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Multi Mode Fiber - Coarse core (50 or 62.5) μ m) It can transmit multiple modes of light. But its intermodal dispersion is relatively large, which limits the frequency of transmitting digital signals and becomes more severe with increasing distance. Therefore, the distance for multimode fiber optic transmission is relatively close, usually only a few kilometers.

 

Single mode optical fibers can only transmit single-mode signals, while multi-mode optical fibers can transmit multi-mode signals,

 

Multimode fiber

As the name suggests, it is a fiber optic that can propagate multiple modes of electromagnetic waves (in this case, light waves); Due to the presence of multiple modes of transmission, there is significant inter mode dispersion, resulting in a smaller capacity for transmitted information; Multimode fiber has a larger core, usually 50um, and a numerical aperture of around 0.2; The number of modes depends on the diameter, numerical aperture, and wavelength of the fiber core.

 

Single-mode optical fiber

Only one mode of signal wave can be transmitted, but it must meet the conditions: it seems to be related to the thing called normalized frequency mentioned in the textbook, and the fiber core needs to be finer, preferably 3 or 4 times the working wavelength; So, in terms of appearance, single-mode optical fibers are much finer than multimode optical fibers; Single mode fiber does not have mode dispersion because it only transmits one mode

Multimode optical fibers are used in small capacity, short distance systems. Single mode optical fibers are used in backbone, high-capacity, and long-distance systems.

 

The core diameter of single-mode fiber is generally 9/125, while that of multimode fiber is 50/125 or 62.5/125.

Single mode and multimode are relative to specific wavelengths, and the same fiber may be single mode or multimode at different wavelengths.

 

There is no distinction between single and multiple modes of light, and there are two types of light sources: single longitudinal mode (dfb) and multiple longitudinal mode (fp).

 

Multimode optical fibers are slightly finer in diameter than single-mode fibers. Single mode 652 has a diameter of 62.5/125, while there are two types of multimode fibers: 50/125 and 62.5/125. In terms of price, multimode fibers are generally 1.5-2 times that of single-mode fibers with the same number of cores. In practical applications, multimode fibers are mainly used in data access optical cables. The biggest disadvantage of multimode fibers compared to single-mode fibers is inter mode dispersion (due to the different rates of the same type of light in different modes).

 

Fiber optic is divided into two categories: multimode fiber and single-mode fiber. The difference between multimode fiber and single-mode fiber mainly lies in the different transmission methods of light, and of course, the bandwidth capacity is also different. Multimode fibers have a larger diameter, and beams of different wavelengths and phases continuously reflect and transmit forward along the fiber wall, causing dispersion and limiting the transmission distance and bandwidth between two repeaters. The bandwidth of multimode fibers is about 2.5Gbps. The diameter of single-mode fiber is relatively thin, and light propagates in a straight line without much reflection, resulting in reduced dispersion, increased bandwidth, and longer transmission distance. However, the accompanying optical terminal equipment is expensive, with a bandwidth of over 10Gbps for single-mode fibers.

 

Multimode optical cable is a communication cable that is equipped with multiple optical fibers (composed of silicon dioxide and quartz glass). Having multiple structural models and two transmission modes, it is a widely used communication optical cable, mainly used in long-distance communication and inter office communication transmission. Multimode optical cable transmission: In terms of transmission, 8-core multimode optical cables are mainly available in two specifications: single-mode and multi-mode. Single mode (inner diameter 9 μ The outer diameter of m is 125 μ m) Multimode (there are two types, one with an inner diameter of 62.5 μ The outer diameter of m is 125 μ M and inner diameter are 50 μ The outer diameter of m is 125 μ m) Single mode is a long-distance transmission mode with wavelengths of 1310 and 1550; Multimode is a mode of short distance transmission (limited to a transmission distance of less than 2000 meters), with wavelengths of 850 and 1300. (Single mode is widely used, while multimode is gradually being phased out)

 

Multi Mode Fiber: The center glass core is thicker (50 or 62.5) μ m) It can transmit multiple modes of light. But its intermodal dispersion is relatively large, which limits the frequency of transmitting digital signals and becomes more severe with increasing distance. For example, a 600MB/KM fiber has only a bandwidth of 300MB at 2KM. Therefore, the distance for multimode fiber optic transmission is relatively close, usually only a few kilometers.

