1566 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 6, JUNE2004 New Methodology to Evaluate the Performance of Ring Resonators Using Optical Low-Coherence Reflectometry Y. Gottesman, E. V. K. Rao, and D. G. Rabus Abstract—This paper describes the efficient implementation of optical low-coherence reflectometry (OLCR) measurements to evaluate the performance of InP-based high-index contrast ring resonators. Using examples of racetrack ring resonators, this paper shows here for the first time that all characteristic parameters relevant to resonator design (coupling coefficient , propagation losses , and optical cavity length ), and ultimately its performance, can be extracted in a straightforward manner. This is accomplished by introducing a new methodology that im- plicates OLCR measurements in transmission and also reflection modes as an alternative approach, in comparison with conven- tional spectral analysis, to extract resonator design parameters. Index Terms—Filter analysis, integrated optics, interferograms and reflectograms, optical low-coherence reflectometry (OLCR), racetrack-shaped resonators, ring design parameters, ring res- onators, transmission and reflection modes, wavelength-division multiplexing (WDM). I. INTRODUCTION S EMICONDUCTOR-BASED high-index contrasted mi- croring resonators undoubtedly offer highly promising and elegant solutions to accomplish a large variety of optical functions crucial to dense-wavelength-division-multiplexing (DWDM) networks. A few examples are the signal routing, wavelength filtering, and more specifically the add/drop func- tion of a wavelength [1]–[3]. In all these applications, the most sought characteristics of a ring resonator are its intrinsic quality or the factor, its finesse , and the free spectral range (FSR). Obviously, these characteristics are critically dependent on the ring-design-related parameters such as propagation loss ( ), coupling coefficient ( ), and the optical cavity length ( ). Conventionally, the performance of ring resonators is eval- uated in the wavelength space (or space) by recording the Manuscript received October 28, 2003; revised March 24, 2004. This work has been supported by the Région Ile de France, SESAME Project 1377, and by the Conseil Générale de l’Essone. Y. Gottesman was with the Laboratoire de Photonique et de Nanostructures (LPN), Centre National de la Recherche Scientifique (CNRS), Marcoussis 91460, France. He is now with the Institut National des Telecommunications, Evry 91011, France. E. V. K. Rao is with the Laboratoire de Photonique et de Nanostructures (LPN), Centre National de la Recherche Scientifique (CNRS), Marcoussis 91460, France. D. G. Rabus was with the Heinrich–Hertz–Institut fuer Nachrichtentechnik, Berlin 10587, Germany. He is now with Forschungszentrum Karlsruhe GmbH, Institut fur Mikrostrukturtechnik, Karlsruhe 76021, Germany. Digital Object Identifier 10.1109/JLT.2004.829216 filter response under very high spectral resolution often in a limited domain. The ring-design-related parameters are subse- quently extracted from a best fit between computed and mea- sured spectral responses [4]–[6]. This task, however, becomes tedious and time consuming for filters with high and high values, since they impose the use of a high-resolution spec- trometer and a finely tunable laser source in addition to remark- able experiment stability. In addition, such high resolutions are not easily accessible to an everyday optical spectrum analyzer (OSA). Furthermore, in the early stages of resonator conception and fabrication, the spectral analysis of filters can be delicate due to the presence of unwanted reflections in the ring and/or in the surrounding optical circuit. In this context, this paper proposes and demonstrates an al- ternative approach that implies the use of optical low-coherence reflectometry (OLCR) technique to extract the ring resonator design parameters almost in a straightforward way by analyzing devices both with and without antireflective (AR) coatings. This paper is partitioned into different sections, each con- taining specific information. Based on the analogy between ring resonators and conventional Fabry–Pérot (FP) filters, Section II first develops a set of arguments that confirm undeniably the suitability of low-coherence reflectometry (which is known to explore filter information in the time or space domain) to analyze ring resonators. As these devices operate exclusively in throughput conditions, the remainder of this section describes a novel measurement procedure using OLCR in transmission mode to facilitate the recording of interferograms. Section III is devoted to the newly developed methodology wherein a procedure to extract resonator design parameters from the OLCR recorded interferograms is detailed. Here, a few examples are provided of interferograms computed with and as parameters deliberately chosen in the range of most probable values. Section IV is completely assigned to the presentation of data—both measured and computed—and their exploitation to extract the ring resonator design parameters ( , , and ). Since OLCR measurements in conventional reflection mode can also provide useful information needed for filter conception, we further included such data in this section together with a specially adapted procedure to extract ring de- sign parameters from experimental reflectograms. Finally, the principal features of this work, namely, the efficient implication of OLCR measurements to analyze high-index contract ring resonators on InP are summarized in Section V. 0733-8724/04$20.00 © 2004 IEEE