An optical system for narrowing the bandwidth of a beam of incident radiation includes an optical mixing unit configured to receive a first beam of radiation and a second beam of radiation. The first and second beams are spatially chirped and the first beam is inverted relative to the second beam. The optical mixing unit includes an optical mixing member, which includes a nonlinear optical medium to mix the first and second beams into an output beam having a narrower bandwidth than the beam of incident radiation.