Efficient fiber-optic structure with applications to sensor arrays

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Abstract
A fiber-optic structure which performs the functions of sensing and telemetry with a minimum of components and with efficient utilization of optical power is described. This structure, referred to as a recursive lattice array, requires N+1 couplers and N fiber sensing loops to realise N sensors. It is shown that for pulsed operation, the duty cycle approaches 100% and the maximum sampling rate is 1/(N+1)T, with T denoting the transit time of a single sensing loop. In the ideal (lossless) case, the power returned to the receiver from any sensor is -10 log 2N referred to the input, compared with previously reported, nonrecursive structures for which this figure-of-merit is -20 log N. Expressions for the optimum coupler tap ratios for two different cases of interest are derived: first, for the case in which all the coupler tap ratios are equal, and second, for the case where they may assume different values. The magnitudes of decaying recirculating terms which add noise to the desired primary returns from each sensor are estimated. Methods for reducing the magnitudes of the undesired terms are outlined.