A fluid-cooled laser amplifier module (100) is disclosed which comprises: a casing; a plurality of slabs (110) of optical gain medium oriented in parallel in the casing for cooling by a fluid stream (154, 156); a polarisation rotator (120) disposed between a first group of one or more slabs (111) of the optical gain medium and a second group of one or more slabs (112) of the optical gain medium; optical windows (150, 152) for receiving an input beam or pulse (130) for amplifying by the slabs and for outputting the amplified beam or pulse (140); and fluid stream ports (155, 157) for receiving and discharging the fluid stream for cooling the slabs.