Researchers have developed fiber-optic sensing with greater resolution over the span

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

At present, Spanish and Swiss researchers have addressed these difficulties by developing a method that detects temperatures in the order of one centimeter at centimeter-level spatial resolution over a 10 km range in a relatively short period of time Or stress change method. The team believes the high resolution of the solution allows it to find its way into long-distance infrastructure monitoring and a more sophisticated biomedical environment.

Signal distortion

The SBS optical fiber sensor encounters a CW wave-propagating continuous-wave (CW) probe laser beam by sending a pulsed laser signal, a pumping pulse, propagating through a length of optical fiber. (In fact, in order to prevent some systematic errors, these systems typically use two CW probe waves and separate the two wave forms by the modulation frequency associated with the properties of the fiber material, the so-called double-sideband method.) The pump pulse and Nonlinear interactions of optical fibers produce SBS, inelastic Stokes and anti-Stokes scattering, which will alter 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 fiber length can be detected by analyzing the Brillouin frequency shift.

A joint research team from Spain and Switzerland envisaged applying its improved optical fiber sensing method to large-scale infrastructure projects and biomedical monitoring. (Source: American Optical Society)

Although SBS-based fiber sensing has found its way in the construction of various infrastructures, it still has some problems. One of the problems is the limited scope of monitoring. Recent analyzes have shown that the power required by a probe that spans several kilometers (and the stress and temperature changes experienced by the fiber) can distort the pump pulse signal and seriously affect the accurate detection of the Brillouin frequency shift.

Another problem is the limited spatial resolution. Because SBS relies on nonlinear light-matter interactions to produce sound waves, there is a small but significant time lag in spatial resolution in time-domain techniques. Other techniques in the frequency and related domains can make up for the shortcomings of SBS, but take longer - measuring about a million points along the fiber takes about an hour or more.