Passive multiplexing of digital fiber-optic sensors.
A technique for passive multiplexing has been previously described that functions in the manner of an optical radar system. By injecting a short duration light pulse into a bus fiber and spacing sensor taps at discrete locations along the bus, a pulse reflected from each sensor location can be made to produce a sequence of time resolved pulses at the receiver end of the fiber. Simple time division demultiplexing techniques then suffice to separate the time delayed pulses returned from each sensor. By periodically injecting light pulses into the fiber, selecting the pulse train returned from a given sensor, and filtering the resultant signal to remove the pulse repetition frequency, parameters such as pressure, temperature, and displacement may be monitored by a sensor designed to modulate the intensity of the incident light in accordance with ambient conditions. Prior work addresses primarily the construction and multiplexing of analog sensors, specifically for optical hydrophones, for which case small changes in the intensity of the returned signal are to be detected. Here we report a variation of the time delay multiplexing technique where more than one time delay tap is assigned to a single sensor device to enable one to transmit a digital representation of the measured parameter. A digital signal is of great interest both because of the ease of handling digital data and because a digital system generally requires a far smaller SNR. The net result is that a practical digital time delay multiplexed system can be constructed and operated with very high multiplexing ratios. Conversely, the relaxation in optical power requirements for a digital system implies that a smaller number of sensors may be multiplexed using only reliable low power LEDs in place of lasers. Figure 1 shows one example of a digital fiber-optic angle position sensor as part of a system that uses time delay multiplexing. Light pulses are injected into a main fiber bus by an LED or pulsed laser source. A directional tap is placed at a preselected point along the bus fiber to extract a small portion of the optical pulse, and the light from the tap is sent to a star coupler to create a number of equal intensity components. Each component is transmitted via a fiber coil of