The basic principle of fiber optic cable
Optical fiber transmission is based on the principle of total reflection of available light at the interface of two media. For mutant fiber, n1 is the refractive index of the core medium, n2 is the refractive index of the cladding medium, n1 is greater than n2, the incident angle when the light entering the core reaches the interface between the core and the cladding (referred to as the core-cladding interface) When it is greater than the critical angle θc of total reflection, total reflection can occur without light energy passing through the core, and the incident light can be transmitted forward at the interface through numerous total reflections. original
When the fiber is bent, the interface normal is turned, and the incident angle is small, so the incident angle of a part of the light becomes smaller than θc and cannot be totally reflected. However, those rays with a larger incident angle can still be totally reflected, so the light can still be transmitted when the fiber is bent, but it will cause energy loss. Generally, when the bending radius is greater than 50-100 mm, the loss can be ignored. Small bends will cause severe "microbend losses".
People often use electromagnetic wave theory to further study the mechanism of optical fiber transmission, and solve the wave equation by the boundary conditions of optical fiber medium waveguide. Light propagating in an optical fiber contains many modes, each of which represents an electromagnetic field distribution and corresponds to a certain ray described in geometrical optics. The conduction modes present in the fiber depend on the normalized frequency ν value of the fiber
where NA is the numerical aperture, which is related to the refractive index of the core and cladding medium. ɑ is the core radius and λ is the wavelength of the transmitted light. When the fiber is bent, mode coupling occurs, and a part of the energy is transferred from the conduction mode to the radiation mode, which is transmitted to the outside of the core and lost.
Performance: The main parameters of optical fiber are attenuation, bandwidth, etc.
