Abstract
Optical fibers, which have a hetero-core structure, that is, a junction of optical fibers with different core
diameters, provide a sensing function to the transmission lines themselves. This simple structure is fabricated
by inserting and fusing optical fibers with different core diameters into a fiber transmission line.
The measurement principle is based on changes in the transmission light intensity. It does not require any
temperature compensation, allowing for the construction of a measurement system with high sensitivity
and low cost among optical fiber measurement technologies. Since hetero-core optical fiber sensors were
invented, research, development, and industrial applications have been carried out in various fields. In
this special issue, we introduce the measurement principles, the latest research and development results,
and practical applications in industry.
