1. Method for converting photoelectric signals
People have long been using copper wire for transmission of sound and information. However, fiber optic cables have begun to be increasingly used recently. Due to cost issues, it is impossible to replace all copper wires with optical fibers at once. However, if you want to convert some cables into optical signals, the most important thing is to convert the electrical signal into optical signals, and then convert the optical signal into electrical signals.
Using two diodes to convert electrical and optical signals, a semiconductor element such as "diode" must be used. However, diodes that convert electrical signals into optical signals and diodes that convert optical signals into electrical signals must be of different types. In other words, two types of diodes need to be used to connect copper wire and optical fiber.
In order to convert the electrical signal transmitted by the copper wire into an optical signal, a "laser diode" is used. Laser diodes are made of semiconductors called "gallium arsenide (GaAs)" and when pressurized, light at a specific wavelength will be generated.
When using copper wire to transmit digital signals such as "1" and "0", the high voltage state and the low voltage state correspond to "1" and "0" respectively. When the transmitted signal voltage is applied to the laser diode and converted into an optical signal, the light will be generated or disappear. If the luminous state and the disappearance state are set to "1" and "0" respectively, digital data can be transmitted through the optical fiber.
If the optical signal transmitted from the optical fiber is reduced to an electrical signal, a "photodiode" must be used. One property of this kind of diode is that it will generate a current proportional to the light intensity after being illuminated. When the optical signal transmitted by the optical fiber passes through the photodiode, it can be directly converted into an electrical signal.
2. Combine optical fiber and coaxial line
With the technology of converting electrical and optical signals to each other, by effectively using optical fibers and copper wires, a high-speed network can be established at a low cost.
CATV is the representative of it. To send TV programs, a coaxial line made of copper wire connects thousands of households with CATV TV stations. Recently, Internet connection and application to city phones, etc., have been started, with higher speeds, more and more channels, and two-way communication has begun.
It is difficult for coaxial lines to transmit high-frequency electrical signals from a long distance. Therefore, if you want to increase the communication distance, you must use a large number of amplifiers, and the cost will also increase accordingly.
If optical fibers are used to replace the coaxial line, the communication speed of the CATV network can be improved without excessive use of amplifiers. But if fiber is to be introduced into every user's home, the cost will increase.
Therefore, a relay device is set up between the CATV TV station and the user's home, and the cable between the CATV TV station and the relay device is replaced with optical fiber. The diode-electric converter (O/E) and the electric-optical converter (E/O) are used in the relay device. The original coaxial cable is still structurally used in the shorter area between the relay device and the user's home.
In this way, the main line part can support broadband transmission and multi-channel, while the relay device to the user's home does not require any changes, so there will be no cost.