The invention relates to sources of sub-picosecond optical pulses based on single-pass or double-pass optical amplifiers with an optical gain bandwidth in the 2-20 nm range. A passive pulse shaping filter is provided in front of the optical amplifier for pre-shaping seed optical pulses so as to passively pre-compensate for the gain narrowing effect in the optical amplifier. The passive pulse shaping filter may be based on a reflective thin film filter, which may be coupled to a mirror in a multi-pass configuration.