o
btained using these crystals.