Successive Asymmetric Abrupt Tapers for Tunable Narrowband Fiber Comb Filters

We experimentally demonstrate wavelength tunable narrowband comb filters with the extinction ratio of 31.8 dB by cascading several abrupt fiber tapers in series in a standard single-mode fiber (SMF) and a highly Er/Yb-codoped fiber (EYDF), respectively. The fiber is slightly inclined and immersed in a micro-flame where the bilateral edges of flame can provide a higher heating temperature than the center of the flame to achieve two asymmetric abrupt tapers. Adjacent to the last two abrupt tapers, this procedure is repeated several times to make successive abrupt tapers. The individual abrupt taper in the grouped asymmetric abrupt tapers has different waist diameters from each other. The grouped abrupt tapers are then immersed in optical liquids for tunable narrowband comb filters.