1). Introduction
G.652 optical fiber is the earliest type of single-mode optical fiber used, and it is also the most widely used optical fiber in communication networks. Whether it is a long-distance network, a local network or an access network, G.652 optical fiber is the absolute protagonist, and its overall usage accounts for more than 95%.
G.652 optical fibers are divided into four subcategories a, b, c, and d. So, what is the difference between each subcategory? This starts with the attenuation characteristics of the optical fiber and the PMD (polarization mode dispersion) coefficient of the optical fiber.
2). Attenuation Characteristics of Optical Fiber
The attenuation coefficient of conventional single-mode fiber varies with wavelength, as shown in the figure below. Due to the influence of hydroxide ions in the fiber material, the attenuation of the fiber at the wavelength of 1383nm is relatively large, and a wave peak will be displayed in the figure, which is usually called "water peak". Therefore, communication systems generally avoid the 1383nm wavelength region.
Conventional single-mode optical fibers have good attenuation characteristics in the wavelength range from 1260nm to 1675nm (excluding the 1380nm region). Therefore, ITU-T divides single-mode optical fiber communication systems into O, E, S, C, L, and U. Optical band, the wavelength range of each band is shown in the figure below.
In the above several bands, except E band, several other bands can be used for communication. This is nothing at all, but there is still a company named Lucent who can't stand it anymore. They invented a kind of optical fiber in 1998. The attenuation curve of this optical fiber in the E-band is flat, as shown in the figure below. This kind of fiber can be used for communication in O, E, S, C, L, U light bands, so this kind of fiber is also called full wave fiber, or low water peak fiber.
3). PMD coefficient of optical fiber
The optical fiber is drawn out through the drawing tower, just like the ramen, the cross-section of the optical fiber is not a completely regular circle, which leads to the fact that when the optical signal is transmitted in the single-mode optical fiber, the two mutually perpendicular polarization modes contained in the fundamental mode will be separated. Propagate at different speeds, so there is a time difference when reaching the other end of the fiber, which is polarization mode dispersion, or PMD for short, as shown in the figure below. The time difference on the fiber unit length is called the PMD coefficient.
When the communication rate is low, PMD is not enough to affect the system transmission. As the transmission rate increases, PMD becomes an important factor affecting the transmission distance. The relationship between PMD coefficient, transmission rate and transmission distance is shown in the table below.
Obviously, the smaller the PMD coefficient of the optical fiber, the better. The PMD coefficient in the current national standard is recommended not to exceed 0.2ps/√km, and the PMD coefficient of actual optical fiber products generally does not exceed 0.1ps/√km.
4). Classification of G.652 optical fiber
The subcategories of G.652 are mainly distinguished from the two dimensions of fiber attenuation characteristics and PMD parameters, as shown in the table below.
5). Application of G.652 optical fiber
The type of fiber with a larger PMD coefficient shows that it cannot meet the higher and higher transmission requirements. Therefore, with the improvement of the fiber manufacturing process, G.652A and G.652C are gradually eliminated by the market.
The current market has demand for both G.652B and G.652D optical fibers. Since the prices of G.652D and G.652B optical fibers are almost the same, the sales ratio of G.652B optical fibers is very low (less than 5% of the total sales of G.652 optical fibers. %).
Although the G.652D optical fiber is a full-wave optical fiber, it seems that there is not much need to use so many wave bands for optical communication. For example, the current DWDM mainly uses 80 waves in the C-band, and the S and L-bands have not been used for many years. Moreover, due to the limitation of the nonlinear effect of optical fibers, the number of channels that can be carried in the WDM system is limited. Facing the application of DWDM, all-wave optical fiber is completely unnecessary.
In order to cooperate with the use of full-wave optical fiber, ITU-T released the CWDM standard in 2002, which divides the full-band of single-mode optical fiber into 18 wavelengths, and the channel interval of each wavelength is 20nm, as shown in the figure below.
But because CWDM has no superiority compared with DWDM, so, nearly 20 years after the release of the G.652D optical fiber and CWDM standards, the E-band has little practical application. Until the past two years, with the widespread use of passive wavelength division using CWDM technology in C-RAN (centralized radio access network) bearer, the advantages of G.652D optical fiber have been fully reflected
