New methods of mode conversion and beam brightness enhancement in a high-power laser

Radially-polarized beams can be strongly amplified without significant birefringence-induced aberrations. However, further improvement of the beam quality and beam brightness is desirable. We demonstrate two new efficient methods for transformation of the radially-polarized Laguerre-Gaussian beam to a nearly-Gaussian beam with much higher beam quality. The methods are based on separation the radially-polarized mode into two degenerate modes and their coherent addition after the phase flattening. We present detailed analysis and compare the two methods. With one of the methods, we transformed a high-power radially-polarized (0, 1)∗ Laguerre-Gaussian beam with M2 = 2.52 and power of 30 W, to a nearly-Gaussian beam with M2 = 1.3. As a result, the laser beam brightness increased by a factor of ∼2.5.