Fiber optic data links and embedded sensors, such as Fabry- Perot and Mach-Zehnders, are important elements in smart structure architectures. Unfortunately, one problem with optical fiber is the inherent limit through which fibers and cables can be looped. A revolutionary, patented technology has been developed which overcomes this problem. Based on processing the fiber into low loss miniature bends, it permits routing the fiber to difficult areas, and minimizing the size of sensors and components. The minimum bend diameter for singlemode fiber is typically over two inches in diameter, to avoid light attenuation and limit stresses which could prematurely break the fiber. With the new miniature bend technology, bend diameters as small as 1 mm are readily achieved. One embodiment is a sub-component with standard singlemode fiber formed into a 180 degree bend and packaged in a glass tube only 1.5 mm OD X 8 mm long, Figure 1. Measured insertion loss is less than 0.2 dB from 1260 nm to 1680 nm. A final processing step which anneals the fiber into the eventual curvature, reduces the internal stress, thereby resulting in long life expectancy with robust immunity to external loading. This paper addresses the optical and physical performance of the sub-component. Particular attention is paid to attenuation spectra, polarization dependent loss, reflectance, thermal cycle, damp heat, and shock tests. Applications are presented which employs the bend technology. Concatenating right angle bends into a 'wire harness' demonstrates the ability to route fiber through a smart engine or satellite structure. Miniature optical coils are proposed for sensors and expansion joints.
We have developed a technology to bend both singlemode and multimode optical fibers in radii as small as 0.45 mm without significant optical loss or breakage. This technology is of tremendous benefit for compact optical sensors in which it may be desired to reverse the direction of travel of the fiber within a very small sensing head. The bends utilize 'air cladding' to guide the light around the tight radius of the bend without loss. The bends can be packaged in small capsules 1.5 mm in diameter and less than 1 cm long. This miniature-bend technology avoids the 50 mm bend diameter required to reverse the direction of travel for conventional fibers. Miniature 180 degrees bends have been demonstrated with very low optical losses across the entire 1250 to 1650 nm spectral band. The same performance can be obtained in the 820 nm wavelength region. This revolutionary 'kernel' can be included within the sensing head, providing a very small cylindrical sensor whose pigtails exist from the same direction. This paper describes the optical and physical performance of the miniature fiber bend. Particular attention is paid to the measured IR attenuation spectra, polarization dependent loss, and reflectance. A long lifetime is estimated as a consequence of the annealing of the bend region during the fabrication procedure.
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