A “mode” is a beam of light that enters the fiber at a specific angular speed. Multi-mode fiber allows multiple beams of light to propagate simultaneously in the fiber, resulting in mode dispersion (because each “mode” of light enters the fiber at a different angle, they arrive at the other end at different times, a feature called mode dispersion.) The mode dispersion technique limits the bandwidth and distance of multi-mode fiber, resulting in a thick core, low transmission speed, short transmission distance, and poor overall transmission performance. However, multi-mode fiber has the advantage of relatively low cost and is commonly used in buildings or geographically adjacent environments. Single-mode fiber only allows one beam of light to propagate, so it does not exhibit mode dispersion characteristics. As a result, single-mode fiber features a correspondingly thin core, wide transmission bandwidth, high capacity, and long transmission distance, but it is more expensive due to the requirement of a laser source.
Single Mode Fiber
Single-mode fiber has only one strand (two strands in most applications) of glass fiber with a core diameter ranging from 8.3 μm to 10 μm. Due to the relatively narrow core diameter, single-mode fiber can only transmit optical signals with wavelengths of 1310 nm or 1550 nm, which makes coupling with optical devices relatively difficult. The bandwidth of single-mode fiber is higher than that of multi-mode fiber, but at the same time this also puts high demands on the spectral width and stability of the light source. In other words, the spectral width should be narrow and the stability should be good.
Due to its low dispersion and transmission of only one mode of light, single-mode fiber can achieve high-capacity, long-distance transmission. In 100Mbps Ethernet to 1G Gigabit networks, single-mode fiber can support transmission distances of more than 5000m. Because the core diameter of single-mode fiber is too small, it is difficult to control the beam transmission, so it requires an extremely expensive laser as the light source. Additionally, with its higher transmission rate compared to multi-mode fiber and the transmission distance at least 50 times greater than that of multi-mode fiber, so the cost of single-mode fiber is higher than that of multi-mode fiber.
For single-mode optical modules, LD(laser diode) or LED(light-emitting diode) with narrower spectral lines are generally used as the light source, and the coupling components are sized to match the single-mode fiber to achieve high-speed transmission.
Compared to multi-mode fibers, single-mode fibers have a much smaller core diameter. The combination of the small core diameter and single-mode transmission enables the transmission of optical signals in single-mode fibers without distortion due to overlapping optical pulses. Among all fiber types, single-mode fiber has the lowest signal attenuation rate and the highest transmission speed.
Single-mode fiber is mostly used in WDM (Wave-Division-Multiplexing) and other multi-frequency data transmission applications, where the multiplexed optical signals can be transmitted using only a single-mode fiber.
Multi Mode Fiber
Multi-mode fiber is another common fiber type with a core diameter of 50μm to 100μm and an operating wavelength of 850nm or 1310nm. It is relatively easy to couple with optical devices. Multi-mode fiber can transmit multiple modes at a given operating wavelength.
Compared to twisted pair, multi-mode fiber can support longer transmission distances, up to 2000 meters in 10mbps and 100mbps Ethernet. Common multi-mode fibers are available in core diameters of 50μm, 62.5μm and 100μm.
Due to the transmission of up to hundreds of modes in the multi-mode fiber, the propagation constants and group velocities of each mode vary, making the fiber narrow bandwidth, large dispersion and loss in the fiber. This limitation restricts the frequency of the transmitted digital signal, so it is only suitable for short and medium distance, small-capacity fiber optic communication systems, generally limited to transmission distances of a few kilometers .
Unlike single-mode optical modules, multi-mode optical modules generally use less expensive LEDs as the light source, and the coupling components are mostly sized to match the multi-mode fiber to achieve the optimal transmission effect.
Compared with single-mode fiber, multi-mode fiber is less costly. Based on practical needs, the majority of the fiber used in most LANs today is multi-mode fiber.
Difference between single mode fiber and multi-mode fiber
1. Single-mode fiber supports single-fiber transceivers, which are implemented by using 1500nm wavelengths for transmission and 1300nm wavelengths for reception at one end, and the opposite at the other end, using 1500nm wavelengths for reception and 1300nm wavelengths for transmission. Some people refer to this as duplex. In fact, it is not entirely accurate to say so, it should be called multiplexing. Multi-mode fiber only supports dual-fiber transceivers because multi-mode uses refraction for transmission and cannot achieve sending two wavelengths in different directions on the fiber . Only one wavelength can be used, and it cannot be multiplexed.
2. The core diameter of single-mode fiber is small (about 10mm), allowing only one mode transmission, with small dispersion, and operates at long wavelengths (1310nm and 1550nm). Coupling with optical devices is relatively difficult. On the other hand, multi-mode fiber has a larger core diameter (62.5m m or 50m m), allowing hundreds of modes transmission, with larger dispersion, and operates at 850nm or 1310nm. Coupling with optical devices is relatively easy.
3. The logo for multi-mode fiber is “MMF”, while the logo for single-mode fiber is “SMF”. When plugging the fiber into a device and examining the inside, if there is light, it means multi-mode fiber, and if there is no light, it indicated single-mode fiber(the principle of single-mode fiber is to use the laser as the light source, while multi-mode fiber uses two excited tubes as the light source).
Choosing single-mode or multi-mode fiber?
In security applications, the most common deciding factor for choosing between multi-mode or single-mode fiber is the distance. If the distance is only a few miles, multi-mode fiber is preferred. This is because LED transmitters/receivers are much cheaper than the laser needed for single-mode fiber. If the distance is exceeds 5 miles, single-mode fiber is the best choice. Another issue to consider is bandwidth. If future applications may include transmitting large bandwidth data signals, then single mode fiber would be the optimal choice.
Summary
Single-mode fiber is suitable for long-distance transmission, but it comes at a higher cost. Multi-mode fiber, on the other hand, is suitable for short-distance transmission and is more affordable. When choosing fiber optic patch cords, it is important not to make choice blindly. First of all, you should be clear about your specific needs, including the type of cabling system you are using and the distance you need to transmit. Based on these factors, you can choose the appropriate fiber optic patch cord that best suits your needs.
