The difference between single mode fiber and multimode fiber - The popular science article

by:FOT     2020-07-14
Fiber optic cabling is divided into two types - — Multimode and single mode. Most people may know that the length of the multimode wiring, shorter than single-mode wiring, so the single mode suitable for long-distance outdoor fiber applications, and multimode is the main choice of the data center and building interior application. However, due to the inherent ability of high bandwidth single-mode fiber popularity in its application in short distances is becoming more and more high, more and more technical personnel face installation single-mode and multimode optical fiber at the same time. But we found that not everyone understand differences between the two types of optical fiber technology. We deeply know the next. What is the model? In the field of optical fiber data transmission, the term 'pattern' is used to describe the transmission of optical signals in optical fiber glass fiber core way - — The pattern is the transmission of light path. Therefore, single-mode light, the light spread along a path; In the multimode optical fiber, optical transmission in the multiple paths. You can imagine: slide down from the top water slides, you continue to bounce between the slide on both sides of the baffle decline. Each Angle is a pattern in the process of decline. The transmission of light in the multimode fiber optic equipment core way same as the above situation. Light to illuminate an Angle to the glass, and then bounce back, and spread along the length of the fiber core. Why not from inside the fiber core light shines out? First of all, with small Angle light irradiation on the glass, glass is like a mirror reflect light. Second, there is a layer cladding on the outside of the fiber core. In order to make the light stay inside the fiber core and cladding with different refractive index properties, the technical parameter determines the amount of reflection and refraction of light into the material. By contrast, in the single mode fiber, light travels along a straight line, because the small size of single-mode optical fiber in the fiber core ( About one over ten) for multiple mode fiber optic equipment core, , the light will not rebound. Bandwidth limitations why single-mode fiber support high bandwidth and long delay? In the single mode to send light can eliminate differential mode delay ( DMD) And the DMD is the main factor of multimode optical fiber bandwidth restrictions. In the multimode fiber travels in a variety of patterns, some light will move along the fiber center, and other light moving along the path near the fiber core cladding. On the edge of the lateral propagation mode is known as the high order mode, close to the center of the fiber core transmission mode is called the low-order mode. High order and low the speed of different pattern, DMD is the propagation time difference. DMD is smaller, the less likely the spread of the light pulse over time, the higher the bandwidth also. The greater the time difference between the pulse, the receiver may not be able to distinguish the pulse. DMD is directly related to distance - — With the increase of fiber length increases. This is why the multimode fiber is much shorter than the distance of the single-mode fiber requirements, multimode fiber up to 500 meters, and the length of the single mode fiber can be up to 10 km. Fiber defect is one of the causes of the DMD, optical fiber manufacturers have mastered by carefully optimized to limit the dmo fiber refractive index distribution. Model of refraction occurs not only on the border of the fiber core and cladding, multimode fiber optic equipment using gradient refractive index distribution, fiber core center to the refractive index of fiber core and cladding boundaries constantly changing. This is a parabola formed ( The symmetrical curve) Path, which leads to the low order model near the optical fiber core is the slower moving over short distances, and the high order mode are close to the edge of the fiber core move more long distances quickly. So it can reduced the time delay between the pulses to the greatest extent, thus reducing the dmo, achieve higher bandwidth. More significant difference between multimode and single mode is more significant difference including cable color, light source and test, etc. Single mode are almost yellow, and multimode usually light green. Different types of multimode also may have a different color - — OM3 is light green, almost all OM4 multimode sometimes USES a called Erika Violet ( Erica violet) Pink, to help with OM3 distinguish, and the latest generation of multimode fiber OM5 celadon. Another key differences is that a light source and transmission equipment related costs. Single-mode fiber with narrow spectral width of the laser light source, so the high cost of receivers. Compared with multimode fiber optic cable, lower the cost of the single mode optical fiber cable, but the cost of the single mode optical fiber receiver is 1 of the multimode receiver. 5 to 4 times. For multimode and single mode test method, it is important to understand these two kinds of optical fiber types cannot be mixed, access line must be compatible with the type of the measured optical fiber. Test multimode fiber also require circular flux ( EF) Test, to how much light is incident to the fiber optic cable to be tested. EF test limit the number of emission patterns to reduce variability, to realize accurate and repeatable test results.
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