Strain sensing in fiber-optic coils with buried Bragg gratings

Results of an application of fiber-optic Bragg gratings for sensing longitudinal strain in a fiberoptic coil with 18 layers is described. Some of the fiber winding parameters in this test coil resembled those typically used in a fiber-optic data link payout dispensers. 9(th) and 10(th) layers of this coil have Bragg gratings in their center with unstrained Bragg wavelengths of 1294.74 and 1285.47 nm respectively. Bragg grating-based technique is used to measure longitudinal stretch or compression in specific layers of optical fiber in the coil, and relate it to the various winding parameters like the number of turns above and below the turn of interest as well as the winding tension in each turn. The observed results are compared to a cable-pack mechanics model (CPMM) that is widely used in the design of such coils.