Optical fiber is an abbreviation of optical fiber, a fiber made of glass or plastic, which can be used as a light transmission tool. The transmission principle is'total reflection of light'. The former presidents of the Chinese University of Hong Kong Gao Kun and George A. Hockham first proposed the idea that optical fiber can be used for communication transmission. For this reason, Gao Kun won the 2009 Nobel Prize in Physics.
introduce
The tiny optical fiber is encapsulated in a plastic sheath so that it can be bent without breaking. Generally, the transmitting device at one end of the optical fiber uses a light emitting diode (LED) or a laser beam to transmit light pulses to the optical fiber, and the receiving device at the other end of the optical fiber uses a photosensitive element to detect the pulses.
In daily life, since the transmission loss of light in optical fibers is much lower than that of electricity in wires, optical fibers are used for long-distance information transmission.
Usually the two terms optical fiber and optical cable are confused. Most optical fibers must be covered by several layers of protective structures before use, and the covered cables are called optical cables. The protective layer and insulating layer on the outer layer of the optical fiber can prevent damage to the optical fiber from the surrounding environment, such as water, fire, and electric shock. Optical cable is divided into: optical fiber, buffer layer and coating. Optical fiber is similar to coaxial cable, except that there is no mesh shield. In the center is the glass core through which light propagates.
In a multimode fiber, the core diameter is 50 μm and 62.5 μm, which are roughly equivalent to the thickness of a human hair. The single-mode fiber core has a diameter of 8 μm to 10 μm. The core is surrounded by a glass envelope with a lower refractive index than the core to keep the light inside the core. On the outside is a thin plastic jacket to protect the envelope. Optical fibers are usually bundled and protected by a casing. The fiber core is usually a double-layer concentric cylinder with a small cross-sectional area made of quartz glass. It is brittle and easy to break, so an external protective layer is needed.
principle
Light and its characteristics
1. Light is an electromagnetic wave
The wavelength range of visible light is 390~760nm (nanometer). The part larger than 760nm is infrared light, and the part smaller than 390nm is ultraviolet light. The optical fiber is used in three types: 850nm, 1310nm, and 1550nm.
2. Refraction, reflection and total reflection of light.
Because the propagation speed of light in different substances is different, when light is emitted from one substance to another, refraction and reflection occur at the interface of the two substances. Moreover, the angle of refracted light changes with the angle of incident light. When the angle of the incident light reaches or exceeds a certain angle, the refracted light will disappear, and all the incident light will be reflected back, which is the total reflection of light. Different materials have different refraction angles for light of the same wavelength (that is, different materials have different refractive indexes), and the same material has different refraction angles for light of different wavelengths. Optical fiber communication is formed based on the above principles.
1. Optical fiber structure:
The bare fiber of optical fiber is generally divided into three layers: the center high refractive index glass core (the core diameter is generally 50 or 62.5μm), the middle is the low refractive index silica glass cladding (the diameter is generally 125μm), and the outermost is the resin coating for reinforcement. Floor.
2. Optical fiber numerical aperture:
The light incident on the end face of the optical fiber cannot all be transmitted by the optical fiber, but only the incident light within a certain angle range. This angle is called the numerical aperture of the fiber. The larger numerical aperture of the optical fiber is beneficial to the butt connection of the optical fiber. Optical fibers produced by different manufacturers have different numerical apertures (AT&T CORNING).
3. Types of optical fiber:
There are many types of optical fibers, and the required functions and performances vary according to different uses. However, the principles of design and manufacture of optical fiber for cable TV and communication are basically the same, such as: ① small loss; ② certain bandwidth and small dispersion; ③ easy wiring; ④ easy integration; ⑤ high reliability; ⑥ manufacturing comparison Simple; ⑦Inexpensive and so on. The classification of optical fiber is mainly summarized from the working wavelength, refractive index distribution, transmission mode, raw material and manufacturing method. Here are examples of various classifications as follows.
(1) Working wavelength: ultraviolet fiber, observable fiber, near-infrared fiber, infrared fiber (0.85μm, 1.3μm, 1.55μm).
