Optical fiber communication is to use light as an information carrier to transmit in the fiber core for communication.
However, not all light is suitable for optical fiber communication. The wavelength of light is different, and the transmission loss in the optical fiber is different.
The core of optical fiber – core (silica fiber)
In order to reduce the loss as much as possible and ensure the transmission effect, researchers have been working hard to find the most suitable light.
In the early 1970s, optical fiber communication started the process of practical implementation. At that time, the main research and development object was multimode optical fiber.
Multimode fiber has a larger core diameter, allowing different modes of light to be transmitted on a single fiber.
The light that was first used was light with a wavelength of 850nm, and this band (band) is also directly called the 850nm band.
Later, in the late 1970s and early 1980s, single-mode fiber began to be used on a large scale.
After testing, the engineers found that the light in the wavelength range of 1260nm~1360nm has the least signal distortion and the lowest loss caused by dispersion.
Therefore, they adopted this wavelength range as the early optical communication band and named it O-band (O-band). O, is “Original (original)” means.
In the following 30 to 40 years, after long exploration and practice, experts gradually concluded a “low loss wavelength region”, that is, the 1260nm~1625nm region. Light in this wavelength range is most suitable for transmission in optical fibers.
This area is further divided into five bands, namely O-band, E-band, S-band, C-band and L-band.
With the continuous evolution of technology, experts have also verified the law between optical fiber transmission loss and light wavelength, as shown in the following figure:
The most commonly used band is called C band (1530nm~1565nm). C, is “conventional (conventional)” means.
C-band exhibits the lowest loss and is widely used in metro, long-haul, ultra-long-haul, and submarine cable systems. In the WDM wavelength division multiplexing system, the C-band is also often used.
The L-band (1565nm~1625nm) next to the C-band is the second lowest loss band and one of the mainstream choices in the industry. When the C-band is not enough to meet the bandwidth requirements, the L-band will also be used as a supplement. L, means “long-wavelength (long wavelength)”.
The S-band (1460nm~1530nm), that is, the “short-wavelength (short wavelength)” band, has higher fiber loss than the O-band. It is often used as a downstream wavelength in PON (Passive Optical Network) systems.
PON is the system for home fiber optic broadband
Its upstream wavelength is 1310nm, and its downstream wavelength is 1490nm
Finally, let’s look at the E-band.
This band is a bit special, it is the least common of the five bands. E, means “extended (extended)”.
When you observe the relationship between wavelength and loss just now, you will find that there is an obvious irregular shock in the E-band.
That’s because hydroxide ions (OH-) absorb in the 1370-1410nm band, so the loss increases dramatically. This is also known as the water peak.
In the early days, due to process limitations, water (OH group) impurities often remained in the optical fiber glass fiber, resulting in the highest attenuation of the E-band, which could not be used normally.
Later, the dehydration technology in the glass making process was invented, and the attenuation of the most commonly used optical fiber (ITU-T G.652.D) in the E-band became lower than that in the O-band. (This type of fiber is also known as low water peak fiber or no water peak fiber.)
However, since many existing fiber optic cables installed before the year 2000 exhibit high attenuation in the E-band, there are still some limitations in the use of the E-band in optical communications.
In addition to the above bands, there is actually another band that will be used, and that is the U-band (ultra-long-wavelength band, ultra-long-wavelength band: 1625-1675 nm). The U-band is mainly used for network monitoring.