Watt-level, efficient and radially polarized Nd:YAG laser

Radially polarized light attracts much attention because of its applications in high-precise metal cutting, optical tweezers and other fields. Many schemes have been developed to force the radial-polarization excitation in laser resonator by employing various intracavity polarization selectors, such as birefringent crystal and/or conical Brewster window [1]. These methods, however, are applicable only in a limited range of laser output power [1], and encounter big challenges in improving the laser efficiency, beam's cylindrical symmetry and polarization purity simultaneously. In recent years the photonic crystal grating (PCG), one kind of three-dimensional sub-wavelength gratings, appeared as an effective polarization-dependent element which can provide the specified reflectivity/transmission to the radial polarization and thus can play as the output coupler of laser resonator for desired polarization. Such grating mirror shows unique superiority when combining with compact and end-pumped microchip laser in providing high polarization purity and excellent modal symmetry as well as high laser efficiency embodied in the radial-polarization emission [2]. However, the thermal lens effect in the end-pumped microchip gain medium remained as an obstacle for further power scaling, and so far the output power from such laser is kept in the hundreds milliwatts level. An efficient, high-power and compact solid state laser that emits radially polarized light with high polarization purity would be extremely desirable.