Optical fiber is an inorganic non-metallic material, divided into many types, some are new and some are not new
Optical fiber is the abbreviation of optical fiber, which is made of glass or plastic. The fiber can be used as a light transmission tool. The transmission principle is 'total reflection of light'. Former presidents of the Chinese University of Hong Kong, Kun Kao and George A. Hockham, first proposed the idea that optical fibers could be used for communication transmission, for which Kao won the 2009 Nobel Prize in Physics.
Basic introduction:
Optical fiber The tiny optical fiber is encapsulated in a plastic sheath, allowing it to bend without breaking. Typically, a 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 a receiving device at the other end of the optical fiber uses a photosensitive element to detect the pulses.
In daily life, because the transmission loss of light in optical fibers is much lower than the loss of electricity in wires, optical fibers are used for long-distance information transmission.
Often the terms optical fiber and optical cable are confused. Most optical fibers must be covered by several layers of protective structures before use. The covered cable is called an optical cable. The protective layer and insulation layer on the outer layer of the optical fiber can prevent damage to the optical fiber from the surrounding environment, such as water, fire, electric shock, etc. Optical cables are divided into: optical fiber, buffer layer and coating. Fiber optic is similar to coaxial cable, except without the mesh shield. At the center is a glass core through which light travels.
In multimode optical fiber, the core diameters are 50μm and 62.5μm, which are roughly equivalent to the thickness of a human hair. The diameter of the single-mode optical fiber core is 8μm~10μm. The core is surrounded by a glass envelope with a lower refractive index than the core to keep light within the core. On the outside is a thin plastic jacket to protect the envelope. Optical fibers are usually bundled into bundles 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 it needs an external protective layer.
Types of principles
Folded light and its characteristics
Optical fiber 1. Light is a kind of electromagnetic wave
The wavelength range of the visible light part is: 390 ~760nm (nanometer). The part greater than 760nm is infrared light, and the part less than 390nm is ultraviolet light. There are three types of optical fibers used: 850nm, 1310nm, and 1550nm.
2. Refraction, reflection and total reflection of light.
Because light propagates at different speeds in different materials, when light is emitted from one material to another, refraction and reflection will occur at the interface between the two materials. Furthermore, the angle at which light is refracted changes as the angle of incident light changes. 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. This is the total reflection of light. Different substances have different refractive angles for light of the same wavelength (that is, different substances have different refractive indexes), and the same substance also has different refractive angles for light of different wavelengths. Optical fiber communication is based on the above principles.
1. Optical fiber structure:
Bare optical fiber is generally divided into three layers: a central high refractive index glass core (the core diameter is generally 50 or 62.5 μm), and a middle low refractive index Silica glass cladding (generally 125μm in diameter), and the outermost is a reinforced resin coating.
2. Fiber numerical aperture:
Not all the light incident on the fiber end face can be transmitted by the fiber, only the incident light within a certain angle range can. This angle is called the numerical aperture of the optical fiber. A larger numerical aperture of the optical fiber is beneficial to the docking of optical fibers. Optical fibers produced by different manufacturers have different numerical apertures (AT&T CORNING).
3. Types of optical fibers:
There are many types of optical fibers, and the required functions and performances vary according to different uses. However, for optical fibers used in cable TV and communications, their design and manufacturing principles are basically the same, such as: ① small loss; ② certain bandwidth and small dispersion; ③ easy wiring; ④ easy to integrate; ⑤ high reliability; ⑥ manufacturing comparison Simple; ⑦ cheap, etc. The classification of optical fibers is mainly summarized from the working wavelength, refractive index distribution, transmission mode, raw materials and manufacturing methods. Examples of various classifications are as follows.
(1) Working wavelength: UV 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, gradient (GI) type fiber, others (such as triangular type, W type, concave type, etc.).
(3) Transmission mode: single-mode fiber (including polarization-maintaining fiber, 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 also be divided into inorganic materials (carbon, etc.), metal materials (copper, nickel, etc.) and plastics.
(5) Manufacturing methods: Pre-molding includes vapor axial deposition (VAD), chemical vapor deposition (CVD), etc., and drawing methods include Rod intube and double crucible methods, etc. .
