(Received April 24, 2006)A frequency stabilized single-mode Q-switched Nd:YAG ring cavity laser was developed for compact andhigh-accuracy spectroscopic sensing systems, such as compact direct-detection Doppler lidars. The averageoutput power was 3.1 W with optical conversion efficiency of 16 % at a pulse repetition frequency of 10 kHz.Injection seeding technique was used for single-mode oscillation, stabilization of the laser frequency wasachieved with the feedback control using a Fabry-Perot interferometer as a frequency discriminator. Highfrequency stability of 0.2 MHz (rms) was realized together with a Fourier limited line width of 4.8 MHz.Key Words : Single frequency, Frequency modulation, Ring laser, Q-switch laser1. IntroductionA laser diode pumped solid state laser exhibits stable and lownoise output characteristics because of its high efficiency andlow thermal loading. Frequency stabilized, single-mode, Q-switched pulsed laser are useful for high-precision spectroscopicsensor systems, such as Dopler laser radars, and for pump sourcesof non-linear optical devices such as optical parametric oscilla-tors. A compact nano-second pulsed output laser systems withhigh-average output power larger than a few W are required forthese applications. A number of techniques have been devel-oped for single frequency oscillation of CW lasers. For example,a high frequency stability of the non-planar ring oscillator (MI-SER) was realized to be a few MHz even in the case of freerunning operation,