Diode-pumped actively Q-switched Nd:GGG laser operating at 938 nm X. Wang a,n , H.J. Eichler a , Zhiguo Zhang b a Institut f¨ ur Optik und Atomare Physik, Technische Universit¨ at Berlin, Straße des 17 Juni 135, Berlin 10623, Germany b Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China article info Article history: Received 9 February 2011 Received in revised form 15 August 2011 Accepted 23 August 2011 Available online 8 September 2011 Keywords: Active Q-switch Nd:GGG Quasi-three-level abstract The stimulated emission cross-section of Nd:GGG crystal in 938 nm transition was measured by the amplifier approach. It is 2.3  10 20 cm 2 . A quasi-continuous-wave diode pumped, actively Q-switched Nd:GGG laser operating at 938 nm was demonstrated. Pumped by laser diodes with 900 W peak power and 300 ms pulse duration, it generated 168 mJ energy in long pulse mode. The slope efficiency was 36%. Q-switched by a KDnP Pockels cell, 41 mJ output pulse energy was obtained. The pulse duration and peak power were 120 ns and 340 kW, respectively. The optical to optical efficiency was 7%. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Gadolinium gallium garnet (GGG) is one of the best crystals suitable for Nd 3 þ doping. It was first grown by Gcusic et al. using the Czochralski method in 1964 [1]. As shown in Table 1, Nd:GGG has some advantages compared with Nd:YAG and Nd:SFAP. It can be grown core-free up to 15 cm in radius with superior optical quality [2]. The concentration of Nd 3 þ in GGG can reach 4% or even higher due to the weak concentration quenching of Nd 3 þ substituting Gd 3 þ . It is indicated by the change of the fluores- cence lifetime, obvious quenching is observed only when concentration is higher than 2% at Nd 3 þ -doping. Moreover, it has broad absorption linewidth and excellent thermal conductivity. GGG crystal has been considered as a good candidate for solid-state heat capacity lasers (SSHCL) [3] because of its high thermal diffusivity, great fracture stress and large diameter (can be cut from the boule). A SSHCL generating 67 kW average power has been demonstrated in 2006 [4]. Although diode-pumped 1.06 and 1.33 mm-Nd:GGG lasers have been widely studied [5,6], the Nd:GGG lasers operating on 4 F 3/2 - 4 I 9/2 transitions, especially pulsed 938 nm-lasers with high peak power, have been rarely demonstrated. It is because of their serious re-absorption loss and very small stimulated emission cross section [12]. 900 nm region lasers attracted much attention recently since their frequency doubling has been recognized as an effective way to realize the blue laser. Furthermore, 938 nm coherent laser sources have a promising application in lidar system. Compared with another crystal—Nd:SFAP, which can be also doped with 2% Nd 3 þ , Nd:GGG has larger emission cross section, much lower re-absorption loss and weaker amplified spontaneous emission (as shown in Table 1). These advantages make Nd:GGG more favorable to operate on quasi-three-level transition. The first laser operation at 938 nm was achieved inside a Nd:GGG waveguide fabricated by pulsed-laser deposition, where a low lasing threshold can be realized [13]. In 2007, continuous-wave laser operation of a diode pumped Nd:GGG laser at 938 nm was reported [14]. Its maximum output power was 3 W [15]. The passive Q-switch of Nd:GGG laser was realized very recently, using V 3 þ :YAG as the saturable absorber [16]. The average output power of 186 mW was achieved with 358 ns pulse width and 116 kHz pulse repetition frequency, which correspond to a peak power of 4.5 W. For applications of frequency doubling and lidar system, high peak power and short pulse duration will benefit the perfor- mance. For these reasons a diode-pumped, actively Q-switched Nd:GGG laser at 938 nm was demonstrated for the first time. Pumped from both ends by quasi-continuous-wave (qcw) laser diodes (LD), 168 mJ output laser energy was obtained with 456 mJ absorbed pump pulse energy. To obtain active Q-switch the combination of a reflected polarizer and a KD n P Pockels cell was used. The maximum pulse energy of the Q-switched laser was 41 mJ, with 120 ns pulse duration and 5 Hz repetition rate. The corresponding peak power was 340 kW, which was much higher compared with previous works [16,17]. 2. Stimulated emission cross-section Stimulated emission cross-section is an important parameter for a laser crystal and can be measured by the amplifier approach Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/optlastec Optics & Laser Technology 0030-3992/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2011.08.017 n Corresponding author. Tel.: þ49 30 31422706; fax: þ49 30 31421079. E-mail address: wangxinnnn@hotmail.com (X. Wang). Optics & Laser Technology 44 (2012) 476–481