Fiber Bragg Grating sensors - Photo-writing processes, and specifications

Discovered at the end of the 1980s, Bragg gratings are diffraction gratings produced in the core of single-mode optical fibers, mostly of the germanosilicate type. A Bragg grating is typically obtained by locally exposing the fiber core to a laser interference pattern, which modulates its refractive index thanks to the photosensitivity of germanium-doped silica.

Once created, when illuminated with a spectrally broad optical source, due to the interference (constructive in reflection) taking place within this index modulation, such so-called "straight short-pitch" – gratings – act as reflectors, each for a fine spectral band centered at their characteristic wavelength λ _B = 2.n _e .Λ (with Λ ~ 0.5 µm the pitch of the interference and therefore of the grating, and n _e the effective index ~ 1.45 of the propagation mode).

Any change in these parameters has the effect of spectrally "shifting" the Bragg line, so fine tracking of the line can be used to trace the inducing parameters. So, beyond their use as spectral filters in the field of optical telecommunications, Bragg gratings also play the role of excellent transducers which, naked or suitably conditioned, become sensors of various parameters (temperature, deformation, pressure, angle, acceleration, refractive index, etc.).

Another advantage of these "Bragg grating sensors" is that, thanks to their spectral response, they can easily be interrogated and wavelength multiplexed by a remote measurement system, thus forming a "distributed sensor network".

Thanks to the remarkable performance of Bragg grating sensors and their measurement-demultiplexing instrumentation, numerous applications have gradually emerged. They are now used in a wide range of applications, addressing the needs and issues of structural monitoring (civil engineering, aeronautics, marine, etc.) and material health control (organic composites, concrete, metallic materials, etc.).

Numerous products are available to end-users on the market, and at the same time, R&D is continuing worldwide, with current challenges focusing on the ability to address extreme measurements, for example. This applies to cryogenic applications (space launch tanks, liquefied natural gas transport, research, etc.), and above all to very high temperatures (steelmaking, aeronautics, nuclear power, etc.), which requires special laser inscription techniques for networks with dedicated internal structures and specific packaging.

The aim of this article is to present the technology of the various types of Bragg gratings (straight, inclined, variable...