Higher Resolution Fiber Optic Sensors Deliver a Resolution of One Centimeter

In order to repair the aging infrastructure and monitor existing bridges, dams and other large buildings, distributed optical fiber sensors need a new type of light source to monitor the stress and temperature changes in the building. However, this common fiber optic sensor, based on stimulated Brillouin scattering (SBS) nonlinear optical phenomena, is limited by the insurmountable spatial extent and resolution.

At present, researchers in Spain and Switzerland have solved these difficulties. They have developed a method capable of detecting a temperature of one millionth of a millionth of a cubic centimeter of spatial resolution in a short time within a range of 10 kilometers. Or the method of stress change. The team believes that the solution's high resolution enables it to find its way into long-distance infrastructure monitoring and more sophisticated biomedical environments.

Signal distortion

The SBS fiber optic sensor communicates with a counter-propagating continuous wave (CW) probe laser beam by transmitting a pulsed laser signal, ie, a pump pulse, through a length of optical fiber. (Actually, in order to prevent certain systematic errors, these systems typically use two CW probe waves and distinguish the two waves by the modulation frequency associated with the fiber's material properties, the so-called double-sideband approach.) Pump Pulses 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 light 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 analyzing the Brillouin frequency shift.

Although SBS-based fiber sensing has found its way into 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 for a probe spanning several kilometers (and the stress and temperature changes experienced by the fiber) can distort the pump pulse signal and severely affect the accurate detection of Brillouin frequency shifts.

Another problem is the limited spatial resolution. Because SBS relies on non-linear light-to-substance interactions to generate sound waves, there is a small but significant time delay in spatial resolution in the time domain. Other techniques in the frequency and related domains can compensate for the shortcomings of SBS, but it takes longer - measuring one million points along the fiber takes about an hour or more.

Questions about scanning

The Spanish and Swiss joint research teams, as well as scientists from the University of Alcalá and the Swiss Federal Institute of Technology in Lausanne (EPFL), seem to have found a solution to these problems. They delve into the Brillouin frequency shift associated with stress or temperature changes by studying the actual details of the signal scan.

In most time-domain SBS-based fiber sensing schemes, the frequency shift is determined by symmetrically scanning the shift of the two sideband probe beams relative to the fixed pumping pulse frequency. However, it turns out that this scanning method is the main source of pulse distortion at high probe power. This is due to the unquantifiable asymmetrical energy transfer between the two probes' sidebands and pump pulses - 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 (associated with the Stokes and anti-Stokes frequencies of the fiber), and the input pump beam is scanned with the associated frequency - This can significantly reduce signal distortion. This method means that the upper bound of the power of the probe beam becomes higher and the span of the optical fiber sensing system becomes longer. In addition, the system also has a higher spatial resolution by eliminating signal distortion in the pump 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) experiment. They found that the method was able to detect a one-million-point Brillouin shift along the fiber with a resolution of up to one centimeter, and was able to detect a 3-centimeter “hot spot” at the far end of the fiber. Moreover, since the system remains in the time domain, this method can implement these functions in 20 minutes, much less than the time spent in using the frequency-dependent domain method.

The research team believes that in addition to the applications in the infrastructure, the technology can also be used in other areas. Alejandro Dominguez-Lopez from the University of Alcalá stated: “Since 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 an aircraft wing.” Researchers also believe that The higher resolution of the system may facilitate the development of certain biomedical applications, such as the detection of temperature biases in breast cancer.

(Original title: Researchers Develop Fiber Optic Sensors with a Higher Span Resolution