244 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 1, JANUARY 1, 2006 Demonstration of All-Fiber WDM for Multimode Fiber Local Area Networks X. J. Gu, Waleed Mohammed, and Peter W. Smith Abstract—We have successfully demonstrated wave- length-division multiplexing in a conventional 62.5- m diameter graded index multimode fiber using a fiber Bragg grating written on a novel multicylindrical shell multimode fiber. The performance parameters of the device, such as insertion loss and interband and intraband isolations, are presented. The issues and improvements of the device are discussed. Index Terms—Add–drop module, fiber Bragg grating (FBG), mux and demux module, wavelength-division multiplexing (WDM), WDM in multimode fiber. I. INTRODUCTION W AVELENGTH-DIVISION multiplexing (WDM) is a technology that has revolutionized telecommunication. Systems with over 80 channels at a 50-GHz channel spacing in C-band have been designed and are commercially available from several equipment manufacturers. Applications of WDM technology are almost exclusively in long-haul fiber-optic net- works that use single-mode fibers. The components of choice in WDM technology are thin-film filters, arrayed waveguides, and fiber Bragg gratings (FBGs), all designed to be compatible with single-mode fibers. Graded-index (GRIN) multimode fibers are used predomi- nantly in local area networks (LANs) because of their relatively low modal group velocity dispersion and large fiber core size. With the increasing traffic in LANs, a number of research groups have proposed innovative designs to realize WDM in multimode fibers in order to achieve gigabits per second and even potentially terabits per second capacities [1]. The wave- length selection devices reported include the waveguide splitter and thin-film filters [2], liquid crystal etalon [3], grating-based monochromator, etc. [1]. Although these designs had achieved the WDM functionality, the adoption of bulk optics compo- nents has the drawback of high cost and high insertion loss in addition to the increased complexity. It is desirable to develop a compact all-fiber component for wavelength selection in a multimode WDM network. The fabrication of FBGs in GRIN multimode fibers has been reported previously [4]. However, due to the spectral spreading of different propagation modes, these FBGs usually display multiple reflection peaks with very low reflectivities that are Manuscript received August 25, 2005; revised October 13, 2005. This work was supported in part by the Natural Sciences and Engineering Research Council of Canada and by Photonics Research, Ontario. X. J. Gu is with the Department of Electrical and Computer Engineering, Ry- erson University, Toronto, ON M5B 2K3 Canada (e-mail: xgu@ee.ryerson.ca). W. Mohammed and P. W. Smith are with the Department of Electrical and Computer Engineering, University of Toronto, Toronto, ON M5S 3G4, Canada. Digital Object Identifier 10.1109/LPT.2005.861538 unsuitable for WDM applications. In 2001, Szkopek et al. pro- posed a novel multishell multimode fiber structure that permits the formation of narrow-band high-reflectivity FBGs [5]. The fabrication of such FBGs in this novel fiber was demonstrated two years later [6]. In this letter, we report the WDM in a 62.5- m diameter GRIN multimode fiber using an FBG fabricated in a multishell multimode fiber. Two multiplexed wavelengths separated by 2.4 nm were demultiplexed by an FBG into a transmission port and a reflection port, respectively. The performance parameters of the WDM, such as insertion loss, interband, and intraband isolation, are discussed. The results clearly demonstrate that all-fiber WDM in a multimode fiber network can be achieved. This low-cost compact design can be expanded for a WDM system with many wavelengths by cascading multimode FBGs fabricated at different wavelengths. The multimode FBG can also be used in spectroscopy to filter out unwanted spectral features, such as the excitation wavelength in the emission spectrum in fluorescence spectroscopy or the silica Raman feature in fiber-based Raman spectroscopy. II. EXPERIMENT The novel multishell fiber used for fabricating narrow-band high-reflectivity FBGs has a refractive index profile that consists of four alternating cylindrical shells of high and lower reflec- tive index [6]. Each high index shell is sufficiently thin so that only one mode can propagate. The structure was designed so the guided modes have approximately the same propagation con- stant at a preselected wavelength. In the multishell fiber used, the thickness of each shell is 2.53 m, the shell separation is 6.33 m, and the inner shell diameter is 25.3 m. The cladding and shell refractive indexes at 1.55 m are 1.4331 and 1.4351, respectively. In this way, when a periodic index is written in the fiber and the Bragg condition is satisfied, only one wavelength will be reflected. The FBGs were fabricated by focusing a collimated KrF ex- cimer laser (Lumonics, model PM844) beam though a phase mask onto a horizontally positioned fiber. The typical energy density of the 248-nm pulses at the fiber was 0.05 J/mm per pulse. The laser pulse rate was set at 30 Hz, while the exposure time was 5–8 min. The index profile of the FBG was apodized with a sinc function of 25 mm in length. The spectrum of the FBG was monitored during UV exposure with an optical spec- trum analyzer (Ando, model AQ-6310B). The multishell fiber was hydrogen loaded for two weeks under a pressure of 100 at- mospheres at room temperature to enhance its photosensitivity. All FBGs were annealed at 150 C for over 15 h to ensure their long-term stability. 1041-1135/$20.00 © 2005 IEEE