1. Preface
Since the optical fiber communication technology was pioneered by the industry, it can be summarized that optical fiber communication has three advantages over traditional copper communication: first, the communication capacity is large; Second, it has good anti-electromagnetic interference and confidentiality; Third, it is light in weight and can save a lot of copper. For example, laying an 8-core cable with a length of 1, kilometers can save 1,1 tons of copper and 3,7 tons of lead compared with laying an 8-core cable with the same length. Therefore, the optical fiber cable was welcomed by the communication industry as soon as it came out, which brought a revolution in the communication field and a round of investment and development boom.
Although glass fiber has the above-mentioned advantages, it has a fatal weakness, such as low strength, poor flex resistance and poor radiation resistance. Therefore, in recent 2 years, the industry has never stopped the research and development of other optical fiber materials, among which the research and development of plastic optical fiber is one of the most interesting research fields in the industry. At present, great progress has been made, and commercial products have been available, which have been widely used in automobiles, CD players, industrial electronic systems, small optical disk systems and personal computers. In the future, plastic optical fibers will be used in many fields, such as sensors and photonic crystal fibers.
II. Advantages of plastic optical fiber
Compared with glass optical fiber, plastic optical fiber is light and flexible, high in impact strength, low in price, radiation-resistant, easy to process, and can be made large, although its light transmission band is generally 3 dB/km in the initial stage and is narrow (limited to the visible region). In addition, the diameter of the central part of the plastic optical fiber through which light passes is about 1 mm, which is 1 times larger than that of the glass optical fiber, and the connection between the fiber and the terminal devices such as personal computers is very easy. Therefore, the installation cost of plastic optical fiber is very low, and it is only necessary to use a very simple alignment connection plug when installing, which can be produced by the existing technology.
iii. brief introduction of research and development of plastic optical fiber products
the research of plastic optical fiber began in the 196s. In 1968, DuPont Company of the United States prepared plastic optical fiber with polymethyl methacrylate as the core material, but the optical loss was large. In 1974, Japan's Mitsubishi Rayon Company developed a plastic optical fiber with PMMA and polystyrene as the core materials and fluoroplastics with low refractive index as the cladding. Its optical loss is 35dB/km, so it is difficult to be used for communication.
in the 198s, some large enterprises and universities in Japan did a lot of research on the preparation of low-loss plastic optical fibers. In 198, Mitsubishi company polymerized PMMA with high purity MMA monomer, which reduced the loss of plastic optical fiber to 1-2dB/km. In 1983, NTT Company began to replace H atom in PMMA with deuterium, which made the lowest optical loss reach 2dB/km and could transmit light waves from near infrared to visible light.
in recent years, companies in Europe, Japan and other countries have made important progress in the development of plastic optical fibers. The optical loss rate of the plastic optical fiber developed by them has dropped to 25 ~ 9 dB/km. Its working wavelength has been extended to 87 microns (near infrared light), which is close to the practical level of Shi Ying glass fiber. A PFX plastic series optical fiber developed in the United States has excellent radiation resistance. In addition, the Opti-Giga plastic optical fiber developed by Boston Optical Fiber Company, Massachusetts, USA is even more eye-catching. It is not only lighter, more flexible and cheaper than glass, but also can transmit data at a speed of 3 megabits per second within 1 meters. This kind of optical fiber can also use the refraction of light or the jumping mode of light in the fiber to achieve higher transmission speed. Now the United States, Europe and Japan have used plastic optical fibers for short-distance transmission, such as automobiles, medical devices and photocopiers.
Japan is the largest producer of plastic optical fiber in the world in terms of current production capacity, but Europe has promoted the development of new application fields of plastic optical fiber and established optical fiber inspection standards. The second half of 21 is an important stage for the development of plastic optical fiber industry in Europe. During this period, a new development policy for inspection and measurement of plastic optical fiber in Europe was established. The world's first dedicated plastic optical fiber application center (POFAC) was completed in Nuremberg, Germany. Germany has successfully developed the multimedia bus system MOST(24Mbit/s) using plastic optical fiber, and several car manufacturers have introduced this system into their own products. German BMW Company (BMW) has created a record of using 1m plastic optical fiber in its new 7 series of products. Europe 21 plastic optical fiber academic exchange conference and European optical fiber communication conference were held simultaneously in Amsterdam, the Netherlands. German automobile industry not only promotes the application of plastic optical fiber, but also promotes the establishment of inspection and measurement standards for plastic optical fiber.
Japan has also established standards for plastic optical fibers, but these standards are invalid for European counterparts. Japanese industrial standards only give the standard of one type of plastic optical fiber, with a numerical aperture of .5 and only one wavelength of 65nm. The standard does not mention different excitation light conditions in plastic optical fiber, nor does it stipulate that balanced mode distribution must be formed in plastic optical fiber.
The previously established inspection method for glass optical fiber is not suitable for inspecting plastic optical fiber because of Rayleigh scattering. Now there is only one instrument for inspecting plastic optical fiber sold by Luciol Instruments Company, a newly established Swiss company.
