We demonstrate a simple technique to provide conventional Brillouin optical time-domain analysis sensors with mitigation for pump pulse attenuation. The technique is based on operating the sensor in loss configuration so that energy is transferred from the probe wave to the pump pulse that becomes amplified as it counter-propagates with the probe wave. Furthermore, the optical frequency of the probe wave is modulated along the fiber so that the pump pulse experiences a flat total gain spectrum that equally amplifies all the spectral components of the pulse, hence, preventing distortion. This frequency modulation of the probe brings additional advantages because it provides increased tolerance to non-local effects and to spontaneous Brillouin scattering noise, so that a probe power above the Brillouin threshold of the fiber can be safely deployed, hence, increasing the signal-to-noise ratio of the measurement. The method is experimentally demonstrated in a 100-km fiber link, obtaining a measurement uncertainty of 1 MHz at the worst-contrast position.

We demonstrate a technique to reduce the sidelobes in optical pulse compression reflectometry for distributed acoustic sensing. The technique is based on using a Gaussian probe pulse with linear frequency modulation. This is shown to improve the sidelobe suppression by 13 dB compared to the use of square pulses without any significant penalty in terms of spatial resolution. In addition, a 2.25 dB enhancement in signal-to-noise ratio is calculated compared to the use of receiver-side windowing. The method is tested by measuring 700 Hz vibrations with a 140  nε amplitude at the end of a 50 km fiber sensing link with 34 cm spatial resolution, giving a record 147,058 spatially resolved points.

Rayleigh scattering-based dynamic strain sensing with high spatial resolution, fast update rate and high sensitivity is highly desired for applications such as structural health monitoring and shape sensing. A key issue in dynamic strain sensing is the trade-off between spatial resolution and the Signal to Noise Ratio (SNR). This trade-off can be greatly relaxed with the use of coding. A sequence of optical pulses is injected into the fiber and the detected backscattered signal is cross-correlated with the original signal. With the use of coding, SNR is indeed improved, but if the sequence is not well chosen, the resulting peak to sidelobe ratio (PSR) can be rather low. An excellent choice of codes are biphase Legendre sequences which offer near perfect periodic autocorrelation (PPA). Other common issues in Rayleigh scattering-based sensing techniques are signal fading and dynamic range. The former issue can occur due to destructive interference between lightwaves that are scattered from the same spatial resolution cell and, in coherent detection schemes, when the polarization states of the backscattered light and the reference light are mismatched. The latter issue is a concern in phase sensitive schemes which require signal jumps not to exceed 2π. In this paper, a biphase Legendre sequence with 6211 pulses is used in conjunction with polarization diversity scheme and a PM fiber. The setup provides two independent measurements of the sensing fiber complex profile and achieves highly sensitive, distributed dynamic strain sensing with very low probability of fading. In addition, the system can handle both very large perturbation signals and very small perturbation signals. The system operated at a scan rate of ∼ 107kHz and achieved spatial resolution of ∼10cm and sensitivity of ∼1.1 mrad/ √ Hz. The ratio between the powers of the maximum and minimum excitations that can be measured by the system is 136 dB.

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.

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.