Transmission difference between single mode fiber and multimode fiber

When the geometric size of the fiber (mainly the core diameter d1) is much larger than the wavelength of the light wave (about 1µm), there will be dozens or even hundreds of propagation modes in the fiber. Different propagation modes have different propagation velocities and phases, resulting in time delay and widening of optical pulses after long-distance transmission. This phenomenon is called the modal dispersion of the fiber (also called intermodal dispersion).

Modal dispersion will narrow the bandwidth of multimode fiber and reduce its transmission capacity, so multimode fiber is only suitable for small-capacity fiber communication.

In short, the multimode fiber has a large core diameter (62.5mm or 50mm), allows hundreds of modes to transmit, has large dispersion, and operates at a wavelength of 850nm.

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 up to 2000 meters of transmission distance; in 1Gbps Gigabit network, multimode fiber It can support up to 550 meters of transmission distance; in the 10Gbps 10 Gigabit network, the 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 the light, such as the core diameter d1 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 a fiber is called a single-mode fiber.

Since it has only one mode of propagation, the problem of modal dispersion is avoided, so the single-mode fiber has a very wide bandwidth, which is especially suitable for large-capacity optical fiber communication. 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 fiber is very small, more stringent requirements are placed on its manufacturing process.

Single-mode fiber transmission distance

The core diameter of the single-mode fiber is 8.3 μm, and the outer diameter of the cladding is 125 μm. The operating wavelengths of the single-mode optical module are 1310 nm and 1550 nm. Compared with the multi-mode fiber, the single-mode fiber can support longer transmission distance. Even 1G Gigabit Ethernet, single-mode fiber can support transmission distances of more than 5000m.

The devices used in single-mode optical modules are twice that of multi-mode optical modules, so the overall cost of single-mode optical modules is higher than that of multi-mode optical modules; the transmission distance of single-mode optical modules can reach 150 to 200km; multi-mode optical modules The transmission distance is only up to 2km.