Fluorescent-based fiber sensors is fabricated by ‘‘writing’’ a fiber grating into the core of a germanium-doped optical fiber. This can be done in a number of ways. One method, illustrated by the fig in the previous post, uses two short-wavelength laser beams that are angled to form an interference pattern through the side of the optical fiber. The interference pattern consists of bright and dark bands that represent local changes in the index of refraction in the core region of the fiber. Exposure time for making these gratings varies from minutes to hours, depending on the dopant concentration in the fiber, the wavelengths used, the optical power level, and the imaging optics.

Other methods that have been used include the use of phase masks as well as interference patterns induced by short, high-energy laser pulses. The short duration pulses have the potential to be used to write fiber gratings into the fiber as it is being drawn. Substantial efforts are being made by laboratories around the world to improve the manufacturability of fiber gratings because they have the potential to be used to support optical communication as well as sensing technology. Once the fiber grating has been fabricated, the next major issue is how to extract information.When used as a strain sensor, the fiber grating is typically attached to, or embedded in, a structure. As the fiber grating is expanded or compressed, the grating period expands or contracts, changing the grating’s spectral response.

For a grating operating at 1300 nm, the change in wavelength is about 10^-3 nm per microstrain. This type of resolution requires the use of spectral demodulation techniques that are much better than those associated with conventional spectrometers. Several demodulation methods have been suggested using fiber gratings, etalons, and interferometers. Figure below illustrates a system that uses a reference fiber grating. The reference fiber grating acts as a modulator filter. By using similar gratings for the reference and signal gratings and adjusting the reference grating to line up with the active grating, one may implement an accurate closed-loop demodulation system. An alternative demodulation system would use fiber etalons. One fiber can be mounted on a piezoelectric and the other moved relative to a second fiber end. The spacing of the fiber ends as well as their reflectivity in turn determine the spectral filtering action of the fiber etalon.

Figure: Fabrication of a fiber grating sensor can be accomplished by imaging to short-wavelength laser beams through the side of the optical fiber to form an interference pattern. The bright and dark fringes imaged on the core of the optical fiber induce an index of refraction variation resulting in a grating along the fiber core.