IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 7, APRIL 1, 2010 465 25-Gb/s Direct Modulation of Implant Confined Holey Vertical-Cavity Surface-Emitting Lasers Chen Chen, Student Member, IEEE, Zhaobing Tian, Student Member, IEEE, Kent D. Choquette, Fellow, IEEE, and David V. Plant, Fellow, IEEE Abstract—A 25-Gb/s direct modulation of an 850-nm implant- confined holey vertical-cavity surface-emitting laser (VCSEL) is demonstrated with a low operation current density of 7.4 KA/cm . The high-speed performance arises from cavity designs that are achieved using standard fabrication and epitaxial materials. The etched holey structure is incorporated into the top mirror of the VCSEL to tailor the size of the optical cavity independent from that of the electrical current aperture, enabling us to achieve high-speed modulation and low operation current density simultaneously. Index Terms—High-speed modulation, optical interconnects, semiconductor lasers, vertical-cavity surface-emitting lasers (VCSELs). I. INTRODUCTION T HE vertical-cavity surface-emitting laser (VCSEL) is a suitable laser source for short-haul optical communica- tions, owing to its ability for low-cost high-volume manufacture, low power consumption, and other unique advantages. High- speed direct modulation of a VCSEL is desired to further the transmission capacity of communication networks, such as Eth- ernet local area networks and board-level optical interconnects. Recently a variety of approaches have been exploited to improve the direct modulation bandwidth of VCSELs in different wave- length regimes [1]–[7]. Among the approaches taken, some en- abling technologies have been identified through the optimiza- tion of VCSEL epitaxial designs, which include the tapered oxide structure [2], the buried tunnel junction [3], [7], and the in- corporation of quantum-well materials with higher differential gain [2]–[6]. However, the incorporation of a photonic crystal or a generalized holey structure into the top distributed Bragg re- flector (DBR) of the VCSEL offers a different optimization di- mension. This approach enables us to engineer the index guiding of the transverse optical modes independent from the electrical (or current injection) aperture. A large modulation bandwidth can be achieved from reducing the optical modal volume and in- creasing the laser efficiency. Meanwhile, a relatively large elec- trical aperture can be maintained to ensure a low operation cur- rent density, enabling us to improve VCSEL reliability at the Manuscript received November 02, 2009; revised December 09, 2009; ac- cepted January 12, 2010. First published January 29, 2010; current version pub- lished March 05, 2010. C. Chen, Z. Tian, and D. V. Plant are with Department of Electrical and Computer Engineering, McGill University, Montreal, QC H3A 2A7, Canada (e-mail: chen.chen4@mail.mcgill.ca; tianzhaobing@gmail.com; david.plant@mcgill.ca). K. D. Choquette is with Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA (e-mail: choquett@illinois.edu). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2010.2040990 (a) (b) Fig. 1. (a) Cross section schematic and (b) optical image of the holey VCSEL. same time [8]. Moreover, the holey VCSEL approach is inde- pendent of the epitaxial design and operation wavelengths [9], and thus can be combined with other methodologies to improve modulation bandwidth. In our prior work, the small- and large-signal modulation characteristics of implant-confined holey VCSELs have been studied [1], [10]. Despite a maximum small-signal bandwidth (or 3-dB electrical bandwidth) of 18 GHz, the large-signal performance of holey VCSELs can be severely hindered by the carrier diffusion effect arising from the size difference be- tween the electrical and optical apertures, making it difficult to predict the large-signal performance of a VCSEL from the small-signal measurements [10]. It was found that a small aper- ture size difference (less than 4 m in diameter) can minimize the carrier diffusion effect and thus achieve large-signal modu- lation at higher data rates [10]. In this work, we demonstrate the 25-Gb/s operation of an implant-confined holey VCSEL with a small aperture size difference of 2.6 m. The operation current density is as low as 7.4 KA/cm . The device design parameters, small-signal characteristics, and bit-error-ratio (BER) measure- ment for a multimode fiber link are also presented. II. DEVICE STRUCTURE AND DC PROPERTIES Fig. 1(a) illustrates the cross section schematic of a holey VCSEL. The implant-confined VCSELs were fabricated from a wafer with a conventional 850-nm epitaxy design. Protons are implanted with 340-keV energy and a dose of cm [11]. Coplanar ground-signal-ground contacts were deposited on planarized polyimide to reduce parasitic capacitance and fa- cilitate on-wafer high-speed measurement. The holey structures were defined using electron beam lithography and etched ap- proximately 16 periods (out of 21 total periods) into the top DBR. The fabrication process was discussed in more details in 1041-1135/$26.00 © 2010 IEEE