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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