Thermal Diffusion Technique for In-Fiber Discrete Waveguide Manipulation and Modification: A Tutorial

Thermal diffusion is the phenomenon of the migration of dopants in an optical fiber during high-temperature treatment. Research on the theory and technique of optical fiber thermal diffusion shows that the thermal diffusion technique (TDT) is a more stable and controllable means than the fusion tapering technique to change the refractive index distribution, which is used to manipulate and modify the optical field of in-fiber integrated discrete waveguides. In this tutorial, after a brief introduction to the basic thermal diffusion concepts, we review the development of TDT. Then we emphasize the basic theory of thermal diffusion, including typical calculation models and boundary conditions, and we describe several typical micro heaters. Based on in-fiber discrete waveguides, we demonstrate TDT to realize several fundamental functions of in-fiber integrated optical devices, including signal exchange, mode field convertors, beam transformers, and phase shapers. In addition, we also discuss methods to evaluate thermal diffused discrete waveguides. This article provides a tutorial introduction, a comprehensive background on this subject as well as a selection of the most important applications.