Illuminant
The output signal of the microcomputer control system is an electrical signal, and the optical fiber system transmits an optical signal. Therefore, in order to transmit the electrical signal generated by the microcomputer system in the optical fiber, the electrical signal must first be converted into an optical signal. The light source is such an electro-optical conversion device.
The light source first converts the electrical signal into an optical signal, and then sends an optical signal to the optical fiber. In the optical fiber system, the light source has a very important position. Incandescent lamps, lasers and semiconductor light sources can be used as fiber optic light sources. Semiconductor light source is the use of semiconductor PN junction to convert electrical energy into light energy, commonly used semiconductor light sources are semiconductor light-emitting diodes (LED) and laser diodes (LD).
Semiconductor light source has been widely used in optical fiber transmission system because of its advantages of small size, light weight, simple structure, easy to use and easy to be compatible with optical fiber.
Photoreceiver
Before the optical signal transmitted in the optical fiber is received by the microcomputer system, it must first be restored to the corresponding electrical signal. This conversion is achieved by means of a light receiver. The role of the optical receiver is to convert the optical signal transmitted by the optical fiber into an electrical signal, and then the electrical signal is processed by the control system. The optical receiver is based on the principle of photoelectric effect, the light irradiates the PN junction of the semiconductor, and the PN junction of the semiconductor will produce a carrier after absorbing light energy, so the photoelectric effect of the PN junction is generated, thus converting the optical signal into an electrical signal. Semiconductor receivers used in optical fiber systems mainly include semiconductor photodiodes, phototriode, photomultiplier tubes and photocells. Photoelectric triode can not only turn the incident light signal into an electrical signal, but also amplify the electrical signal, which can be well matched with the interface circuit of the control system, so the photoelectric triode is the most widely used.
Optical fiber
Optical fiber is the transmission channel of optical signal and the key material of optical fiber communication.
The optical fiber consists of a core, cladding, coating layer and jacket, which is a symmetrical cylinder with multi-layer dielectric structure. The main body of the fiber core is silica, which is mixed with trace amounts of other materials to improve the optical refractive index of the material. There is a cladding layer outside the fiber core, and the cladding and the fiber core have different optical refractive index, and the optical refractive index of the fiber core is higher to ensure that the optical signal is mainly transmitted in the fiber core. Outside the cladding is a layer of paint, mainly used to increase the mechanical strength of the fiber, so that the fiber is not subject to external damage. The outermost layer of the optical fiber is the jacket, which also plays a protective role.
The two main characteristics of optical fibers are attenuation and dispersion. Loss is the attenuation or dispersion of the optical signal in unit length, expressed in db/km, the parameter is related to the transmission distance of the optical signal, the greater the loss, the shorter the transmission distance. The multi-microcomputer elevator control system generally has a short transmission distance, so in order to reduce costs, most of them choose plastic optical fiber. The dispersion of fiber is mainly related to pulse broadening. In Mitsubishi elevator control system, optical fiber communication is mainly used for data transmission between group control and single ladder and data transmission between two parallel single ladder. The optical fiber device used by Mitsubishi elevator is mainly composed of a light source, an optical receiver and an optical fiber, wherein the light source and the optical receiver are packaged in a fixed plug of the optical fiber connector, and the optical fiber is connected to the dynamic plug.
Optical wavelength division multiplexing technology
Wavelength Division Multiplexing (WDM) technology refers to the use of multiple lasers to transmit multiple different wavelengths of light simultaneously on the same optical fiber. It can greatly improve the transmission capacity of optical fiber transmission system. The 1.6 Tbit/s WDM system has been commercialized on a large scale. In order to further improve the capacity of optical fiber transmission, DWDM (Dense Wavelength Division Multiplexing) has become a major international research object since 1995, and Lucent Bell Laboratories believes that the capacity of commercial DWDM systems can reach up to 100 Tbit/s. The 10 Gbit/s based DWDM has gradually become the core network mainstream in many operators in our country. In addition to the increasing number of wavelengths and transmission capacity of the DWDM system, the optical transmission distance has also increased from 600 km to more than 2000 km. In addition, Coarse Wavelength Division Multiplexing (CWDM) has also emerged in the expansion of metro optical transport network, with advantages of large capacity, short distance transmission and low cost. The researchers also found that WDM multiplexing of multiple optical time division OTDM signals can greatly improve transmission capacity. As long as it is properly combined, it can achieve transmission above Tbit/s, therefore, it has also become the development direction of future optical fiber communication. Most transmission experiments in the lab over 3 Tbit/s are done this way.
Optical soliton communication technology
Light is a special kind of ultra-short light pulse on the order of ps, and its waveform and speed remain unchanged after long distance transmission through optical fibers. Optical soliton communication is the use of optical soliton as a carrier to achieve long distance distortion-free communication, and the information transmission can reach thousands of miles under the condition of zero error code. Many tests have shown that it can be used for submarine optical cable communication, etc., and is suitable for combining with WDM system to form ultra-high-speed large-capacity optical communication, when the single channel rate reaches 40 Gbit/s or more, the advantages of optical soliton communication can be fully reflected.
Optical fiber access technology
Fiber access using PON technology can be combined with a variety of technologies, such as ATM\SDH and Ethernet, respectively, to generate APON, GPON, EPON. In contrast, EPON inherits the advantages of Ethernet and the cost is relatively low, after combining with optical fiber technology, EPON is not only limited to local area networks, but also extended to metropolitan area networks, and even wide area networks. Optical fiber to the home also uses EPON technology; GPON has the most advantages in circuit switching service support and can make full use of the existing SDH technology, but the technology is complicated and the cost is high. APON will be used to implement FTTH schemes.
Application field
The application field of optical fiber communication is very wide, mainly used in the local trunk line, the advantages of optical fiber communication can be fully played here, gradually replace the cable, and be widely used. It is also used for long-distance trunk line communication, which mainly relies on cable, microwave and satellite communication in the past, and now gradually uses optical fiber communication and forms the world's dominant bit transmission method. For global communication networks, public telecommunications networks in various countries (such as China's national primary trunk, provincial secondary trunk and sub-county branch lines); It is also used for high-quality color television transmission, industrial production site monitoring and scheduling, traffic surveillance control and command, town cable television networks, shared antenna (CATV) systems, for fiber optic local area networks and other uses such as in aircraft, spacecraft, ships, underground mines, power sector, military and corrosion and radiation.
Optical fiber transmission system is mainly composed of optical transmitter, optical receiver, optical cable transmission line, optical repeater and various passive optical devices. In order to achieve communication, the baseband signal must also be processed by the electrical terminal and sent to the optical fiber transmission system to complete the communication process.
It is suitable for optical fiber analog communication system, but also suitable for optical fiber digital communication system and data communication system. In the optical fiber analog communication system, the electrical signal processing refers to the baseband signal amplification, pre-modulation and other processing, and the electrical signal inverse processing is the inverse process of the originating processing, that is, demodulation, amplification and other processing. In optical fiber digital communication systems, electrical signal processing refers to the amplification, sampling, quantization of baseband signals, that is, pulse code modulation (PCM) and line code type coding processing, and electrical signal inverse processing is also the inverse process of the origin. For data optical fiber communication, the electrical signal processing mainly includes the amplification of the signal, and the digital communication system is different from it does not require code transformation.
