Mode-Selective Characteristics of an Optical Fiber With a High-Index Core and a Photonic Bandgap Cladding

The modal characteristics of an optical fiber consisting of a high-index core and a photonic bandgap cladding surrounded by a high-index outer ring are investigated. The cladding of the fiber is formed with periodically arranged high-index rods that support cladding modes with effective indexes crowded over a narrow region above the bandgap. With this fiber structure, the core modes with effective indexes lower or higher than those of the cladding modes can be transmitted at a low loss, while the core modes with effective indexes matched to those of the cladding modes suffer from a high loss. Numerical results are presented to demonstrate the operation principle of the fiber and the possibility of selectively eliminating the fundamental mode or a high-order mode in a few-mode fiber over a wide operating wavelength range. This special fiber can be employed to realize effective mode filters for mode-sensitive applications, such as mode-division multiplexing.