​Bragg gratings—patterns etched into the axis of an optical fiber—have the capacity to reflect light emitted by a laser. The wavelength of the reflected light depends on the temperature and on any deformations in the light's path. Embedding Bragg gratings deep inside metal structures can provide valuable information about the effects of strain. Things like local deformation can be measured during and after manufacturing, for instance.

Given the high temperatures involved in AM, the researchers at CEA-List —in partnership with CEA-DES via the Samanta metal AM platform— used particularly heat-resistant Bragg gratings etched using a femtosecond laser, which forms strings of microbubbles inside the fiber. This technique eliminates conventional Bragg gratings' tendency to fade under high temperatures.

The fibers were positioned deep inside the part during additive manufacturing, and temperatures of nearly 700 °C were measured tens of microns from the fusion bath at a rate of 5 kHz. The result? In situ dynamic monitoring of the manufacturing process. The fiber Bragg grating sensors, which remain in place, are interrogated over the part's lifetime, monitoring its health under exposure to vibrations and high temperatures and checking for any deformations.

The technology will be of interest for the structural health monitoring of parts used in critical environments in the aeronautics, nuclear, oil & gas, and other industries.