368 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 2, FEBRUARY 2004 Single Fundamental-Mode Output Power Exceeding 6 mW From VCSELs With a Shallow Surface Relief Å. Haglund, J. S. Gustavsson, J. Vukuˇ sic ´, P. Modh, Member, IEEE, and A. Larsson, Member, IEEE Abstract—A single fundamental-mode output power of 6.5 mW was achieved from an 850-nm vertical-cavity surface-emit- ting laser (VCSEL) with a shallow surface relief, the highest single-mode power ever reported using this technique. The VCSELs were fabricated from epitaxial material grown to yield an antiphase reflection from the topmost layer. A circular surface relief, acting as a mode discriminator, was etched in the center to reduce the mirror loss for the fundamental mode. This “inverted” surface-relief technique offers relaxed etch depth control and, therefore, improves reproducibility and yield. Index Terms—Single mode, surface relief, vertical-cavity sur- face-emitting laser (VCSEL). I. INTRODUCTION T HE vertical-cavity surface-emitting laser (VCSEL) has proven to be a low-cost light source with attractive prop- erties such as surface emission, a circular and low divergent output beam, and simple integration in one- and two-dimen- sional arrays. These features, together with its high-speed performance, have made it an established transmitter in, for example, short-distance parallel fiber-optic interconnects. New applications are continuously arising, such as in spec- troscopy, laser printing, optical storage, and in longer distance communication [1]. Many of these applications require stable single-mode output powers of several milliwatts. Unfortunately, the VCSEL has a tendency to lase at multiple wavelengths. This is due to its relatively large transverse dimensions, which support several transverse modes. To enable an impact on new market areas, considerable effort has been invested to achieve high single-mode output power. Several methods have been developed that affect the transverse guiding and/or introduce mode-selective loss or gain [2]. A single-mode output power of 6.0 mW has been achieved with a monolithic coupled resonator VCSEL using proton implantation [3]. Another approach is to use an antiresonant reflecting optical waveguide VCSEL, which has produced a single-mode output power of 7.1 mW [4]. The drawback with these devices is that their manufacture involves a more complex epitaxial growth and fabrication as compared to standard VCSELs, which, in the end, hampers a low-cost and high yield manufacturing. Looking for less complex solutions, the most straightforward technique for achieving single-mode emission is simply to de- Manuscript received August 26, 2003; revised October 3, 2003. This work was supported by the Foundation for Strategic Research through Chalmers Center for High-Speed Electronics and Photonics. The authors are with the Photonics Laboratory, Department of Microtech- nology and Nanoscience, Chalmers University of Technology, SE-412 96 Göte- borg, Sweden (e-mail: aasa@elm.chalmers.se). Digital Object Identifier 10.1109/LPT.2003.821085 crease the index-guiding aperture of a standard oxide-confined VCSEL until higher order modes are no longer supported by the waveguide or prevented from lasing due to high diffraction losses. A single-mode power of 4.8 mW has been demonstrated using an oxide aperture of diameter 3.5 m [5]. This method limits the output power due to a small current confining re- gion, which increases the differential resistance and, thereby, the self-heating, which makes the output power rollover at lower drive currents. The increased self-heating also degrades the reli- ability. A further problem is the difficulty in reproducing a small oxide aperture, and thereby, reproducing the performance. An- other less complex solution is the surface-relief technique [6], which allows for higher output power by a larger current aper- ture, and thereby, a lower self-heating. This method only in- volves a slight modification to standard VCSEL processing. By etching a shallow surface relief into the top mirror, lateral differ- ences in the mirror loss can be achieved. The modal losses can then be engineered to give higher mirror losses for the higher order transverse modes, which prevent or delay their onset. In this letter, we present record performance of VCSELs using an “inverted” surface-relief technique. II. SURFACE-RELIEF TECHNIQUE The surface-relief technique can be applied in two different ways. The first and most pursued way is to etch a shallow struc- ture, in the form of a donut, in the top layer of an ordinary VCSEL structure, thereby, increasing the losses of higher order modes [6]. The second way is to add an extra -thick layer on top of an ordinary VCSEL structure during epitaxial growth [7]. If this topmost layer has a higher refractive index than the under- lying layer, both the reflection at the interface between these two layers and the reflection at the topmost layer–air interface will be in antiphase with reflections further down in the mirror stack. If it has a lower refractive index, only reflections at the topmost layer–air interface will produce an antiphase reflection. A top- most layer with a high refractive index is, therefore, preferable. To lower the mirror loss of the fundamental mode, a circular disk-shaped structure is then etched in the center of the device. By choosing an appropriate etch depth, a relatively high mirror loss contrast between etched and unetched areas can be achieved (see Fig. 1). The advantage of the second approach is that it utilizes the high thickness precision in the epitaxial growth to reach a narrow local maximum in the mirror losses. This will then relax the required etch depth precision since a local min- imum in the mirror loss is much broader (see Fig. 1). When designing an oxide-confined VCSEL with a mode-se- lecting surface relief, there exists an optimal combination of re- lief diameter, oxide aperture diameter, and etch depth, to pro- 1041-1135/04$20.00 © 2004 IEEE