- One missing component in the next generation optical communications link is an ultra-fast tunable filter. This is a crucial element in dense wavelength division multiplexing systems. Here, a novel fiber loop configuration is proposed, which operates as such a filter. The filter implementation is based upon currently available technologies. It is shown that with the suggested technique, optical selective elements can be generated with a tuning speed of a few nanoseconds. This is three orders of magnitude faster than other currently available filters. The filter can be designed as compact and is immunized to external instabilities. Using compound configurations, the filter can be designed with a very wide range of filter finesse and free spectral range.