A highly efficient end-pumped microchip laser has for the first time been engineered to emit radially polarized and pulsed light

　 Researchers at Shanghai Institute of Optics and Fines Mechanics (SIOM/China) and University of Electro-Communications (UEC/Japan) have generated radially polarized pulse from an end-pumped and passively Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) microchip laser for the first time. According to the group, the end-pumped microchip configuration provides high laser efficiency and excellent modal symmetry control as well as the energetic pulse when being combined with Q-switching technique. [Optics Letters 33 (22), 2686, 2008].

　 Radially polarized beams are useful in optical tweezers as well as materials processing applications such as cutting and drilling. At the mention of the application in metal cutting, it is well known that the radially polarized beam can introduce doubled efficiency compared with linearly or circularly polarized beam. Such feature of it makes that the researchers are attracted and fascinated with the prospect to construct the new laser source emitting radially polarized pulse with high peak power and short duration.

　 "We have developed a simple and compact laser system that emits Q-switching pulse with radial polarization. This is the first time that the (radial) polarization control technique is unified with the passive Q-switch technique into the end-pumped microchip configuration" said by Prof. Jianlang Li, the team leader at the Shanghai Institute of Optics and Fine Mechanics, "Under non-optimized cavity condition, the radially polarized and Q-switched pulse has the width of 86 ns, the peak power of 759 W at 6,7kHz repetition rate. The laser's slope efficiency is nearly 30.2%. The polarization purity of radially polarized light is as high as 97.4%."

　 The laser consists of a 1.9 mm thick Nd:YAG ceramic microchip, a chromium-doped YAG saturable absorber and a polarization-selective concentric grating mirror (PCG). The laser cavity measures 5.5 cm in length and is end-pumped using a fibre-coupled 808 nm laser diode. The grating mirror is composed of alternating layers of high and low refractive-index materials (Nb₂O₅/SiO₂) shaped into triangular concentric corrugations with sub-wavelength layer thickness. When the pump power exceeds the lasing threshold, the system emits an annular beam with a dark centre, which can be radially polarized and Q-switched.

　 Future work for the group includes implementing mode-locked operation and solving the thermal-lensing effect at higher pump powers in order to obtain higher output.