Enhancement of the Mode Area and Modal Discrimination of Microchip Lasers Using Angularly Selective Mirrors

Angularly selective mirrors (ASMs) are proposed as a means to expand the mode area and modal discrimination of microchip lasers. ASMs used as output couplers selectively reflect incoming k vectors over a narrow angular range, while they transmit more inclined components. The eigenvalue problem of a microchip resonator equipped with a Gaussian ASM is solved analytically in the paraxial optics approximation using the ABCD matrix formalism. The narrow angular distribution of the reflected beam produces, through the laws of diffraction, a significant increase of the mode size and improved transverse mode discrimination, at the expense of higher oscillation threshold due to larger output coupling losses. Simulations performed using the parameters of Yb³⁺-doped YAG material show that one order of magnitude increase of the mode area can reasonably be achieved without causing overheating and thermal fracture. ASMs can be directly deposited on the active material in the form of a resonant grating mirror. This technology involves only planar batch processes that retain the mass production advantage of microchip lasers. The significant increase of brightness of microchips expected from this innovation will give rise to more effective and more compact devices and new applications.