(2) Refractive index distribution: step (SI) type fiber, near-step type fiber, graded (GI) type fiber, others (such as triangle type, W type, recessed type, etc.).
(3) Transmission mode: single-mode fiber (including polarization-maintaining fiber and non-polarization-maintaining fiber), multi-mode fiber.
(4) Raw materials: quartz optical fiber, multi-component glass optical fiber, plastic optical fiber, composite optical fiber (such as plastic cladding, liquid core, etc.), infrared materials, etc. According to the coating material, it can be divided into inorganic materials (carbon, etc.), metal materials (copper, nickel, etc.) and plastics.
(5) Manufacturing methods: Pre-plasticizing includes vapor phase axial deposition (VAD), chemical vapor deposition (CVD), etc., and wire drawing methods include Rod intube and double crucible methods.
Silica Optical Fiber
Silica Fiber is an optical fiber in which silicon dioxide (SiO2) is the main raw material, and the refractive index distribution of the core and cladding is controlled according to different doping amounts. Quartz (glass) series optical fibers have the characteristics of low power consumption and broadband, and are now widely used in cable television and communication systems.
The advantage of quartz glass optical fiber is low loss. When the light wavelength is 1.0～1.7μm (around 1.4μm), the loss is only 1dB/km, and the lowest at 1.55μm is only 0.2dB/km.
Fluorine-doped fiber
Fluorine Doped Fiber is one of the typical products of silica fiber. Generally, in the 1.3μm waveband communication optical fiber, the dopant controlling the core is germanium dioxide (GeO2), and the cladding is made of SiO2. However, most of the cores of fluorine-connected fibers use SiO2, but fluorine is doped in the cladding. Because Rayleigh scattering loss is a phenomenon of light scattering caused by changes in refractive index. Therefore, it is desirable to form dopants of refractive index fluctuation factors, and less is better. The main effect of fluorine is to reduce the refractive index of SIO2. Therefore, it is often used for the doping of the cladding.
Compared with optical fibers of other raw materials, quartz optical fiber also has a broad spectrum of light transmission from ultraviolet light to near-infrared light. In addition to communication purposes, it can also be used in fields such as light guide and image transmission.
Infrared fiber
As the working wavelength of the quartz series optical fiber developed in the field of optical communication, although it is used in a shorter transmission distance, it can only be used in 2μm. For this reason, it can work in the field of longer infrared wavelengths, and the developed optical fiber is called infrared optical fiber. Infrared Optical Fiber is mainly used for light energy transmission. For example: temperature measurement, thermal image transmission, laser scalpel medical treatment, thermal energy processing, etc. The penetration rate is still low.
Composite fiber
Compound fiber is made of SiO2 raw material, and then appropriately mixed oxides such as sodium oxide (Na2O), boron oxide (B2O3), potassium oxide (K2O) and other oxides to make a multi-component glass fiber, which is characterized by multi-component glass It has a lower softening point than quartz glass and a large difference in refractive index between the core and the cladding. Fiber optic endoscopes mainly used in medical services.
CFC fiber
Fluoride Fiber Chloride Fiber (Fluoride Fiber) is an optical fiber made of fluoride glass. This optical fiber material is also referred to as ZBLAN (that is, fluoride glass materials such as ZrF2), barium fluoride (BaF2), lanthanum fluoride (LaF3), aluminum fluoride (AlF3), and sodium fluoride (NaF) are simplified into The abbreviation of, mainly works in the optical transmission service of 2～10μm wavelength. Because ZBLAN has the possibility of ultra-low loss fiber, the feasibility development for long-distance communication fiber is underway, for example: its theoretical lowest loss, in It can reach 10-2～10-3dB/km at 3μm wavelength, while quartz fiber is between 0.15-0.16dB/Km at 1.55μm. At present, ZBLAN fiber can only be used at 2.4～2.7 due to the difficulty of reducing scattering loss. μm temperature sensors and thermal image transmission have not yet been widely used. Recently, in order to use ZBLAN for long-distance transmission, a 1.3 μm praseodymium-doped fiber amplifier (PDFA) is being developed.
