optical fibre transmission, that is, data and signal transmission using optical fiber as the medium. Optical fiber can not only be used to transmit analog signals and digital signals, but also can meet the needs of video transmission. optical fibre transmission is generally carried out by optical cable, and the data transmission rate of a single optical fiber can reach several Gbps. Without the use of optical repeaters, the optical fibre transmission distance can reach tens of kilometers.
(1) Twisted pair stage
At this stage, voice cannot be mixed with large-scale data communication, and it is also suitable for such data communication.
(2) Cable + twisted pair
It can meet the needs of users for a large number of data transmission and video, but it needs more access equipment. Its cost is relatively higher, and it is not easy to expand the demand in the future.
(3) Optical fiber stage
This is what we call the final stage of optical fibre transmission. At this time, the corresponding ancillary equipment is more perfect, the data processing capacity is stronger, and the expansibility is better. The development is also very fast, and the price of access equipment has been adjusted, which can be said to be a comprehensive communication stage in one step. Two theories can be used to analyze the transmission of light in optical fiber: the theory of ray optics and the theory of wave optics. The theory of ray optics is a method of using light rays to replace the transmission route of light energy. This theory is easy to get simple and intuitive analysis results for multimode fiber whose optical wave is smaller than the size of optical waveguide. But for complex problems, ray optics can only give a rough concept.
Wave optics treats the light in the optical fiber as a classical electromagnetic field, so the light field must obey the Maxwell equations and all the boundary conditions. Based on the boundary conditions of wave equation and electromagnetic field, comprehensive and correct analytical or numerical results can be obtained, and the allowable field structure in the waveguide can be given.
According to the number of optical fibre transmission modes, optical fiber can be divided into multimode fiber and single-mode fiber. At a certain working wavelength, multimode optical fiber can transmit many modes of dielectric waveguides, while single-mode optical fiber can only transmit fundamental modes.
Only the fundamental mode is transmitted in the single-mode fiber, so roughly speaking, the mode field diameter is the diameter of the fundamental mode spot on the receiving end of the single-mode fiber, and it can also be roughly considered that the mode field diameter is similar to the core diameter of the single-mode fiber.
When the normalized frequency of the optical fiber is less than its normalized cutoff frequency, the single-mode transmission can be realized, that is, only the fundamental mode is transmitted in the optical fiber, and all the other high-order modes are cut off. That is to say, in addition to the parameters of the optical fiber, such as the core radius and the numerical aperture that satisfies certain conditions, the optical wavelength must be greater than a certain value in order to achieve single-mode transmission, which is called the cutoff wavelength of single-mode fiber.
Therefore, the cutoff wavelength means the minimum working wavelength that enables the optical fiber to achieve single-mode transmission. In other words, even if all other conditions are satisfied, it is still impossible to achieve single-mode transmission if the optical wavelength is not greater than the cutoff wavelength of single-mode fiber.