Gain flattening filter for optical fibre amplifiers

A gain flattening filter for compensation of at least a portion of the spectral gain profile of an optical amplifier comprises an optical substrate having at least a first gain flattening filter disposed on a first surface of the optical substrate and a second gain flattening filter disposed on a second surface of the optical substrate. The first gain flattening filter has a transmission curve with a spectral loss profile for optical signals in the optical wavelength range, and has a net insertion loss error function relative to the spectral gain profile of an optical amplifier. The spectral loss profile of the second gain-flattening filter corresponds to the net insertion error function of the first gain-flattening filter.