An optical amplifier comprises a printed circuit board (PCB) having at least one support layer of electrically non-conducting material and at least one conductive layer of electrically conducting material, and a foil heater in the form of a conductive track 20 on the PCB. A specified length of erbium optical fibre is wound around an aluminium spool mounted on the PCB and clamped by a clamp with the interposition of a thermal gasket, so that the optical fibre can be heated by the heater. Cutouts are provided in the PCB surrounding the heater to minimise loss of heat from the heater. The form of the heater enables it to be produced during fabrication of the optical fibre without utilising additional costly fabrication steps.