Single-clad fiber laser is the earliest type of fiber laser, which can be traced back to the 1960s. The gain materials used include Nd-doped silicate glass, Qin-doped optical fiber, rare earth-doped optical fiber and fluoride glass optical fiber. Double-clad fiber laser (DCFIL) is a kind of fiber laser developed at the end of 1980s, and it is the focus and hotspot of research and development at present. Due to the change of pumping mode, the laser output power of this fiber laser is obviously improved, reaching several watts to nearly 100 watts. The gain fiber used includes the time-sensitive fiber doped with rare earth elements (such as Er'X, Yb3X, Nd3+, etc.). ), fluoride glass fiber doped with rare earth elements (ZBLAN), photonic crystal fiber (PCF) and so on. In order to improve the output power, symmetrical circle, eccentric circle, D-shape, rectangle, hexagon and quincunx are designed.
Flower-shaped inner cladding structure, in which rectangular inner cladding structure has the highest conversion efficiency. It is reported that the output power of erbium-doped double-clad fiber laser is 103W, the wavelength is 1565nm, and the pulse width of mode-locked erbium-doped fiber laser reaches 3fs, which lays the foundation for realizing all-fiber high-speed communication. At present, this kind of fiber laser has expanded from the mature optical fiber communication field to the laser application fields such as industrial processing, medicine, printing and national defense.
Fiber laser with fiber structure is a new pumping method proposed in recent years. In fact, it is an improvement of cladding end pumping mode. It injects pump light from the side of the cladding, thus forming the concept of "arbitrary shape" fiber laser, and realizing kilowatt-class high-power fiber laser. At present, the output power of Yb-doped fiber laser is 2000W, and the lasing wavelength is 1.060um. There are many ways of cladding side pumping, such as V-groove side pumping, fully spliced side pumping and fiber bundle side pumping. Using optical phase array technology, high energy fiber laser can be obtained. This kind of fiber laser has very important applications in national defense and military fields, such as laser weapon system, photoelectric countermeasure, laser active jamming and so on. The United States, Germany and other countries have corresponding development plans and implementation projects for military high-power fiber lasers. At present, the cavity structures of fiber lasers are mainly Fabry-Perot (F-P) cavity, circular cavity, V-shaped cavity, figure-eight cavity, Fox-Smith cavity and some compound cavities. Fiber laser is a new type of laser. The research and development of fiber laser will push optical fiber and related technologies including optical fiber communication to a new height. Compared with semiconductor lasers, fiber lasers are better coupled with optical fiber communication systems and networks, at least structurally.
Fiber laser is an all-fiber light source and will gradually become an important candidate light source in the field of optical fiber communication. In addition, superfluorescence fiber light source (SFS) and resonant cavity-free photonic crystal fiber laser (PCFL) are also one of the research hotspots in recent years. The successful development of erbium-doped fiber amplifier (EDFA) is a major technical breakthrough in the field of optical fiber communication in 1980s and 1990s, which is of great significance. In recent years, with the continuous improvement and development of fiber amplifier technology and the integration with WDM technology, long (ultra-long) distance, ultra-large capacity, ultra-high speed and dense wavelength division multiplexing (DWDM) of optical fiber communication are becoming the main technical development directions in the fields of long-distance high-speed optical fiber communication and overseas optical fiber communication. Optical fiber amplifiers include doped optical fiber amplifiers (doped with rare earth elements, such as EDFA, PDFA, YDFA, etc. ), nonlinear fiber amplifiers (Raman fiber amplifier (RFA), Brillouin fiber amplifier (BFA), fiber parametric amplifier (OPA), plastic fiber amplifier (POFA) and erbium-doped optical waveguide amplifier (EDWA). The main technical indicators are bandwidth characteristics, noise characteristics, gain characteristics and so on. EDFA is the earliest developed and most widely used optical fiber amplifier, which has been completely commercialized. It has the characteristics of high gain, high power, wide frequency band, low noise, gain characteristics independent of polarization, transparency to data rate and format, low insertion loss and low crosstalk of multi-channel amplification. The wavelengths of pump light are mainly 980nm (three-level system) and 1480nm (two-level system), and there are three basic pumping modes: co-direction, reverse direction and bidirectional. Cascading EDFA can form a multi-stage EDFA system. Ordinary timely EDFA works in 1535- 1565nm band (G band), with a general gain of more than 30dB, a gain bandwidth of 20-40 nm, an output power of about +20dBm and a noise figure (NF) of less than 5 dB. EDFA can take the form of line (trunk), power supply, prefix, local area network, etc. In order to further improve the performance of EDFA, other doping elements can be added to silicon-based erbium-doped glass fiber. For example, Al, Sm, Yb, N, P and Sb are doped to improve the gain bandwidth and flattening characteristics of the amplifier. In recent years, F-EDFA, Te-EDFA, Bi-EDFA, fluoride-doped glass fiber, silicate glass fiber and phosphate glass fiber (Tm) have been applied to L-band, while iron-doped fiber amplifier (TDFA) applied to S-band has become the research focus of optical fiber amplifier. Qin-doped fiber amplifier () and Er-doped fiber amplifier () can work at the wavelength of 13 10nm, which is of great practical significance to improve and improve the performance of existing optical fiber communication systems. Both NDFA and PDFA use Qin (nd)-doped and Pr-doped fluoride glass fibers as amplification gain media. However, due to the limitation of amplified spontaneous emission (ASE), NDFA is not easy to become a high gain 13 10nm amplifier, and the pumping wavelength is 795nm. PDFA has low amplification efficiency and unstable operation. A PDFA with maximum gain of 40dB, noise figure of 5dB and output power of +20dBm is developed. The structural performance and reliability of NDFA and PDFA need to be further improved and improved in order to be fully commercialized. Raman fiber amplifier (RFA) uses Raman effect in optical fiber to amplify optical signals.
