Hollow-core photonic bandgap fibers.
Scanning-electron-microscope image of a low-loss hollow-core photonic bandgap fiber.
Hollow-core photonic bandgap fibers can transport light in the core with very low loss because the cladding prohibits light transmission throughout a range of wavelengths called the bandgap. They are useful for high-power, ultrafast, and nonlinear optics applications. Some uses require both a wide bandgap and low attenuation, but a typical tradeoff for reducing attenuation in the fiber had the effect of decreasing the width of the bandgap.
J.C. Knight’s group at the University of Bath, U.K., found a way around this tradeoff: They carefully designed the fiber so that the cladding structure ends at what would be the natural edge of cells that make up the core (Opt. Express 16, 1142). The core in their fiber is a hole the size of seven cells. This results in a fiber with a wide bandwidth and low attenuation over the entire bandgap. The researchers measured a loss of less than 50 dB/km over 300 nm for a fiber operating at 1,550 nm.
Unlike previous fibers of this type, Knight’s group made the silica fibers with a periodic array of holes surrounding the core without an extra tube for the core, which simplifies the manufacturing process.
[ Yvonne Carts-Powell is a freelance science writer who specializes in optics and photonics. ]