Precision Dynamic Sensing With Ultra-Weak Fiber Bragg Grating Arrays by Wavelength to Frequency Transform

Abstract

A high-speed interrogator using wavelength to frequency transform is proposed for high-precision demodulation of ultra-weak fiber Bragg gratings (UWFBGs). An amplified spontaneous emission source, modulated by a frequency-swept microwave signal, is adopted as the light source. The signal reflected by the UWFBGs is temporally broadened by a dispersion element, which is then detected and mixed with the driven microwave signal. Through wavelength-dependent time delay introduced by the dispersion element and the frequency sweeping of the microwave signal, the wavelength of the fiber Bragg grating is converted to a beat frequency by the mixing process. Due to the ambient temperature fluctuation, the effective refractive index of the sensing fiber varies over time. Consequently, the induced time delay varies with temperature, generating errors in the wavelength demodulation. By using a dispersion compensating fiber and a single-mode fiber, which introduce opposite group-velocity dispersion (GVD), such error is effectively reduced. Moreover, a dispersion correction model, which considers the wavelength dependence of GVD, is established to replace the traditional dispersion model. Experimental demonstration verifies that the interrogator, operating at 80 kHz, shows high wavelength demodulation accuracy of 6.96 pm.