Differences and Choices between Single Mode Fiber and Multimode Fiber

1. What are single-mode and multimode fibers? What is their difference?

The concepts of single-mode and multi-mode refer to the classification of optical fibers according to their propagation modes - the concepts of multi-mode fibers and single-mode fiber propagation modes. We know that light is an electromagnetic wave with a very high frequency (3 × 1014Hz). When it propagates in optical fibers, theoretical discoveries such as wave optics, electromagnetic fields, and solving Maxwell's equations have been made:

When the geometric size of the fiber core is much larger than the wavelength of light, light will propagate in dozens or even hundreds of propagation modes in the fiber, such as TMmn mode, TEmn mode, HEmn mode, etc. (where m, n=0, 1, 2, 3,...).

The HE11 mode is referred to as the fundamental mode, while the rest are referred to as higher-order modes.

1) Multimode fiber

When the geometric size of the fiber (mainly the core diameter d1) is much larger than the wavelength of the light (about 1 µ m), there will be dozens or even hundreds of propagation modes in the fiber. Different propagation modes have different propagation speeds and phases, resulting in delay and broadening of optical pulses after long-distance transmission. This phenomenon is called mode dispersion (also known as intermodal dispersion) in optical fibers.

Modal dispersion can narrow the bandwidth of multimode fibers, reducing their transmission capacity. Therefore, multimode fibers are only suitable for smaller capacity fiber optic communication.

The refractive index distribution of multimode optical fibers is mostly a parabolic distribution, also known as a gradient refractive index distribution. Its fiber core diameter is about 50 µ m.

2) Single-mode optical fiber

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

Due to its unique mode propagation and avoidance of mode dispersion issues, single-mode fibers have an extremely wide bandwidth, making them particularly suitable for high-capacity fiber optic communication. Therefore, in order to achieve single-mode transmission, it is necessary to satisfy certain conditions for the parameters of the optical fiber. According to the formula calculation, for the optical fiber with NA=0.12, it is necessary to λ= When achieving single-mode transmission above 1.3 µ m, the radius of the fiber core should be ≤ 4.2 µ m, that is, its core diameter d1 should be ≤ 8.4 µ m.

Due to the extremely small core diameter of single-mode optical fibers, more stringent requirements are placed on their manufacturing process.

2. What are the advantages of using fiber optics?

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

2) Without relay support, the length can reach tens to hundreds of kilometers, and the copper wire is only a few hundred meters.

3) Not affected by electromagnetic fields and radiation.

4) Lightweight and small in size.

5) Fiber optic communication is not electrified and can be used safely in flammable, explosive and other places.

6) Wide range of ambient temperature for use.

7) Long service life.

3. How to choose fiber optic cables?

The selection of optical cables should not only be based on the number of fiber cores and types of fibers, but also on the structure and outer sheath of the cable according to its usage environment.

1) When burying outdoor optical cables directly, it is advisable to choose loose sheathed armored optical cables. When overhead,