Two-dimensional coherently coupled vertical cavity laser arrays

Abstract

Two-dimensional coherently coupled vertical cavity surface emitting laser (VCSEL) arrays have long been pursued for narrow divergence and high brightness applications. We discuss both top and bottom-emitting coherent VCSEL arrays that can operate in the in-phase mode. The key to their operation is strongly coupled leaky-mode operation, which can be achieved using ion implantation to define a periodically arrayed gain regions, and a 2D photonic crystal hole pattern etched into the top facet to control the lateral modes [1,2]. The brightness of VCSEL arrays typically suffer from either incoherence between elements [3] or an out-of-phase far field profile [4], either of which can arise from periphery current injection. Bottom-emitting arrays can circumvent these difficulties by allowing direct current injection into all laser elements. Previous pulsed coherent bottom emitting arrays have employed reflectivity modulation to select the supermode, which necessarily leads to out-of-phase emission [5]. Here we report a simple self-aligned process to demonstrate inphase continuous wave (CW) emission in both top and bottom emitting coherently coupled VCSEL arrays. We also show for top emitting coherently coupled arrays that the phase can be varied by differential current injection over a full range of π which enables beam steering at modulation speeds up to 100 MHz [6].The VCSEL arrays under study are arranged in a 2x1 or 2x2 configuration, where the fabrication procedure has been described in previous work [1,2]. The top emitting arrays emit at 850 nm while the bottom (substrate) emitting arrays emit at 980 nm. The photonic crystal pattern etched into the top mirror limits the number of lasing modes and provides stable index guiding. The defects in the hole pattern create the array elements and are aligned with implant apertures approximately the same size. The latter confines the current to the individual elements, as shown in Fig. 1 for either top emitting or bottom emitting geometry. The top metal contact and uppermost semiconductor layer is etched with a focused ion beam for electrical isolation of each defect cavity to allow preferential current injection. For the bottom emitting arrays, a Au layer with 3 µm thick 7x7 µ m Au squares with a 9 µm pixel pitch is deposited on the high reflectivity DBR mirror. The thick Au squares block subsequent proton implantation, providing self-aligned current confinement to the array elements. The bottom emitting arrays shown here do not have photonic crystal mode control.Shown in Fig. 2 are near and far field profiles from in-phase operation of the arrays. The near field modal pattern of the 2x2 top emitting array in Fig. 2(a) exhibits fringes between the elements [1], which is indicative of the leaky-mode behavior [2]. The current injection path defined by the implantation suppresses the index which enables in-phase operation, as shown in Fig. 2(b). However due to current crowding issues, larger arrays only exhibit out-of-phase operation (with an on-axis far field null) or the inn...