There are many ways to classify fiber optics, such as quartz-based fiber, multi-component glass fiber, plastic-clad quartz-core fiber, all-plastic fiber, and fluoride fiber.
The mode of light transmission can be divided into multi-mode fiber (Multi-mode Fiber) and single-mode fiber (Single-mode Fiber).
The operating wavelength of the optical fiber can be divided into short-wavelength fiber, long-wavelength fiber, and ultra-long-wavelength fiber.
If classified according to the best transmission frequency window can be divided into conventional single-mode fiber and dispersion-shifted single-mode fiber.
If classified according to the refractive index distribution, it can be divided into step-type and gradient-type fibers.
Here we only focus on single-mode fiber and multi-mode fiber.
(1) single-mode fiber: single-mode fiber with a weak central glass core (core diameter is generally 9 or 10 μm) can only transmit in a single mode at a given wavelength, with the advantage of wide transmission bandwidth and large capacity.
Because only one light mode can be transmitted, its inter-mode dispersion is small and suitable for long-distance communication. Still, there is also material dispersion and waveguide dispersion, so single-mode fiber has high requirements for the spectral width and stability of the light source, i.e., the spectral width should be narrow, and the peace should be good.
Later, it was found that at 1.31 μm, the material dispersion and waveguide dispersion of single-mode fiber is positive and negative, and their sizes are equal. This means that the total distribution of single-mode fiber is zero at 1.31 μm wavelength. In terms of the loss characteristics of the fiber, 1.31 μm is a low-loss window of the fiber.
In this way, the 1.31μm wavelength region becomes an ideal operating window for fiber optic communication and is now the principal working band for practical fiber optic communication systems. The International Telecommunication Union ITU-T determined 1.31μm conventional single-mode fiber’s main parameters in the G.652 recommendation, so this fiber is also known as G.652 fiber.
(2) Multi-mode fiber: It can transmit simultaneously at a given operating wavelength in multiple modes. Multi-mode transmission, due to different ways of light communication along the line of different speeds, will produce phase differences that will lead to transmission distortion, thus making its transmission band subject.
The center glass core is thicker (50 or 62.5 μm) and can transmit various light modes. However, its inter-mode dispersion is large, which limits the frequency of transmitted digital signals, and will be more severe with the increase in distance.
Therefore, the transmission performance of multi-mode fiber is poor compared to single-mode fiber. For example, a 600MB/KM fiber has only 300MB of bandwidth at 2KM. Therefore, the transmission distance of multimode fiber is closer, usually only a few kilometers.
The fiber core size can easily distinguish single-mode and multi-mode fiber. In general, short-wave optical modules use multi-mode fiber to ensure data transmission accuracy and long-wave optical modules use single-mode fiber. Patch cords for single-mode fibers are generally indicated in yellow, with blue or green connectors and protective sleeves.
The jumper of multi-mode fiber is generally indicated in orange, or some in gray, with beige or black connectors and protective sleeves. Fiber optic patch cords at both ends of the optical module must be the same wavelength. That is to say, both lots of the fiber must be the same wavelength optical module. A simple way to distinguish is for the color of the optical module to be consistent.