In the fiber optic communication moment affects people’s life nowadays, hanging over our heads on the overhead fiber optic cable can be realized in a moment tens of kilometers of network signal transmission, buried in the underground fiber optic cable backbone network laying, realize the optical fiber to the home, FTTP, FTTB, FTTO and FTTD to make a great contribution. Across tens of thousands of kilometers laid undersea fiber optic cable to the world’s various continents of exchange and communication closely linked together. A hair size of the fiber is how to achieve communication and information transmission, we will unveil the principle of fiber optic communication.
What Is Fiber Optics
Optical fiber, or fiber optics for short, is a type of fiber made of glass or plastic that uses the principle of total internal reflection of light in these fibers to transmit light as a conduction medium. In order to protect the hair-thick fiber from breaking due to excessive bending, a coating is added to the outer layer of the fiber and it is placed in a plastic protective sleeve.
The most widely used field of optical fiber is communication. Currently, the optical fiber used for communication is basically a quartz fiber, whose main component is high-purity quartz glass, i.e., silicon dioxide (SiO2). Because the information in the optical fiber transmission loss is much lower than electricity in the wire conduction loss, but also because the main raw material for the production of silicon, great reserves, easier to mine, so the price is very cheap, prompting the optical fiber is used as a long-distance information transfer medium.
Typically, a transmitting device at one end of an optical fiber uses a light-emitting diode or a laser beam to send pulses of light into the fiber, and a receiving device at the other end of the fiber detects the pulses using a photosensitive component. Optical fibers are used to transmit light waves carrying information for the purpose of communication.
Fiber Optics Work On The Principle Of Total Reflection Of Light
Dispersion Of Optical Fiber
- The cause of dispersion
In the optical fiber, the optical signal is composed of many different components, due to the signal of each frequency component or each mode component of the propagation speed of the signal is different, after a distance through the optical fiber transmission, the different components of the time delay difference between the different components, resulting in the transmission of the signal waveform distortion, the pulse broadening, a phenomenon known as the optical fiber dispersion. - The impact of dispersion
The presence of optical fiber dispersion makes the transmitted signal pulse distortion and broadening, thus generating inter-code interference. In order to ensure the quality of communication, it is necessary to increase the inter-code spacing, that is, to reduce the transmission rate of the signal, which limits the communication capacity and transmission distance of the fiber optic system. - Classification of dispersion
According to the cause of dispersion, fiber dispersion can be divided into mode dispersion, material dispersion, waveguide dispersion and polarization dispersion.
Optical Fiber Loss
The loss of optical fiber refers to the reduction of optical power caused by absorption, scattering and other reasons after the optical signal is transmitted through the fiber.
- Absorption loss
Intrinsic absorption: the absorption effect inherent in the fiber material itself.Impurity absorption: impurities in the optical fiber on the absorption of light. - Scattering loss
Linear scattering; nonlinear scattering; structural imperfections scattering. - Other losses
Microbending attenuation, etc.
Because the fiber is soft, can be bent, but after bending to a certain extent, although the fiber can guide light, but will make the light transmission path change. At this time, the transmission mode is converted to radiation mode, so that part of the light energy penetrates into the cladding or through the cladding to become a radiation mode outward leakage loss, thus generating losses. When the bending radius is greater than 5 to 10cm, the loss caused by bending can be ignored.
Conventional Bands For Fiber Optic Communications
Optical fiber is used as a transmission medium for communication. However, not all light is suitable for fiber optic communication. Light with different wavelengths (which can be simply understood as light with different colors) has different transmission losses in an optical fiber. Light with high transmission loss has no way to carry information in the fiber.
After a long period of research by scientists, it was first discovered that light with a wavelength of 850nm can be used as light for optical communication, and this band is also directly known as the 850nm band. However, the transmission loss in the wavelength region of the 850nm band is relatively large, and there is no suitable fiber optic amplifier. Therefore 850nm band is only suitable for short-range transmission.
Later, scientists have explored the “low-loss wavelength region” light band, that is, 1260nm ~ 1625nm region of light, the most suitable for transmission in the optical fiber. The relationship between transmission loss and optical wavelength is shown in the figure below.
The 1260nm~1625nm region is subdivided into five bands: O-band, E-band, S-band, C-band and L-band.
