Transmission advantages
Until 1960, the American scientist Maiman invented the world's first laser, which provided a good light source for optical communications. After more than two decades, people have made research on optical transmission media and finally made low-loss optical fibers, thus laying the cornerstone of optical communications. Since then, optical communications has entered a stage of rapid development.
Optical fiber transmission has many outstanding advantages:
Frequency bandwidth
The width of the frequency band represents the size of the transmission capacity. The higher the frequency of the carrier, the greater the bandwidth of the signal that can be transmitted. In the VHF frequency band, the carrier frequency is 48.5MHz～300Mhz. With a bandwidth of about 250MHz, it can only transmit 27 TV sets and dozens of FM broadcasts. The frequency of visible light reaches 100,000 GHz, which is more than one million times higher than the VHF frequency band. Although the optical fiber has different losses for different frequencies of light, the bandwidth is affected, but the bandwidth in the lowest loss region can also reach 30,000 GHz. At present, the bandwidth of a single light source only occupies a small part of it (the frequency band of multi-mode fiber is about several hundred MHz, and a good single-mode fiber can reach more than 10 GHz). The use of advanced coherent optical communication can arrange 2,000 lights in the range of 30,000 GHz. Carrier, wavelength division multiplexing, can accommodate millions of channels.
Low loss
In a system composed of coaxial cables, the best cable has a loss of more than 40dB per kilometer when transmitting 800MHz signals. In contrast, the loss of optical fiber is much smaller, the transmission of 1.31um of light, the loss per kilometer is below 0.35dB, if the transmission of 1.55um of light, the loss per kilometer is smaller, up to 0.2dB or less. This is 100 million times smaller than the power loss of a coaxial cable, making it possible to transmit at a much longer distance. In addition, optical fiber transmission loss has two characteristics. One is that it has the same loss in all cable TV channels, and there is no need to introduce an equalizer for equalization like a cable trunk; the other is that its loss hardly changes with temperature, so you don’t need to worry about it. Changes in ambient temperature cause mains level fluctuations.
Light weight
Because the optical fiber is very thin, the diameter of the single-mode fiber core wire is generally 4um ~ 10um, and the outer diameter is only 125um. With waterproof layer, reinforcing ribs, sheath, etc., the diameter of an optical cable composed of 4 to 48 optical fibers is less than 13mm. It is much smaller than the standard coaxial cable with a diameter of 47mm. In addition, the optical fiber is a glass fiber with a small specific gravity, which makes it have the characteristics of small diameter and light weight, and it is very convenient to install.
Strong anti-interference ability
Because the basic component of optical fiber is quartz, it only transmits light, does not conduct electricity, and is not affected by electromagnetic fields. The optical signals transmitted in it are not affected by electromagnetic fields. Therefore, optical fiber transmission has strong resistance to electromagnetic interference and industrial interference. Precisely because of this, the signal transmitted in the optical fiber is not easy to be eavesdropped, which is conducive to confidentiality.
High fidelity
Because optical fiber transmission generally does not require relay amplification, it will not introduce new nonlinear distortions due to amplification. As long as the linearity of the laser is good, the TV signal can be transmitted with high fidelity. The actual test shows that the carrier combination triple beat ratio C/CTB of a good AM fiber system is more than 70dB, and the intermodulation index cM is also more than 60dB, which is much higher than the nonlinear distortion index of the general cable trunk system.
Reliable working performance
We know that the reliability of a system is related to the number of devices that make up the system. The more equipment, the greater the chance of failure. Because the number of equipment contained in the optical fiber system is small (unlike a cable system that requires dozens of amplifiers), the reliability is naturally high. In addition, the life of the optical fiber equipment is very long, and the trouble-free working time is 500,000 to 750,000 hours. Among them, the shortest life span is the laser in the optical transmitter, and the lowest life span is more than 100,000 hours. Therefore, the working performance of a well-designed, correctly installed and debugged optical fiber system is very reliable.
Cost continues to fall
At present, some people have proposed a new Moore's law, also called the optical law (Optical Law). The law states that the bandwidth of optical fiber transmission of information doubles every 6 months, while the price is doubled. The development of optical communication technology has laid a very good foundation for the development of Internet broadband technology. This cleared the last obstacle for large-scale cable television systems to adopt optical fiber transmission methods. Since the source of the material (quartz) for the optical fiber is very abundant, with the advancement of technology, the cost will be further reduced; while the copper material required for the cable is limited, the price will be higher and higher. Obviously, optical fiber transmission will have an absolute advantage in the future, and it will become the most important transmission method for the establishment of cable TV networks in the whole province and even the whole country.
Structure principle
The optical fiber is composed of two layers of glass with different refractive indexes. The inner layer is an optical inner core with a diameter of several micrometers to several tens of micrometers, and the diameter of the outer layer is 0.1 to 0.2 mm. Generally, the refractive index of the inner core glass is 1% larger than that of the outer glass. According to the principle of light refraction and total reflection, when the angle at which the light hits the interface between the inner core and the outer layer is greater than the critical angle for total reflection, the light cannot pass through the interface and is completely reflected.
Fiber attenuation
The main factors that cause fiber attenuation are: intrinsic, bend, squeeze, impurities, unevenness and butt joints, etc.
Intrinsic
It is the inherent loss of optical fiber, including: Rayleigh scattering, inherent absorption, etc.
bending
When the optical fiber is bent, part of the light in the optical fiber will be lost due to scattering, resulting in loss.
extrusion
Loss caused by slight bending when the optical fiber is squeezed.
Impurity
Impurities in the optical fiber absorb and scatter the light propagating in the optical fiber, causing loss.
