High-quality laser emission is one of the most critical parameters for obtaining high-intensity focusing in a nonlinear medium,1,2 but this requirement can be met through a suitable design of the laser resonator. Our study is aimed at producing efficient second-harmonic generation (SHG) of copper vapor laser (CVL) radiation in β-barium-borate (BBO). In our experiments, we have arranged a self-filtering unstable resonator (SFUR), which provides diffraction-limited operation (110 µrad at 511 nm in a 12-mm-diameter beam) with 50% average power extraction (2.5 W respect to 5 W of the stable resonator) and 100% peak power extraction (25 kW).3 The coupling geometry was based on the single-pass generation scheme: the green component (511 nm) of the SFUR beam, polarized in the horizontal plane by an intracavity plate polarizer and selected by two dichroic beam splitters, was focused into a BBO crystal of 4 × 5 × 6.5-mm, sealed in a cell with antireflection-coated quartz windows. Focusing was tested with lenses of different focal lengths, ranging from 20-100 cm. An ultraviolet-grade, uncoated quartz prism was used to separate the fundamental and harmonic beams. Thermal detuning or stresses were induced in the crystal by relatively high average power. To investigate SHG's prevention of these disturbance effects the pump repetition rate was externally reduced (by means of a chopper) from the typical operative value of 5 kHz down to 30 Hz. In this condition, we monitored the energy and the pulse shape of both fundamental and SHG beams to evaluate the conversion efficiency in terms of pulse energy and peak power.

© 1991 Optical Society of America

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