O band
The wavelength range of O band is 1260nm~1360nm, the signal distortion caused by optical dispersion in this wavelength band is the smallest and the loss is the lowest, so it is the early optical communication wavelength band. Therefore, it is named O-band, where O means “Orignal”.
E band
The wavelength range of E band is 1360nm~1460nm. E-band is the least common of the five bands. It refers to “extended”. From the above transmission loss and optical band relationship chart, you can see that the E band has an obvious irregular transmission loss bump. This transmission loss bump is due to the 1370nm ~ 1410nm wavelength light is absorbed by hydroxide ions (OH-), which leads to a sharp increase in transmission loss, this bump is also known as the water peak.
Due to early fiber optic process limitations, in the fiber optic glass fiber, often residual water (OH-based) impurities, resulting in the highest attenuation of E band light transmission in the fiber, can not be normally used for transmission communication use. With the improvement of optical fiber processing technology, ITU-T G.652.D optical fiber appeared, making the transmission attenuation of E band light become lower than Q band. The problem of water peaks in E-band light was solved.
S band
S band wavelength range: 1460nm ~ 1530nm. S refers to “short-wavelength (short wavelength)”. S band optical transmission loss is lower than O-band, often used in PON (Passive Optical Network) system downlink wavelength.
C band
C band wavelength range: 1530nm ~ 1565nm, C refers to “conventional (conventional)”, C band optical transmission loss is the lowest widely used in metropolitan area networks, long-distance, ultra-long-distance and submarine cable systems. C band is also frequently used in WDM networks.
L Band
L band wavelength range: 1565nm~1625nm, L means “long-wavelength”, L band header transmission loss is the second lowest. When C band light is insufficient to meet bandwidth requirements, L band light is used as a supplement to optical networks.
U band
In addition to the above five wavelengths, there is actually another wavelength that will be used, that is, U band. U band wavelength range is 1625nm ~ 1675nm, U refers to “ultra-long-wavelength (ultra-long-wavelength)”. U band is mainly used for network monitoring.
Here is a short summary of these traditional bands.
Rookie Bands For Fiber Optic Communications
From the previous analysis of conventional bands, it can be seen that in order to widen the C band used for optical communications, support can be sought from the adjacent short-wavelength bands (S band) and long-wavelength bands (L band). This is like trying to widen an existing road by looking at the availability of wasteland on both sides of the road, and widening the road if there is wasteland.
Next, let’s look at the rookie band CE/Cpp/C+L, what resources are borrowed from the S and L bands?
CE Band
The CE (C Extended) band is also known as the C+ band. What wavelength range is “+” in CE band compared to C band?
We can divide the C-band resources into 80 pathways to transmit information, where each pathway occupies 0.4nm of band range resources. Therefore, the C-band is also known as C80 band. CE band borrows part of the wavelength resources of the L-band (i.e., long-wavelength band), and the wavelength range is extended to 1529.16 nm ~ 1567.14 nm, and the CE band resources can be divided into 96 channels to transmit the information, that is, the C96 band.
The transmission capacity of CE band is increased by 20% compared with C band.
Cpp Band
Cpp (C plus plus) band is also known as C++ band. cpp band not only borrows wavelength resources from L band like CE band but also borrows resources from S band at the same time, and the wavelength range is extended to 1524.30 nm~1572.27 nm. according to the division of resources of each pathway occupying 0.4 nm of the wavelength range, the resources of the wavelength band can be divided into 120 pathways to transmit the information. information.
Therefore, Cpp band is also called C good wave: the transmission capacity of Cpp band is increased by 50% compared with that of C band.
C+L Band
C+L band can be literally understood that both C-band and L-band resources are used for optical communications. Also divided according to the band range resources of 0.4 nm occupied by each pathway, there are three common transmission schemes for C+L band as follows.
C120+L80: Cpp band (120 paths) + L band (80 paths) to realize 200-wave system. The L-band is actually L+ band, with a wavelength range of 1575.16nm~1617.66nm. The transmission capacity of the C120+L80 transmission scheme is increased by 1.5 times compared to the C-band.
C96+L96: CE band (96 paths) + L band (96 paths) to realize 192-wave system. The L-band is the actual L++ band, with a wavelength range of 1575.16nm~1626.43nm. The transmission capacity of the C96+L96 transmission scheme is increased by more than a factor of 1 compared to the C-band.