Plastic coated optical fiber
Plastic Clad Fiber (Plastic Clad Fiber) is a step-type fiber in which high-purity silica glass is used as the core, and plastic with a refractive index slightly lower than that of silica, such as silica gel, is used as the cladding. Compared with silica fiber, it has the characteristics of core rent and high numerical aperture (NA). Therefore, it is easy to combine with the light-emitting diode LED light source, and the loss is small. Therefore, it is very suitable for local area network (LAN) and short-distance communication.
Plastic Optical Fiber
This is an optical fiber in which both the core and the cladding are made of plastic (polymer). Early products were mainly used in optical communications for decoration and light-guided lighting and short-distance optical bond circuits. The raw materials are mainly organic glass (PMMA), polystyrene (PS) and polycarbonate (PC). Loss is restricted by the inherent C-H combined structure of plastics, generally up to tens of dB per km. In order to reduce the loss, fluorine series plastics are being developed and applied. Since the core diameter of the Plastic Optical fiber is 1000μm, which is 100 times larger than the single-mode quartz fiber, the connection is simple, and it is easy to bend and construct. In recent years, with the progress of broadbandization, the development of multimode plastic optical fiber with graded (GI) refractive index has received social attention. Recently, the application is relatively fast in the car's internal LAN, and it may also be used in the home LAN in the future.
Single mode fiber
Single-mode fiber This refers to the fiber that can only transmit one propagation mode in the working wavelength, usually referred to as single-mode fiber (SMF: Single Mode Fiber). At present, it is the most widely used optical fiber in cable television and optical communications. Because the core of the fiber is very thin (about 10μm) and the refractive index is in a step-like distribution, when the normalized frequency V parameter is less than 2.4, theoretically, only single-mode transmission can be formed. In addition, SMF does not have multi-mode dispersion. Not only is the transmission frequency band wider than the fiber with more mode, but also the material dispersion and structural dispersion of SMF are added and offset, and its synthesis characteristic happens to form the characteristic of zero dispersion, which makes the transmission frequency band wider. In SMF, there are many types due to differences in dopants and manufacturing methods. DePr-essed Clad Fiber (DePr-essed Clad Fiber), its cladding forms a double structure, and the cladding adjacent to the core has a lower refractive index than the outer inverted cladding.
Multimode fiber
Multimode fiber refers to the fiber in which the possible propagation mode of the fiber is multiple modes according to the working wavelength, called multimode fiber (MMF: MULti ModeFiber). The core diameter is 50μm, and because the transmission mode can reach several hundred, compared with SMF, the transmission bandwidth is mainly dominated by modal dispersion. Historically, it has been used for short-distance transmission in cable television and communication systems. Since the emergence of SMF fiber, it seems to have formed a historical product. But in fact, because MMF has a larger core diameter than SMF and is easier to combine with light sources such as LEDs, it has more advantages in many LANs. Therefore, MMF is still receiving attention again in the field of short-distance communication. When MMF is classified according to the refractive index distribution, there are two types: gradient (GI) type and step (SI) type. The refractive index of the GI type is the highest in the center of the core, and gradually decreases along the cladding. As the SI type light wave is reflected in the optical fiber, the time difference of each light path is generated, which causes the emitted light wave to be distorted and the color shock is large. As a result, the transmission bandwidth is narrowed, and there are currently fewer SI-type MMF applications.
Dispersion shifted fiber
When the operating wavelength of a single-mode fiber is 1.3Pm, the mode field diameter is about 9Pm, and its transmission loss is about 0.3dB/km. At this time, the zero-dispersion wavelength is exactly at 1.3pm. Among the quartz optical fibers, the transmission loss in the 1.55pm section is the smallest (about 0.2dB/km) from the raw material. Since the practical erbium-doped fiber amplifier (EDFA) works in the 1.55pm band, if zero dispersion can be achieved in this band, it will be more conducive to the application of long-distance transmission in the 1.55pm band. Therefore, by cleverly using the composite offset characteristics of the dispersion of the quartz material in the fiber material and the dispersion of the core structure, the original zero dispersion of the 1.3Pm section can be shifted to the 1.55pm section to constitute zero dispersion. Therefore, it is named Dispersion Shifted Fiber (DSF: DispersionShifted Fiber). The method of increasing the structural dispersion is mainly to improve the refractive index distribution performance of the core. In the long-distance transmission of optical communication, zero fiber dispersion is important, but not the only one. Other properties include low loss, easy connection, cable formation or small changes in characteristics during work (including the effects of bending, stretching and environmental changes). DSF is designed to consider these factors comprehensively.
