An erbium doped block of glass has input port and reflective end faces arranged such that a signal is launched into the block and is amplified as it traverses the block following a zig-zag path. A laser diode pump is focused to excite erbium ions within the block thereby amplifying the input signal light traversing the block numerous times. A gain flattening filter flattens the gain of the signal being amplified numerous times as the filter is within the path upon each pass across the block.