Broadband single-mode operation of standard optical fibers by using a sub-wavelength optical wire filter

We report the use of a sub-wavelength optical wire ( SOW) with a specifically designed transition region as an efficient tool to filter higher-order modes in multimode waveguides. Higher-order modes are effectively suppressed by controlling the transition taper profile and the diameter of the sub-wavelength optical wire. As a practical example, single-mode operation of a standard telecom optical fiber over a broad spectral window ( 400 similar to 1700 nm) was demonstrated with a 1 mu m SOW. The ability to obtain robust and stable single-mode operation over a very broad range of wavelengths offers new possibilities for mode control within fiber devices and is relevant to a range of application sectors including high performance fiber lasers, sensors, photolithography, and optical coherence tomography systems. (C) 2008 Optical Society of America.