G.652 fiber is the earliest type of single mode fiber used and is currently the most widely used fiber in communication networks. Whether it is long distance network, local network or access network, G.652 fiber is the absolute protagonist, accounting for more than 95% of its overall usage.
G.652 is a dispersion non-shifted single mode fiber. Its cutoff wavelength is the shortest and can be used for both 1550nm and 1310nm, but the best working wavelength is in the 1310nm where the dispersion is zero, and the attenuation is 0.3~0.4dB/km, and the dispersion coefficient is 0~3.5ps/nm.km; And the loss is minimal when the wavelength is 1550nm, the attenuation is 0.19~0.25dB/km, and the dispersion coefficient is 15~18ps/nm.km, but the dispersion coefficient in the 1550nm band is large at 17ps/(nm.km), which is not suitable for long-distance applications above 2.5 Gb/s.
G.652 fiber is divided into four subcategories: a, b, c, and d. What are the differences between each subcategory? This should explain from the attenuation characteristics and the PMD (polarization mode dispersion) coefficient of the fiber.
The Fiber Attenuation
The loss coefficient of conventional single-mode fiber changes with wavelength, as shown in the figure 1 below. Due to the influence of hydroxyl 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 the “water peak”. Therefore, communication systems generally avoid the 1383nm wavelength area.
Conventional single mode fiber has good attenuation characteristics in the wavelength range from 1260nm to 1675nm (excluding the 1380nm region). Therefore, ITU-T divides single mode fiber communication systems into O, E, S, C, L, and U band, the wavelength range of each band is shown in the figure below.
Among the above bands, except E band, several other bands can be used for communication. This was nothing at first, but a company called Lucent couldn’t stand it anymore. In 1998, they invented an fiber whose attenuation curve in the E-band is flat, as shown in the figure 3 below. It can be used for communication in the O, E, S, C, L, and U bands. Therefore, this kind of fiber is also called full-wave fiber, or low water peak fiber.
Fiber PMD
The fiber is pulled out through a drawing pulley, just like noodles. The cross-section of the fiber is not a completely regular circle. This causes the two mutually perpendicular polarization modes contained in the fundamental mode to be transmitted at different rates when the optical signal is transmitted in a single mode fiber, so there is a time difference when the optical signal reach the other end of the fiber. This is called polarization mode dispersion or PMD for short, as shown in the figure 4 below. The time difference per unit length of the fiber is called the PMD coefficient.
When the communication rate is low, PMD is not enough to affect 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.
| PMD (ps/√km) | 2.5Gb/s (km) | 10Gb/s (km) | 40Gb/s (km) |
| 3 | 180 | 11 | 1 |
| 1 | 1600 | 100 | 6 |
| 0.5 | 6400 | 400 | 25 |
| 0.2 | 40000 | 2500 | 156 |
| 0.1 | 160000 | 10000 | 625 |
Obviously, the smaller the PMD coefficient, the better. The current national standard recommends that the PMD coefficient not exceed 0.2ps/√km, and the PMD coefficient of actual fiber products generally does not exceed 0.1ps/√km.
Classification of G.652 fiber
Each subcategory of G.652 is mainly distinguished from the two dimensions: fiber loss characteristics and PMD parameters, as shown in the following table.
| Item | Conventional attenuation | Low water peak |
| PMD ≤0.5 | G.652A | G.652C |
| PMD ≤0.2 | G.652B | G.652D |
From the table, the main difference between G.652A, G.652B, G.652C, and G.652D is PMD polarization mode dispersion. A/B are basic single mode fibers, and C/D are low water peak single mode fibers. Among them, G652D is the most commonly used. Since its fiber dispersion is very small when operating at a wavelength of 1300nm, the transmission distance of the system is only limited by loss.
The difference between these four types of fiber:
The G652A fiber supports transmission distances of 400 km in 10 Gbit/s systems, 40 km in 10 Gbit/s Ethernet systems, and 2 km in 40 Gbit/s systems. Available in D, E, S, C and L5 bands, it can work in the entire operating wavelength range of 1260-1625nm. It has better bending performance and more precise geometric and dimensional technical requirements.
G652B optical fiber supports a transmission distance of 3000 km in a 10 Gbit/s system and 80km in a 40 Gbit/s system.
The properties and application scope of G652C fiber are similar to G652A fiber. However, the coefficient of G652C fiber at the wavelength of 1550nm is lower, and the water absorption peak near 1380nm is eliminated, that is, the system can work in the 1360~1530nm band (extended frequency band E-band and short-band S-band).
The properties of G.652D fiber are basically the same as G.652B fiber, and the attenuation coefficient is the same as G.652C fiber, that is, the system can work in the 1360~1530nm band. G.652.D has the most stringent indicators among all G.652 levels. It is completely backward compatible and has no structural difference from ordinary G.652 fiber. It is currently the most advanced non-dispersion shifted fiber for metropolitan area networks. When talking about G652 fiber, most of the time it means G652D. G652D fiber is widely used in many applications.
| Fiber type | Attenuation | PMD (Polarization mode dispersion) | Bending loss |
| G652A | ≤0.5/0.4dB at 1310/1550nm | ≤0.5 ps/sqrt(km) | ≤0.5 dB at 1550nm |
| G652B | ≤0.4/0.35dB at 1310/1550/1625nm | ≤0.2 ps/sqrt(km) | ≤0.5 dB at 1625nm |
| G652C | ≤0.4dB frm 1310nm to 1625nm≤0.3dB frm at 1550nm &1383nm | ≤0.5 ps/sqrt(km) | ≤0.5 dB at 1625nm |
| G652D | ≤0.4dB from 1310nm to 1625nm≤0.3dB from at 1550nm &1383nm | ≤0.2 ps/sqrt(km) | ≤0.5 dB at 1625nm |
G.652 Fiber Application
Optical fiber with larger PMD coefficients are unable to meet the increasingly high-speed transmission requirements. Therefore, as the fiber manufacturing process improves, G.652A and G.652C are gradually eliminated from the market.
There is current market demand for both G.652B and G.652D fibers. Since the prices of G.652D and G.652B are almost the same, the sales proportion of G.652B fibers is very low (less than 5% of the total sales of G.652 fibers).
Although G.652D is a full-wave fiber, it seems that there is not much need to use so many bands for optical communication. For example, the current DWDM mainly operates 80 waves in the C band. For many years, the S and L bands have not been used. Moreover, due to the limitation of the nonlinear effect of fiber, the number of channels that can be carried in the wavelength division system is limited. For DWDM applications, full-wave optical fiber is completely unnecessary.
In order to cooperate with full wave fiber usage, ITU-T released the CWDM standard in 2002, which divided the full-band of single mode fiber into 18 wavelengths, and the channel spacing of each wavelength is 20nm, as shown in the figure 5 below.
However, since CWDM has no advantages over DWDM, nearly 20 years after the release of the G.652D fiber and CWDM standards, there have been few practical applications in the E-band. It was not until the past two years that the advantages of G.652D fiber were fully demonstrated with the widespread use of passive wavelength division using CWDM technology in C-RAN (centralized radio access network) transport.
