We demonstrate a Brillouin optical time domain analysis sensor based on a phase-modulated probe wave and RF demodulation that provides measurements tolerant to frequency-dependent variations of the pump pulse power induced by non-local effects. The tolerance to non-local effects is based on the special characteristics of the detection process, which provides an RF phase-shift signal that is largely independent of the Brillouin gain magnitude. Proof-of-concept experiments performed over a 20-km-long fiber demonstrate that the measured RF phase-shift spectrum remains unaltered for large frequency-dependent deformations of the pump pulse power. Therefore, it allows the use of a higher optical power of the probe wave, which leads to an enhancement of the detected signal to noise ratio. This can be used to extend the sensing distance, to improve the accuracy of the Brillouin frequency shift measurements, and to reduce the measurement time.

Brillouin optical time-domain analysis (BOTDA) sensors have great potential to provide distributed measurements of temperature and strain over large structures with high spatial resolution and measurement precision. However, their performance ultimately depends on the amount of probe and pump pulse power that can be injected into the sensing fiber, which determines the signal-to-noise ratio of the detected measurement signal. The probe wave power is constrained by the generation of noise induced by spontaneous Brillouin scattering and at lower power by the so-called non-local effects. In this work, we focus on the latter. We review the physical origins of non-local effects and analyze the performance impairments that they bring. In addition, we discuss the different methods that have been proposed to counteract these effects comparing their relative merits and ultimate performance. Particularly, we focus on a technique that we have devised to compensate non-local effects which is based on introducing an optical frequency modulation or dithering to the probe wave. This method is shown to provide a comprehensive solution to most of the impairments associated with non-local effects and also to enable some side benefits, such as amplification of the pump pulses to compensate the attenuation of the fiber.

We report on the effects of large pump pulse powers on Brillouin optical time-domain analysis (BOTDA) sensors based on phase-modulated probe wave and coherent self-heterodyne detection. These sensors are particularly suitable to perform dynamic strain and temperature measurements because the radio-frequency (RF) signal that is obtained when the probe wave is detected has a phase-shift spectrum that is independent to first order of the Brillouin gain. Therefore, a fixed optical frequency separation between pump and probe wave can be deployed and uses the stable RF phase-shift spectrum to obtain the Brillouin frequency shift (BFS) from measured changes in the probe RF phase-shift. However, in this paper, it is found that there is a narrowing of the RF phase-shift spectrum that depends on the Brillouin gain. This effect becomes significant when very high power pulses are used and the resultant large gain induces a narrowing of the RF phase-shift spectrum. This narrowing leads to a BFS measurement error when the sensor is configured for dynamic measurements. We analyze, theoretically and experimentally, the origins and the magnitude of the narrowing of RF phase-shift spectra for high pump pulses in a coherent BOTDA sensor. Furthermore, this spectral shape change is compared to the broadening of the gain spectrum that has been recently discovered in conventional direct-detection BOTDA sensors, which is linearly dependent on the pulse peak power injected to the fiber, finding that the spectral shape change is less significant in coherent BOTDA sensors. Finally, we quantify the BFS measurement error that it can induce and find the trade-offs to keep it below a certain threshold. It is found that, from a practical point of view, this effect is significant for short fibers, where nonlinear effects are negligible and large pump pulses can be used.

We demonstrate a technique to mitigate the residual second-order non-local effects in Brillouin optical time-domain analysis (BOTDA) sensors in which the Brillouin frequency shift (BFS) profile is not uniform along the fiber. It is based on adding a wavelength modulation to the probe wave that makes it track the average BFS found along its way. Using this method we are able to inject a total probe wave power of 15 dBm in a 120-km sensing fiber link, which, to the best of our knowledge, is the highest probe power ever demonstrated in a long-range BOTDA sensing fiber link. The enhancement in the detected signal-to-noise ratio brought by the use of such power provides 2-MHz BFS measurement precision at the end of the 120-km sensing link with 3-m spatial resolution, all without the need to resort to additional means such as the use of coding or Raman gain. © (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.