The authors of the current paper belong to the leading group in the integration of electronic and photonic devices into an optical fiber. The MIT group has shown that it is possible to draw optical fibers potentially in kilometer lengths, incorporating metals, semiconductors, and isolating materials. They have shown in previous work transistors, photodiodes, and other functional devices in fiber. In this research, the authors introduce a new degree of freedom to be exploited in fiber devices with electrodes, starting from an ~1-m-long fiber that behaves as a distributed (1-m long) photodetector. They create a position-dependent voltage bias along the fiber, exploiting the different resistivities of a semiconductor core, a thin film of semiconductor, and a conducting polycarbonate. Each fiber segment is thus subjected to a different potential in the open-circuit bias condition (one power supply) and when both fiber extremes are biased with different power supplies. The electrical response of the fiber to external perturbation—in this case illumination of the fiber from the side—allows determining where the perturbation occurs. The interesting aspect of this development is that one does not need to have an electrical contact for every fiber segment. In this paper, the authors demonstrate the detection of up to three illuminated points simultaneously. Through the original use of a nonuniform bias along the fiber, the authors are able to avoid time-resolving the reading signal to extract the spatial information desired. In contrast to (electrical) time-domain reflectometry, the current technique does not use high-speed electronics and is not affected by dispersion.
The principle of the position-dependent voltage bias to determine the location of the perturbation can be possibly extended to other applications. In piezoelectric fibers, as the ones demonstrated by the authors, one could determine the position of a mechanical perturbation in a distributed way. They introduce a new way to treat a fiber as composed of a sequence of short-length elements, without the need to address each one with its own contacts. This paper adds a powerful tool to help in integrating many short-length devices in a single optical fiber.
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