A fiber optic sensor system includes an optical source to output a first optical signal to launch into an optical fiber, and a coherent detector to mix a coherent Rayleigh backscatter signal generated by the optical fiber in response to the first optical signal with a second optical signal output by the optical source and to generate a mixed output signal. A phase detection and acquisition system determines a phase difference between first and second locations along the optical fiber based on phase information extracted from the mixed output signal and combines the phase information extracted from multiple acquisitions to detect strain on the optical fiber sensor.