Problem of increasing the brightness in doubling the frequency of laser radiation

The brightness of the output radiation obtained as a result of efficient power conversion to the second harmonic is shown to be governed by the ratio of the geometric component of the beam divergence (i.e., of the characteristic divergence angle of the wave normals) to the diffraction-limit divergence at the entry to a nonlinear crystal where the second harmonic is generated. Experimental results are used to demonstrate that 88% of the power is converted in a KDP crystal and that two regimes are possible (depending on the wavefront curvature): conservation of the radiation brightness and its increase by a factor of 3.5. A simple laser system is described for demonstrating this effect and capable of producing a second harmonic of 5 × 10¹⁷ W · cm^–2 · sr^–1 brightness.