The main advantages of RFA are low noise figure, full-band amplification, insensitivity to temperature and online amplification. RFA can be divided into discrete type and distributed type to meet different requirements. Discrete RFA mainly uses special optical fibers with high Raman gain (such as high germanium doped optical fibers, etc.). ), the length is about 1-2km, the pumping power is several watts, and the three-stage stake line generated by the laser with the pumping wavelength of 1.06um can pump and amplify the optical signal with the wavelength of 1.3t.m; 1.55rlm wavelength optical fiber communication system can use 1.48t.m pump laser. The discrete RFA can generate a small signal gain of more than 40dB, and the saturated output power is about +25dBm. As a high-gain and high-power amplifier, it is mainly used in communication systems that need high gain and are easy to control. Distributed RFA directly uses transmission fiber as amplification gain medium, which has the characteristics of distributed amplification, low noise figure and system upgrade. It is mainly used for distributed compensation amplification of optical fiber systems, and can be used in remote pumping, broadband and long-distance 1.3pm and 1.55f4m optical fiber transmission systems and networks. The noise figure (NF) of RFA is obviously smaller than that of EDFA, and the NF of distributed RFA is generally between 0.5- 1dB. Compared with EDFA, RFA has obvious advantages in broadband characteristics, gain characteristics, optical signal-to-noise ratio (QSNR) and configuration flexibility, and is more suitable for transmission systems and networks with large capacity, high speed and long distance. In addition, there is a trend to combine RFA and EDFA to form hybrid fiber amplifier (HFA), which absorbs the advantages of RFA and EDFA and further improves the performance of fiber amplifier. (1) when it thunders, it is easy to produce high accidental overvoltage on the power supply line and destroy the amplifier. A lightning arrester can be installed on the power supply of the amplifier to protect the amplifier from being burned out.
② The power supply voltage of the amplifier is too low, or the contact of the power socket is poor, and the amplifier burns out after working under high current for a long time. The measures taken are to select the amplifier of switching power supply, broaden the power supply range, ensure the normal operation of the amplifier and keep the signal quality unchanged, especially in rural areas, the power supply lines are irregular, the voltage is low and unstable, and the amplifier is damaged. Switching power amplifiers can also be selected in these places.
(3) Because the power load of each phase of the power supply line is unbalanced, there is current in the center line. Once the center line is burnt out, the phase voltage becomes the line voltage, reaching about 380V V V. Due to the different loads of each phase, the phase-to-phase voltage will also change, and the operational amplifier will be burnt out due to the voltage increase.
(4) The filter capacitor in the power amplifier is dried up, aged and its capacity is reduced due to long-term work, resulting in a 50Hz alternating current crossover line moving up and down on the TV screen, which makes the picture unclean. This is a common problem in maintenance.
⑤ The internal heat dissipation of the amplifier is not good, which makes the level fluctuate. The reason is that the adjustable part is metal sheet. When the temperature of the amplifier is too high, it is easy to make the adjustable metal sheet expand with heat and contract with cold, resulting in poor contact and unstable level.
⑥ The indicators of amplifiers that have been used for a certain number of years will deteriorate, and the components will age, especially the amplifiers in the first few stages must be replaced, so as not to affect the transmission quality; The failed and repaired amplifier can not be used in the first few stages of trunk and branch lines; It can be used in terminals with few families, and it will not affect large-scale users to watch TV once it breaks down.
⑦ The temperature of the power amplifier itself is too high, and the installation location is in direct sunlight, which may easily lead to the power amplifier fusing and interrupt the TV signal. ① When lightning strikes, the feeder voltage rises, which will not only burn out the feeder, but also burn out all the feeder amplifiers in the line. The measure is to install lightning overvoltage protector in front of feeder, and the amplifier is selected with lightning protection.
(2) A section of cable in the trunk line is short-circuited due to man-made or other reasons, so that the amplifier and feeder between the short-circuit point and the feed power supply are burnt out. In planning and design, effective measures should be taken to protect the feeder from short circuit.
(3) Due to frequent power outages, the generated surge instantly impacts the voltage, which is easy to burn the fuse of the amplifier. Some fuses are not broken, and the power supply or module components of the power amplifier are also burned due to high instantaneous voltage. An overvoltage eliminator can be installed in the amplifier, which can play a certain role.
③ The aging of the power cord and poor contact of the power plug of the power amplifier will make the power amplifier work intermittently, resulting in trouble-free TV signals sometimes.
4. Amplifier level control
In the access network, it is very important to control the level of the amplifier's inlet and outlet, because the level is very sensitive to the change of environmental temperature. In summer, the temperature rises, the cable loss increases and the incoming line level decreases, which leads to the deterioration of the carrier-to-noise ratio of the system. At the same time, the gain of the amplifier will also decrease with the increase of temperature, thus reducing the transmission level and affecting the user's viewing. When the temperature drops, the cable loss will decrease and the output level of the amplifier will increase more. If it exceeds the allowable range, it will cause the system to change the intermodulation ratio. Moire and wiper intermodulation interference appear on the screen, which affects the viewing effect of the terminal. In severe cases, it is impossible to watch. Therefore, maintenance technicians urgently need to strictly control the liquid level at the inlet and outlet of the amplifier and take effective measures to make up for the serious instability of the liquid level caused by temperature difference, which must be paid attention to.