Introduction to G651,G652,G653,G654,G655,G656,G657 Fiber
There are seven kinds of optic fiber according to ITU standard: G651, G652, G653, G654, G655, G656, G657; But do you know what is the feature of each kind? How to choose them when you want to design a fiber optic network?
Let’s begin with the structure of the optic fiber, shown as below:
It consists of three parts: Core, Cladding, Coating.
G651 is Multimode fiber, all others(G652~657) are single mode fiber. The diameter of multimode fiber core is 50~100nm. Most of them are 50 or 62.5 as we knew, we always say 50/125 or 62.5/125 when talking about multimode fiber, 125 is the diameter of cladding. The core diameter is affecting transmission performance. Performance is better when the diameter is small. There are many transmission modes through the multimode fiber.
The smaller the diameter, the less transmission mode. when the diameter becomes small enough, it is only one transmission mode. Yes, it is single mode fiber. we always use 9/125 or 10/125 to indicate single mode fiber.
The attenuation of multimode fiber(850nm) is high as shown in the graph below. That is why multimode only used for a short distance.
G652 is single mode fiber, used massively in different kinds of applications. There are two important parameters of single mode optical fiber. One is Mode field diameter, another is attenuation.
Mode Field Diameter is important parameters for optical fiber. the mode field diameter (MFD) is an expression of distribution of the irradiance, i.e., the optical power per unit area, across the end face of a single-mode fiber.
For a Gaussian intensity (i.e., power density, W/m2) distribution in a single-mode optical fiber, the mode field diameter is that at which the electric and magnetic field strengths are reduced to 1/e of their maximum values, i.e., the diameter at which power density is reduced to 1/e2 of the maximum power density, because the power density is proportional to the square of the field strength.
The MFD indicate the optical fiber transmission performance. MFD is not a fixed value and will be changed according to wavelength.
The attenuation of optic fiber is another important parameter. attenuation affects the propagation of waves and signals, it is related to wavelength. From the chart below, you can find that the attenuation is small at 1310 & 1550. That is why we always focus on 1310 or 1550 when talking about G652.
When the transmission rate becomes higher and higher, dispersion is a problem need to solve. Dispersion will affect the performance obviously.
dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency.
In optics, one important and familiar consequence of dispersion is the change in the angle of refraction of different colors of light, as seen in the spectrum produced by a dispersive prism and in chromatic aberration of lenses.
Dispersion is related to wavelength.
The G652 attenuation is low at 1550nm but the dispersion is high. Then G653 is developed which is 0 dispersion at 1550. It seems perfect. But this fiber is not suitable for WDM system since 0 dispersion.
G654 is used under the sea. Two improvements are made to meet the long-distance and large capacity requirements:
1. Reduce the attenuation 0.22db/km to 0.19db/km, it will allow transmitting longer.
2. Increase the mode field diameter, it will enlarge the capacity. How to make the MFD bigger, The one way is to make the fiber core diameter bigger. The G652 cutoff wavelength is 1260, while the G654 cutoff wavelength is 1530. From the cutoff wavelength, you can know the G654 core diameter is bigger than G652’s.
G653 dispersion is 0 at 1550nm, not suitable for WDM system. G655 is come out to solve the problem. G655 dispersion is low at 1550nm and can be used in WDM system.
G655 is popular before 2005. but it falls into disuse. Since the Dispersion compensation technology are improved for G652. G655 is replaced by G652.
G.655 attenuation is low at 1460nm～1625nm（S+C+L band), But when the wavelength is less than 1530, the dispersion is too low for WDM system. So in order to use 1460~1530, G656 come out.
G657 is not sensitive to bending, the bending radius is small. It is usually used in FTTH socket. It is very popular due to the better performance while the cost is just a little higher comparing to G652D.
G657 include G657A&G657B. the G657A is compatible with G652, but G657B is not compatible with G652. it means the loss will be big if splice G652 with G657B.