534 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 8, APRIL 15, 2009 Simple VCSEL Base-Station Configuration for Hybrid Fiber-Wireless Access Networks Elaine Wong, Member, IEEE, Angulugaha Gamage Prasanna, Student Member, IEEE, Christina Lim, Senior Member, IEEE, Ka Lun Lee, Member, IEEE, and Ampalavanapillai Nirmalathas, Senior Member, IEEE Abstract—A simple and compact base-station (BS) configu- ration that exploits the optical properties of a vertical-cavity surface-emitting laser (VCSEL) for use in hybrid fiber-wireless access networks is proposed and experimentally demonstrated. Compared to existing configurations in the literature, the proposed configuration eliminates the need for external and custom-designed optical elements. Experiments demonstrate that radio-frequency-over-fiber channels can be successfully reflected, added, and rewritten at the BS with low penalty, thereby high- lighting the potential of a single VCSEL as an integrated reflector for “thru” channels and a transmitter to “add” and “rewrite” channels. Index Terms—Access networks, antenna base station (BS), fiber- wireless networks, vertical-cavity surface-emitting laser (VCSEL), wavelength reuse. I. INTRODUCTION T HE hybrid fiber-wireless network is a promising network technology that can meet the anticipated demand of future broadband interactive services. Such a network is illustrated in Fig. 1 where central offices (COs) in a metropolitan area ring network topology act as a gateway to an optical trunk network while serving widely distributed antenna base stations (BSs) configured in point-to-multipoint (P2MP) and ring topologies. Wavelength-division multiplexing (WDM) is incorporated to transport a large number of fiber radio channels to the BSs through a single CO. In order to support gigabit-per-second services in the wireless segment, a large number of antenna BSs with pico-cell coverage are required to cover a certain geographical area [1]. One of the deployment aims of such a network is, therefore, to install functionally simple and compact BSs with minimal optical and electronics inventory. All active switching, processing, and wavelength routing functionalities are also to be migrated to the CO in a centralized control architecture to further lessen the complexity of BSs. To date, much of the effort on simplifying BS design combines the use of external and custom-designed optical components, such as looped-back arrayed waveguide gratings, multiport circulators, and fiber Bragg gratings (FBGs), to achieve functionalities such as optical add–drop multiplexing and carrier reuse [2], Manuscript received December 19, 2007; revised October 28, 2008. First pub- lished February 03, 2009; current version published April 01, 2009. The authors are with the ARC Special Research Center of Ultra-Broadband Information Networks, Department of Electrical and Electronic Engineering, University of Melbourne, Melbourne, VIC 3010, Australia (e-mail: e.wong@ee. unimelb.edu.au; pgamagea@ee.unimelb.edu.au; c.lim@ee.unimelb.edu.au; k.lee@ee.unimelb.edu.au; a.nirmlathas@ee.unimelb.edu.au). 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.2009.2014393 Fig. 1. Hybrid fiber-wireless network with widely distributed antenna BSs in P2MP and ring topologies. [3]. These components are implemented in addition to optical transceivers and radio-frequency (RF) interfaces at the BSs. In this letter, a simple BS configuration using a single ver- tical-cavity surface-emitting laser (VCSEL) to serve as a di- rectly modulated transmitter to “add” and “rewrite” channels and an integrated optical reflector to bypass “thru” channels, is proposed. In a P2MP topology, the remote node demultiplexes the fiber radio channels and routes each of them to a designated BS. In addition to receiving the downlink radio channel and transmitting the uplink radio channel on a different carrier wave- length, the optical rewrite property may be used to rewrite down- link data with uplink data to conserve the total number of WDM channels on the network. In a ring topology, an additional func- tionality of optical add–drop multiplexing is required at each BS so incoming radio channels not designated for a particular BS are routed to downstream BSs. Our proposed BS configuration eliminates additional optical filtering components to perform si- multaneous optical add–drop and rewrite functions. To the best of our knowledge, this is the first proposal and proof-of-concept demonstration exploiting the integrated reflector, transmitter, and optical rewrite properties of VCSEL to realize a simple, compact, and cost-efficient antenna BS for fiber-wireless appli- cations. II. PROPOSED VCSEL BS DESIGN VCSELs are highly suitable to be deployed in BSs as they are well known for low-cost fabrication, compactness, and low power consumption [4]. The active layer of a VCSEL is sand- wiched between two planar distributed Bragg reflector (DBR) mirrors in which reflection spectrum of the top DBR mirror is shown in the top diagram of Fig. 2. The reflectivity which is de- signed to be exceedingly high ( 99.5%) to compensate for the 1041-1135/$25.00 © 2009 IEEE