Dispersion Flat Fiber
Dispersion shifted fiber (DSF) is a single-mode fiber designed with zero dispersion in the 1.55pm band. The dispersion flattened fiber (DFF: Dispersion Flattened Fiber) has a wide wavelength range from 1.3Pm to 1.55pm. The dispersion can be made very low, and the fiber that achieves almost zero dispersion is called DFF. Because DFF has to reduce the dispersion in the range of 1.3pm to 1.55pm. It is necessary to carry out a complicated design for the refractive index distribution of the optical fiber. However, this kind of fiber is very suitable for wavelength division multiplexing (WDM) lines. Because the process of DFF fiber is more complicated, the cost is more expensive. In the future, as output increases, prices will also decrease.
Dispersion Compensation Fiber
For trunk systems using single-mode fibers, most of them are constructed using fibers with zero dispersion in the 1.3pm band. However, now the smallest loss is 1.55pm. Due to the practical use of EDFA, it would be very beneficial if the 1.55pm wavelength can be operated on a 1.3pm zero-dispersion fiber. Because, in the 1.3Pm zero-dispersion fiber, the dispersion in the 1.55Pm band is about 16ps/km/nm. If a section of fiber with the opposite sign of the dispersion is inserted in this optical fiber line, the dispersion of the entire optical line can be made zero. The fiber used for this purpose is called Dispersion Compensation Fiber (DCF: DisPersion Compe-nsation Fiber). Compared with the standard 1.3pm zero-dispersion fiber, DCF has a thinner core diameter and a larger refractive index difference. DCF is also an important part of WDM optical lines.
Polarization maintaining fiber
The light waves propagating in the optical fiber have the properties of electromagnetic waves, so in addition to the basic light wave single mode, there are essentially two orthogonal modes of electromagnetic field (TE, TM) distribution. Generally, because the structure of the fiber section is circularly symmetrical, the propagation constants of the two polarization modes are equal, and the two polarized lights do not interfere with each other. However, in fact, the fiber is not completely circularly symmetrical. The combining factors between the polarization modes are irregularly distributed on the optical axis. The dispersion caused by this change in polarized light is called Polarization Mode Dispersion (PMD). For cable TV, which mainly distributes images, the impact is not too big, but for some services that have special requirements for ultra-wideband in the future, such as:
① When heterodyne detection is used in coherent communication, when light wave polarization is required to be more stable;
②When the input and output characteristics of optical equipment are related to polarization;
③When making polarization-maintaining optical couplers and polarizers or depolarizers, etc.;
④ Make optical fiber sensors that use light interference, etc.,
Where the polarization is required to be kept constant, the fiber that has been modified to make the polarization state unchanged is called polarization maintaining fiber (PMF: Polarization Maintaining fiber), or fixed polarization fiber.
Birefringent fiber
Birefringent fiber refers to a single-mode fiber that can transmit two inherent polarization modes that are orthogonal to each other. The phenomenon that the refractive index varies with the direction of the deflection is called birefringence. It is also called PANDA fiber, that is, Polarization-maintai-ning AND Absorption-reducing fiber. It is arranged in two transverse directions of the core, with a glass part with a large thermal expansion coefficient and a circular cross-section. In the high-temperature fiber drawing process, these parts shrink, which results in stretching in the y-direction of the core, and at the same time compressive stress in the x-direction. This results in a photoelastic effect of the fiber material, and a difference in refractive index in the X direction and the y direction. According to this principle, the effect of keeping the polarization constant is achieved.