 

Multimode fibers allow multiple beams of light to propagate simultaneously in the light, resulting in mode dispersion (because each mode light enters the fiber at different angles and reaches the other end at different times, this feature is called mode dispersion). Mode dispersion technology limits the bandwidth and distance of multimode optical fibers. Therefore, the core wire of multimode optical fibers is thick, the transmission speed is low, the distance is short, and the overall transmission performance is poor. However, its cost is relatively low compared to other products, and it is generally used in buildings or geographically adjacent environments.

 

In practical use, laser and multimode fiber coupling can be recommended according to the Gbit/s Ethernet standard:

① Bias injection

To avoid bandwidth degradation caused by direct injection of the aforementioned laser into the multimode fiber, the standard stipulates the use of Mode Conditioning Patch Cord MCP to couple the laser output into the multimode fiber. The mode adjustment cable is a short single-mode fiber, with one end coupled to the laser and the other end coupled to the multi-mode fiber. The standard stipulates that the output spot of single-mode optical fiber intentionally deviates from the axis of multi-mode optical fiber by a certain distance, and the allowable deviation range is 17-24 μ m. Its purpose is to avoid the central refractive index depression without deviating too far, only selectively exciting a small group of lower order modes.

② Central injection

For multi-mode fibers with ideal refractive index distribution and no central depression, central injection can be used without mode adjustment of the connection. The advantage of doing so is that it can effectively increase the laser bandwidth of multimode fibers, reduce the complexity of network systems, and lower system costs. A mode adjustment connection is about 80-100 US dollars. InfiniCor CL 1000 (62.5 μ M core diameter and InfiniCor CL 2000 (50 μ M-core diameter is the first type of multimode fiber in Gigabit Ethernet where 1300nm wavelength laser is directly injected without the need for mode adjustment wiring.

 

Additionally, it should be noted that when it comes to 10 Gigabit multimode optical cables, it is important to note that fiber optic systems cannot transmit optical signals without optical transceivers and fibers. Due to the fact that traditional multimode fibers can only support tens of meters of 10G transmission, a new type of optical transceiver was adopted to match 10G applications. ISO/IEC 11801 established a new multimode fiber standard level, namely the OM3 category, which was officially promulgated in September 2002. OM3 fiber has been optimized for both LED and laser bandwidth modes, and requires strict DMD testing and certification. The adoption of new standards for fiber optic cabling systems can support at least 10 Gigabit transmission up to 300 meters in multimode mode, and can reach over 10 kilometers in single-mode mode mode (1550nm can also support 40 kilometers of transmission).

 

The bandwidth of light is the product of the modulation frequency of the maximum modulation frequency pulse that a fiber can pass through and the length of the fiber. It is a comprehensive indicator that characterizes the optical characteristics of multimode fibers. For example, the optical transmitter of ordinary optical cables is generally LED (light-emitting diode), which requires the fiber to have a bandwidth of at least 200MHz at 850nm. The optical transmitter of OM3 optical cables is VCSEL (vertical cavity surface emitting laser), Require the optical cable to reach 2000MHz at 850nm. Due to various factors such as light source, coupling method, waveguide structure, and receiver performance, aside from other influencing factors, the bandwidth of the fiber itself is determined by the dispersion characteristics of the multimode fiber. Considering that there are very few optical transmission paths within single-mode optical fibers, the intermode dispersion is small and even negligible, and the laser dispersion of the matched LD optical transmitter is small, the bandwidth of the optical fiber is limited to multi-mode optical fibers. The transmission bandwidth of single-mode optical fibers can be understood as infinite, but due to their high requirements for connectors, single-mode optical cables are generally chosen for long-distance.

 

 

 

 

 

 

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