Highly wavelength-dependent loss for short-pass edge filters with high cutoff slope

The discrepancy of refractive index dispersions between core and cladding materials in optical fibers can give rise to highly wavelength-dependent loss through fundamental-mode cutoff effect to serve as short-pass edge filters. A high cutoff efficiency including the good performance of a high cutoff slope and a large transmission loss for stopband is important to clearly separate the passband and stopband wavelengths. In this work, an SMF-28 fiber is heated and stretched until its diameter is reduced to approach a few tens of micrometers. An optical liquid, with its dispersion is highly different from that of silica fiber, is subsequently employed surrounding the tapered fiber to cause widely tunable fundamental-mode cutoff and the short-pass edge filter is thus obtained. The cutoff slope is found to be mainly controlled by the size of the shrunk core and can be optimized by choosing proper core and cladding size for tapered fibers.