Quartz Fiber
Silica Fiber uses silicon dioxide (SiO2) as the main raw material, and controls the core and cladding properties according to different doping amounts. Fiber with refractive index profile. Quartz (glass) series optical fibers have the characteristics of low consumption and broadband, and are now widely used in cable TV and communication systems.
The advantage of quartz glass optical fiber is low loss. When the light wavelength is 1.0~1.7μm (about 1.4μm), the loss is only 1dB/km, and the lowest at 1.55μm, only 0.2dB/km.
Fluorine-doped fiber
Fluorine Doped Fiber is one of the typical products of quartz fiber. Generally, in communication optical fibers in the 1.3 μm wave domain, the dopant controlling the core is germanium dioxide (GeO2), and the cladding is made of SiO2. However, most of the cores of fluorine-connected optical fibers use SiO2, while fluorine is incorporated into the cladding. Because Rayleigh scattering loss is a light scattering phenomenon caused by changes in the refractive index. Therefore, it is desirable that the dopant causing the refractive index change factor be as small as possible. The main function of fluorine is to reduce the refractive index of SIO2. Therefore, it is often used for doping of cladding.
Compared with optical fibers made of other raw materials, quartz optical fiber also has a broad light transmission spectrum 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 optical fiber
As a working wavelength of quartz series optical fiber developed in the field of optical communication, although it is used for shorter transmission distances, it can only be used for 2μm. For this reason, the optical fiber developed can work in the longer infrared wavelength field and is called infrared optical fiber. 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., but the penetration rate is still low.
Compound fiber
Compound fiber is made by mixing SiO2 raw materials appropriately, such as sodium oxide (Na2O), boron oxide (B2O3), potassium oxide (K2O), etc. Oxide is made into multi-component glass optical fiber, which is characterized by the fact that the softening point of multi-component glass is lower than that of quartz glass and the refractive index difference between the core and the cladding is very large. Fiber optic endoscopes mainly used in the medical business.
Fluoride fiber
Fluoride fiber Fluoride fiber is an optical fiber made of fluoride glass. This optical fiber raw material is also referred to as ZBLAN (that is, chloride glass raw materials such as ZrF2, BaF2, LaF3, AlF3, and NaF) are simplified into The abbreviation of ZBLAN mainly operates in the optical transmission business of 2~10μm wavelength. Due to the possibility of ultra-low loss fiber, ZBLAN is being developed for the feasibility of long-distance communication fiber. For example, its theoretical minimum loss is At 3μm wavelength, it can reach 10-2~10-3dB/km, while at 1.55μm, quartz fiber is between 0.15-0.16dB/Km. Currently, ZBLAN fiber can only be used at 2.4~2.7 due to difficulty in 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 Clad Fiber
Plastic Clad Fiber is made of high-purity quartz glass as the fiber core, and plastics such as silica gel with a slightly lower refractive index than quartz are used as the fiber core. Cladding step fiber. Compared with quartz optical fiber, it has the characteristics of high fiber core 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 networks (LAN) and short-range communications.
Plastic optical fiber
This is an optical fiber whose core and cladding are made of plastic (polymer). Early products were mainly used in decorative and light-guiding lighting and optical communications for short-distance optical bonding. The main raw materials are organic glass (PMMA), polystyrene (PS) and polycarbonate (PC). The loss is restricted by the inherent C-H bonding structure of plastic, and generally can reach tens of dB per km. In order to reduce losses, fluorosol series plastics are being developed and applied. Since the core diameter of plastic optical fiber is 1000 μm, which is 100 times larger than single-mode quartz fiber, it is simple to connect, easy to bend and easy to construct. In recent years, coupled with the progress of broadband, the development of multi-mode plastic optical fiber with gradient type (GI) refractive index has attracted social attention. Recently, it has been rapidly adopted in automobile internal LANs, and it may also be applied in home LANs in the future.
