Chirped-pulse Phase-sensitive Reflectometer Assisted by First Order Raman Amplification

Abstract

The use of linearly chirped probe pulses in phase sensitive-(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 PhiOTDR, 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 nepsilon 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 20 dB (with only 300 ms analysis window and no post-processing) 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 reveals that, for proper values of peak power, the pulse does not suffer any major distortion and therefore the system performance is not compromised.