The first implementation of this technology required a microwave synthesizer to frequency sweep the optical source, coherent detection, and a 3-hour measurement time. More recently, other groups have demonstrated the same functionality with direct-detection (no local oscillator), but still with microwave-synthesized frequency sweeps and a data analysis requiring a large number of C-OTDR traces (4 s measurement time), or with chirped pulses requiring high bandwidth detectors. In the demonstration by Liehr et al., only two wavelengths are used (generated by current-modulation of an inexpensive diode laser) in an alternating pattern, and the data is extracted from time correlation of adjacent C-OTDR traces measured with a low-bandwidth p-i-n photodiode. This method allows for fast measurement times limited by the fiber length, though in the experimental results presented, the traces are averaged over milliseconds and the correlation is determined over a 2.5 minute time window. Still, the experiment is simple to implement and shows promise for practical structural health monitoring.
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