Anti-bad environment fiber
The normal working environment temperature of optical fiber for communication can be between -40℃ and +60℃, and the design is also based on the premise that it is not exposed to a large amount of radiation. In contrast, for the lower temperature or higher temperature and the harsh environment that can be subjected to high pressure or external force, and exposed to radiation, the fiber that can also work is called the Hard Condition Resistant Fiber (Hard Condition Resistant Fiber). Generally, in order to mechanically protect the surface of the optical fiber, an extra layer of plastic is coated. However, as the temperature increases, the protective function of the plastic decreases, which limits the use temperature. If you switch to heat-resistant plastics, such as Teflon (Teflon) and other resins, you can work at 300°C. There are also metals such as nickel (Ni) and aluminum (Al) coated on the surface of quartz glass. This kind of fiber is called Heat Resistant Fiber (Heat Resistant Fiber). In addition, when the optical fiber is irradiated by radiation, the optical loss will increase. This is because when quartz glass is exposed to radiation, structural defects (also called color center: Colour Center) will appear in the glass, and the loss will increase especially at the wavelength of 0.4～0.7pm. The prevention method is to switch to quartz glass doped with OH or F element, which can suppress the loss defects caused by radiation. This kind of fiber is called Radiation Resistant Fiber, and it is mostly used in optical fiber mirrors for nuclear power station monitoring.
Hermetic coated fiber
In order to maintain the long-term stability of the mechanical strength and loss of the optical fiber, the glass surface is coated with inorganic materials such as silicon carbide (SiC), titanium carbide (TiC), and carbon (C) to prevent water and hydrogen from coming from the outside. Diffusion of the manufactured optical fiber (HCF Hermetically Coated Fiber). At present, it is commonly used in the production process of chemical vapor deposition (CVD) to use a carbon layer to accumulate at a high speed to achieve a sufficient sealing effect. This carbon-coated optical fiber (CCF) can effectively cut off the intrusion of the optical fiber from external hydrogen molecules. It is reported that it can be maintained for 20 years without increasing loss in a hydrogen environment at room temperature. Of course, its fatigue coefficient (Fatigue Parameter) can reach more than 200 in preventing the intrusion of moisture and delaying the fatigue process of mechanical strength. Therefore, HCF is used in systems that require high reliability in harsh environments, such as submarine optical cables.
Carbon coated fiber
An optical fiber coated with a carbon film on the surface of a quartz optical fiber is called Carbon Coated Fiber (CCF: Carbon Coated Fiber). The mechanism is to use a dense carbon film to isolate the surface of the optical fiber from the outside world to improve the mechanical fatigue loss of the optical fiber and increase the loss of hydrogen molecules. CCF is a type of hermetic coated optical fiber (HCF).
Metal coated optical fiber
Metal Coated Fiber (Metal Coated Fiber) is an optical fiber coated with a metal layer such as Ni, Cu, Al, etc. on the surface of the optical fiber. There are also plastic coatings on the outside of the metal layer for the purpose of improving heat resistance and being available for energization and welding. It is one of the anti-bad environment optical fibers, and can also be used as a component of electronic circuits. Early products were made by coating molten metal during the drawing process. Because this method has too much difference in expansion coefficient between the glass and the metal, it will increase the small bending loss, and the practical rate is not high. Recently, due to the success of the low-loss non-electrolytic coating method on the surface of the glass optical fiber, the performance has been greatly improved.
Rare earth doped fiber
In the fiber core, the fiber is doped with rare earth elements such as Er, Nd, and Pr. In 1985, Payne of the University of Southampton in the United Kingdom first discovered that Rare Earth DoPed Fiber (Rare Earth DoPed Fiber) had the phenomenon of laser oscillation and light amplification. Therefore, since then, the veil of light amplification such as bait has been unveiled. The 1.55pm EDFA that is now practical is to use bait-doped single-mode fiber and use 1.47pm laser for excitation to obtain 1.55pm optical signal amplification. In addition, error-doped fluoride fiber amplifiers (PDFA) are under development.
