A pulsed laser is disclosed comprising an active lasing medium (1), an output coupler (5) and a rear cavity mirror (6). The rear cavity mirror (6) is attached to a mount (21) via an intermediate attachment (15). Heating elements are embedded within the attachment (15). The attachment (15) is heated in order to vary the physical length of the laser cavity in a manner which seeks to stabilise the output energy and wavelength of the laser pulses emitted from the laser cavity.