Miniaturized Strain-Free Fiber Bragg Grating Temperature Sensors

This work reports the design, fabrication, and characterization of a miniaturized metallic package for optical fiber Bragg grating (FBG) sensors. The aim of the package is to enhance the temperature sensitivity of the grating and eliminate the cross-sensitivities to mechanical effects, i.e., strain or impulsive forces. The package is simple in its design and developed using off-the shelf components: the FBG is first encapsulated into a steel tube, which is in turn placed inside another larger steel tube, where the fiber is kept loose to prevent that any mechanical effect is transferred to the grating. The package has a length of 25 mm and a diameter of about 2 mm, hence it is very compact. Concerning the performance, the packaged FBG has a linear temperature response in the range 5–50 °C with a sensitivity of 28.9 pm/°C, that is about three times that of a standard FBG. Whereas, concerning the longitudinal force sensitivity, it is decreased by two orders of magnitude in comparison to an unpackaged FBG. The temporal evolution of the device is also evaluated when subjected to a temperature step of about 20 °C, demonstrating a fast response time of 5 s. Finally, the effect of an impulsive force is evaluated by means of a falling mass attached to the fiber, demonstrating a negligible response and a strong resistance of the packaged FBG in comparison to unprotected one, which broke after the same test.