IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 40, NO. 9, SEPTEMBER 2004 1155 Vertical-Cavity Surface-Emitting Laser Active Regions for Enhanced Performance With Optical Pumping Jon Geske, Student Member, IEEE, Kian-Giap Gan, Yae L. Okuno, Joachim Piprek, Senior Member, IEEE, and John E. Bowers, Fellow, IEEE Abstract—We have developed an improved active region design for optically pumped vertical-cavity surface-emitting lasers. The design makes use of carrier-blocking layers to segment the absorber and promote uniform carrier populations in the quantum wells with pump efficiencies near 75%. A model to calculate the carrier distribution in the active region and a design methodology are presented along with a metric to describe the car- rier uniformity in the quantum wells. Experimental verification of the performance improvements shows an over 50% reduction in device thresholds and an increase of 20 C in maximum operating temperatures. Index Terms—Optical pumping, semiconductor lasers, surface- emitting lasers, wafer bonding. I. INTRODUCTION V ERTICAL-CAVITY surface-emitting lasers (VCSELs) are of great interest due to their advantages in low-cost manufacturing and packaging. In the pursuit of commer- cial-quality VCSELs operating at 1310 and 1550 nm, optically pumped VCSELs have been used not only as a development tools, but also in commercial products [1], [2]. In the design of optically pumped active regions it is desirable to make use of a long pump absorber region for high pumping efficiency. A long absorber is achieved by using the quantum-well barrier region as the absorber region. Carriers generated in the barrier diffuse to the quantum wells and provide gain for the lasing mode. Because no external bias is applied to an optically pumped device, there is little driving force to distribute the carriers among clusters of wells. Also, in VCSEL active regions it is advantageous for all quantum wells to be positioned as close as possible to the VCSEL cavity electric field standing wave peaks. For these reasons, research using optically pumped ac- tive regions with long absorbers has made use of active region designs with periodically spaced single quantum wells or clus- ters of quantum wells, each positioned on a standing wave peak of the longitudinal field the VCSEL oscillator [3], [4]. These periodic gain designs allow carriers optically generated in long barrier regions to diffuse directly to the nearest quantum-well region without passing over other clusters of wells. Manuscript received March 17, 2004; revised June 8, 2004. The authors are with the Electrical and Computer Engineering Depart- ment, University of California, Santa Barbara, CA 93106 USA (e-mail: geske@ece.ucsb.edu). Digital Object Identifier 10.1109/JQE.2004.833234 Inherent in the periodic gain design is the exponential decay of the pump beam throughout the length of the barrier. Be- cause of the exponential decay in the carrier generation rate, the quantum wells are not uniformly populated and the first quantum well is pumped harder than the last well. Material gain does not increase linearly with quantum-well carrier density so the first well produces less gain per carrier that the last well, resulting in a reduction in device efficiency and higher device thresholds and lower peak operating temperatures. To improve the performance of optically pumped VCSELs and our recently demonstrated ultra-wideband wavelength division multiplexed VCSEL arrays [5], an improved optically pumped active region design has been developed and used for improved device performance [6]. We incorporate precisely placed carrier-blocking layers in the barrier to segment the absorber and control the diffusion of carriers to specific quantum-well regions. This improved segmented-absorber periodic gain (SAPG) active region design results in high pump efficiencies while simultaneously achieving uniform filling of the periodically spaced quantum-well regions. In Section II of this paper, a model for the carrier distribution in the traditional and new SAPG design is built. Section III then outlines a simplified design technique and presents traditional and SAPG active region designs for further numerical and experimental analysis. In Section IV, the proposed designs from Section III are analyzed using the model developed in Section II. The quantum-well injection uniformity is defined as a metric to compare the carrier uniformity in the quantum wells and is cal- culated for the new SAPG and traditional active region designs. In Section V, the results of this modeling and design work are then experimentally verified by building and comparing two optically pumped 1540-nm VCSEL structures utilizing each active region design. Section VI will discuss and compare the model and the experimental results. II. OPTICALLY-PUMPED VCSEL ACTIVE REGION MODEL Fig. 1 shows the conduction band diagram for a traditional optically pumped active region structure and for the new SAPG active region design concept. Both designs utilize a long bar- rier material designed to efficiently absorb the incident pump power. Fig. 1 illustrates the decay of a 980-nm pump beam in- cident upon each active region for a barrier material of 1.22-Q InGaAsP with an 18 000 cm absorption coefficient. Between 0018-9197/04$20.00 © 2004 IEEE