Exposed-core single-mode-fiber channel-dropping filter using a high-index overlay waveguide.

Using the coupling between a standard single-mode fiber and single higher-order modes in a high-index overlay waveguide, we demonstrate a wavelength-filtering device with a channel-dropping response. Two types of filter are described. One uses a thin overlay waveguide, giving a single tunable dropped band response in the 1.2-1.6-microm wavelength region with rejection greater than 20 dB and an insertion loss of about 0.5 dB. The other uses a thick overlay waveguide and gives a comb-filter response with a dropped channel spacing of 13 nm with 20-dB rejection and an insertion loss of between 1 and 4 dB.