Advantages and disadvantages of single-mode fiber and multimode fiber

What are the advantages and disadvantages of single-mode fiber and multimode fiber? For multimode fiber, 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 the mode of the fiber

Dispersion (also known as 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 optical fiber communication.

The refractive index distribution of multimode fiber is mostly parabolic distribution, that is, graded refractive index distribution. Its core diameter is about 50µm.

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) in the

Among them, the rest of the high-order modes are all cut off, and such a fiber is called a single-mode fiber.

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. because

Therefore, in order to achieve single-mode transmission, the parameters of the fiber must meet certain conditions. Calculated by the formula, for the fiber with NA=0.12, the fiber must be above λ=1.3µm

When realizing single-mode transmission, the radius of the fiber core should be ≤4.2µm, that is, the 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.

What are the advantages of using fiber optics?

1) The passband of optical fiber is very wide, theoretically up to 30T.

2) The length supported by no relay can reach tens to hundreds of kilometers, and the copper wire is only a few hundred meters.

3) Not affected by electromagnetic fields and electromagnetic radiation.

4) Light weight and small size.

5) Optical fiber communication is not charged, safe to use and can be used in flammable, violent and other places.

6) The ambient temperature range is wide.

7) Long service life.

How to choose fiber optic cable?

In addition to the number of optical fiber cores and types of optical fibers, the selection of optical cables also depends on the structure and outer sheath of the optical cable according to the environment in which the optical cable is used.

1. For direct burial of optical cables for outdoor use, it is advisable to use loose sleeve armored optical cables. When overhead, a loose-tube optical cable with two or more ribs and a black PE outer sheath can be selected.

2. When selecting the optical cable used in the building, the tight sleeve optical cable should be selected and its flame retardant, poisonous and smoke characteristics should be paid attention to. Generally, the resistance can be selected in the pipeline or in the forced ventilation.

Combustible but smoked type (Plenum) or flammable and non-toxic type (LSZH), and the flame-retardant, non-toxic and smoke-free type (Riser) should be selected in the exposed environment.

3. When laying cables vertically or horizontally in the building, you can choose tight sleeve optical cables, distribution optical cables or branch optical cables that are common in buildings.

4. Select single-mode and multi-mode optical cables according to network application and optical cable application parameters. Usually, indoor and short-distance applications use multi-mode optical cables, and outdoor and long-distance applications mainly use single-mode optical cables.

In the connection of optical fiber, how to choose the different applications of fixed connection and active connection?

The active connection of optical fibers is realized through optical fiber connectors. An active connection point in an optical link is a definite separation interface. In the choice of active connection and fixed connection, the advantages of fixed connection are reflected in lower cost and less optical loss, but poor flexibility, while active connection is the opposite.

When designing the network, it is necessary to flexibly choose the use of active and fixed connections according to the entire link situation to ensure both flexibility and stability, so as to give full play to their respective advantages. The active connection interface is an important test, maintenance, and change interface. Compared with the fixed connection, the active connection is relatively easier to find the fault point in the link, which increases the convenience of the replacement of the faulty device, thereby improving system maintainability and reducing maintenance costs.