The so-called “mode” refers to a beam of light entering the fiber at a certain angular velocity. Multimode fibers allow 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 and takes different times to reach the other end, a feature called mode dispersion.), Mode dispersion technology limits the bandwidth and distance of multimode fibers. Therefore, multimode fibers have thick core wires, low transmission speed, short distances, and poor overall transmission performance, but their cost is relatively low, and they are generally used in buildings or geographically. in an adjacent environment. Single-mode fiber can only allow one beam of light to propagate, so single-mode fiber has no mode dispersion characteristics. Therefore, the core of single-mode fiber is relatively thin, the transmission frequency bandwidth is large, the capacity is large, and the transmission distance is long, but because it requires a laser source, higher cost.
Single Mode Fiber
Single-mode fibers have only one strand (two strands in most applications) of glass fiber with a core diameter of 8.3 μm to 10 μm. Due to the relatively narrow core diameter, single-mode fibers can only transmit optical signals with wavelengths of 1310nm or 1550nm, and it is relatively difficult to couple with optical devices. The bandwidth of single-mode fiber is higher than that of multi-mode fiber, but at the same time, it also puts forward higher requirements for the spectral width and stability of the light source: that is, the spectral width is narrower and the stability is better.
Due to the small dispersion of single-mode fiber, only one mode of light is transmitted, so large-capacity and long-distance transmission can be realized. In 100Mbps Ethernet and even 1G Gigabit network, single-mode fiber can support transmission distance of more than 5000m. Because the core diameter of the single-mode fiber is too small, it is difficult to control the beam transmission, so an extremely expensive laser is required as the light source body, and its transmission rate is higher than that of the multi-mode fiber, and the transmission distance is at least 50 times higher than that of the multi-mode fiber. The cost of fiber is higher than that of multimode fiber.
For single-mode optical modules, LDs or LEDs with narrow spectral lines are generally used as light sources, and the size of the coupling components is matched with the single-mode optical fibers to achieve high-speed transmission.
Compared with multi-mode fiber, the core diameter of single-mode fiber is much smaller, and the characteristics of small core diameter and single-mode transmission make the optical signal transmitted in single-mode fiber not distorted 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 multi-frequency data transmission application systems such as wavelength division multiplexing (WDM, Wave-Division-Multiplexing). The multiplexed optical signal only needs a single-mode fiber to achieve data transmission.
Multi Mode Fiber
Multimode fiber is another common fiber type with a core diameter of 50μm~100μm and an operating wavelength of 850nm or 1310nm, which is relatively easy to couple with optical devices. It can transmit multiple modes at a given operating wavelength.
Compared with twisted pair, multimode fiber can support longer transmission distance. In 10mbps and 100mbps Ethernet, multimode fiber can support up to 2000 meters of transmission distance. The core diameters of common multimode fibers are 50 μm, 62.5 μm and 100 μm.
Since there are hundreds of modes transmitted in multimode fiber, the propagation constants and group rates of each mode are different, so that the bandwidth of the fiber is narrow, the dispersion is large, and the loss is also large, which limits the frequency of transmitting digital signals, so it is only suitable for For short- and medium-distance, small-capacity optical fiber communication systems, generally only a few kilometers can be transmitted.
Different from single-mode optical modules, multi-mode optical modules generally use low-cost LEDs as light sources, and the size of the coupling components is mostly matched with multi-mode optical fibers to achieve ideal transmission effects.
Compared to single-mode fiber, multimode fiber is less expensive. Based on actual needs, most of the optical fibers used in most local area networks are multimode fibers.
The difference between single mode fiber and multimode fiber
1. Single-mode fiber supports single-fiber transceiver. Its implementation is that one end uses a wavelength of 1500 to transmit and a wavelength of 1300 to receive, while the other end is opposite, one end uses a wavelength of 1500 to receive and a wavelength of 1300 to transmit. Some people call this duplex. In fact, it is not accurate to say that, it should be called reuse. Multimode fiber only supports dual-fiber transmission and reception. Since multimode is transmitted by means of refraction, it is not possible to transmit two wavelengths in different directions on the fiber. Only one wavelength can be used and cannot be multiplexed.
2. The core diameter of single-mode fiber is small (about 10mm), only one mode is allowed to transmit, the dispersion is small, and it works at long wavelengths (1310nm and 1550nm), which is relatively difficult to couple with optical devices; multimode fiber has a large core diameter (62.5mm or 50mm), allowing hundreds of modes to transmit, large dispersion, working at 850nm or 1310nm. Coupling with optical devices is relatively easy.
3. The logo of multi-mode is “MMF”, and the logo of single-mode is “SMF”; if you insert it into the device, look inside, if there is light, it means multi-mode, no light means single-mode (the principle is that single-mode is used The laser is used as the light source, and the multi-mode is based on the light-emitting diode as the light source)
Choose single-mode fiber or multi-mode fiber?
In security applications, the most common determinant of choosing between multimode or singlemode is distance. If only a few miles, multimode is preferred. Because the LED transmitter/receiver is much cheaper than the laser required for single mode. For distances greater than 5 miles, single-mode fiber is best. Another issue to consider is broadband, and if future applications may include the transmission of large-bandwidth data signals, then single-mode would be the best choice.
Single-mode fiber is suitable for long-distance transmission; multi-mode fiber is suitable for short-distance transmission. When choosing a fiber optic patch cords, you can’t choose blindly. First, you must know your own needs, what kind of wiring system it is used for, and how far the transmission distance is required, and choose the fiber that suits you according to your needs.