We report the scanning near-field optical microscopy (SNOM) characterization of a 4×4 multimode interference (MMI) device working at a wavelength of 1.55 µm and designed for astronomical signal recombination. A comprehensive analysis of the mapped propagating field is presented. We compare SNOM measurements with beam-propagation-method simulations and thus are able to determine the MMI structure's refractive-index contrast and show that the measured value is higher than the expected value. Further investigation allows us to demonstrate that good care must be taken with the refractive-index profile used in simulation when one deals with low-index contrast structures. We show evidence that a step-index contrast is not suitable for adequate simulation of our structure and present a model that permits good agreement between measured and simulated propagating fields.
© 2005 Optical Society of America
(120.3180) Instrumentation, measurement, and metrology : Interferometry
(130.3120) Integrated optics : Integrated optics devices
(180.5810) Microscopy : Scanning microscopy
(230.7370) Optical devices : Waveguides
Matthieu J. Martin, Taha Benyattou, Régis Orobtchouk, Frédéric Rooms, Alain Morand, Isabelle Schanen, and Pierre Benech, "Scanning near-field optical microscopy as a tool for the characterization of multimode interference devices," Appl. Opt. 44, 2558-2563 (2005)