We have achieved stable Fourier-transform-limited single-axial-mode operation of an unstable-resonator Nd:YAG oscillator. The 10-Hz repetition-rate, 150-mJ, 9-nsec Q-switched Nd:YAG source operated with a measured 50-MHz bandwidth at i5-IvHz frequency jitter when injected with a low-power single-frequency pulse from aTEM 0 oomode Nd:YAG source. The Q-switched Nd:YAG laser operating in a single axial mode with a Fourier-transform-limited bandwidth is a useful source for nonlinear and high-resolution spectroscopic studies. In the past, single-axial-mode operation of the Nd:YAG laser received considerable attention,' and single-mode operation of a TEMNoo-mode Q-switched oscillator has been reported. 2 ' 3 More recently, single-mode operation of the unstable-resonator Nd:YAG source, 4 with its advantage of high peak output power, was obtained 5 for a fraction of the generated

We have invented a quasi-Gaussian profile-transmittance filter based on radially varying the phase retardation in a birefringent element. The radial birefringent element has been applied to resonator design and has demonstrated its usefulness in generating an improved resonator spatial-mode profile.