Miniature fiber optic loop for compact optical sensors

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.