Beam Quality Management in Multi-Stage Side-Pumped Nd:YAG MOPA Laser Systems

A method of beam quality management in multi-stage Master-oscillator power-amplifier (MOPA) laser systems has been presented. Beam quality always gets worse when a laser beam passes though an amplifier due to the thermally induced aberrations in the intensively pumped gain medium. Numerical calculations show that the spherical aberration (SA) coefficient of the aberrated beam wavefront from the amplifier can change the sign in free space propagation. The wavefront can thus be compensated by another amplifier, which has nearly identical pumping parameters with the first amplifier. A single-pass two-stage side-pumped Nd:YAG amplifier is developed in experiments. The seed beam from the oscillator with 42 W output power and beam quality factor of M² = 1.21 is scaled up to 74 W by the first stage of the amplifier, while the beam quality is degraded to M² = 1.80. The beam quality is improved to M² = 1.30 after the second stage by properly wavefront compensation and the output power is 101 W. Experiments are also carried out with a double-pass one-stage amplifier. The output beam with 84 W power and M² = 1.29 is achieved. To our best knowledge, it is the first time that beam quality enhancement in a side-pumped MOPA is demonstrated by SA compensation.