Fibre-optic tapped delay line filter employing coherent optical processing

The letter describes a novel form of optical fibre tapped delay line signal processor in which the signal taps are achieved without introducing couplers into the fibre, by the use of external piezoelectric phase modulators located along the single-mode fibre. This enables the system to operate as a coherent processor and the concept has a number of attractive features including reduced optical losses, increased dynamic range and capability for electronic control of fibre taps. A simple laboratory demonstration of a two-tap filter is described to confirm the operation of the system.