Injection seeded single frequency Nd:YAG MOPA Q-switched laser

Abstract

This paper discusses the realization of an injection-seeded single frequency Q-switched Nd:YAG ring laser with pulse width adjustability from several nanoseconds to 20 ns and an output energy in excess of 100 mJ from a ring cavity slave oscillator. In the cavity configuration under study, the slave ring oscillator was injected by a pulse formed from a CW single frequency non-planar ring oscillator (NPRO) Nd:YAG laser with a Pockels cell modulator. The injected pulse duration is slightly shorter than the emission round trip time inside the slave laser cavity. The use of this injection seeding results in unidirectional Q-switched laser oscillation with suppression of bidirectional Q-switched oscillation that otherwise may be initiated from spontaneous emission. The advantage of these regime is in stable laser operation with no need in adjustment of the seeded light wavelength and the mode of the cavity because there are never two waves of the same polarization existing inside the cavity at the same location and the same time. The cavity contains two Pockels-cells, in which the first cell serves to open the oscillator cavity and the second one performs cavity dumping, generating a pulse with optimized characteristics and enables variation of the duration of the Q-switched output pulse. Proper selection of the pump parameters and Pockels-cell gating enables further operation of the system in a mode when the Q-switched pulse can be formed only under the seeding conditions. It is found that the wavelength of the Q-switched laser radiation matches well to the injected seed NPRO laser longitudinal mode. By using two-stage amplifiers, an output energy better than 300 mJ has been achieved in a MOPA configuration without active control of the cavity.