A laser for generating laser light pulses comprises a cavity containing an active optical gain medium (102) and a spectral filtering device (104), a delay device (110) for delaying light by a predetermined delay time (Δt), means (106) for extracting a portion of laser light from the cavity, launching said portion into said delay means (110) and returning the delayed portion to the optical gain medium (102), control means (116) operable to activate the gain medium for a first time period (C1) to produce a first laser light pulse (LP1) having a duration that is less than the delay time (Δt), and activate the gain medium for a second time period (C2) while a said delayed portion of the first light pulse that has been delayed by the delay means (UO) is traversing the gain medium (102), thereby to produce a second laser pulse (LP2) having a shorter duration and faster risetime than the first laser light pulse (LP1), and output means (108) for outputting the second laser light pulse (LP2).