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FBG Sensors
CMS monitors structural health and machinery using FBG sensors and the best FBG-interrogation instrumentation available.
FBGs or Fiber Bragg Gratings are well established and are now the most popular fiber optic sensors for health monitoring. FBGs are fabricated on single
mode optical fiber (figure 1). The FBGs are written on to a small section of the fibers core with a powerful laser (figure 2).
How It Works
- Light enters fiber nearly parallel to its axis and travels almost 100% in core by internally bouncing off the
cladding, because:
- Cladding refractive index (nd-cladding) < core refractive index (nd-core).
- Internal angle of the light < critical reflection angle (per Snell’s Law).
- Light can travel kilometers with acceptable loss.
(Figure 3)
Grouping
- Typically can monitor up to +/-2500 micro strain (em)/FBG with up to 40 FBGs written-on or spliced-to each fiber*, each FBG having a unique
lBragg :
- Interrogator scans for and finds each unique-lBragg peak.
- Software analyses each unique-lBragg peak and numerically logs its precise lBragg value.
- Can spread the FBG locations over long distances of fiber—even kilometers.
- Greatly simplifies cabling, instrumentation, and installation.
*Using a 160nm-range interrogator. Larger numbers of FBG strain sensors possible for maximum strains < 2500 micro strain (em). >200 FBG ambient
temperature sensors per fiber possible if all temperatures move similarly.
(Figure 4)
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Figure 1
Figure2
Figure 3
Figure4
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