Researchers have developed fiber optic sensors with larger spans and higher resolution

To repair aging infrastructure and monitor existing bridges, dams, and other large structures, 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 – a nonlinear optical phenomenon based on stimulated Brillouin scattering (SBS) – is limited
by insurmountable spatial scope and resolution.

Researchers in Spain and Switzerland have solved these difficulties by developing a way
to detect changes in temperature or stress in a relatively short period of time at a spatial resolution of one part per million at a spatial resolution of centimeters over a range of 10 kilometers.
The team believes that the solution's high resolution allows it to find its place in long-distance infrastructure monitoring and more sophisticated biomedical environments
.

Signal distortion

SBS fiber optic sensors meet a counterpropagating continuous wave (CW) probe laser beam by sending a pulsed laser signal, i.
e
.
pumping pulse, propagating through a fiber of a certain length.
(In fact, to prevent certain systematic errors, these systems typically use two CW probe waves and distinguish the two columns with modulation frequencies related to the characteristics of the fiber material, the so-called bilateral band method
.
) The nonlinear interaction of the pumping pulse with 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 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 shift
.

A joint Spanish-Swiss research team envisions applying its improved fiber-optic sensing method to large-scale infrastructure projects and biomedical monitoring
.

(Image source: Optical Society of America)

Although SBS-based fiber optic sensing has found its place in the construction of various infrastructures, it still has some problems
.
One such problem is the limited
scope of monitoring.
Recent analyses have shown that the power required for transducers spanning several kilometers (as well as the stresses and temperature variations on the fiber) can distort the pumped pulse signal, severely impacting the accurate detection
of Brillouin shifts.

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