We report the application of a dual polarization distributed feedback (DFB) fiber laser as a strain and temperature sensor. By measurement of the absolute wavelength of one polarization as well as the polarization beat frequency, strain and temperature were determined simultaneously. The sensor has an accuracy of +3 microepsilon and +/-0.04 degrees C. Self-heating of the DFB fiber laser as a function of pump power was measured with this sensor.
Contributions to the acoustic signal sensitivity of fiber distributed-feedback (DFB) lasers in air are investigated both theoretically and experimentally. The theoretical results show that the dominant contribution to the laser frequency shift comes from adiabatic temperature shifts in the surrounding air at lower frequencies and from pressure at higher frequencies. The transition frequency was found to be between 5 and 20 kHz, depending on the elastic boundary conditions of the fiber laser. The acoustically induced frequency shifts of two fiber DFB lasers were measured, and the sensitivities varied from 0.61 MHz/Pa at a 100-Hz acoustic frequency to 0.34 kHz/Pa at a 15-kHz acoustic frequency.
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.
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