Definition of Birefringent Fiber and Harsh Environment Fiber

birefringent fiber
Birefringent fiber refers to a single-mode fiber that can transmit two inherent polarization modes that are orthogonal to each other. The phenomenon that the refractive index varies with the polarization direction is called birefringence. It is also known as PANDA fiber, that is, polarization maintaining and absorption reducing fiber (Polarization-maintain-ning AND Absorption- reducing fiber). It is a glass part with a large thermal expansion coefficient and a circular cross-section in the lateral direction of the fiber core. During the high temperature fiber drawing process, these parts shrink, resulting in tension in the y-direction of the fiber core and compressive stress in the x-direction. Resulting in the photoelastic effect of the fiber material, the refractive index is different in the X direction and the y direction. According to this principle, the effect of keeping the polarization constant is achieved.

Harsh environment fiber
The normal working environment temperature of optical fiber for communication can be between -40 and +60 °C, and the design is also based on the premise that it will not be irradiated by a large amount of radiation. In contrast, optical fibers that can work at lower or higher temperatures and in harsh environments that are affected by high pressure or external forces and exposed to radiation are called Hard Condition Resistant Fibers. Generally, in order to mechanically protect the surface of the optical fiber, an additional layer of plastic is coated. However, as the temperature rises, the protective function of the plastic decreases, resulting in limited operating temperature. If you use heat-resistant plastics, such as polytetrafluoroethylene (Teflon) and other resins, you can work in a 300 ℃ environment. There are also metals such as nickel (Ni) and aluminum (Al) that are coated on the surface of quartz glass. This kind of fiber is called heat resistant fiber (Heat Resistant Fiber). Additionally, optical losses increase when the fiber is exposed to radiation. This is because when quartz glass encounters radiation exposure, structural defects (also called color centers: Colour Center) will appear in the glass, and the loss will increase especially when the wavelength is 0.4-0.7pm. The prevention method is to use quartz glass doped with OH or F, which can suppress the loss defects caused by radiation. This kind of fiber is called Radiation Resistant Fiber, which is mostly used for monitoring fiber optic mirrors in nuclear power stations.