Modes of optical fiber - single mode and multimode

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         When we encounter optical fiber , There will always be ‘ multimode ’ and ‘ Monomode ’ This concept . After collecting network information , I always feel that there are some shortcomings . Lack of intuitive explanation . For a layman , It's too abstract to understand .

         Let's start with logic ,‘ multimode ’ and ‘ Monomode ’ Inside ‘ single ’ and ‘ many ’ yes ‘ model ’ Quantifiers in English , Let's start with ‘ model ’ Start with this concept .‘ model ’ It means ‘ Pattern ’, and ‘ Pattern ’ Namely ‘ Mode of optical transmission ’, Since it involves transmission , It must be stable , That is, stable transmission characteristic signal ： Its phase and amplitude must be kept constant . Optical fiber as waveguide path , How many modes of stable transmission , It defines optical fiber ‘ Pattern ’ The amount of ;

        according to Popular science cloud platform of Chinese Academy of Sciences ,《 Propagation mode of optical fiber 》 Described in the ：“ Simple, he said , In the light receiving angle of the optical fiber , Enter the end face of the optical fiber at a certain angle , And can be in the core of optical fiber - Propagating light that produces total reflection at the cladding interface , It can be called a transmission mode of light . When the core diameter of the optical fiber is large , It is within the light receiving angle of the optical fiber , Light waves can be allowed to enter the end face of the optical fiber at multiple specific angles , And propagate in optical fiber , here , It is said that there are multiple modes in the optical fiber . This kind of fiber that can transmit multiple modes is called multimode fiber .” That is, how many modes , It is the number of light waves that are allowed to transmit stably at several incident angles .

        According to AGCO labs 《 Single mode fiber 》 and 《 Multimode fiber 》, In combination with the above introduction , Multimode fiber allows multi angle optical transmission , This leads to mutual interference in small optical fibers , The characteristic of light is the interference of light , Superposition of multiple modes , Cause spots on the output ：

        In the process of multimode fiber transmission , Try to keep up LP01, But the spots will get bigger , The larger spots on the cross section means that the light seen from the interface tailed longer , And for example, in 0 and 1 When it represents the state of no light and light respectively ,1 The intensity of light will affect 0 The state of , Serious interference between signals will occur . If it continues to deteriorate , There will be more serious patterns , Cause the light intensity to disperse sharply .

        Single mode fiber , There is only one mode state , When there is a problem with the incident angle , It will be adjusted in time , As can be seen from the figure below ：1500nm wavelength , The change of the beam when there is a small misalignment of the light incident at the left end of the optical fiber . Start some light into cladding mode , Then it is attenuated . After a distance , Most of the light in the guided mode is still in the optical fiber . 

         When the wavelength is changed to 1000nm You will find ： Due to the interference between modes, it can be seen that the field distribution oscillates periodically ;

        When the wavelength is changed to 500nm You will find ： The form of communication is more complex , Because it supports more modes .

“ For short distance optical fiber communication , Multimode fiber is usually used instead of single-mode fiber , Because a simple light source can be effectively coupled into multimode fiber （ For example, led ,LEDs）, And the requirements for alignment are not very high （ for example , In optical fiber connectors ）. However, the data transmission rate and transmission distance using this kind of optical fiber are limited , Mainly because of the phenomenon of intermodal dispersion ： The group velocity is related to the propagation mode , Therefore, the propagation of ultrashort pulses in multimode fiber will be divided into several pulses with different propagation speeds , It may eliminate the transmitted signal .”

“ If long-distance data transmission , It is more suitable to use single-mode fiber , Because there is no intermodal dispersion , But the cost of the system will be much higher . ”