Optical fiber transmission is now a fast developing way of transmission. Here, we introduce the principle of optical fiber transmission, but before that, we first need to understand the composition of optical transmission system, that is, light source, transmission medium and detector. As a habit, a light pulse represents bit 1, while no light pulse represents bit 0. The transmission medium is very thin glass fiber. When the light is illuminated to the detector, it produces an electric pulse. At the one end of the fiber, we put the light source at the one end and put the detector on the other end. We have a one-way transmission system, which receives an electrical signal and transmits it to the optical pulse and transmits it. Then the receiver converts the optical pulse to the electrical signal.
In fact, if not the use of the principle of a physical, the fiber communication system because it will be leaked and no practical value. When light enters another medium through a medium, for example, when light is injected into silicon dioxide from air, there will be refraction (bending) on the interface between them. The reason is that the refractive index of two substances is different from light. The incident light is shot by angle alpha 1 on the boundary and is ejected with angle beta 1. The refraction depends on the characteristics of the two media (the refractive index of the two). If the incident angle is greater than a critical value, the light will be fully reflected back to the silicon dioxide and will not leak into the air. Therefore, when the incident angle of the light is greater than or equal to the critical value, the light will be completely confined to the optical fiber, and the loss free propagation is a few kilometers.
Because any incident light at a critical value will be completely reflected back to the medium at the boundary of the medium, so different light will propagate in the medium with different reflection angles. It is considered that each beam has a different mode (mode), and the fiber with this characteristic is called the multimode fiber (multi-mode fiber). However, if the diameter of the fiber is reduced to the same wavelength as that of the light wave, the optical fiber is like a waveguide. The light does not reflect and propagate along the straight line. This is the single-mode fiber (single-mode fiber). Single mode fiber is very expensive, but the transmission distance is far away. The current optical fiber system can transmit 30km far at the rate of 1Gb/s. In the laboratory, a higher transmission rate has been achieved on a short distance. The experiment also showed that high power laser can drive the optical fiber to transmit 100km, although the rate is lower, but no repeater is needed.