一、multimode optical fiber

When the geometric size of the optical fiber (mainly the core diameter d1) is far larger than the wavelength of light wave (about 1m), there will be dozens or even hundreds of propagation modes in the optical 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 mode dispersion (also called inter-mode dispersion) of optical fiber.

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

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

二、single-mode fiber

When the geometric size (mainly the core diameter) of the optical fiber can be close to the optical wavelength, such as the core diameter d1 in the range of 5 ~ 10m, the optical fiber only allows one mode (fundamental mode HE11) to propagate in it, and all other higher-order modes are cut off. Such an optical fiber is called a single-mode optical fiber.

Because it has only one mode propagation and avoids the problem of mode dispersion, single-mode fiber has a very wide bandwidth, which is especially suitable for large-capacity optical fiber communication. Therefore, in order to realize single-mode transmission, the parameters of the optical fiber must meet certain conditions. Through the formula calculation, it is concluded that when the single-mode transmission of the optical fiber with NA=0.12 is over λ = 1.3m, the radius of the optical fiber core should be ≤ 4.2m, that is, its core diameter D1 ≤ 8.4m.

Because the core diameter of single-mode optical fiber is very small, it puts forward more stringent requirements for its manufacturing process.

三、 What are the advantages of using optical fiber?

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

2) The length of trunkless support can reach tens to hundreds of kilometers, and the copper wire is only a few hundred meters.

3) Not affected by electromagnetic field and electromagnetic radiation.

4) Light weight and small volume.

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

6) Wide use environment temperature range.

7) Long service life.