Communication technology is developing rapidly, and the single-core capacity in optical communication is also growing rapidly at a rate of doubling every 3 to 5 years. However, the main types of optical fibers used for communication and the main transmission indicators of fibers have not changed much for many years. For example, the widely used G.652D fiber has been used for nearly 20 years.
In recent years, a new type of G.654E fiber has been used in some long distance trunk cables, and has achieved good results. So, what is G.654E fiber? Will G.654E fiber replace the traditional G.652D fiber?
History of G.654 fiber development
In the mid-1980s, in order to meet the needs of long distance communication of submarine cables, a pure quartz core single mode fiber for 1550nm wavelength was developed. Its attenuation near this wavelength is more than 10% lower than that of G.652 fiber. This fiber is defined as G.654 fiber, and its name at the time was “1550nm wavelength attenuation minimum single mode fiber”.
In the 1990s, WDM technology began to be used in submarine communication systems. WDM technology enables dozens or even hundreds of optical channels to be transmitted simultaneously in one fiber, and with the use of fiber amplifiers, high-power multi-wavelength optical signals are coupled into one fiber and gathered on a very small interface, and the fiber begins to show nonlinear characteristics. Due to the nonlinear effect of fiber, when the input optical power exceeds a certain value, the transmission performance of the system will gradually decrease as the input optical power increases.
The nonlinear effect of fiber is related to the optical power density of the fiber core. When the input optical power remains unchanged, the impact of nonlinear effect on transmission performance can be reduced by increasing the effective area of the fiber and reducing the optical power density of the core. Therefore, G.654 fiber began to increase the effective area.
The increase in the effective area of the fiber will lead to an increase in the cut-off wavelength, but the increase in the cut-off wavelength must be controlled to avoid affecting the use of the fiber in the C band (1530nm~1565nm); therefore, the cut-off wavelength of G.654 fiber is set at 1530nm. In 2000, when the ITU revised the G.654 fiber standard, it changed the name to “cut-off wavelength shifted single-mode fiber”.
At this point, G.654 fiber has two characteristics: low attenuation and large effective area. Afterwards, G.654 fiber used for submarine cable communication was also optimized mainly around attenuation and effective area, and gradually developed into four subcategories: A/B/C/D.
Characteristics of G.654E fiber
The optical fiber type used in terrestrial trunk transmission lines is mainly G.652D. As the single-carrier rate of WDM systems exceeds 100G, the nonlinear effect of fiber has a more serious impact on transmission performance. It is natural for researchers to transplant G.654 fiber to terrestrial long-distance trunk transmission systems.
Compared with submarine use, the macrobending loss requirements of G.654 fiber for land use are much stricter (the macrobending loss is consistent with G.652D), while the effective area and attenuation index of the fiber are more extensive than those for submarine use, thus forming the standard of G.654E fiber. The main transmission index differences of each subclass of G.654 fiber are shown in the table below.
| Type | ITU-T G.654 | ||||||
| A | B | C | D | E | |||
| Application | submarine cable | Land cable | |||||
| Mode field diameter MFD@1550nm (um) | 9.5~10.5 | 9.5~13.0 | 9.5~10.5 | 11.5~15.0 | 11.5~12.5 | ||
| 1550nm Max.attenuation coefficient (dB/km) | 0.22 | 0.2 | 0.23 | ||||
| Macro bend | R30mm-100rolls@1625nm (dB) | ≤0.5 | ≤2 | ≤0.1 | |||
| Max. PMD coefficient (ps/km1/2) | 0.5 | 0.2 | |||||
| Dispersion | 1550nm dispersion coefficient (ps/nm*km) | ≤20 | ≤22 | ≤20 | ≤23 | 17~23 | |
| 1550nm dispersion slope (ps/nm2*km) | ≤0.07 | 0.05~0.07 | |||||
Advantages and disadvantages of G.654E compared to G.652D
(1) Advantages of G.654E fiber
For single-carrier WDM systems exceeding 100G, as the single-carrier rate increases, the system’s OSNR tolerance requirements become higher. OSNR is related to factors such as the input optical power and the attenuation of the optical amplifier section. The large effective area and low attenuation characteristics of G.654E fiber can effectively improve OSNR.
The effective area of G.654E fiber is mainly divided into 110 um2 (A110) and 130 um2 (A130). The trunk lines built in China between 2015 and 2018 used A110 and A130 fibers, and the trunk lines built after 2018 only used A130 optical fibers. The effective area of G.654E (A130) fiber is about 47% higher than that of G.652D (A80) fiber. Under the condition that the nonlinear effect remains unchanged, the optimal optical input power can be increased by about 1.7dB.
The typical attenuation value of G.654E fiber is about 0.02dB/km lower than that of G.652D fiber. For an 80km long optical amplification section, the attenuation of G.654E fiber is about 1.6dB lower than that of G.652D fiber.
Since the location of optical amplification stations in terrestrial trunk transmission systems is often fixed, the increase in optical input power and the reduction in fiber attenuation cannot significantly reduce the number of optical amplification stations. With the optical amplifier station settings basically unchanged, the OSNR of G.654E fiber can be improved by about 3dB compared to G.652 fiber.
(2) Disadvantages of G.654E fiber
The cutoff wavelength of G.654E fiber is 1530nm, which limits the use of G.654E fiber at wavelengths below 1530nm. Currently, the single optical module over 100G transmission system in the metropolitan area network mostly works around the 1310nm wavelength (O band), such as the core layer and aggregation layer system of 5G backhaul. Therefore, G.654E fiber is not suitable for use in the metropolitan area network.
The market size of G.654E fiber is far less than that of G.652D fiber, which also leads to the high price of G.654E fiber. Currently, the unit price of G.654E bare fiber is about 10 times that of G.652D fiber.
Usage scenarios of G.654E fiber
Currently, the optical cables used by various operators in the construction of inter-provincial and intra-provincial trunk lines are nearly 15,000 kilometers long with G.654E fiber, and the use effect is basically consistent with the above analysis. This is enough to show the necessity of using G.654E fiber in inter-provincial trunk lines.
Compared with inter-provincial trunk lines, the single carrier rate of intra-provincial trunk lines is usually lower, the number of optical amplification sections in the multiplexing section is much lower, and the system has a correspondingly lower tolerance requirement for OSNR. Therefore, the necessity of using G.654E fiber in provincial trunk lines is not high. It is recommended to use G.652D low-loss optical fiber (the unit price of bare fiber is about 1.5 to 2.0 times that of ordinary G.652D).
In metropolitan area networks, the wavelengths used by some optical transmission systems are within the cutoff wavelength range of G.654E fiber, so G.654E fiber is not suitable for use in metropolitan area transmission.
