Contents I. Introduction
The use of multimode fiber (MMF) for the last mile access networks has attracted much attention in the last years due to the MMF's dominant position in already installed fiber infrastructure inside buildings. Research efforts have been mainly dedicated to enhance baseband digital transmission performance of MMF links [1], as well as to develop low-cost radio-over-fiber (RoF) techniques for short MMF distances [2]. For the emerging broadband wireless systems operating at carrier frequencies in the microwave/millimeter wave bands, the theoretical modal bandwidth limit imposed by the well-known bandwidth-distance product, confines the use of MMF to very short links. Exploiting the passband transmission region of MMF, microwave signals can be transmitted over MMF links [3], but the passband region differs with fiber length variations, which might not be practical in the rollout of a radio-over-MMF-based infrastructure. Another method to overcome this theoretical bandwidth limitation is the optical frequency multiplication (OFM) principle [4]. Preliminary experiments have demonstrated the remote delivery of microwave signals up to 18 GHz over an MMF link [5]. However, to our best knowledge, the fact of surpassing this theoretical modal bandwidth has not been explained accurately yet. In this letter, we investigate the OFM principle and its application to MMF transmission with some theoretical models. In addition, we present the results of an experimental analysis of the OFM performance for the generation of microwave carriers up to 40GHz and their transmission over an MMF link. Upconversion and transmission of M-ary quadrature-amplitude-modulated (M-QAM) radio signals in the 24- to 30-GHz band is also demonstrated and compliant with the requirements of wireless standard IEEE 802.11a.
