To repair aging infrastructure and monitor existing bridges, dams, and other large buildings, distributed fiber optic sensors require a new type of light source to monitor the stress and temperature changes experienced by the building. However, this common fiber optic sensor, based on the nonlinear optical phenomenon of stimulated Brillouin scattering (SBS), is limited by the insurmountable spatial range and resolution.
At present, researchers in Spain and Switzerland have solved these difficulties. They have developed a temperature that can detect a millionth of a millimeter of spatial resolution in a range of 10 kilometers in a short period of time. Or a method of stress change. The team believes that the high resolution of the solution enables it to find its place in long-distance infrastructure monitoring and more sophisticated biomedical environments.
Signal distortion
The SBS fiber optic sensor meets the backpropagating continuous wave (CW) probe laser beam by transmitting a pulsed laser signal, that is, a pumping pulse, through a length of fiber. (In fact, to prevent some systematic errors, these systems typically use two CW probe waves and distinguish the two columns of waves by the modulation frequency associated with the material properties of the fiber, the so-called double-side band method.) Pumping pulses and The nonlinear interaction of the fiber produces stimulated Brillouin scattering (SBS), inelastic Stokes and anti-Stokes scattering, which will change the frequency distribution of the pulsed optical signal. This so-called Brillouin frequency shift depends on the material properties of the fiber as a function of stress and temperature; therefore, changes in those parameters along the length of the fiber can be detected by analysis of the Brillouin frequency shift.
Although SBS-based fiber optic sensing has found its place in the construction of various infrastructures, it still has some problems. One of the problems is the limited scope of monitoring. Recent analysis has shown that the power required to probe a few kilometers (and the stress and temperature variations of the fiber) can distort the pump pulse signal, seriously affecting the accurate detection of the Brillouin frequency shift.
Another problem is the limited spatial resolution. Because SBS relies on nonlinear light-substance interactions to produce sound waves, there is a small but very significant time lag in spatial resolution in time domain techniques. Other techniques in the frequency and related domains can compensate for the shortcomings of SBS, but take longer—measuring it takes about an hour or more to measure a million points along the fiber.
Question about scanning
The joint research team between Spain and Switzerland, as well as scientists from the University of Alcala in Spain and the Federal Institute of Technology in Lausanne (EPFL) in Switzerland, seem to have found a solution to these problems. They delve into the actual details of the signal scan to derive the Brillouin frequency shift associated with stress or temperature changes.
In most time domain SBS based fiber sensing schemes, the frequency shift is determined by symmetrically scanning the offset of the two sideband probe beams relative to the fixed pumping pulse frequency. However, this scanning method has proven to be a major source of pulse distortion at high probe power. This is due to the difficulty in quantifying the asymmetric energy transfer between the two probe sidebands and the pumping pulse - an effect that increases as the probe power increases.
The joint research team found that by changing the scanning method, the sideband probe beam maintains a fixed frequency difference (correlated with the Stokes and anti-Stokes frequencies of the fiber) and scans the input pump beam with the associated frequency— This can significantly reduce signal distortion. This approach means that the probe beam power upper limit becomes higher and the span of the fiber optic sensing system becomes longer. In addition, the system also has a higher spatial resolution by eliminating signal distortion in the pumping pulses.
Resolution up to one centimeter
The researchers tested a 10 km long single-mode fiber using a differential pulse width pair, Brillouin Optical Time Domain Analysis (DPP-BOTDA). They found that the method was able to detect Brillouin shifts of one million points distributed along the fiber, with a resolution of up to one centimeter, and was able to detect a 3 cm "hot spot" at the far end of the fiber. Moreover, since the system remains in the time domain, the method is able to implement these functions in less than 20 minutes, much less than the time spent using the frequency dependent domain approach.
The research team believes that in addition to applications in the infrastructure, the technology can be used in other areas. Alejandro Dominguez-Lopez of the University of Alcala stated: “Because we have such a large monitoring point density, sensors can also be used in areas such as avionics and aerospace to monitor every inch of aircraft wings.” The researchers also believe that The higher resolution of the system may facilitate the development of certain biomedical applications, such as detecting temperature deviations present in breast cancer.
