Enhancement of Dynamic Range of Optical Fiber Sensor Using Fiber Bragg Grating Fabry-Perot Interferometer With Pulse-Position Modulation Scheme: Compensation of Source Wavelength-Sweep Nonlinearity

A fiber optic vibration sensor using a fiber Bragg grating Fabry-Perot interferometer with a wavelength-swept laser diode source, which works well for the case of small strain-amplitude vibration, exhibits inaccuracies in the interrogation caused by the nonlinearity of the wavelength modulation in the source when the strain amplitude due to vibration is large. With increasing the frequency of the wavelength sweep, the span of the wavelength sweep is reduced and the nonlinearity is pronounced. We experimentally characterize the nonlinearity, and by using a fiber Fabry-Perot interferometer as a wavelength reference, we compensate it to enhance the accuracy of the interrogation for large strain-amplitude vibration. The proposed sensor achieves vibration measurement with a strain resolution of 0.3 n epsilon/root Hz, measurement time of 10 mu s, and maximum measurable value of 34 mu epsilon at the wavelength-sweep frequency of 100 kHz under the present condition. The measurement time is limited by the capability of the laser diode driver, and the maximum measurable strain would be easily enhanced by increasing the wavelength-modulation current amplitude.