Coding-nhanced Ultrafast and Distributed Brillouin Dynamic Gratings Sensing Using Coherent Detection

Probe coding can significantly improve the signal to noise ratio and measurement speed in Brillouin Dynamic Grating (BDG) fiber-optic distributed sensing, but only if coherent reception is employed. This is because both the amplitude and phase of the BDG impulse response vary along the fiber due to fiber nonuniformities, as well as the effect of the distributed measurand. As a result, the backreflections from individual code chips add coherently and, therefore, the total backscattered return cannot be decoded by a direct detection receiver (excluding some rare scenarios). Here, following a theoretical derivation of the BDG impulse response, a fully coherent BDG-based sensor is reported. Using a 64-chip Golay bipolar (phase) code, an eightfold sensitivity improvement (over single pulse interrogation) is demonstrated. With a code repetition rate of 1 MHz, basically determined by the length of the fiber, 750-Hz strain oscillations were measured with no averaging, with a spatial resolution of 20 cm.