High peak power Nd:YAG/Cr:YAG ceramic microchip laser with unstable resonator

A doughnut mode microchip laser was demonstrated by introducing a monolithic ceramic Nd:YAG/Cr⁴⁺:YAG chip in an unstable resonator to deliver laser pulses with an energy of 13.2 mJ and a pulse width of 476 ps, corresponding to a record peak power of 27.7 MW. The laser beam quality was characterized by M2∼6 at 10 Hz repetition rate. No significant degradation or change of beam pattern, pulse width, and M2 was confirmed during energy scaling in the case of the unstable cavity, promising for further brightness improving. In comparison with a flat-flat cavity, pulse broadening and M2 increase was observed up to ∼1.2 ns and ∼10, respectively, during energy scaling up to 18 mJ due to the beam pattern degradation. The doughnut beam was observed to have an Airy disk at the focal point, which was suitable for laser induced breakdown in air. The measured breakdown threshold of doughnut beam was comparable to a near-Gaussian beam (M2=1.3).