Multiplexed fiber optic Bragg-grating strain sensor system for use in marine vehicle testing

A novel approach for the interrogation of multiplexed fiber optic Bragg-grating strain sensors is presented. It is a ti,, domain multiplexing technique based on using pulsed radio frequency modulation combined with slow wavelength tuning of a distributed feedback laser diode. A derivative-type signal for each sensor is generated in the receiver and the instantaneous Bragg-wavelength, proportional to the applied strain, is determined from the timing of the zero-crossing of the derivative signal. The Bragg-gratings, all with the same Bragg wavelength, are coupled to the output of a 1 x N fiber optic coupler with appropriate relative fiber delays between each sensor to enable time-division multiplexing. A four channel system has been designed, implemented, and tested in a laboratory environment, and later mounted on the hull of a scaled catamaran model and tested at a ship testing facility. The strain measurements were compared to measurements using standard strain gauges. The system had a dynamic range of about 680 microstrain, limited by the laser diode current tuning, and a resolution of approximately 1 microstrain, limited by interferometric background signals. The bandwidth was limited to about 50 Hz due to the laser diode tuning response.