Monolithic ring resonator with PPLN crystal for efficient cw SHG of 976 nm emitted by a diode laser Danilo Skoczowsky* a , Andreas Jechow a , Herbert Stürmer b , Torsten Poßner b , Sandra Stry c , Joachim Sacher c , Ralf Menzel a a University of Potsdam, Chair of Photonics, Karl-Liebknecht Str.24-25, 14476 Potsdam, Germany; b GRINTECH GmbH, Schillerstraße 1, 07745 Jena, Germany; b Sacher Lasertechnik GmbH, Rudolf Breitscheid Str. 1-5, 35037 Marburg, Germany ABSTRACT A new setup for efficient blue light generation that consists of two passively coupled optical resonators is presented. The first resonator is based on a broad area laser diode (BAL) in a Littrow external cavity with a special off-axis design. This external cavity diode laser provides more than 450 mW diffraction limited and narrow bandwidth emission at 976 nm. A compact cavity design with 40 mm length could be realized. The second resonator is a monolithic high finesse ring cavity containing a 10 mm bulk periodically poled lithium niobate (PPLN) crystal for resonant second harmonic generation. This ring resonator consists of four small mirrors with appropriate reflectivities and two GRIN lenses for stability reasons. All parts of this ring cavity are mounted monolithically on a glass substrate with a size of 19.5 mm x 8.5 mm. First experiments showed good passive matching of both cavities without any active closed-loop control. With this setup efficient SHG was achieved. A maximum optical output power of 70 mW blue light at 488 nm was obtained. The conversion efficiency was better than 15%. Keywords: diode lasers, laser beam shaping, external cavity diode lasers, broad area diode lasers, second harmonic generation, resonant SHG, ring resonators 1. INTRODUCTION Compact, highly efficient and cheap coherent light sources in the visible spectral region are needed for various applications in spectroscopy, digital printing and medical diagnostics [1, 2]. Especially wavelengths in the blue and green spectral region are of high interest, because of the countless number of applications that have been established around the strong emission lines of the Argon ion laser at 488 nm and 514.5 nm. Since those gas lasers are mostly very bulky, show very low efficiencies and suffer on high operation costs and short service intervals, they are highly desired to be replaced. Diode lasers are very small and deliver coherent light with high output powers along with very good wall plug efficiencies. Nowadays even direct emitters in the blue spectrum based on GaN with medium output powers up to 50 mW are available. Unfortunately, these blue laser diodes still suffer on limited lifetimes, have a broad spectral emission and do not cover the demanded wavelength range. Especially between 480 nm and 635 nm no direct emitting diodes are available, yet. Another way to generate visible light is second harmonic generation (SHG) of (near-) infrared light. Diode pumped solid state lasers (DPSSL) are very common sources for SHG to the green spectrum. So far, high single pass conversion efficiencies of more than 50% have been shown [3]. Due to the fixed laser transitions of the solid state materials those lasers are restricted to certain wavelengths. Further DPSSL systems are limited in tunability and the wall plug efficiency of those lasers is rather low. Since the introduction of periodically poled (PP) crystals that utilize the effect of quasi phase matching (QPM) edge emitting diode lasers are used for efficient single pass frequency conversion into the visible. Several approaches for the generation of visible light with high brightness laser diodes and PP materials have been presented, including the use of waveguides [5-8] and bulk nonlinear crystals [9, 10]. Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications VIII, edited by Peter E. Powers, Proc. of SPIE Vol. 7197, 71970K · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.809242 Proc. of SPIE Vol. 7197 71970K-1