The rapid development of optical fiber communication technology accelerates the arrival of "light speed economy". In order to meet the requirements of the high-speed development of communication technology and Internet for the communication system with ultra-high code speed, ultra wide bandwidth and ultra large capacity, in addition to the need to develop better passive and active optical fiber components, it is also necessary to develop new optical fiber materials with ultra-low loss and long wavelength working window, as well as more reasonable new optical fiber structure and excellent manufacturing process. CVD (chemical vapor deposition) method in tube, CVD (chemical vapor deposition) method in rod, PCVD (plasma chemical vapor deposition) method and VaD (axial vapor deposition) method are all correct optical fiber fabrication methods.
Optical fiber materials
The optical fiber based on SiO2 works in the near infrared band of 0.8 μ m-1.6 μ M. the lowest theoretical loss that can be achieved is 0.16db/km at the wavelength of 1550nm, which is close to the theoretical minimum loss limit of quartz optical fiber. If the working wavelength is increased, the attenuation constant will increase due to the influence of infrared absorption. Therefore, many scientists have been looking for ultra long wavelength (more than 2 μ m) window optical fiber materials. There are two kinds of materials, i.e. non quartz glass material and crystal material. Crystal fiber materials mainly include AgC1, AgBr, KBr, CSBR and krs-5. At present, the minimum loss of AgC1 single crystal fiber is 0.1db/km at the wavelength of 10.6 μ M. Therefore, it is necessary to find a new type of optical fiber based on the substrate material to meet the needs of ultra wide band, ultra-low loss and high code speed communication.
Fluoride glass fiber is the most studied ultra-low loss far-infrared fiber at present. It is a multi-component glass fiber based on zrf4-baf2 and hff4-baf2 systems. Its minimum loss is 1 × 10 (negative cubic) dB / km near 2.5 μ m, and its non relay distance can reach more than 1 × 10 (5 cubic) km. In 1989, the loss of 2.5 μ m fluoride glass fiber developed by NTT company in Japan was only 0.01db/km. At present, the loss of ZrF4 glass fiber at 2.3 μ m reaches 0.7db/km, which is far away from the theoretical minimum loss of 1 × 10 (negative cubic) dB / km of fluoride glass fiber, and there is still considerable potential to be explored. Whether we can develop a better optical fiber in this field is of far-reaching significance for the development of ultra long wavelength communication window.
Sulfide glass fiber has a wide infrared transparent region (1.2-12 μ m), which is conducive to multichannel multiplexing. Moreover, sulfide glass fiber has a wide optical gap, less energy absorption caused by free electron transition, and the influence of temperature on the loss is small. Its loss level is 0.2db/km at 6 μ m wavelength, which is a very promising fiber. Moreover, the sulfide glass fiber has a large nonlinear coefficient. The nonlinear devices made of it can effectively improve the speed of optical switch, which can reach hundreds of GB / s or more.
Heavy metal oxide glass fiber has excellent chemical stability and mechanical and physical properties, but the infrared properties are not as good as halide glass, the regional permeability is poor, and the scattering is also large. However, if the advantages of halide glass and heavy metal oxide glass are combined, it is of great significance to make a good performance halide heavy metal oxide glass fiber. The loss of the geo2-sb2o3 system optical fiber produced by VAD process in Furukawa electronic company of Japan is up to 13dB / km at the wavelength of 2.05 μ M. if it is further de-oh treated, it can reach 0.1db/km.
Polymer optical fiber has made great progress since it was first invented by DuPont company in 1860s. In 1968, DuPont company developed the step plastic optical fiber (Si POF) of polymethylmethacrylate (PMMA), with a loss of 1000db / km. In 1983, the loss of NTT's pdpmma plastic fiber at 650nm wavelength was reduced to 20dB / km. Since the harmonic absorption of C-F bond is almost nonexistent in the visible region, its intensity is less than 1dB / km even when it extends to the wavelength of 1500nm. The theoretical limit of perfluorinated PMMA fiber loss is 0.25db/km at 1300nm and 0.1db/km at 1500nm, which has great potential. In recent years, y.koike and others, using MMA monomer and tfpma (tetrafluoropropyl methyl acrylate) as the main raw materials, made the graded index polymer preform by centrifugal technology, and then drawn into GI POF (graded index polymer fiber), which has a very wide bandwidth (> 1GHz. Km) and a attenuation of 56dB / km at 688nm wavelength, and is suitable for short-distance communication. In China, MMA, BB and BP were used as the main raw materials, and IGP technology was successfully used to prepare graded plastic optical fiber. NTT company of Japan recently developed fluorinated polyimide material (fulpi) which has high transmittance in near-infrared light, and has the advantages of adjustable refractive index, heat resistance and moisture resistance. It solves the problem of poor transmittance of polyimide, and has been used in light transmission. The research of polycarbonate and polystyrene is also in progress. It is believed that polymer optical fiber materials with better performance will be developed and utilized in the near future.
Special environment has special requirements for optical fiber. The core and cladding materials of quartz optical fiber have good heat resistance. The heat resistance temperature reaches 400-500 ℃, so the use temperature of optical fiber depends on the coating materials of optical fiber. At present, the heat curing temperature of LSP coating is more than 400 ℃, and its optical transmission and mechanical properties are still very good at 600 ℃. The non-uniform nucleation thermochemical reaction (HNTD) was carried out on the surface of the hot optical fiber by using the cold organism, and then the carbon black was produced by cracking on the surface of the optical fiber, that is, the carbon coated optical fiber. Carbon coated fiber has good surface compactness, very low diffusion coefficient, and can eliminate microcracks on the surface of the fiber, which solves the problem of "fatigue" of the fiber.
