According to different user requirements, different types of services, and the development of technology at different stages, the form of optical fiber communication systems can be diverse.
At present, a relatively large number of system forms are used for optical fiber digital communication systems of intensity modulation / direct detection (IM / DD). The principle block diagram of this system is shown in Figure 1. As can be seen from the figure, the optical fiber digital communication system is mainly composed of an optical transmitter, an optical fiber, and an optical receiver.
Figure 1 Schematic diagram of optical fiber digital communication system
In the point-to-point optical fiber communication system, the signal transmission process: the input signal sent to the optical transmitter terminal is transformed into a code structure suitable for transmission in the optical fiber after the pattern conversion, and the intensity of the light source is directly driven by the drive circuit Modulation, so that the optical power output by the light source changes with the input signal current, that is, the light source completes the electrical / optical conversion and sends the corresponding optical power signal to the optical fiber for transmission; on the lines of the communication system, currently, single-mode optical fiber This is due to its better transmission characteristics; after the signal reaches the receiving end, the input optical signal is first detected directly by a photodetector to complete the optical / electrical conversion, and then amplified, equalized, and judged. A series of processing to restore it to the original electrical signal, thereby completing the entire transmission process.
In order to ensure the communication quality, an optical repeater must be provided at an appropriate distance between the transceivers. There are two main types of optical repeaters in optical fiber communication, one is a repeater in the form of optical-electrical-optical conversion, and the other is an optical amplifier that directly amplifies the optical signal.
In optical fiber communication systems, the main factors that determine the relay distance are the loss of optical fiber and the transmission bandwidth.
Generally, the attenuation of a fiber per unit length of transmission in the fiber is used to represent the loss of the fiber, and its unit is dB / km. At present, the practical silica-based optical fiber has a loss of about 2 dB / km in the 0.8 to 0.9 μm band; a loss of 5 dB / km at 1.31 μm; and at 1.55 μm, the loss can be reduced to 0.2 dB / km, which is close to The theoretical limit of SiO2 fiber loss. Traditionally, 0.85 μm is called the short-wavelength of fiber optic communication; 1.31 μm and 1.55 μm are called the long-wavelength of optical fiber communication. They are three practical low-loss working windows in optical fiber communication.
In digital optical fiber communication, information is transmitted by the presence or absence of optical signals in each time slot. Therefore, the relay distance is also limited by the fiber transmission bandwidth. Generally, MHz.km is used as the unit of the transmission bandwidth per unit length of fiber. If the bandwidth of a certain fiber is given as 100MHz.km, it means that only 100MHz bandwidth signals are allowed to be transmitted on each kilometer of fiber. The longer the distance and the smaller the transmission bandwidth, the smaller the communication capacity.