The German Institute of Engineers and the Research Group of the Institute of Electronic Engineering have specified in detail the measurement methods of numerical aperture, attenuation, transmission and mechanical characteristics, environment and life of plastic optical fiber. The establishment of inspection methods and standards for plastic optical fiber will certainly promote the development of international plastic optical fiber trade and eliminate misunderstandings in trade.
Japan attaches great importance to the application of plastic optical fiber. As early as a few years ago, 45 manufacturers of optical communication and multimedia products, including NEC, Fujitsu and Sumitomo Electric Industrial Co., Ltd., jointly announced that they would realize the practical application of plastic optical fiber that has been successfully developed in Japan. Plastic optical fiber is considered as the key technology to introduce multimedia into the home because of its low cost, and then some manufacturers set about establishing production lines. ?
In p>1986, Japan's F Fujitsu Company developed the SI-type heat-resistant POF with PC as the core material. The heat-resistant temperature can reach 135 degrees Celsius and the attenuation can reach 45 dB/km.
In p>199, Professor Koike Sukehiro of Keio University in Japan successfully developed a plastic optical fiber with graded refractive index. The core material was fluorine-containing PMMA, and the cladding was fluorine-containing, and it was manufactured by interfacial gel technology. The attenuation of this plastic fiber is below 6db/km, the light source is 65-13nm, the bandwidth of 1m is 3GHz, and the transmission rate is 1Gb/s, which exceeds that of GI-type Shi Ying fiber and is widely regarded as a new optical communication medium in fttp in the era of high-speed multimedia.
in p>1996, people suggested to build ATM physical layer of user network with very low cost based on plastic optical fiber. In 1997, NEC Corporation of Japan carried out the test of ATM and LAN with 155 Mbit/s.
at the OFC conference in 2, ASAHI GLASS company of Japan reported that the attenuation coefficient of fluorinated gradient plastic fiber was 41dB/km at 85nm, 33dB/km at 13nm, and the bandwidth reached 1MHz.km .. The high-speed transmission test of 5m and 2.5Gbit/s and the long-term thermal aging test of 7 degrees Celsius were successfully carried out with this fiber. The experimental conclusion is that fluorinated gradient plastic optical fiber can completely meet the requirements of short-distance communication.
From the research and development of plastic optical fiber, the research focus of plastic optical fiber is mainly focused on the following three aspects:
1. Reducing optical loss;
2. increase the bandwidth (from SI type to GI type);
3. Improve heat resistance. (Polycarbonate (PC), silicone resin, cross-linked acrylic acid and * * * polymer can improve the heat resistance to 125-15 degrees Celsius)
The latest practical progress in attenuation and bandwidth of plastic optical fiber is that Japan's ASAHI GLASS Company said in July 2 that it commercialized Keio University's GI-POF technology and manufactured GI optical fiber with fully fluorinated polymer CYTOP, which was named GI-GOF.
the latest practical progress in heat resistance of plastic optical fiber is that Japan JSR and Asahi Kasumi Co., Ltd. jointly developed the SI-POF made of heat-resistant transparent resin ARTON(norbornene), which is heat-resistant to 17 degrees Celsius and is expected to be available to the automobile market in the first half of 21.
IV. Key points of research and development of plastic optical fiber products
1. Optical fiber structure
As the name implies, plastic optical fiber is composed of plastic materials, namely, the core and cladding of optical fiber. Compared with Shi Ying glass multimode optical fibers with large core diameters of 5/125 μ m and 62.5/125 μ m, the core diameter of plastic optical fiber is as high as 2-1 μ m, and the cheap injection-molded plastic connector without fiber positioning sleeve can be used for its connection, even if the deviation of core alignment in fiber connection will not affect the coupling loss. It is the plastic optical fiber structure that gives it the advantages of fast construction and low connection cost. In addition, if the core diameter is 1μm or more, the multimode noise existing in Shi Ying glass multimode fiber can be eliminated.
2. Optical fiber materials
When choosing plastic optical fiber materials, people should focus on the following issues: low attenuation, small dispersion, good stability, simple manufacture and low price.
The core materials selected for plastic optical fiber are: polymethyl methacrylate, polystyrene polycarbonate, fluorinated polymethyl methacrylate, perfluororesin, etc. The cladding of plastic optical fiber is polymethyl methacrylate, fluoroplastics, silicone resin, etc. The reasons are as follows: ① these polymers have good light transmittance, uniform optics and convenient refractive index adjustment; ② It can be purified by vacuum distillation in the presence of monomer; ③ Strong ability to form optical fiber; ④ Good processing and chemical stability and low price;
3. Manufacturing technology
At present, the two methods used to manufacture plastic optical fibers in the industry: extrusion method and interfacial gel method are both evolved from plastic production and processing technology.
extrusion method is mainly used to manufacture plastic optical fiber with step refractive index distribution. The process steps are roughly as follows: firstly, the monomer methyl methacrylate as the fiber core is purified by vacuum distillation, and then sent into a polymerization container together with a polymerization initiator and a chain transfer agent < P >, then the container is heated in an electric oven and left for a certain period of time to completely polymerize the monomer, and finally, the container containing the fully polymerized methyl methacrylate is heated to the drawing temperature. The melted polymer is pressurized by dry nitrogen from the upper end of the container, and a plastic optical fiber core is extruded from the nozzle at the bottom of the container, and the extruded fiber core is coated with a layer of polymer with low refractive index, thus making the step-type plastic optical fiber.
