The difference between single-mode fiber and multi-mode fiber transmission is that when the geometric size of the fiber (mainly the core diameter d1) is much larger than the wavelength of light (about 1µm), there will be dozens or even hundreds of propagation modes in the fiber. Different propagation modes have different propagation speeds and phases, resulting in time delay and widening of optical pulses after long-distance transmission. This phenomenon is called modal dispersion (also known as intermodal dispersion) of optical fiber.
Modal dispersion will narrow the bandwidth of multimode fiber and reduce its transmission capacity, so multimode fiber is only suitable for small-capacity optical fiber communication.
In short, the multimode fiber has a large core diameter (62.5mm or 50mm), allowing hundreds of modes of transmission, large dispersion, and an operating wavelength of 850nm.
The transmission distance of multimode fiber
Compared with twisted pair, multimode fiber can support longer transmission distance;
In 10mbps and 100mbps Ethernet, multimode fiber can support a transmission distance of up to 2000 meters;
In a 1GbpS Gigabit network, multimode fiber can support a transmission distance of up to 550 meters;
In 10Gbps 10 Gigabit network, multimode fiber OM3 can reach 300 meters, and OM4 can reach 500 meters;
What is single mode fiber
When the geometric size of the fiber (mainly the core diameter) can be close to the wavelength of light, for example, the core diameter d1 is in the range of 5-10µm, the fiber only allows one mode (fundamental mode HE11) to propagate in it, and the rest of the higher-order modes are all cut off , such fibers are called single-mode fibers.
Since it has only one mode of propagation and avoids the problem of mode dispersion, single-mode fiber has an extremely wide bandwidth and is especially suitable for high-capacity fiber optic communications. Therefore, in order to achieve single-mode transmission, the parameters of the fiber must meet certain conditions. It is calculated by the formula that for a fiber with NA=0.12 to achieve single-mode transmission above λ=1.3µm, the radius of the fiber core should be ≤4.2µm, that is, its core diameter d1≤8.4µm.
Since the core diameter of the single-mode optical fiber is very small, more stringent requirements are put forward for its manufacturing process.
Transmission distance of single-mode fiber
The core diameter of the single-mode optical fiber is 8.3 μm, and the outer diameter of the cladding is 125 μm. The working wavelength of the single-mode optical module is 1310nm and 1550nm. Compared with the multi-mode optical fiber, the single-mode optical fiber can support a longer transmission distance. As well as 1G Gigabit network, single-mode fiber can support a transmission distance of more than 5000m.
The devices used in the single-mode optical module are twice that of the multi-mode optical module, so the overall cost of the single-mode optical module is higher than that of the multi-mode optical module; the transmission distance of the single-mode optical module can reach 150 to 200km; the multi-mode optical module The transmission distance can only reach 2km.
