Stable single-axial-mode operation of an unstable-resonator Nd:YAG oscillator by injection locking.

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 pulses and was utilized in measurements.6 However, in previous work, single-mode operation was not obtained at the Fourier-transform limit or with long-term frequency stability. In this Letter we report successful operation of a Q-switched unstable-resonator Nd:YAG oscillator in a stable single axial mode with 50-MHz bandwidth and E5-MHz frequency stability. Output at 150 mJ per pulse at 10-Hz repetition rate in a 9-nsec pulse for a peak power of 16.7 MW was obtained. The key to achieving stable single-axial-mode operation was the application of injection locking in place of the more usual approach of using spectral-selective elements within the resonator cavity. Before the use of injection locking, wve attempted to obtain stable single-mode operation by using intracavity 6talons aided by electronic control of the Q switch. Electronic line narrowing5 permits improved spectral selectivity by providing multiple transits through the line-narrowing 6talons during the controlled microsecond buildup time. Careful attention to 6talon selection and transverse-mode control led to singleaxial-mode operation of the unstable-resonator Nd: YAG oscillator for 95% of the pulses but at a reduced output-pulse energy of 50 mJ.6 Two factors severely limited the axial-mode selection within the unstable-resonator oscillator. The first, a result of the unstable-resonator geometry, is 6taloninduced loss and lack of full spectral selectivity. The second, a result of high pump levels required for efficient unstable-resonator operation, is the thermally induced birefringence in the Nd:YAG medium. The unstable resonator has a spherically diverging wave in all sections of the resonator. The diverging wave is partially attenuated by the tilted 6talon, leading