We demonstrate distributed acoustic sensing (DAS) by interrogation of Rayleigh backscattering from fibers with long linearly frequency modulated pulses and coherent detection. This system provides sustained real-time phase demodulation without inline amplification over a range of 148 km in standard single mode fiber and up to 171 km in low-loss OFS TeraWave SCUBA 125 fiber. This is the longest reported range for DAS measurements. The optical dynamic range of the recording is 57 dB. With a 10 km fiber, we obtain a record-low interrogation noise above 50 Hz (rms average over position) of 134 and 89 µrad/√Hz with gauge lengths (equal to spatial resolution) of 10 and 34 m, respectively. A total harmonic distortion of −42 dB (rms average over position) is demonstrated with a gauge length of 10 m.
The layer-peeling method for reconstruction of fiber and waveguide gratings is extended to the case of birefringent reflective gratings with polarization-dependent background index and polarization-dependent effective index contrast. Using a discrete grating model, we characterize the set of possible reflection and transmission Jones matrices and show that for a given wavelength, the total structure can be represented by a discrete reflector sandwiched between two retardation sections. In reflection the discrete reflector acts as a partial polarizer. A method for designing birefringent gratings is developed and tested numerically.
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