Uneven
Loss caused by the non-uniform refractive index of the optical fiber material.
Docking
The loss caused by fiber butt, such as: different axis (single-mode fiber coaxiality is required to be less than 0.8μm), the end face is not perpendicular to the axis, the end face is not flat, the butt core diameter is not matched, and the splicing quality is poor.
Artificial attenuation
In actual work, it is sometimes necessary to perform artificial optical fiber attenuation, such as optical fiber attenuators used in optical communication systems to debug optical power performance, debug optical fiber instrument calibration, and optical fiber signal attenuation.
production method
At present, the optical fiber used in communication is generally a silica optical fiber. The chemical name of quartz is silicon dioxide (SiO2), which has the same main composition as the sand we use to build houses. However, optical fibers made of ordinary quartz materials cannot be used for communication. Communication optical fiber must be composed of extremely high-purity materials; however, adding a small amount of dopant into the main material can make the refractive index of the core and cladding slightly different, which is beneficial to communication.
There are many methods for manufacturing optical fiber preform by VAD method. At present, there are mainly: in-tube CVD (chemical vapor deposition) method, in-rod CVD method, PCVD (plasma chemical vapor deposition) method and VAD (axial vapor deposition) method . But no matter which method is used, the preform must be made at high temperature first, and then heated and softened in a high-temperature furnace, drawn into a filament, and then coated and molded to become an optical fiber core wire. The manufacturing of optical fibers requires that every process must be commensurately precise and controlled by a computer. In the process of manufacturing optical fiber, we should pay attention to:
Optical fiber preform made by VAD method
①The purity of optical fiber raw materials must be very high.
②It is necessary to prevent impurity contamination and air bubbles from entering the optical fiber.
③To accurately control the distribution of refractive index;
④ Correctly control the structural size of the optical fiber;
⑤ Minimize the scar damage on the surface of the optical fiber and improve the mechanical strength of the optical fiber.
Tube stick method
Insert the inner core glass rod into the outer glass tube (as close as possible), melt and draw the wire;
Double crucible method
In two concentric platinum crucibles, put the inner core and outer glass frit into the inner and outer crucibles respectively;
Molecular filling method
The microporous silica glass rod is immersed in the additive solution with high refractive index to obtain the cross-sectional structure of the required refractive index distribution, and then the drawing operation is performed. The process is more complicated. In optical fiber communication, internal and external vapor deposition methods can also be used to ensure that optical fibers with low optical loss rate can be manufactured.
Space fusion
Put the fiber drawing device in the microgravity environment of space to pull it, and you can get the ultra-long high-quality light guide fiber that is not available on the earth.
Fiber classification
According to the classification method of different optical fiber classification standards, the same optical fiber will have different names.
Classified by fiber material
According to the material of the optical fiber, the types of optical fiber can be divided into quartz optical fiber and all-plastic optical fiber.
Silica fiber generally refers to an optical fiber composed of a doped silica core and a doped silica cladding. This fiber has very low loss and moderate dispersion. At present, the vast majority of optical fibers for communication are quartz optical fibers.
All-plastic optical fiber is a new type of optical fiber for communication, which is still in the development and trial stage. All-plastic fiber has the characteristics of large loss, thick core (100-600μm in diameter), large numerical aperture (NA) (usually 0.3-0.5, which can be coupled with light sources with larger light spots) and low manufacturing cost. At present, all-plastic optical fiber is suitable for shorter length applications, such as indoor computer networking and communication in ships.
Classification by fiber profile refractive index distribution
According to the different refractive index distribution of the fiber profile, the types of fibers can be divided into step-type fibers and graded-type fibers.
Classified by transmission mode
According to the number of optical fiber transmission modes, the types of optical fibers can be divided into multi-mode optical fibers and single-mode optical fibers.
Single-mode fiber is a fiber that can only transmit one mode. Single-mode fiber can only transmit the fundamental mode (lowest-order mode), there is no inter-mode delay difference, and has a much larger bandwidth than multi-mode fiber, which is very important for high-speed transmission. The mode field diameter of a single-mode fiber is only a few microns (μm), and its bandwidth is generally one or two orders of magnitude higher than that of a graded multimode fiber. Therefore, it is suitable for large-capacity, long-distance communications.
Classification according to international standards (classification according to ITU-T recommendations)
In order to make the optical fiber have a unified international standard, the International Telecommunication Union (ITU-T) has formulated a unified optical fiber standard (G standard). According to the ITU-T recommendations on optical fibers, the types of optical fibers can be divided into:
G.651 fiber (50/125μm multimode graded index fiber)
G.652 fiber (non-dispersion shifted fiber)
G.653 fiber (dispersion shifted fiber DSF)
G.654 fiber (cut-off wavelength shift fiber)
G.655 fiber (non-zero dispersion shifted fiber).
In order to meet the needs of the development of new technologies, the current G.652 fiber has been further divided into three sub-categories G.652A, G.652B, and G.652C, and G.655 fiber is further divided into G.655A and G.655B. Subcategories.
According to the IEC standard classification, the IEC standard divides the types of optical fibers into
Type A multimode fiber:
A1a Multimode fiber (50/125μm type multimode fiber)
A1b multimode fiber (62.5/125μm type multimode fiber)
A1d multimode fiber (100/140μm type multimode fiber)
Class B single-mode fiber:
B1.1 corresponds to G652 fiber, and B1.3 fiber is added to correspond to G652C fiber
B1.2 corresponds to G654 fiber
B2 fiber corresponds to G.653 fiber
B4 fiber corresponds to G.655 fiber
