Accurate tuning of mismatched twin-core fiber filters

We have demonstrated a reproducible and accurate way of tuning the coupling wavelength of a mismatched twin-core filter. This method allows high quality all-fiber filters to be implemented with low loss at any desired wavelength over a range of few hundred nanometers from the same fiber. An similar to 500-nm tuning range is demonstrated, limited only by the measurement setup and not by the technique itself. Highly accurate control of the filtering wavelength in combination with control of filtering strength by adjustment of the length of the twin-core fibers allows complex filter profiles to be implemented by cascading of several different filters. These filters are also very stable to a change in temperature (similar to 0.26 nm/100 degrees C at 1.55 mu m) and strain (less than 0.3 nm/mstrain at 1.55 mu m), permitting easy packaging. (C) 1998 Optical Society of America.