Patent US 5,331,648

A laser system (10) comprises a microchip laser (11) longitudinally pumped by a laser diode (17). Lens system (18) focusses the pump radiation onto the lasing medium (12) which is in the form of a plate carrying a mirrored surface (13) forming one of the mirrors of the laser (11). The other mirror of laser (11) is formed on a substrate (14) mounted on a piezo-electric ceramic mount (15). Part (21) of the output of the laser (11) is delivered to a cavity (23) which functions as a interferometer the output of which is opto-electric converted at (26) and demodulated at (28) to provide a phase-shifted reference frequency with respect to an oscillator (16) operating at one of the mechanical reference frequencies of the unit formed by components (14) and (15). The reference frequency signal is amplified at (29) and fed with the oscillator output through a coupler (19) to drive the mount (15) so that it is both a frequency modulating and a frequency correcting element.