Learn about optical repeater transmission system in minutes

“The emergence of optical fiber amplifiers is an important milestone in the history of optical fiber communications.”

Optical signal; Photodetector; Low noise preamplifier; High gain main amplifier; Equalization circuit; Decision regeneration circuit; Optical modulator; light source; AGC circuit; Clock extraction circuit; Automatic power control;

  The transmission distance of any optical fiber communication system is limited by the factors such as optical fiber loss and dispersion. The optical fiber communication network system, like the telecommunication network, must add a regenerative relay station at a certain distance to complete the transmission of signals during long-distance long-distance transmission. The amplification and regeneration to ensure signal transmission quality. In optical fiber transmission links, in addition to using various active devices with different functions, the quality of optical passive devices has an impact on the performance of optical fiber networks.

Optical-electrical-optical repeater structure diagram

Learn about types of Optical-electrical-optical repeater

  Absorption and dispersion of the optical fiber result in attenuation of the optical signal and waveform distortion, and therefore, the quality of information transmission is reduced, the bit error rate is increased, and the communication distance is limited. In order to meet long-distance communication requirements, there are usually two methods for the relay transmission of optical signals: First, an optical-electric hybrid repeater used in the early days, and an optical-electrical-optical conversion method is adopted, and its structure is shown in the following figure. Attenuation and distortion of the optical signal will be received by the optical receiver, and converted to electrical signals for processing, and then modulate the optical transmitter light source, convert the optical signal to continue transmission; Another method is to use optical amplifier to light The signal is amplified directly and transmitted.

Optical-electrical-optical repeater structure

  Early (and still widely used) optical fiber relays use optical-electrical-optical conversion. The structure of a typical digital optical repeater is shown in the figure below, mainly consisting of a photodetector, an electrical signal amplifier (a low noise preamplifier and a high gain main amplifier), an equalizer circuit, an automatic gain controller (AGC) circuit, and a decision Regeneration circuit, light modulation circuit and light source.

FOT Typical digital optical repeater structure diagram

Light-electric-light conversion process

  The photodetector is used to sense the received optical signal, convert it into an electrical pulse signal, and then through an electrical signal amplifier for amplification, regeneration decision processing, etc., to restore the same digital signal stream as the transmission end, and then pass The light modulator modulates the light source, converts it into an optical signal and enters the optical fiber to continue transmission. That is, each relay station processes the transmitted optical signal using an optical-electrical-optical conversion method.

Optical-electric-optical repeater functional modules

  From the composition of the repeater, the main functional modules can be summarized as follows.

  1. Balanced magnification. It means that the distorted weak electrical signal that is input is equalized, compensated, shaped and amplified to a certain extent to meet the signal processing requirements.
  2. Timing extraction circuit. It refers to extracting the clock frequency from the input electric pulse signal sequence to obtain the timing pulse for use in circuits such as synchronous demodulation.
  3. Decision regeneration circuit. It refers to regenerating the transmission distortion waveform to obtain the same electrical pulse sequence shape process as the transmission end.

  The most important feature of this repeater is that it can shape and regenerate the pulse signal so that the distortion of the waveform will not be accumulated. The inadequacies are complex equipment, high costs, and inconvenient maintenance.

All-optical repeater

  The optical-electrical-optical conversion repeater technology is relatively mature, but its disadvantage is that the equipment is complex and the cost is high, and it is also a bottleneck of the signal transmission link (the electric signal bandwidth is much narrower than the optical signal bandwidth). With the development of optical device technology, it has been developed to directly amplify and relay optical signals.

Fiber Amplifier Overview

FOT EDFA-1550H

FOT EDFA-1550H

  Prior to the advent of optical fiber amplifiers, repeaters of optical fiber communication systems used optical-electrical-optical conversion methods without exception. This resulted in the complexity of equipment in communication systems, which resulted in high costs and reduced capacity. As a result, the efficiency of the system was reduced. Increased network costs and other issues. Therefore, scientists have long been devoted to the study of all-optical relays, that is, direct optical amplification repeaters that do not require optical-electrical-optical conversion. The emergence of optical fiber amplifiers is an important milestone in the history of optical fiber communications. The development trend of optical fiber communication systems is to realize all-optical networks. Optical amplifiers are directly used to amplify optical signals. The amplification process is shown in the figure.

fiber optical amplifier working principle (from fiber optic tutorial)

Optical amplifier type

  An all-optical repeater, ie, an optical amplifier, is characterized by directly amplifying the optical signal and has a high degree of transparency to the signal pair format and rate (because the optical amplifier only amplifies the received signal, so it can support various bit rates and any The format of the signal) makes the system structure simple and flexible.
  Optical amplifiers mainly include semiconductor optical amplifiers (SOAs) and fiber amplifiers. The title optical amplifier refers to an optical amplifier device made of a semiconductor material. If the reflection film at both ends of the semiconductor laser is removed, that is, a semiconductor traveling wave optical amplifier without feedback, it can amplify light of different wavelengths. Fiber amplifiers include two types: nonlinear fiber amplifiers and doped fiber amplifiers. The doped fiber amplifier is an optical amplifier developed in recent years. It utilizes a rare-earth metal ion doped (Er), neodymium (Nd), praseodymium (Pr), ytterbium (Tm), etc. doped with a rare- The ions are incorporated into the optical fiber, and the pump light source is externally applied to meet certain conditions to constitute an optical amplifier. Common doped fiber amplifiers (1.55 μm operating band), Erbium doped fiber amplifiers (1.3 μm working band), Erbium-doped fiber amplifiers (1.55 μm working band), etc.

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