Abstract

In coherent optical transmission systems and sensors, passive quadrature detection1 is frequently used as it facilitates unambiguous measurement of amplitude and phase and avoids the need for an optical phase-locked loop. This requires the use of some kind of multiport network with the appropriate phase shifts. A network to produce three outputs 120° apart is easily implemented in a three-fiber fused coupler.2 However, this arrangement is not the optimum tor sensitivity or rejection of local oscillator intensity noise. Implementation of a multiport network to produce true quadrature (90°) and balanced outputs has proved more difficult, requiring precise control of differential phase delays.1 Various techniques have recently been proposed3,4 which achieve this by birefringence or mode coupling in a multiport coupler, and in all these the phase relationship of the outputs is determined by coupling in the fiber system.

© 1987 Optical Society of America

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