Laser apparatus, comprising: a laser resonator including a multilayer semiconductor gain-structure, the gain-structure being energized by optical pump radiation, thereby causing fundamental-frequency radiation to circulate in the laser resonator, the fundamental frequency being one of a range of possible oscillating frequencies dependent on an instant length of the laser-resonator; an optically nonlinear crystal located in the laser-resonator and arranged for type-II frequency doubling of the circulating fundamental-frequency radiation, the optically nonlinear crystal having an acceptance bandwidth in a predetermined temperature range; a heating element arranged to selectively vary the temperature of the crystal within the predetermined temperature range; a first birefringent filter located in the laser resonator, configured and arranged to restrict the range of possible fundamental frequencies that can oscillate within the gain-bandwidth to a range thereof within the acceptance bandwidth of the optically nonlinear crystal and wherein the optically nonlinear crystal functions as a second birefringent filter having a transmission peak dependent on the crystal temperature, characterized in that said laser apparatus further comprises: a detector arranged to monitor fundamental-frequency radiation reflected from the first birefringent filter, the reflected radiation being at a minimum when a transmission peak of the second birefringent filter is at an instant one of the possible oscillating frequencies; and a controller configured to receive error signals from the detector and configured to control the heating element in response thereto in a manner to maintain the monitored reflection at about a minimum, thereby maintaining the transmission peak of the second birefringent filter at about the instant oscillating frequency. a laser resonator including a multilayer semiconductor gain-structure, the gain-structure being energized by optical pump radiation, thereby causing fundamental-frequency radiation to circulate in the laser resonator, the fundamental frequency being one of a range of possible oscillating frequencies dependent on an instant length of the laser-resonator; an optically nonlinear crystal located in the laser-resonator and arranged for type-II frequency doubling of the circulating fundamental-frequency radiation, the optically nonlinear crystal having an acceptance bandwidth in a predetermined temperature range; a heating element arranged to selectively vary the temperature of the crystal within the predetermined temperature range; a first birefringent filter located in the laser resonator, configured and arranged to restrict the range of possible fundamental frequencies that can oscillate within the gain-bandwidth to a range thereof within the acceptance bandwidth of the optically nonlinear crystal and wherein the optically nonlinear crystal functions as a second birefringent filter having a transmission peak dependent on the crystal temperature, characterized in that said laser apparatus further comprises: a detector arranged to monitor fundamental-frequency radiation reflected from the first birefringent filter, the reflected radiation being at a minimum when a transmission peak of the second birefringent filter is at an instant one of the possible oscillating frequencies; and a controller configured to receive error signals from the detector and configured to control the heating element in response thereto in a manner to maintain the monitored reflection at about a minimum, thereby maintaining the transmission peak of the second birefringent filter at about the instant oscillating frequency.