Fabrication of high N.A. graded‐index optical fibers by mcvd method

Cracks in the host material have posed a problem in the fabrication of high‐M.A. silica fibers. This paper examines a high‐NA fiber with a graded refractive index profile to prevent these cracks. A high‐N.A. (= 0.39) graded fiber was fabricated by the MCVD method, in which the accumulation temperature is varied in accordance with the difference in composition of the core glass film. A method was developed for generating a host material with high‐N.A. and low losses. We realized a loss of 2.49 dB/km (at the measured wavelength of 1.67 μm) and a relative index of refraction difference Δ = 3.3%. For the core materials GeO 2 ‐ P 2 O 5 ‐ SiO 2 and P2O 5 ‐ SiO 2 we experimentally found a relation between the relative index difference Δ and the loss. Rayleigh scattering, structural imperfection scattering and OH absorption were examined for the propagation loss characteristics. At longer wavelengths, the Rayleigh scattering and the OH absorption can be reduced and the high N.A. fiber is more practical. When Δ is larger than 3%, the structural imperfection loss increases rapidly and it is necessary to improve the processing. For the bending loss the number of windings, the bending radius and the Δ dependence are derived. For a bend diameter R, the bending loss was found to be (1/R) 1.31 exp (−k Δ).