Vertical-Cavity Surface-Emitting Lasers: Mbe Growth and Optical Information Processing Applications
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VERTICAL-CAVITY SURFACE-EMITTING LASERS: MBE GROWTH AND OPTICAL INFORMATION PROCESSING APPLICATIONS
J.P. HARBISON', LT. FLOREZ*, A. SCHERER', C.J. CHANG-HASNAIN*, A.C. VON LEHMEN*, W.K. CHAN*, E.M. CLAUSEN, JR*, M. ORENSTEIN*, AND J.L JEWELL** * Bellcore, Red Bank, New Jersey 07701-7040 •* AT&T Bell Laboratories, Holmdel, New Jersey 07733 ABSTRACT The basic issues involved in the growth by molecular beam epitaxy (MBE) of vertical-cavity surface-emitting lasers (VCSELs) are discussed. Successful VCSEL optical information processing applications demonstrated to date are discussed, including twodimensional arrays, holographic memory retrieval, optical addressing, wavelength division multiplexing, and tunable VCSEL operation. 1. INTRODUCTION Recent advances in the past few years in the field of vertical-cavity surface-emitting lasers, or VCSELs, have spanned the entire range from the first demonstration of lowthreshold operation [1] to a wide variety of demonstrated practical applications, some of which will be discussed later in this paper, all in the short period of two to three years. In this paper we will briefly review the concepts involved in the growth and fabrication of these lasers and give a brief summary of some of the progress which has been made in integrating these novel devices into real-world applications. The concept of a VCSEL is quite straightforward. It involves a central semiconductor light-emitting active region, similar to that found in conventional edgeemitting semiconductor diode lasers, surrounded on the top and bottom by highlyreflective mirror stacks which provide a lasing cavity which is vertical to the semiconductor wafer surface. The concept has been pursued since the late 1970's in the pioneering work of K. Iga and his group at the Tokyo Institute of Technology [2,3]. From the inception of such work, it was realized that increased functionality could be achieved by such a vertical geometry, as it paved the way for the possibility of two dimensional laser arrays, as well as allowing the cost benefits of multiple processing of many individual lasers in parallel in analogy with the cost savings per transistor inherent in large scale integrated electronic circuits. The other potential advantage of decreased current threshold, which should in theory be possible due to the substantially reduced volume of the active region of the material (a decrease of as much as 100 or more when compared to conventional edge-emitters where long stripe geometries are required), proved to be quite elusive, however. The key to the success of VCSELs lays in the fabrication of appropriate highly reflecting mirrors. Such high reflectivity is required to compensate for the drastically reduced path length through active material dictated by the vertical geometry, resulting in substantially reduced optical gain per pass. Whereas conventional horizontal-cavity edgeemitting lasers with cleaved-facet mirrors can operate with mirror reflectivities of 30% or so, the much shorter cavity VCSELs must have mirror reflectiviti
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