Optical fiber with new structure
The structure of optical fiber determines the transmission performance of optical fiber. Reasonable refractive index distribution can reduce the attenuation and dispersion of light. In order to improve the waveguide performance of optical fiber, especially to achieve low loss and low dispersion, to meet the requirements of long-distance and large capacity communication, the structure of optical fiber can be designed to control the distribution of refractive index. For example, the structure of triangle refractive index distribution is adopted: area distribution cladding, concave cladding and four cladding structure. The dispersion of waveguide is increased so that the wavelength of zero dispersion is shifted. DSF (dispersion shifted fiber), i.e. G.653 fiber, is designed. It moves the wavelength of zero dispersion to the lowest loss window of 1550nm, which optimizes the combination of loss characteristics and dispersion characteristics of fiber and improves fiber communication Transmission performance of the system.
The dispersion of G653 optical fiber at 1550nm is zero, which brings serious FWM (four wave mixing) effect to WDM (wavelength division multiplexing) system. In order to overcome the shortcomings of DSF, people have improved DSF. By designing the refractive index profile, the zero dispersion point is shifted, so that it is in the range of 1530-1565nm, and the absolute dispersion value is 1.0-6.0ps / (nm. Km), maintaining a sufficient dispersion value to restrain FWM At the same time, the dispersion value is small enough to ensure the transmission rate of single channel is 10Gb / s, and the dispersion compensation is not needed when the transmission distance is more than 250km. This kind of fiber is NZDSF (non-zero dispersion displacement fiber), which ITU-T calls G.655 fiber.
The first generation of G.655 fiber is mainly designed for C-band (1530-1565nm) communication window, mainly including true wave of Lucent company and smf-ls fiber of Corning company in the United States. Their dispersion slope is relatively large. With the development of broad band optical amplifier (BofA), WDM system has been extended to L-band (1565-1620nm). In this case, if the dispersion slope remains the original value (0.07-0.10ps / (Nm2 · km)), the dispersion difference between short wavelength and long wavelength will increase with the increase of distance in long-distance transmission, which will inevitably result in high Rayleigh dispersion in L-band, affect the transmission distance of 10Gb / s and above high code speed signals, or adopt high-cost dispersion compensation measures; However, the dispersion at the low band end is too small to suppress the nonlinear effects such as FWM, SPM, XPM, etc. in multi wavelength transmission. Therefore, it is of great practical value to develop a fiber with low dispersion slope.
The second generation of G.655 fiber meets the above requirements, has a lower dispersion slope, and better meets the requirements of DWDM (dense wavelength division multiplexing). The second generation of G.655 fiber mainly includes true wave RS fiber and true wave XL fiber of Lucent company in the United States. Its dispersion slope is reduced to below 0.05ps / (Nm2 · km). Leaf (large effective area fiber) of Corning company and FREELIGHT fiber newly launched by Pirelli company expand the work window to 1625nm. Recently, Lucent has developed LazrSPEED multimode fiber. The second generation of G.655 optical fiber successfully overcomes the transmission damage caused by optical fiber nonlinearity and greatly improves the transmission performance of optical fiber communication system.
With the rapid development of optical fiber communication system, DFF (dispersion flattened fiber) appears again. It uses special double or multiple cladding structure to form narrow and deep refractive index notch and strengthen the dispersion of waveguide, so as to obtain zero dispersion at 1300nm and 1550nm, make the total dispersion of optical fiber near flat in the wavelength range of 1300-1600nm, and expand the bandwidth of optical fiber, which is conducive to DWDM and phase The development of dry optical communication.
DWDM system hopes to be able to carry out wavelength division multiplexing in the widest possible available band, allocate various services of different speed and nature to different wavelengths, and carry out path selection and insertion in the optical path. However, the existence of hydroxyl (OH -) absorption peak near 1385nm in the available band causes serious loss of optical power and restricts the use of 1350-1450nm band. Therefore, each company is committed to eliminate the oh absorption peak and develop "water free fiber", so as to realize the practical application of the fifth window of 1350-1450nm. All wave optical fiber developed by Lucent company of the United States overcomes the harmonic absorption of OH -, thus realizing the utilization of complete wave band in the range of 1280-1625nm. The increase of the effective working wavelength range is beneficial to reduce the requirements of OPD (optical passive device) and oad (optical active device) by increasing the distance between the wavelength channels, greatly reducing the cost of the communication system, and at the same time, the large capacity transmission of the optical fiber communication system can be realized by increasing the density of wavelength division multiplexing.
Intensity modulation, a direct detection communication system, can achieve high code speed and large capacity transmission, and has the advantages of easy modulation, but it is essentially a "noise communication system", while coherent optical communication heterodyne communication system has the advantages of long relay and high transmission rate, which uses the phase and polarization of light to transmit information. In order to meet the requirements of coherent communication system, we have developed "Panda", "bow" and "flat" high birefringence polarization maintaining fiber, as well as "side pit" single-mode polarization maintaining fiber, which lays the foundation for future all-optical communication.（https://www.shhjnet.com/index.html）