C120+L96: Cpp band (120 paths) + L band (96 paths) to realize 216-wave system. The actual wavelength range of L-band is 1575.16nm~1626.43nm. The transmission capacity of C120+L96 transmission scheme is increased by about 2 times compared to C-band for L++ band.
Types Of Fiber Optics
According to the length of the transmission distance, the transmission wavelength, communication optical fiber can be divided into:
Multimode optical fiber
It can transmit multiple modes of light. However, its intermode dispersion is large, which limits the frequency of transmitting digital signals and becomes more severe with distance.
Singlemode fiber
Only one mode of light can be transmitted, so its intermode dispersion is small and it is suitable for long-distance communication.
According to the International Telecommunication Union ITU-T recommendations, communication optical fibers are divided into seven categories, G.651-G.657, of which G.651 is a multimode fiber and G.652 to G.657 are single-mode fibers.
G.651 multimode optical fiber
Suitable for light wavelength of 850nm/1310nm short distance transmission (Ethernet, LAN).
G.652 dispersion non-shifted single mode fiber
Conventional single mode fiber, suitable for access network applications with wavelengths of 1310nm-1550nm. Suitable for fiber-to-the-home, long-haul network laying, metropolitan area network.
G.653 dispersion-shifted single mode optical fiber
Suitable for light wavelength of 1550nm long-distance transmission, as long as the application of the backbone network, submarine fiber optic cable, and soon out of the history stage.
G.654 cut-off wavelength-shifted single mode fiber
Suitable for optical wavelength of 1550nm long-distance transmission of submarine cables and 5G bearer network, but does not support DWDM.
G.655 non-zero dispersion shifted single mode fiber
Suitable for light wavelength of 1550nm long-distance transmission of the backbone network, submarine fiber optic cables, support for DWDM, only for long-distance line laying use.
G.656 low-slope non-zero dispersion shifted single mode fiber
A type of non-zero dispersion shifted fiber, with strict requirements on the speed of dispersion, ensuring transmission performance over a larger wavelength range in DWDM systems. The possibility of mass production is low.
G.657 bend-resistant single-mode fiber
A new product based on the demand of FTTx technology and assembly application, G.657 fiber is the latest fiber variety developed on the basis of G.652 fiber in order to realize Fiber-to-the-Home (FTTH) standard. It is more suitable for the realization of FTTH information transmission, suitable for installation in indoor or building and other narrow places.Â
Advantages Of Fiber Optic Communication
Huge communication capacity
Theoretically, an optical fiber can simultaneously transmit 10 billion voice paths, the current simultaneous transmission of 500,000 voice paths of the test has been successful, thousands or even hundreds of thousands of times higher than the traditional coaxial cable, microwave and so on. With the development of hollow-core optical fiber and the success of 800G data transmission test network, the capacity and transmission rate of optical fiber communication will continue to break through.
Long relay distance
Optical fiber has extremely low attenuation coefficient, with appropriate optical transmission, optical receiving equipment, optical amplifiers, forward error correction and RZ code modulation technology, etc., which can make its relay distance of thousands of kilometers or more, while the traditional cable can only be transmitted 1.5km, microwave 50km, which is simply incomparable with it.
Good confidentiality
Because the basic composition of optical fiber is quartz, only transmits light, does not conduct electricity, is not subject to the role of electromagnetic fields, in which the transmission of light signals are not affected by electromagnetic fields, so optical fiber transmission of electromagnetic interference, industrial interference has a strong ability to resist. Because of this, the signals transmitted in optical fiber are not easily eavesdropped, thus facilitating confidentiality.
Strong adaptability
It has the advantage of not being afraid of the interference of strong external electromagnetic field and corrosion resistance.
In Summary
Due to the small size, light weight, abundant raw material sources, low price and other characteristics, according to the different needs of the use of the environment, optical fiber is added tightly wrapped/semi-tightly wrapped jacket, Kevlar and plastic outer jacket made of indoor optical fiber cables, and then terminated fiber optic connectors, used as fiber optic patch cords to link a variety of indoor passive devices.
In addition, due to the superior long-distance transmission and low-loss characteristics of single-mode fiber, the coating layer of the fiber is colored and reinforced with aramid yarns, water-blocking yarns, metal armor, steel wires and other reinforcing bonds and PE sheaths to be made into outdoor fiber optic cables to meet the requirements of different laying methods, such as overhead, underground, submarine, and air blowing, to complete the construction of backbone networks, metropolitan area networks, LANs and other network construction.