IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 5, MARCH 1, 2011 323 2-D Stacked External Cavity Multiwavelength Surface-Emitting Laser Array Yigit O. Yilmaz, Oleg V. Smolski, Viktor O. Smolski, Joshua K. Lockerman, and Eric G. Johnson Abstract—In this letter, we present a multiwavelength two-dimensional array based on vertically stacked grating coupled surface-emitting laser bars. The emitters in the array were individually wavelength stabilized using external Littrow gratings fabricated on silicon-based spacers. In pulse operation, lasing with a 0.3-nm linewidth from each device and maximum total peak power of 120 W was achieved. Index Terms—External cavity lasers, laser arrays, semicon- ductor lasers, surface-emitting lasers. I. INTRODUCTION D EVELOPMENT of high power light sources based on one- and two-dimensional (2-D) laser diode arrays are still in strong demand for efficient pumping of solid-state, fiber lasers, and for nonlinear second harmonic generation [1]. There has been significant efforts in designing novel 2-D laser diode arrays such as stair-step stacked multiple single emitters [2] and surface emitters with total internal reflection beam deflec- tors [3]. One disadvantage of the 2-D stacks based on edge- emitter devices is the complex alignment of the collimation optics in the fast- axis; however, vertical cavity surface emit- ting (VCSEL) 2-D arrays offer an attractive alternative to tra- ditional laser diode stacks by providing high output power den- sity and simplified integration with external optical components. Although VCSELs provide numerous advantages, their power conversion efficiency is relatively low when compared to edge emitters [4]. Grating coupled surface emitting lasers (GCSELs) with broad stripes have a large emitting area, competitive con- version efficiencies, and scalable for 2-D array configurations [5]. Another desirable feature of 2-D arrays is wavelength stabi- lization of the array or wavelength selection for each individual emitter. The wavelength stabilization in broad area GCSELs can be performed by an internal mechanism such as dual grating re- flector (DGR) [6]. The DGR section of a GCSEL device con- sists of a grating coupler on the p-side and a feedback grating on the n-side in a Littrow configuration. This approach can be realized with an alternative scheme, where wavelength stabi- lization is based on the same concept, but is achieved with a Littrow grating flip chip bonded to the substrate. This approach offers an integrated solution to the wavelength selectivity in a single die or in array form. In fact, this approach enables one to Manuscript received October 07, 2010; revised December 15, 2010; accepted January 01, 2011. Date of publication January 10, 2011; date of current version February 24, 2011. This work was supported by the Office of Naval Research under Grant ONR N00014-09-1-0498. The authors are with the University of North Carolina at Charlotte, Charlotte, NC 28223 USA (e-mail: egjohnso@uncc.edu). Digital Object Identifier 10.1109/LPT.2011.2104943 Fig. 1. (Top) Cross-section schematic of a single GCSEL device; (bottom) cross-section schematic of the multiwavelength 2-D array based on vertical staking grating coupled emitter bars and Si dies with the processed Littrow grat- ings. stack arrays of these chips as shown in Fig. 1. In this integrated external cavity configuration the fabrication tolerances on the feedback grating are less stringent and the substrate material can be varied according to the desired application. Moreover, the process can easily be scaled to large arrays, with each in- dividual emitter locked to the same wavelength or an arbitrary array of wavelengths defined by the period of the Littrow grat- ings for each element. In this letter, we present a multiwavelength 2-D GCSEL array based on an integrated external cavity configuration. In this design, the one-dimensional (1-D) GCSEL bars are stacked in a vertical configuration with a wavelength selective element integrated into the spacers. In recent work, we have shown the possibility of vertical stacking in 1-D with multiwavelength GCSEL/DGR single emitters, in which the Littrow grating is monolithically fabricated on the GCSEL chip [7]. In the current 2-D-array design, the wavelength locking element used is a Littrow grating fabricated on external silicon die which is bonded on the n-side of the GCSEL bars. As shown on Fig. 1, the Littrow grating provided wavelength- selective feedback to the active medium in combination with 1041-1135/$26.00 © 2011 IEEE