Fiber Bragg Grating sensors - Demultiplexing and Measuring techniques, applications

Bragg gratings are diffractive microstructures obtained by photo-inscribing laser interference within the core of single-mode optical fibers. Short-period Bragg gratings consist of a uniform, submicron modulation of the refractive index of the fiber core. In fact, a Bragg grating a few millimeters long has several thousand Λ "steps", and acts as a reflector for a fine spectral band (0.2 to 0.3 nm) centered at ${\text{λ}}_B=2\cdot {\text{n}}_e\cdot \text{Λ}$, with Λ (~ 0.5 µm for example) and n_e (~ 1.45) the effective index of the mode. Any change in Λ or n_e therefore "moves" λ_B whose tracking provides the inducing parameter. Of course, Bragg gratings can be multiplexed. To do this, it is sufficient, for example, to cascade several of them with slightly different pitches Λ_i and probe them each around their characteristic wavelength, with incident light reflected respectively at λ_B1, λ_B2... λ_BN. The poorly resolved measurement of λ_Bi indicates the address of the ith network, while its accurate analysis provides the desired measurement.

In addition to this brief description of Bragg grating transducers, the reader may wish to refer to the article [R 6 735] in Techniques de l'Ingénieur, which provides a complete and detailed analysis of the different types of Bragg gratings, their inscription processes and their metrological characteristics. The aforementioned article also contains an analysis of sensors based on these translating Bragg gratings, for the measurement of various parameters, and for numerous fields of application (e.g. extensometers for civil engineering concrete, deformation or temperature sensors for composite materials, etc.).

This article deals with the complementary part of Bragg grating sensors, i.e. systems for measuring and demultiplexing spectral information, most often in the context of multisensor approaches, i.e. fiber optic sensor arrays – RCFO –, before turning to applications. For each application, the choice of measurement system is just as important as the choice of sensor, and indeed inseparable from it. Indeed, while certain metrological characteristics (sensitivity, linearity, dynamics, etc.) naturally depend on the quality of the sensor(s) and their transducer Bragg gratings, others are closely...