Single-mode fiber
Single-mode fiber refers to an optical fiber that can only transmit one propagation mode in the operating wavelength. It is usually referred to as single-mode fiber (SMF: Single Mode Fiber). Currently, it is the most widely used optical fiber in cable TV and optical communications. Since the core of the optical fiber is very thin (about 10 μm) and the refractive index is distributed in a step-like manner, when the normalized frequency V parameter is <2.4, theoretically, only single-mode transmission can be formed. In addition, SMF has no multi-mode dispersion. Not only does the transmission frequency band be wider than that of multi-mode fiber, but also the material dispersion and structural dispersion of SMF are additively offset, and its synthetic characteristics just form zero dispersion characteristics, making the transmission frequency band wider. There are many types of SMF due to differences in dopants and manufacturing methods. The cladding of DePr-essed Clad Fiber forms a double structure. The cladding adjacent to the core has a lower refractive index than the outer inverted cladding.
Multi-mode optical fiber
Multi-mode optical fiber is an optical fiber that has multiple propagation modes according to its working wavelength and its possible propagation modes is called multi-mode optical fiber (MMF: MUlti ModeFiber). The core diameter is 50 μm. Since the transmission modes can reach hundreds, the transmission bandwidth is mainly dominated by mode dispersion compared with SMF. Historically used for short-distance transmission in cable television and communications systems. Since the emergence of SMF optical 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 renewed attention in the field of short-distance communications. 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 highest at the core center and gradually decreases along the cladding. Due to the reflection and progression of SI-type light waves in the optical fiber, time differences occur in each light path, resulting in distortion of the emitted light wave and large color excitation. As a result, the transmission bandwidth becomes narrower. Currently, SI type MMF is rarely used.
Dispersion-shifted fiber
When the operating wavelength of single-mode fiber is 1.3Pm, the mode field diameter is about 9Pm. The transmission loss is about 0.3dB/km. At this time, the zero dispersion wavelength is exactly at 1.3pm. Among quartz optical fibers, the 1.55pm segment has the smallest transmission loss (about 0.2dB/km) in terms of raw materials. Since the now practical erbium-doped fiber amplifier (EDFA) operates 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 utilizing the synthetic cancellation characteristics of the quartz material dispersion in the optical fiber material and the core structure dispersion, the original zero dispersion in the 1.3pm segment can be shifted to the 1.55pm segment to form zero dispersion. Therefore, it is named dispersion shifted fiber (DSF: DispersionShifted Fiber). The method of increasing structural dispersion is mainly to improve the refractive index distribution performance of the fiber core. In long-distance transmission of optical communications, zero fiber dispersion is important, but not the only one.
Other properties include low loss, easy splicing, and little change in characteristics during cabling or operation (including the effects of bending, stretching, and environmental changes). DSF takes these factors into consideration comprehensively during design.
Dispersion flat fiber
Dispersion-shifted fiber (DSF) is a single-mode fiber designed with zero dispersion in the 1.55pm band. Dispersion Flattened Fiber (DFF: Dispersion Flattened Fiber) is a fiber that can achieve very low dispersion in a wide band from 1.3Pm to 1.55pm, almost zero dispersion. It is called DFF. Because DFF needs to reduce dispersion in the range of 1.3pm to 1.55pm. This requires complex design of the refractive index distribution of the optical fiber. However, this fiber is very suitable for wavelength division multiplexing (WDM) lines. Because the process of DFF fiber is more complicated and more expensive. As production increases in the future, prices will also decrease.
Dispersion compensation fiber
For trunk systems using single-mode fiber, most of them are composed of fibers with zero dispersion in the 1.3pm band. However, 1.55pm, which has the smallest loss now, will be very beneficial if the 1.55pm wavelength can also be operated on the 1.3pm zero-dispersion fiber due to the practical use of EDFA. Because, in the 1.3Pm zero-dispersion optical fiber, the dispersion in the 1.55Pm band is about 16ps/km/nm. If a section of optical fiber with the opposite sign of dispersion is inserted into this optical fiber line, the dispersion of the entire optical line can be zero. The optical fiber used for this purpose is called dispersion compensating fiber (DCF: DisPersion Compe-nsation Fiber). Compared with 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 optical fiber
Because the light waves propagating in the optical fiber have the properties of electromagnetic waves, in addition to the basic single mode of light waves, there are actually electromagnetic fields (TE, TM) distribution of two orthogonal modes. Usually, since the structure of the optical fiber cross-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 optical fiber is not completely circularly symmetrical. For example, if there is a curved part, two polarization modes will appear. 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 currently focuses on distributing images, the impact is not too great, but for some future ultra-wideband services that have special requirements, such as:
① Heterodyne detection is used in coherent communications, requiring light wave polarization When it is more stable;
② When the input and output characteristics of optical machines and other requirements are related to polarization;
③ When making polarization-maintaining optical couplers, polarizers or depolarizers, etc.;
④Producing optical fiber sensors that utilize light interference, etc.
