Raman-assisted BOTDA sensors

Abstract

We review some recent results on the application of distributed Raman amplification schemes, including ultralong lasers, to the extension of the operating range and contrast in Brillouin optical time domain analysis (BOTDA) distributed sensing systems.Brillouin optical time domain analysis (BOTDA) is a widely-used technique for the distributed measurement along optical fibers. The main interest of BOTDA lies in the possibility of using it to develop sensors for the continuous monitoring of temperature and/or strain in optical fibers [1][2][3][4][5]. Additionally, BOTDA has been used to perform power distribution measurements along optical fibers, enabling the measurement of fiber attenuation [6], chromatic dispersion [7] and parametric amplification [8]. The measurement range of BOTDA systems is typically shorter than 50 km [9] and generally limited to 20-30 km with a spatial resolution between 1-2 meters. The measurement range limitation is basically due to fiber attenuation. The losses in the fiber cause a drop of signal contrast with distance and a growth in the measurement uncertainty, as a simple result of the pump power reduction that critically scales the actual Brillouin gain. In these systems there is also a trade-off between resolution and measurement range: while it is desirable to have the highest possible resolution, this requires the use of short optical pulses, so the effective distance for amplification is reduced accordingly. This increases the difficulty in detection because the signal-to-noise ratio (SNR) is reduced. To some extent, one can compensate this by raising the pump power. However, the pump power cannot be increased indefinitely since other competing nonlinear effects (modulation instability and Raman) and also significant pump depletion start to take place. Thus, depending on system requirements, one has to choose between short pulses for increased resolution, or longer pulses for increased range. Several studies have been realized to extend the system resolution without impairing the measurement range. These rely on signal processing methods [10,11], distributed amplification along the fiber to overcome the fiber loss [12][13][14], or a combination of both [15]. In this talk we will review some of our recent results in the application of Raman-amplified schemes, including advanced higher-order amplification based on the ultralong laser architecture [16], to the improvement of the performance of BOTDA distributed sensors.