With the rapid development of 5G networks, the demand for network data transmission has increased exponentially. As the underlying transport network, the transmission capabilities of optical networks are crucial to the development of 5G networks.
A magic weapon to expand the transmission capacity of optical networks is to continuously dig into the available band resources of optical fiber, that is, to continuously expand the transmission path width of optical networks. As the transmission path becomes wider, the transmission capacity of the optical network will naturally increase.
Recently, three new wave bands, CE, Cpp, and C+L, have emerged in optical networks, adding to the expansion of optical network transmission capabilities.
Original band
As the name suggests, optical fiber communication is communication in which light is used as an information carrier and optical fiber is used as a transmission medium. However, not all light is suitable for fiber optic communications. Different wavelengths of light (which can be simply understood as light of different colors) have different transmission losses in optical fibers. Light with high transmission loss cannot carry information and transmit it in the optical fiber.
After long-term research by scientists, it was first discovered that light with a wavelength of 850nm can be used as light for optical communications. This band is also directly called the 850nm band. However, the transmission loss in the wavelength region of the 850nm band is relatively large, and there is no suitable fiber amplifier. Therefore, the 850nm band is only suitable for short-range transmission.
Then, scientists explored the “low-loss wavelength region” optical band, that is, the light in the 1260nm~1625nm region, which is most suitable for transmission in optical fibers. The relationship between transmission loss and optical band is shown in the figure below.
The 1260nm~1625nm area is subdivided into five bands: O-band, E-band, S-band, C-band and L-band.
O band
The O-band wavelength range is: 1260nm~1360nm. The optical dispersion in this band causes the smallest signal distortion and the lowest loss, making it an early optical communication band. Therefore, it is named O-band (O-band), where O refers to “Original (original)”.
E-band
The E-band wavelength range is: 1360nm~1460nm. The E-band is the least common band among the five bands. E stands for “extended”. From the above diagram of the relationship between transmission loss and optical band, you can see that the E-band has an obvious irregular transmission loss bump. This transmission loss bump is because light with a wavelength of 1370nm~1410nm is absorbed by hydroxide ions (OH-), which causes a sharp increase in transmission loss. This bump is also called a water peak.
Due to the limitations of early optical fiber technology, water (OH-based) impurities often remain in the optical fiber glass fiber, causing the E-band light to have the highest attenuation when transmitted in the optical fiber and cannot be used for normal transmission and communication.
With the improvement of optical fiber processing technology, ITU-T G.652.D optical fiber appeared, which made the transmission attenuation of E-band light lower than that of O-band, solving the water peak problem of E-band light.
S band
The S-band wavelength range is: 1460nm~1530nm. S refers to “short-wavelength (short wavelength)”. The transmission loss of S-band light is lower than that of O-band, and it is often used as the downlink wavelength of PON (passive optical network) systems.
C-band
The C-band wavelength range is: 1530nm~1565nm. C stands for “conventional”. C-band light has the lowest transmission loss and is widely used in metropolitan area networks, long-distance, ultra-long-distance and submarine optical cable systems. C-band is also often used in wavelength division networks.
L band
L-band wavelength range is: 1565nm~1625nm. L refers to “long-wavelength”. L-band light has the second lowest transmission loss. When C-band light is not enough to meet bandwidth requirements, L-band light will be used as a supplement for optical networks.
U-band
In addition to the above five bands, there is actually another band that will be used, and that is the U-band. The wavelength range of U-band is 1625nm~1675nm. U refers to “ultra-long-wavelength (ultra-long wavelength)”. U-band is mainly used for network monitoring.
