This Small Business Innovative Research (STTR) Phase I project will demonstrate the feasibility of an innovative optical chiral fiber sensor (CFS) technology. A double helix structure with pitch greatly exceeding the wavelength will be imposed by twisting glass fibers with noncircular cores as they pass through a miniature oven in a "twisting tower" with enhanced temperature and motion control. The chiral long period grating (CLPG) will couple core and cladding modes to produce a series of dips in transmission of the core mode. The central wavelengths and depths of these dips are sensitive measures of the strain, pressure, torque, and temperature of the fiber and of the refraction index of the material surrounding the fiber. The CPLG design will be guided by an innovative combination of analytical calculations assisted by 2-D hybrid transverse finite elements electromagnetic simulations and fully three-dimensional finite vector elements simulations of wave propagation. These simulations will yield the coupling strength between the core and cladding modes. The results of transmission measurements with varying temperature, elongation and surrounding refractive index will be used to refine the assumptions of the calculations. The improved model will be used to fabricate the CLPG and to design a transducer for a CFS.
The innovative CLPGs present a clear advantage of a broad choice of glass materials, which may be selected for resistance to harsh environments at high temperatures and/or radiation levels such as those in oil wells, nuclear reactors or outer space. In contrast, conventional fiber gratings created in UV sensitive glass fibers are significantly degraded in such harsh environments. The versatile fabrication approach allows for the flexible production of a full suite of sensors functioning over a broad frequency range by a single tool by changing the computer controlled twist and draw rates. This manufacturing process will make it possible to fabricate highly sensitive uniform CLPGs while dramatically lowering the production cost relative to conventional fiber gratings, which require precise patterning of UV radiation. The improved manufacturing process will also be used to produce other devices based on chiral fibers for filter, laser, sensor and polarizer applications. The computational model developed will enhance the understanding of optical interactions with chiral fibers and thereby facilitate the development of new chiral fiber devices.
