1840 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 24, DECEMBER 15, 2009 Impedance Characteristics and Parasitic Speed Limitations of High-Speed 850-nm VCSELs Y. Ou, J. S. Gustavsson, P. Westbergh, Å. Haglund, A. Larsson, and A. Joel Abstract—The impedance characteristics of high-speed oxide- confined 850-nm vertical-cavity surface-emitting lasers have been studied with the aim of identifying the importance of device par- asitics for the modulation bandwidth. Through equivalent circuit modeling, it is confirmed that device parasitics have a major impact on the bandwidth and the importance of each individual circuit element has been investigated. According to the extrapolation of the parameters derived from S measurements below 20 GHz to- wards higher frequencies and assuming that the mesa capacitance can be reduced by adding a few extra oxide layers without signifi- cantly affecting series resistance, our model predicts that the 3-dB parasitic frequency can be increased from 22 to above 30 GHz. Ac- counting also for bandwidth limitations due to thermal effects, we expect an increase of the modulation bandwidth of several giga- hertz which may enable direct current modulation at 40 Gb/s. Index Terms—Electrical parasitics, high speed, impedance, modulation bandwidth, vertical-cavity surface-emitting laser (VCSEL). I. INTRODUCTION T HE vertical-cavity surface-emitting laser (VCSEL) is an ideal light source for short reach optical communication links and interconnects owing to cost-effective fabrication, low power consumption, high modulation speed at low currents, and good beam quality. With electrical interfaces for serial transmis- sion becoming standardized for speeds of several tens of giga- bits/s for a variety of applications, there is a need for the devel- opment of VCSELs that can operate much in excess of today’s 10 Gb/s. For VCSELs emitting at 850 nm, the standard wavelength in short reach communication links (where high-speed multimode fiber is available), operation at 30 Gb/s was first demonstrated by Johnsson et al. [1]. To date, the highest speed error-free operation demonstrated at this wavelength is 38 Gb/s in a back-to-back configuration [2]. Recently, we reported on 850-nm VCSELs with a modulation bandwidth in excess of 20 GHz [3], [4] and demonstrated transmission at 32 Gb/s over 50 m of multimode fiber under direct current modulation [5]. Manuscript received June 29, 2009; revised September 27, 2009. First published October 23, 2009; current version published December 01, 2009. This work was supported by the European project VISIT (FP7-224211) and the Swedish Foundation for Strategic Research. Y. Ou, J. S. Gustavsson, P. Westbergh, Å. Haglund, and A. Larsson are with the Photonics Laboratory, Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, SE-412 96 Göteborg, Sweden (e-mail: petter.westbergh@chalmers.se). A. Joel is with the IQE Europe Ltd., Pascal Close, St. Mellons, Cardiff, CF3 0LW, U.K. 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.2009.2034618 The oxide confined VCSELs are multimode with a relatively large oxide aperture (9 m) and operate at a current density of only 10 kA/cm , which is the same current density at which today’s highly reliable 10-Gb/s VCSELs operate. The modulation bandwidth of the VCSEL is limited by intrinsic and extrinsic effects. With increasing bias current, both the resonance frequency and the damping of the resonant carrier-photon interaction increase. Initially, the modulation bandwidth increases, but eventually the damping becomes large enough to limit the bandwidth. This defines the maximum intrinsic modulation bandwidth. With proper design of the active region, using strained quantum wells (QWs), an intrinsic modulation bandwidth in excess of 30 GHz was achieved [4]. With increasing bias current also the temperature of the active region increases due to self-heating. This leads to an increase of the threshold current and a reduction of the differential gain, eventually resulting in a saturation of the photon density and therefore the modulation bandwidth. This defines the maximum bandwidth due to thermal effects. With proper design for low heat generation (low resistance) and low thermal impedance, a thermally limited bandwidth of about 30 GHz was achieved [4]. Finally, the modulation bandwidth is limited by parasitics (resistances and capacitances) associated with the device struc- ture and geometry, forming a low-pass -filter which shunts the modulation current outside the active region at high fre- quencies. With a measured small-signal modulation bandwidth of 20 GHz, and intrinsic and thermally limited bandwidths at or above 30 GHz [4], there are reasons to believe that parasitics play a major role in limiting the modulation bandwidth. It is the purpose of this work to clarify the role of parasitics of these high-speed VCSELs. Impedance measurements are used to provide values for the various elements of an equivalent cir- cuit used to represent the VCSEL. The importance of the various elements and design modifications for improving the extrinsic modulation bandwidth are indentified. II. VCSEL DESIGN AND EQUIVALENT CIRCUIT Fig. 1 shows a schematic of the oxide-confined 850-nm VCSEL under study [4]. To reduce capacitance, two oxide layers, a thick layer of benzocyclobutene (BCB) under the p-bond pad, and an undoped substrate [6] are used. Resistance is minimized using modulation doping and graded interfaces in the distributed Bragg reflectors (DBRs) and thermal impedance is lowered by using binary alloys in the major part of the lower DBR. The use of strained InGaAs QWs improves differential gain and thereby the intrinsic and thermally limited bandwidths. Also shown in Fig. 1 is the circuit used to represent the elec- trical equivalence of the VCSEL [7]. The capacitance repre- 1041-1135/$26.00 © 2009 IEEE