MULTIPLEXED INTERFEROMETRIC FIBEROPTIC SENSORS WITH DIGITAL SIGNAL-PROCESSING

A microcontroller-based digital signal processing measuring pressure i n internal combustion engine source provides optical power for four in terferometric sensors. The laser current is repetitively modulated so that its optical frequency is nearly a linear function of time over mo st of a cycle. The interferometer phase shift is proportional to the e lapsed time from the initiation of a sawtooth until the sensor output signal level crosses a threshold value proportional to the laser outpu t power. This elapsed time, assumed to vary linearly with the combusti on chamber pressure, is determined by the use of a digital timer-count er. The system has been used with fiber Fabry-Perot interferometer tra nsducers for in-cylinder pressure measurement on a four-cylinder gasol ine-powered engine.