New in-line fiber band pass filters using high silica dispersive optical fibers

New types of in-line fiber filters whose core and cladding are composed of binary silica doped with either boron or fluorine are proposed. Utilizing dispersive characteristics of SiO₂-B₂O₃ and SiO₂-F systems, we demonstrate that an optical fiber, with borosilicate core and fluorosilicate cladding, shows short-pass filter response. Experimentally, we fabricated a short-pass filter fiber that could be used as a 1310/1550 nm WDM filter. We also propose theoretically that a long-pass filter could be obtained in a fiber structure whose core is fluorosilicate with borosilicate cladding. In a special long-pass filter structure, we found that the normalized frequency can be kept uniform over a wide wavelength range from 1200 to 1700 nm, which will be applied in wideband wavelength division multiplexing devices.