Figure 1 (IMAGE)
Caption
(a) Working principle of the TE-?OTDR technique. The fiber under test is probed by an optical frequency comb with a tooth spacing and a random spectral phase profile. The impulse response of the fiber is encoded on the backscattered signal generated by the propagation of the probe comb. This signal is beaten with a local oscillator, which is another optical frequency comb with the same random spectral phase profile. The LO and the probe comb are composed of the same number of lines, but the line spacing of the LO is slightly higher by an amount . The detection stage consists in a balanced photodetector followed by an electrical low-pass filter. The beating between lines of the probe comb and the neighboring lines of the LO comb results in a radiofrequency comb with a tooth spacing that is given by . This entails a down-conversion of the optical bandwidth, being the compression factor CF the ratio between and . Alternatively, the above process can be understood in the time domain as a large time expansion of the detected signal. (b) Temperature map of a hot point with 2 cm of length measured by the TE-OTDR scheme. A perturbation of 0.2 Hz is recovered. (c) Dynamic strain map around a 4 cm of length obtained by means of the range-extended TE-OTDR scheme. A perturbation of 5 Hz is recovered in this case.
Credit
by Miguel Soriano-Amat, Hugo F. Martins, Vicente Durán, Luis Costa, Sonia Martin-Lopez, Miguel Gonzalez-Herraez and María R. Fernández-Ruiz
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