The manufacturing method of gradient refractive index distribution plastic optical fiber is interfacial gel method, and the process steps of interfacial gel method are roughly as follows: firstly, high refractive index dopant is put into core monomer to make core mixed solution, secondly, initiator and chain transfer agent for controlling polymerization speed and polymer molecular weight are put into core mixed solution, then this solution is put into a hollow tube selected as cladding material, and finally, the PMMA tube filled with core mixed solution is put into an oven, at a certain temperature and at a certain temperature. During the polymerization process, the PMMA tube was gradually swollen by the mixed solution, thus forming a gel phase on the inner wall of the PMMA tube. When the molecular movement speed of the gel phase slows down, the polymerization reaction is accelerated due to the "gel action", and the thickness of the polymer gradually thickens, and the polymerization ends at the center of the PMMA tube, thus obtaining an optical fiber preform with a gradient refractive index distribution along the radial direction, and finally sending the plastic optical fiber preform into a heating furnace to be heated and drawn into a plastic optical fiber with gradient refractive index distribution;
4. Optical fiber performance
The research focuses on attenuation, dispersion and thermal stability of plastic optical fiber.
(1) attenuation
The attenuation of plastic optical fiber is mainly limited by the absorption loss and dispersion loss of the core-coated plastic material. People choose plastic materials with low refractive index and low isothermal compression rate, and reduce structural defects (such as core diameter fluctuation, core-cladding interface defects, etc.) by stabilizing the manufacturing process of plastic optical fibers, so that plastic optical fibers can obtain small scattering loss, and the absorption loss of plastic materials is caused by the stretching vibration absorption of molecular bonds (hydrocarbons, fluorocarbon, etc.) and electron jump absorption.
in plastic materials with carbon-hydrogen bonds as the basic skeleton, the attenuation coefficient at the wavelength of 65nm is about 12 db/km. If fluorine atoms are used to replace hydrogen in carbon-hydrogen bonds, not only the intrinsic attenuation is small, but also the dispersion is reduced. The gradient index plastic optical fiber made of fluorinated plastics has no absorption loss caused by atomic vibration in the infrared region. Therefore, the attenuation in the range of visible light to infrared is very small, that is, the attenuation coefficient is 41 dB/km at the wavelength of .85 μ m and the gradient refractive index distribution is 33 dB/km at the wavelength of 1.3 μ m..
(2) Bandwidth
Plastic optical fibers used as short-distance optical transmission media can be divided into two types according to their refractive index distribution shapes: step refractive index distribution plastic optical fibers and gradient refractive index distribution plastic optical fibers. Due to the mode dispersion, the incident light is repeatedly reflected, and the emitted waveform is broadened relative to the incident waveform, so its transmission bandwidth is only tens to hundreds of MHz．km. The fluorinated plastic fiber with gradient refractive index distribution can select its refractive index distribution index as 2.7-2.33 in the wavelength range of .85-1.3 μ m by selecting low dispersion materials and optimizing gradient refractive index distribution means, so as to suppress the mode dispersion and control the broadening effect of the outgoing light wave relative to the incident light wave, and then the plastic fiber with gradient refractive index distribution with transmission bandwidth as high as several hundred MHz. km to 1 GHz. km can be manufactured.
(3) Thermal stability
Because plastic optical fiber is made of plastic material, it will be oxidized and degraded when working in high temperature environment. Oxidative degradation is formed by carbonyl group, double bond and crosslinking in optical fiber core material. Oxidative degradation will accelerate the electron transition, which will lead to the increase of fiber loss. In order to improve the thermal stability of plastic optical fiber, the usual methods are as follows: ① selecting plastic materials containing fluorine or silicon to manufacture plastic optical fiber; ② The working wavelength of the light source of plastic optical fiber is selected to be greater than 66nm, so as to ensure the long-term and reliable thermal stability of plastic optical fiber.
V. Technical Keys
At present, there are two key technical problems for plastic optical fiber products: one is to design new light-transmitting materials and sheath materials. Plastic optical fiber, like Shi Ying glass fiber, consists of two parts: one is the core material, and the other is the sheath. It is very important to manufacture high-quality optical fiber. The core material of optical fiber requires the higher transparency and refractive index, while the sheath requires the refractive index to be smaller than the core material, and the greater the difference between them, the better. However, it is difficult to improve the refractive index of the core material, and there is still potential to reduce the refractive index of the cortex, mainly concentrated in fluorine-containing polymers. The second key point is the process conditions, and how to control the molecular weight and uniformity of the core polymer and improve the transparency of the new optical fiber technology is studied to further improve the light transmission efficiency and reduce the light loss rate. Once these two questions are solved,