Overcoming Nonlocal Effects and Brillouin Threshold Limitations in Brillouin Optical Time-Domain Sensors

Non-Local Effects in Brillouin Optical Time-Domain Analysis Sensors

“…In this way, as is portrayed in Figure 2b,c, when the frequency difference does not match the BFS of the fiber, the Brillouin gain and loss spectra generated by both probe waves over the pump pulse do not overlap perfectly, leading to linear distortion of the pump pulse spectrum. As is depicted in Figure 3, the distortion of the pulse spectrum leads to a distortion of the measured gain spectra and BFS measurement error [14]. It is found that this limits the maximum probe wave power to around −3 dBm for typical standard single-mode fiber parameters.…”

“…As is depicted in Figure 4a, the optical frequency of the probe waves is modulated in the time domain following a saw-tooth shape [21]. Alternatively, sinusoidal or a triangular shape could be deployed, which have been shown to have a similar performance [14,22]. Additionally, this frequency modulation (FM) is synchronized to the pump pulses so that a series of pulses experience the same optical frequency of the probe waves at any given location.…”

“…Furthermore, as highlighted in Figure 4a, the Brillouin interaction induced by both probe waves upon the optical pulse spreads over a large frequency range. Therefore, the net gain and loss spectra induced by both probe waves broadens so that no distortion is introduced in the pump pulse spectrum and second-order non-local effects are suppressed [14]. In addition, the gain and loss spectra mutually compensate, so there is no depletion of the pulse.…”

“…It is important to point out that, as it is depicted in Figure 4b, the FM of the probe wave is also used to perform the frequency scan of the Brillouin spectra, instead of doing it by a conventional frequency sweep. At each location of the fiber, the pump pulse is made to interact with different probe wave frequencies simply by changing the relative delay between the probe wave FM modulation and the pump pulse [14]. Note that the number of relative delay steps is given by the relation between the peak-to-peak frequency deviation of the FM, which defines the range of frequencies to scan, and the desired frequency step; hence, it is exactly the same as in a conventional BOTDA scheme.…”

“…To do this, all measured data are introduced in a matrix, so the trace measured for each pulse delay is stored in a different row of the matrix. The method is simply to apply a shift to the elements of the columns of the matrix containing the probe wave samples (rows) for each relative delay between the FM modulation and the pump pulses [14].…”