1258 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 11, JUNE 1, 2006 Compact Modal Interferometer Built With Tapered Microstructured Optical Fiber Joel Villatoro, Vladimir P. Minkovich, Member, IEEE, and David Monzón-Hernández, Member, IEEE Abstract—A modal interferometer was built with tapered large- mode-area microstructured optical fiber (MOF). The tapering was introduced by slowly stretching the fiber while it was heated with a high temperature flame torch. With this “slow-and-hot” tapering process, the MOF air holes collapse and the taper waist is trans- formed into a solid unclad multimode fiber. This allows the cou- pling between the fundamental HE MOF mode and the HE modes of the solid fiber. The beating of the HE modes makes the transmission spectra of the taper to exhibit an oscillatory pat- tern. The influence of the taper geometry and wavelength on the interference pattern is discussed. The interferometer can be used for diverse applications. Index Terms—Microstructured optical fibers (MOFs), modal interference, optical fiber interferometers, optical fiber sensors, tapered fibers, variable attenuator. I. INTRODUCTION A LL-FIBER interferometers are important devices since they are useful in diverse applications; ultrahigh-resolution metrology and light modulation are just two examples. Fiber interferometers can also be key devices in modern instruments such as gyroscopes. So far, different approaches have been proposed to construct interferometers with conventional optical fibers [1]–[6]. The most compact ones are modal interferome- ters since they can be constructed in a short section of optical fiber [3]–[6]. Modal interferometers exploit the relative phase displacement of the first two fiber modes HE and HE (or LP and LP ). Some drawbacks of these interferometers are, among others, the critical launching conditions, the polarization and temperature dependence, and the limited wavelength range ( 150 nm) in which they operate. The advent of microstructured optical fibers (MOFs) enables new possibilities to construct interferometers that can be impor- tant for many new application areas. By cascading two iden- tical long-period gratings or by using an MOF with air holes of different diameters, novel modal interferometers can be con- structed [7], [8]. An advantage of these approaches is the broad operating range ( 650 nm) of the interferometers. However, the requisite of two identical gratings, the critical launching condi- Manuscript received November 29, 2005; revised March 7, 2006. This work was supported by the Consejo National de Ciencia y Tecnologia, Mexico, under Grant 42 986-F and Grant P46972-F, and by the “Ramón and Cajal” Program of the Ministerio de Educación y Ciencia, Spain. J. Villatoro is with the ICFO-Institut de Ciències Fotòniques, 08860 Castelldefels, Barcelona, Spain (e-mail: joel.villatoro@icfo.es). V. P. Minkovich and D. Monzón-Hernández are with Centro de Investiga- ciones en Optica A. C., 37150 León GTO, Mexico (e-mail: vladimir@cio.mx; dmonzon@cio.mx). Digital Object Identifier 10.1109/LPT.2006.875520 Fig. 1. Schematic representation of a modal interferometer based on tapered MOF. and are, respectively, the length and diameter of the solid waist. The shadowed area represents the polymer coating of the MOF. tions, or the polarization and temperature dependences are still an inconvenient. Here we demonstrate that a tapered MOF with collapsed air holes over a localized region is suitable for the construction of compact modal interferometers (see Fig. 1). By collapsing the air holes, a zone of the MOF is transformed into a solid unclad multimode optical fiber. As a consequence, the funda- mental HE mode of the holey fiber is coupled to the HE modes of the solid fiber. The beating between the modes makes the transmission of the taper versus wavelength to exhibit an oscillatory pattern. The influences of the taper diameter, length, and wavelength on the performance of the interferometer are discussed. As an application, we demonstrate an in-line vari- able attenuator for the wavelength of 1550 nm. II. DEVICE FABRICATION AND WORKING MECHANISM A large-mode-area MOF, with solid core and four full rings of air holes in the cladding [9], was employed to construct a modal interferometer. Such a fiber had the following parame- ters: core diameter of 11 m, average hole diameter of 2.7 m, average hole spacing (pitch) of 5.45 m, and outside diameter of 125 m. This MOF guides light by means of the modified total internal reflection effect and it is single mode from 600 nm [9]. The fiber was tapered by gently elongating it while a zone of length was heated with a high temperature oscillating flame torch. With this “slow-and-hot” tapering process, one gets a uniform-waist tapered MOF in which the air holes collapse (see Fig. 1). Our process is the opposite of the “fast-and-cold” method proposed previously for tapering MOFs, preserving the holey structure (see, for example, [10]). 1041-1135/$20.00 © 2006 IEEE