Abstract

For lensless in-line optical devices, thermally diffused expanded- core (TEC) fibers have been proposed by Kawakami,¹ and the application of the TEC technique to isolators and the low-loss splicing of high-N.A. single-mode fibers have been actively studied.⁸ In a TEC fiber the mode-field diameter (MFD) is gradually increased along the fiber axis with the thermal diffusion of the dopants induced by heat treatment. However, the heating process for a dopant diffusion requires either a high temperature or a long duration. In this paper we propose and demonstrate a MFD conversion technique that uses thermal relaxation of the residual stress in the core.

© 1993 Optical Society of America