Abstract

The use of linearly chirped probe pulses in phase-sensitive optical time-domain reflectometry (Phi OTDR) technology has been recently demonstrated to allow for high-resolution, quantitative, and dynamic temperature or strain variation measurements in a simple and very robust manner. This new sensing technology, known as chirped-pulse Phi OTDR, had a maximum reported sensing range of 11 km. In this paper, a 75-km sensing range with 10-m spatial resolution is demonstrated by using bidirectional first-order Raman amplification. The system is capable of performing truly linear, single-shot measurements of strain perturbations with an update rate of 1-kHz and 1-n epsilon resolution. The time-domain trace of the sensor exhibits a signal-to-noise ratio (SNR) in the worst point of > 3 dB, allowing to monitor vibrations up to 500 Hz with remarkable accuracy. To demonstrate the capabilities of the proposed system, we apply 100 n epsilon vibrations in the noisiest point of the fiber, with a frequency modulated from 70 to 150 Hz over a period of 10 s. The results obtained in these conditions demonstrate a vibration detection SNR of > 20 dB (with only 300-ms analysis window and no postprocessing) and no evidence of nonlinearity in the acoustic response. The optical nonlinear effects that the probe pulse could suffer along the sensing fiber are thoroughly studied, paying special attention to potential distortions of the pulse shape, particularly in its instantaneous frequency profile. Our analysis revealsthat, for proper values of peak power, the pulse does not suffer any major.