Let’s summarize these traditional bands below.
| Wavelength | Name | Wavelength range |
| 850nm | 850nm-band | 850nm(770nm~910nm) |
| O | Original band | 1260nm~1360nm |
| E | Extended band | 1360nm ~ 1460nm |
| S | Short-wavelength band | 1460nm~1530nm |
| C | Conventional band | 1530nm~1565nm |
| L | Long-wavelength band | 1565nm~1625nm |
| U | Ultra-long-wavelength band | 1625nm~1675nm |
CE/Cpp/C+L band
Commonly used wavelength bands for optical communications are: the traditional C-band wavelength range of 1529.16nm ~ 1560.61nm. The new band CE/Cpp/C+L mentioned here refers to the new band resources introduced by current optical communications to expand traditional C-band transmission resources.
From the previous traditional band analysis, we can see that if we want to expand the C-band used in optical communications, we can seek support from the adjacent short-wavelength bands (S-band) and long-wavelength bands (L-band). This is like if you want to expand an existing road, you can only see if the wasteland on both sides of the road is available. If there is wasteland, you can widen the road.
Next, let’s take a look at the rookie bands CE/Cpp/C+L. What resources have been borrowed from the S and L bands?
CE band
CE (C Extended) band is also called C+ band. So what wavelength range is “+” CE band compared to C band? We can divide the C-band resources into 80 channels to transmit information, each of which occupies 0.4 nm of band range resources. Therefore, C-band is also called C80 band. The CE band borrows part of the wavelength resources of the L band (i.e. long wavelength band), and the wavelength range is extended to 1529.16nm ~ 1567.14nm. The CE band resources can be divided into 96 channels to transmit information, which is the C96 band. Compared with the C-band, the transmission capacity of the CE band has increased by 20%.
Cpp band
Cpp (C plus plus) band is also called C++ band. The Cpp band not only borrows wavelength resources from the L-band like the CE band, but also borrows resources from the S-band, with the wavelength range extended to 1524.30nm ~ 1572.27nm. According to the band range resource division of each channel occupying 0.4 nm, the band resources can be divided into 120 channels to transmit information. Therefore, the Cpp band is also called the C120 band. The transmission capacity of the Cpp band has increased by 50% compared to the C band.
C+L band
C+L band can be understood literally. Both C-band and L-band resources are used for optical communications. Similarly, according to the band range resource division of each channel occupying 0.4 nm, there are three common transmission schemes in the C+L band as follows.
- C120+L80: Cpp band (120 channels) + L band (80 channels), realizing a 200-wave system. The L band is actually the L+ band, with a wavelength range of 1575.16nm ~ 1617.66nm. The transmission capacity of the C120+L80 transmission solution is 1.5 times higher than that of the C-band.
- C96+L96: CE band (96 channels) + L band (96 channels), realizing a 192-wave system. The L band is actually the L++ band, with a wavelength range of 1575.16nm ~ 1626.43nm. The transmission capacity of the C96+L96 transmission solution is more than doubled compared to the C-band.
- C120+L96: Cpp band (120 channels) + L band (96 channels), realizing a 216-wave system. The L band is actually the L++ band, with a wavelength range of 1575.16nm ~ 1626.43nm. The transmission capacity of the C120+L96 transmission solution is approximately twice that of the C-band.
Finally, a picture shows these three rookie bands.
Conclusion
In short, scientists have expanded the available wavelength resources of optical fiber to a very large range. However, whether these band resources can be truly applied to communication systems such as 5G is also affected by the following factors.
Due to the limitations of optical devices, for example, the following optical devices cannot directly support the newly expanded band range and need to be upgraded.
- Erbium-doped fiber amplifier (EDFA)
- Modulators and other active devices
- Wavelength Selective Switch (WSS) Passive Components
For L-band, the degradation of transmission performance will increase the complexity of operation and maintenance, thereby increasing cost investment.
The good news is that operators have made full use of existing optical fiber resources, expanded the available optical fiber band resources, and improved transmission capacity as the goal of future optical communication network development. Currently, some operators have begun to deploy Cpp band optical networks.
With the rapid advancement of technology, we will definitely see optical communication networks using C+L band solutions in the future.