Raman fiber
Raman effect means that when monochromatic light of frequency f is projected into a substance, scattered light of frequency f±fR and f±2fR other than frequency f will appear in the scattered light. This phenomenon is called Raman effect. . Because it is produced by the energy exchange between the molecular motion of the substance and the lattice motion. When a substance absorbs energy, the number of vibrations of light becomes smaller, and the scattered light is called stokes line. Conversely, the scattered light that obtains energy from matter and increases the number of vibrations is called anti-Stokes line. Therefore, the deviation FR of the vibration number reflects the energy level and can show the value inherent in the substance. The fiber made by using this nonlinear medium is called Raman Fiber (RF: Raman Fiber). In order to confine the light in the small fiber core for long-distance propagation, the interaction effect between light and matter will appear, which can make the signal waveform undistorted and realize long-distance transmission. When the input light is enhanced, coherent induced scattered light will be obtained. Raman fiber lasers are used for sensing Raman scattered light, which can be used as power sources for spectroscopic measurement and fiber dispersion testing. In addition, induced Raman scattering, in the long-distance communication of optical fiber, is under study as an optical amplifier.
Eccentric fiber
The core of the standard optical fiber is set in the center of the cladding, and the cross-sectional shape of the core and the cladding is concentric. However, due to different uses, there are also cases where the core position, core shape, and cladding shape are made into different states or the cladding is perforated to form a special-shaped structure. Compared with standard optical fibers, these optical fibers are called special-shaped optical fibers. Excentric Core Fiber (Excentric Core Fiber), it is a kind of special-shaped fiber. The core is set off-center and close to the eccentric position of the outer line of the cladding. Since the core is close to the surface, part of the light field will spread over the cladding (called this as the Evanescent Wave). Using this phenomenon, the presence or absence of attached substances and changes in refractive index can be detected. Eccentric fiber (ECF) is mainly used as an optical fiber sensor for detecting substances. Combined with the optical time domain reflectometer (OTDR) test method, it can also be used as a distribution sensor.
Luminous fiber
Use optical fiber made of fluorescent material. It is a part of the fluorescence generated when it is irradiated by light waves such as radiation, ultraviolet rays, etc., which can be transmitted through the optical fiber by closing the optical fiber. Luminescent Fiber (Luminescent Fiber) can be used to detect radiation and ultraviolet rays, as well as wavelength conversion, or as a temperature sensor, chemical sensor. It is also called Scintillation Fiber in the detection of radiation. From the perspective of fluorescent materials and doping, plastic optical fibers are being developed.
Multi-core fiber
A normal optical fiber is composed of a core region and a cladding region surrounding it. However, Multi Core Fiber has multiple cores in a common cladding area. Due to the closeness of the cores to each other, there are two functions. One is that the core spacing is large, that is, there is no optical coupling structure. This kind of optical fiber can increase the integration density per unit area of the transmission line. In optical communications, ribbon cables with multiple cores can be made, while in non-communication fields, as optical fiber image bundles, there are thousands of cores made. The second is to make the distance between the cores close, which can produce light wave coupling. Using this principle, a dual-core sensor or optical circuit device is being developed.
Hollow fiber
The optical fiber is made into a hollow core to form a cylindrical space. The optical fiber used for light transmission is called a hollow fiber (Hollow Fiber). Hollow optical fiber is mainly used for energy transmission, and can be used for X-ray, ultraviolet and far infrared light energy transmission. There are two types of hollow fiber structures: one is to make glass into a cylindrical shape, and the core and cladding principles are the same as those of the step type. Use the total reflection of light between the air and the glass to spread. Since most of the light can be transmitted in the air without loss, it has the function of spreading a certain distance. The second is to make the reflectance of the inner surface of the cylinder close to 1, to reduce reflection loss. In order to improve the reflectivity, a dielectric is set in the lamp to reduce the loss in the working wavelength range. For example, the loss of wavelength 10.6pm can reach several dB/m.
