3118 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 19, OCTOBER 1, 2012 Grating Couplers for Fiber-to-Fiber Characterizations of Stand-Alone Dielectric Loaded Surface Plasmon Waveguide Components Michael G. Nielsen, Jean-Claude Weeber, Karim Hassan, Julien Fatome, Christophe Finot, Serkan Kaya, Laurent Markey, Ole Albrektsen, Sergey I. Bozhevolnyi, Guy Millot, and Alain Dereux Abstract—Dielectric loaded gratings (DLGs) comprised of polymer gratings lying on a thin gold film are used to couple light at telecommunication frequencies in and out of plasmonic waveg- uides featuring sub-micron cross-sections. The grating couplers are found to be efficient and easy to implement to perform direct fiber-to-fiber telecommunication characterizations of dielectric loaded surface plasmon polariton waveguide (DLSPPW) compo- nents. By analyzing the dispersion of the plasmonic Bloch modes supported by DLGs as a function of the period and the filling factor of the gratings, efficient couplers comprised of gratings with a filling factor around 0.5 are designed and fabricated by a simple one-step electron beam lithography process. Typical losses in the range of dB per coupler are obtained for gratings designed to operate at normal and 30 -tilted incidence. The performance of the couplers for normal incidence can be further improved by adding a back-reflecting Bragg mirror. We demonstrate the transmission of a 10 Gbits/s signal along a 75 m-long DLSPPW by using DLG couplers for light injection and extraction. A power penalty below below 0.4 dB on the bit-error-rate has been measured over the entire C-band demonstrating the suitability of DLSPPWs for Wavelength-Division-Multiplexed high bit rate traffic and the efficiency of DLG couplers for fiber-to-fiber char- acterizations of stand alone DLSPPW components. Index Terms—Dielectric loaded waveguides, grating couplers, surface plasmon Bloch mode. I. INTRODUCTION D IELECTRIC loaded surface plasmon polariton waveg- uides (DLSPPWs) are comprised of dielectric ridges de- posited on top of metallic films [1] or strips [2], [3]. Most often, the dielectric material in DLSPPWs is a polymer that can be en- Manuscript received February 27, 2012; revised August 02, 2012; accepted August 03, 2012. Date of publication August 08, 2012; date of current version September 26, 2012. This work is part of the European FP7 research program PLATON, under Contract 249135. This work was supported in part by the PI- CASSO platform and the Conseil Régional de Bourgogne and Ministère de l’En- seignement Supérieur et de la Recherche. M. G. Nielsen is with Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS-Université de Bourgogne, BP 47870, F-21078 Dijon, France, and also with the Institute of Technology and Innovation (ITI), University of Southern Denmark, DK-5230 Odense M, Denmark (e-mail: mgni@iti.sdu.dk). J.-C. Weeber, K. Hassan, J. Fatome, C. Finot, S. Kaya, L. Markey, G. Millot, and A. Dereux are with Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS-Université de Bourgogne, BP 47870, F-21078 Dijon, France (e-mail: jcweeber@u-bourgogne.fr). O. Albrektsen and S. I. Bozhevolnyi are with the Institute of Technology and Innovation (ITI), University of Southern Denmark, DK-5230 Odense M, Den- mark. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2012.2212418 gineered to be of interest for specific purposes such as passive plasmonic components [4]–[7], doped polymers [8] for plasmon losses compensation [9], thermo-optical routing [10]–[12] or switching of high-bit rate signals [13], [14]. Recently, DLSPPW components have been successfully integrated within a passive silicon optical circuitry [15] making the use of on-chip plasmon- based optical functionalities for datacom applications realistic. Although of great importance for fiber in and out coupling of on-chip plasmonic components, the use of a dielectric circuitry for the optical addressing of DLSPPWs is far from being op- timum for the development of plasmonic devices. Indeed, in this approach, the plasmonic components need to be fabricated onto chips equipped with dielectric waveguides, a solution that is nei- ther cost-effective nor efficient when many parameters of the plasmonic system need to be changed. In order to circumvent these limitations, the use of polymer waveguides for addressing DLSPPW components has been demonstrated [16]. However, with this approach, the use of specific substrates coated with a thick low refractive index layer is mandatory and secondly the modal mismatch between the bus polymer waveguides and the DLSPPW structures leads to quite large insertion losses. The so- lution consisting of using grating couplers to achieve the optical addressing of DLSPPW devices is not expected to be subject to the same drawbacks. As already shown in the context of sil- icon photonics, grating couplers are very efficient to perform the fiber-to-fiber optical interfacing of miniaturized optical waveg- uides [17], [18]. For silicon waveguides the grating couplers are often excited by a cleaved single-mode optical fiber. Given that the total length of plasmonic devices can be significantly smaller than the diameter of a typical optical fiber, cleaved fibers cannot be used for our purpose. In this work, we demonstrate an efficient fiber-to-fiber in and out coupling of DLSPPW components by using polymer grat- ings engraved on top of a metal film, denoted hereafter as di- electric loaded gratings (DLGs) and excited by moderately fo- cused spots. Unlike grating couplers engraved into the metal film [19], our approach can be used for fast prototyping of DL- SPPW components since it requires a single step micro-fabri- cation process. Typical losses in the range of 10 dB per coupler are obtained, making possible standard fiber-to-fiber telecom- munication characterizations of stand-alone DLSPPW devices. In the second section of this work, we discuss the properties of the DLG plasmonic Bloch modes. In particular, we estab- lish the dispersion of these Bloch modes as a function of the filling factor of the gratings. The role of the filling factor is ana- lyzed on the basis of approximate analytical results providing a 0733-8724/$31.00 © 2012 IEEE