A Q-switched laser comprising: a) a resonant cavity, containing therein a gain medium; b) a means for pumping the gain medium to increase an optical gain within the gain medium to produce an optical circulating power at a fundamental optical frequency with a cavity round trip time within the resonant cavity; c) a signal generator producing a gate signal having a gate width, and modulated at a pulse repetition frequency; d) an optical switch within the resonant cavity, responsive to the gate signal received from the signal generator to change the optical switch from a high loss state to a low loss state for a duration of the gate width for producing an optical pulse with an optical pulse width; wherein in the high loss state, energy accumulates within the gain medium to produce an initial optical gain, while the optical circulating power approaches an initial optical circulating power; and wherein in the low loss state, the optical circulating power increases to a peak optical circulating power, thereafter decaying as the energy accumulated within the gain medium diminishes and the optical gain is depleted; e) a nonlinear coupler (97, 107) within the resonant cavity for converting a portion of the optical circulating power at the fundamental optical frequency into a converted optical power, wherein the converted optical power has a converted optical frequency with a converted optical pulse width; wherein the converted optical frequency is a second harmonic of the fundamental optical frequency; characterised in that a nonlinear coupling level, denoted by α, defined as a coefficient of P 2 in a rate equation T RT dP dt = gP − α P 2 satisfies the relationship α gi / 2 < α < α gi ∗ 2, wherein α gi, is defined by the relationship ∝ gi = T RT / E sat ∗ 20 wherein T RT is the cavity round trip time; E sat is an effective saturation energy; g is optical gain, P is the optical circulating power and dP/dt is a first derivative of P with respect to time; and f) a means for coupling the converted optical power out of the resonant cavity. a) a resonant cavity, containing therein a gain medium; b) a means for pumping the gain medium to increase an optical gain within the gain medium to produce an optical circulating power at a fundamental optical frequency with a cavity round trip time within the resonant cavity; c) a signal generator producing a gate signal having a gate width, and modulated at a pulse repetition frequency; d) an optical switch within the resonant cavity, responsive to the gate signal received from the signal generator to change the optical switch from a high loss state to a low loss state for a duration of the gate width for producing an optical pulse with an optical pulse width; wherein in the high loss state, energy accumulates within the gain medium to produce an initial optical gain, while the optical circulating power approaches an initial optical circulating power; and wherein in the low loss state, the optical circulating power increases to a peak optical circulating power, thereafter decaying as the energy accumulated within the gain medium diminishes and the optical gain is depleted; e) a nonlinear coupler (97, 107) within the resonant cavity for converting a portion of the optical circulating power at the fundamental optical frequency into a converted optical power, wherein the converted optical power has a converted optical frequency with a converted optical pulse width; wherein the converted optical frequency is a second harmonic of the fundamental optical frequency; characterised in that a nonlinear coupling level, denoted by α, defined as a coefficient of P 2 in a rate equation T RT dP dt = gP − α P 2 satisfies the relationship α gi / 2 < α < α gi ∗ 2, wherein α gi, is defined by the relationship ∝ gi = T RT / E sat ∗ 20 wherein T RT is the cavity round trip time; E sat is an effective saturation energy; g is optical gain, P is the optical circulating power and dP/dt is a first derivative of P with respect to time; and f) a means for coupling the converted optical power out of the resonant cavity.