Polymer
According to the material, there are inorganic optical fiber and polymer optical fiber. The former is widely used in industry. Inorganic optical fiber materials are divided into two types: single-component and multi-component. The single component is quartz, and the main raw materials are silicon tetrachloride, phosphorus oxychloride and boron tribromide. Its purity requires that the impurity content of transition metal ions such as copper, iron, cobalt, nickel, manganese, chromium, and vanadium is less than 10ppb. In addition, the OH-ion requirement is less than 10ppb. Quartz fiber has been widely used. There are many multi-component raw materials, mainly silicon dioxide, boron trioxide, sodium nitrate, thallium oxide and so on. This material is not yet popular. The polymer optical fiber is an optical fiber made of transparent polymer, which is composed of a fiber core material and a sheath material. The core material is a fiber made of high-purity and high-transmitting polymethyl methacrylate or polystyrene, and the outer layer is a fluorine-containing polymer or organic silicon polymer.
The optical loss of polymer optical fiber is relatively high. In 1982, Japan Telegraph and Telegraph Company used deuterated methyl methacrylate polymer filament as the core material, and the optical loss rate was reduced to 20dB/km. However, the characteristic of polymer optical fiber is that it can make large size, large numerical aperture optical fiber, high coupling efficiency of light source, good flexibility, slight bending does not affect the light guiding ability, easy arrangement and bonding, easy to use, and low cost. However, the optical loss is large, and it can only be used in short distances. Optical fiber with optical loss of 10～100dB/km can transmit hundreds of meters
Polarization Maintaining Fiber
Polarization maintaining fiber: Polarization maintaining fiber transmits linearly polarized light, which is widely used in various fields of national economy such as aerospace, aviation, navigation, industrial manufacturing technology and communications. In the interferometric fiber sensor based on optical coherent detection, the use of polarization-maintaining fiber can ensure that the linear polarization direction remains unchanged, improve the coherent signal-to-noise ratio, and achieve high-precision measurement of physical quantities. As a special type of optical fiber, polarization maintaining fiber is mainly used in sensors such as fiber optic gyroscopes, fiber optic hydrophones, and fiber optic communication systems such as DWDM and EDFA. Because fiber optic gyroscopes and fiber optic hydrophones can be used in military inertial navigation and sonar, they are high-tech products, and polarization-maintaining fiber is its core component, so polarization-maintaining fiber has been included in the list of embargoes against China by western developed countries. In the drawing process of polarization-maintaining fiber, due to structural defects generated inside the fiber, the polarization-maintaining performance will decrease. That is, when linearly polarized light is transmitted along a characteristic axis of the fiber, part of the optical signal will be coupled into another The characteristic axis ultimately results in a decrease in the polarization extinction ratio of the output polarized light signal. This defect affects the birefringence effect in the fiber. In a polarization maintaining fiber, the stronger the birefringence effect and the shorter the wavelength, the better to maintain the polarization state of the transmitted light.
Application and future development direction of polarization maintaining fiber
Polarization-maintaining optical fiber will have greater market demand in the next few years. With the rapid development of new technologies in the world and the continuous development of new products, polarization-maintaining optical fibers will develop in the following directions:
(1) Use the new technology of photonic crystal fiber to manufacture a new type of high-performance polarization-maintaining fiber;
(2) Develop temperature-adaptive polarization-maintaining optical fiber to meet the requirements of aerospace and other fields;
(3) Develop various rare earth-doped polarization-maintaining fibers to meet the needs of optical amplifiers and other device applications;
(4) Develop fluoride polarization-maintaining fiber to promote the development of fiber optic interference technology in the field of infrared astronomy technology;
(5) Low-attenuation polarization-maintaining fiber: With the continuous improvement of single-mode fiber technology, loss, material dispersion and waveguide dispersion are no longer the main factors affecting fiber communication, and the polarization mode dispersion (PMD) of single-mode fiber has gradually become a limitation The most serious bottleneck of optical fiber communication quality is particularly prominent in high-speed optical fiber communication systems of 10 Gbit/s and above.
(6) Use Kerr effect and Faraday rotation effect to manufacture polarized light devices.
In addition, according to the different fiber heads, there are: C-Lens. G-Lens. Green lens
Folding common optical fiber specifications
Single mode: 8/125μm, 9/125μm, 10/125μm
Multimode: 50/125μm, European standard
62.5/125μm, American standard
Industrial, medical and low-speed networks: 100/140μm, 200/230μm
Plastic: 98/1000μm, used for automobile control
