Optical Fiber Transmission Has Many Outstanding Advantages
Bandwidth
The width of the frequency band represents the size of the transmission capacity. The higher the frequency of the carrier, the larger the frequency bandwidth over which the signal can be transmitted. In the VHF frequency band, the carrier frequency is 48.5MHz～300Mhz. The bandwidth is about 250MHz, which can only transmit 27 sets of TV sets and dozens of sets 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 frequency bandwidth is affected due to the different losses of optical fibers to different frequencies of light, the frequency bandwidth in the lowest loss region can also reach 30,000 GHz. The bandwidth of a single light source only accounts for a small part of it (the frequency band of multi-mode fiber is about several hundred megahertz, and a good single-mode fiber can reach more than 10 GHz). Using advanced coherent optical communication, 2,000 optical carriers can be arranged in the range of 30,000 GHz. , for wavelength division multiplexing, which 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 loss per kilometer is less than 0.35dB when transmitting light of 1.31um. That's 100 million times less power loss than coaxial cable, allowing it to travel much farther.
In addition, there are two characteristics of optical fiber transmission loss,
One is to have the same loss in all cable TV channels, and it is not necessary to introduce an equalizer for equalization like a cable trunk;
The second is that its loss hardly changes with temperature, so there is no need to worry about fluctuations in mains level caused by changes in ambient temperature.
light weight
Because the optical fiber is very thin, the diameter of the single-mode optical fiber core wire is generally 4um to 10um, and the outer diameter is only 125um. With the addition of waterproof layer, reinforcing ribs, sheath, etc., the diameter of the optical cable composed of 4 to 48 optical fibers is less than 13mm. It is much smaller than the standard coaxial cable's diameter of 47mm. In addition, the optical fiber is glass fiber and has a small specific gravity. It has 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 signal transmitted in it is not affected by electromagnetic fields. Therefore, optical fiber transmission has strong resistance to electromagnetic interference and industrial interference. Because of this, the signal transmitted in the optical fiber is not easy to be eavesdropped, so it is conducive to confidentiality.
high fidelity
Because optical fiber transmission generally does not require repeater amplification, no new nonlinear distortion will be introduced due to amplification. As long as the linearity of the laser is good, TV signals can be transmitted with high fidelity. The actual test shows that the carrier combination three beat ratio C/CTB of a good AM fiber system is above 70dB, and the intermodulation index cM is also above 60dB, which is much higher than the nonlinear distortion index of the general cable trunk system.
Reliable performance
We know that the reliability of a system is related to the number of devices that make up the system. The more devices there are, the greater the chance of failure. Because the fiber optic system contains a small number of devices (does not require dozens of amplifiers like the cable system), the reliability is naturally high, and the life of the fiber optic equipment is very long, with a trouble-free working time of 500,000 to 750,000 hours. Among them, the laser in the optical transmitter has the shortest lifespan, and the minimum lifespan is also more than 100,000 hours. Therefore, the working performance of a well-designed, correctly installed and debugged optical fiber system is very reliable.
Costs keep falling
Someone proposed a new Moore's Law, also known as the Optical Law. The law states that the bandwidth of optical fiber transmission information doubles every 6 months, while the price decreases by 1 times. The development of optical communication technology has laid a very good foundation for the development of Internet broadband technology. This clears the last obstacle for large-scale cable television systems to adopt optical fiber transmission. Due to the abundant sources of materials (quartz) for making optical fibers, with the advancement of technology, the cost will be further reduced; while the copper raw materials required for cables are limited, the price will be higher and higher. Obviously, optical fiber transmission will occupy an absolute advantage in the future, and become the most important transmission method for establishing the cable television network in the whole province and even the whole country.
