https://phys.org/news/2018-07-optical-fibers-materials.html

A sensor that doesn't disturb the light

No wider than a strand of hair, an optical fiber made of glass transmits light that varies according to four parameters: intensity, phase, polarization and wavelength. These parameters are altered when the fiber is stretched or the temperature changes, enabling the fiber to act like a sensor by detecting cracks in structures or abnormal temperatures. But up to now it was not possible to determine what was happening around the fiber without having light escape the fibre, which disrupts its path.