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One of the stringent requirements on the receivers of distributed industrial fiber optic systems is fast and accurate adaptive thresholding and a wide dynamic range. This is due to the existence of variable link lengths and geometry, especially in multipleaccess situations, coupled with unipolar optical signaling. This paper analyzes the application of a simple spread-spectrum system in such fiber-optic data links. It proposes a detector model that yields a tractable expression for the probability of error. The optimum threshold and the probability of error are then related to the received optical power, bit rate, and spectrum-spreading factor. A procedure is indicated by which, based on the minimum expected received power, a spreading facor and a threshold are chosen to insure a given maximum error rate at all the receivers.