1694 JOURNAL OF LIGHTWAVE TECHNOLOGY,VOL. 22, NO. 7, JULY2004 RF Transmission Over Multimode Fibers Using VCSELs—Comparing Standard and High-Bandwidth Multimode Fibers Christina Carlsson, Anders Larsson, Member, IEEE, and Arne Alping, Member, IEEE Abstract—Fiber-optic radio-frequency links have been assem- bled using oxide-confined vertical-cavity surface-emitting lasers (VCSELs) and multimode fibers. Links with single and multimode VCSELs and with standard and high-bandwidth fibers have been evaluated and compared in the frequency range of 0.1–10 GHz. The best results were obtained for links with a multimode VCSEL and a high-bandwidth fiber. For a 500-m-long link, a spurious free dynamic range of 104 dB Hz at 2 GHz and 100 dB Hz at 5 GHz were obtained while allowing for a VCSEL–fiber mis- alignment of 12 m. Corresponding numbers for the intrinsic link gain and noise figure are 29 and 33 dB, and 39 and 42 dB at 2 and 5 GHz, respectively. Inferior performance was observed for the standard fiber link due to a larger variation in modal group velocities. This paper also presents a detailed link analysis to identify performance limitations and to suggest modifications for improved performance. Index Terms—Analog modulation, distortion, noise, semicon- ductor lasers, vertical-cavity surface-emitting lasers (VCSELs). I. INTRODUCTION T HERE is a need for high-performance fiber-optic radio-frequency (RF) links in fiber-fed distributed antenna systems (DASs). Such cost-sensitive applications require a low-cost technology, and therefore multimode fibers and vertical-cavity surface-emitting lasers (VCSELs) are of interest [1], [2]. The VCSEL has inherent advantages over edge-emitting lasers in terms of fabrication cost and fiber coupling performance. Multimode fibers enjoy the benefit of more relaxed alignment tolerances and therefore simplified packaging. On the other hand, due to modal dispersion, the bandwidth-length product of a multimode fiber is small com- pared with a single-mode fiber. However, new high-bandwidth multimode fibers, with bandwidth-length products one order of magnitude higher than standard multimode fibers, have recently been introduced [3]. These fibers are optimized for 850 nm, where low-cost VCSELs are readily available. Earlier reports have shown that multimode VCSELs can provide the dynamic range required for high-performance Manuscript received November 10, 2003; revised March 31, 2004. This work was supported in part by the Swedish Defence Material Administration (FMV) Microwave Photonics program under Grant 60304-LB103937 and in part by the Foundation for Strategic Research (SSF) through Chalmers Center for High Speed Electronics and Photonics (HSEP). C. Carlsson and A. Larsson are with the Photonics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-412 96 Göteborg, Sweden (e-mail: christina@elm.chalmers.se). A. Alping is with Microwave and High Speed Electronics Research Center (MHSERC), Ericsson AB, Mölndal SE-43184, Sweden. Digital Object Identifier 10.1109/JLT.2004.831163 fiber-optic RF and microwave links [4]. Preliminary experi- ments also indicate that links with VCSELs and high-bandwidth multimode fibers can fulfill the requirements for a number of fiber-fed DAS [5]. This paper presents the results of a more comprehensive study that compares the performance (dynamic range, link gain, noise figure (NF), and misalignment toler- ance) of links with standard and high-bandwidth graded-index multimode fibers, with lengths up to 500 m, using directly modulated 840-nm VCSELs as light sources. Links with single and multimode VCSELs are also compared and the effects of VCSEL–fiber misalignments on link gain and dynamic range are investigated. Frequencies of primary interest are in the low-gigahertz range, including those used in mobile communication systems, such as Global System Mobil (GSM) and Universal Mobile Telecommunication Sytsem (UMTS) (1–2 GHz) and wireless local area networks such as HiperLAN and IEEE 802.11 (2–5 GHz). Finally, the paper analyzes the measured link gain, NF, and dynamic range in order to pinpoint major limitations and suggest modifications for improved link performance. II. LINK COMPONENTS A. Vertical-Cavity Surface-Emitting Lasers The VCSELs used in the link experiments are 840-nm oxide- confined VCSELs of the same design as those that previously underwent a comprehensive study of their analog modulation behavior [4], including the small-signal modulation response , the frequency-dependent impedance, the relative inten- sity noise (RIN), and the spurious free dynamic range (SFDR). This study showed that, for fundamental reasons related to re- laxation oscillations, spatial hole burning, and mode competi- tion, multimode VCSELs with an intermediate oxide aperture diameter of 6–8 m have the highest SFDR. For the present experiments, we therefore use two oxide-con- fined 840-nm VCSELs, one with an oxide aperture diameter of 6 m (multimode) and the other 2 m (single mode), to be able to also investigate the dependence of link performance on VCSEL-mode characteristics. Important VCSEL parame- ters are listed in Table I. More detailed information on typical VCSEL characteristics, including dependencies on oxide ape- ture size and bias current, can be found in [4]. The VCSELs are butt-coupled to the fibers. To be able to pre- cisely control the laser–fiber alignment, and to investigate ef- fects of misalignments, the fiber was attached to a piezoelectric positioner. 0733-8724/04$20.00 © 2004 IEEE