John Wallace
To a degree that depends on its makeup, glass is photosensitive to ultraviolet (UV) radiation. Passing UV light though a periodic mask onto an optical fiber makes possible the fabrication of long-period optical-fiber gratings, which can serve as narrow-band optical filters, mode converters, or sensors. However, only specialized glasses have sufficient photosensitivity to make effective devices, whereas the most commonly used optical fibers are of pure or low-doped fused silica.
Researchers at Waseda University (Tokyo, Japan) and the Université de Montréal (Montreal, Quebec, Canada) are bypassing the photosensitivity problem by using ion implantation, which changes the fiber`s refractive index by glass compaction. Starting with a standard single-mode fiber etched to 53-µm cladding diameter, the researchers use the Université de Montréal tandem accelerator to pass helium ions through a metal amplitude mask containing 29 periods of a 170-µm-period grating with 60-µm openings. The ions implant themselves to a depth of 24 µm, reaching the fiber`s core. The result for 1410-nm light passing through the fiber is a sharp 15-dB coupling of the core mode to a cladding mode. A side-effect-high background loss-should be alleviated by using a mask with smaller openings. Fiber Bragg gratings could be made using a short-pitch mask. For information, contact Yoshimichi Ohki at [email protected].