An optical amplifier comprises an erbium-doped, silica-based optical fibre (6) coupled to a second optical waveguide, comprising a fibre (4) having a D-shaped cross-section. The second optical waveguide (4) has an overlaid grating (10) formed on a silica wafer (9) which is constructed and configured so that the waveguide filter provides loss in transmission substantially at the wavelength of the gain peak of the amplifier. The resonant reflection wavelength is chosen to be substantially equal to, or greater than, the wavelength of the gain peak of the amplified gain spectrum. Transmission losses at wavelengths shorter than the resonant reflection wavelength of the grating act to reduce the gain peak thereby flattening the gain spectrum of the amplifier so providing a larger useful gain bandwidth for optical fibre systems having a cascade of such fibre amplifiers.
