We report high-energy, high-efficiency second harmonic generation in a near-infrared all-solid-state burst-mode picosecond laser at a repetition rate of 1 kHz with four pulses per burst using a type-I noncritical phase-matching lithium triborate crystal. The pulses in each burst have the same time delay (
${\sim}1~\text{ns}$
), the same pulse duration (
${\sim}100~\text{ps}$
) and different relative amplitudes that can be adjusted separately. A mode-locked beam from a semiconductor saturable absorber mirror is pulse-stretched, split into seed pulses and injected into a Nd:YAG regenerative amplifier. After the beam is reshaped by aspheric lenses, a two-stage master oscillator power amplifier and 4f imaging systems are applied to obtain a high power of
${\sim}100~\text{W}$
. The 532 nm green laser has a maximum conversion efficiency of 68%, an average power of up to 50 W and a beam quality factor
$M^{2}$
of 3.5.
5-kW-level Bi-directional High-efficiency Pump and Signal Combiner with Negligible Beam Quality Degradation