Fiber Bragg grating sensing is a relatively mature fiber optic sensor technology currently being used in structural health monitoring systems. Therefore, there are significant benefits to using this technology as a platform for other sensing modalities. In this work, a side polished fiber Bragg sensor is described for sensing refractive index changes. The effective refractive index of a fiber Bragg grating is a function of the refractive index of the media surrounding it, and its sensitivity may be optimized with appropriate design. As the external refractive index changes, the wavelength at which incident light experiences a maximum reflection from the grating will shift. The sensitivity of a fiber Bragg grating to external refractive index changes increases when the grating is polished on one side. This side-polishing technique enables the Bragg grating to preserve a greater portion of its mechanical strength compared with other techniques such as chemical etching. This work utilizes side-polished fiber Bragg grating sensors centered at a 1542.9 nm wavelength with cladding thickness values of approximately 1-2 μm. The response of these sensors to small refractive index changes was studied. Previous work on fiber Bragg grating sensors has shown that the peak wavelengths can be measured with 3 pm repeatability. With this repeatability, this study demonstrated that a 0.001 refractive index change can be observed. By using materials that change index with moisture or pH, this technique can be used to construct both pH and moisture sensors.
A wavelength measurement system based on a quantum well electroabsorption photodetector is demonstrated and used to monitor a Bragg fiber optic sensor. A bandwidth limited wavelength resolution of +/- 3.62 pm/(root)Hz was achieved, corresponding to a strain resolution of +/- 3.02 (mu) (epsilon) /(root)Hz for a 1553.5 nm BFOS. A total accessible wavelength range of 90 nm was demonstrated for the system, making the system well suited for wavelength multiplexing applications.
Smart structure technology could revolutionize engineering by the 21st Century. Improvements in safety, reductions in cost, and enhancement of the environment are some of the expected benefits of this technology. A major impediment to the implementation of this technology in practical structures is going to be extracting the optical information from the structurally integrated sensing system. The answer to this problem lies in: demodulating, processing, and multiplexing the raw optical data from an array of sensors and transmitting these data from the structure in a single channel by means of an unobtrusive optoelectronic interface that is part of the structure. The single output channel from this optoelectronic smart structure interface could take several user-friendly forms, the most versatile involves free- space propagation and leads to the `optical synapse' concept. We have demonstrated that intracore Bragg grating laser sensors could form the basis of an OSSI and have shown that optical multiplexing is possible in the case of Bragg grating fiber laser sensors.
The development of a fiber laser sensor which permits efficient interrogation of Bragg grating sensors is reported. The fiber laser is linewidth-narrowed and tuned by a remotely located, sensing Bragg grating that is surface adhered to a structure under test. The Bragg grating- tuned fiber laser is used in conjunction with a passive wavelength demodulation system (WDS) to form a fiber laser strain sensor system (FLS3), which was used to track both static and dynamic strains on an aluminum beam. The FLS3 could measure strains with a resolution of approximately 4 (mu) (epsilon) and a bandwidth of 13.0 kHz. The viability of the laser strain sensor concept lends itself to the development of a compact, potentially embeddable smart sensor that would output demodulated sensing data directly to the user.
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