Non-phase-matched tunable band rejection in an all-solid photonic bandgap fiber with high-index rods on graded-index pedestals.

A new type of an all-solid photonic bandgap fiber for a non-phase-matched tunable band-rejection filter was proposed and fabricated by introducing a hexagonal array of high-index rods surrounded by graded-index pedestals in silica cladding. Due to the graded index and subsequent weak confinement of light, the proposed fiber showed two contrasting transmission spectra: flat transmission for a long fiber segment of ~1 m in contrast to typical bandgap transmission in a short fiber segment of ~10 cm. For the 120-cm-long fiber, we observed unique band-rejection transmission without any requirement of phase-matching conditions, whose rejection strength was tunable by mechanical perturbations such as bending and twisting. Detailed device principles, fiber design, fabrication, and transmission characteristics are discussed in both theory and experiment.