To be submitted to OFC 2003 FBG-based Multi-Channel Low Dispersion WDM Filters A. Doyle, C. Juignet, Y. Painchaud, M. Brown, N. Chummun-Courbet, É. Pelletier, and M. Guy TeraXion, 20-360 Franquet St., Sainte-Foy, Québec, Canada, G1P 4N3 Phone: (418) 658-9500; Fax: (418) 658-9595; e-mail address: adoyle@teraxion.com Abstract: Multi-channel FBGs for use as low dispersion WDM filters are reported. The filters are realized in a single section of optical fiber using superposition techniques and are designed for use in high speed networks (10-40Gbit/s) where they may be employed in OADM applications. 1. Introduction Fiber Bragg gratings (FBGs) are a well established technology for the fabrication of components for optical telecommunications applications such as Wavelength-Division-Multiplexing (WDM) [1]. In simple terms a Bragg grating allows light propagating along an optical fiber to be reflected back when its wavelength is equal to 2n eff Λ where Λ is the grating period and n eff is the effective refractive index of the fiber core at the center wavelength. This paper presents details as to the superposition of multiple low dispersion WDM filters in a section of optical fiber resulting in a multi-channel low dispersion WDM filter. We demonstrate that these filters have attractive optical performances which render them useful for many telecommunications applications such as low-loss optical add-drop multiplexing (OADM). 2. Motivation Low dispersion WDM filters based on FBGs have attracted increased interest over the past few years [2]. These filters are attractive, not only because of their flat top spectral response, steep spectral roll off and low insertion loss, but also due to their flattened in-band group delay profile which permits their use in high speed networks (≥ 10 Gbit/sec). Low dispersion filters (LDF) typically exhibit up to a 10-fold reduction in group delay variation across a reflected band when compared with standard FBG-based WDM filters. The in-band group delay profile for a standard WDM filter is typically a parabolic function with a variation of up to 100ps, this compares poorly with the maximum variation of < 15ps typically observed for a LDF. Fig. 1a illustrates the difference in in-band group delay for a LDF and a standard 50GHz WDM filter. The net effect of such a reduction in in-band group delay variation is that for a given filter configuration a LDF has a wider usable bandwidth available for error-free high speed (≥ 10 Gbit/sec) data transport than a standard WDM filter (Fig. 1b). -30 -25 -20 -15 -10 -5 0 1544.6 1544.7 1544.8 1544.9 1545.0 1545.1 1545.2 1545.3 1545.4 Wavelength (nm) Reflectivity (dB) 50 75 100 125 150 175 200 Group Delay (ps) 1.E-10 1.E-09 1.E-08 1.E-07 1.E-06 -15 -10 -5 0 5 10 15 20 Dentuning of Laser from Filter Central Wavelength (GHz) Bit Error Rate Low Dispersion Filter Standard Filter Standard Filter Usable Passband Low Dispersion Filter Usable Passband Fig. 1. a.) Reflection spectrum and corresponding group delay for a low dispersion 50GHz WDM filter and a standard WDM 50GHz filter. b.) BER Plot illustrating improvement on bandwidth utilization for low dispersion 50GHz WDM filter versus standard 50GHz WDM filter at 10GBit/s. A LDF is normally produced by writing a grating with a complex apodization profile in an optical fiber. The apodization profile serves, not only, to reduce the sidelobes observed in the grating’s reflection spectrum but also to equalize the in-band group delay variation by introducing a series of phase shifts along the grating’s length [2].