Sinusoidal frequency scan OFDR with fast processing algorithm for distributed acoustic sensing

Recently it was shown that sinusoidal frequency scan optical frequency domain reflectometry (SFS-OFDR) can achieve remarkable performance in applications of distributed acoustic sensing (DAS). The main advantage of SFS-OFDR is the simplicity with which highly accurate sinusoidal frequency scans can be generated (in comparison with linear frequency scans). One drawback of SFS-OFDR has been the computationally intensive algorithm it required for processing of the measured backscatter data. The complexity of this algorithm was O(N²) where N is the number of backscatter samples. In this work a fast processing algorithm for SFS-OFDR, with computational complexity O (N log N), is derived and its performance and limitations are studied in details. The new algorithm facilitated highly sensitive DAS operation over a sensing fiber of 64km, with 6.5m resolution and scan rate of 400Hz. The high sensitivity of the system was demonstrated in a field trial where it successfully detected human footsteps near the end of the fiber with excellent SNR.