Whenever the polarization wave is required to remain constant, an optical fiber that has been modified to keep the polarization state unchanged is called polarization maintaining Optical fiber (PMF: Polarization Maintaining fiber), also known as fixed polarization fiber.
Birefringent fiber
Birefringent fiber refers to a fiber that can transmit two mutually orthogonal intrinsic polarization modes in a single-mode fiber. The phenomenon that the refractive index varies with the direction of deflection is called birefringence. It is also called PANDA fiber, which is Polarization-maintai-ning AND Absorption-reducing fiber. It is a glass part with a large thermal expansion coefficient and a circular cross-section on both sides of the fiber core. During the high-temperature fiber drawing process, these parts shrink, resulting in tension in the y-direction of the core and compressive stress in the x-direction. This causes a photoelastic effect in the fiber material, causing a difference in refractive index between the X and y directions. According to this principle, the effect of maintaining constant polarization is achieved.
Hard-environment-resistant optical fiber
The normal working environment temperature of communication optical fiber can be between -40 and +60°C, and the design is also based on the premise that it will not be exposed to a large amount of radiation. .
In contrast, optical fibers that can operate at lower or higher temperatures and in harsh environments where they are subjected to high pressure or external force or exposed to radiation are called Hard Condition Resistant Fibers. Generally, in order to mechanically protect the optical fiber surface, an extra layer of plastic is coated. However, as the temperature rises, the protective function of plastic decreases, resulting in restrictions on the use temperature. If you use heat-resistant plastics, such as polytetrafluoroethylene (Teflon) and other resins, you can work in a 300°C environment. There are also quartz glass surfaces coated with metals such as nickel (Ni) and aluminum (Al). This kind of optical fiber is called Heat Resistant Fiber. In addition, when optical fiber is illuminated by radiation, optical loss increases. This is because when quartz glass is exposed to radiation, structural defects (also called color centers) will appear in the glass, and the loss increases especially at wavelengths of 0.4 to 0.7pm. The preventive method is to use quartz glass doped with OH or F, which can suppress the loss defects caused by radiation. This type of optical fiber is called Radiation Resistant Fiber and is mostly used in fiber optic mirrors for monitoring nuclear power plants.
Sealed coated optical fiber
In order to maintain the mechanical strength of the optical fiber and the long-term stability of the loss, the glass surface is coated with silicon carbide (SiC), titanium carbide (TiC), carbon (C) and other inorganic materials are used to prevent the diffusion of water and hydrogen from the outside (HCF Hermetically Coated Fiber). At present, it is common to use high-speed deposition of carbon layers during the chemical vapor deposition (CVD) production process to achieve a sufficient sealing effect. This kind of carbon-coated fiber (CCF) can effectively cut off the intrusion of hydrogen molecules from the fiber and the outside. It is reported that it can last for 20 years in a hydrogen environment at room temperature without increasing loss. Of course, it prevents moisture intrusion and delays the fatigue process of mechanical strength, and its fatigue coefficient (Fatigue Parameter) can reach more than 200. Therefore, HCF is used in systems that require high reliability in harsh environments, such as submarine optical cables.
Carbon Coated Fiber
An optical fiber that is coated with a carbon film on the surface of a quartz fiber is called a carbon-coated fiber (CCF: Carbon CoatedFiber). The mechanism is to use a dense film layer of carbon to isolate the optical fiber surface from the outside world to improve the mechanical fatigue loss of the optical fiber and the increase in the loss of hydrogen molecules. CCF is a type of hermetically coated fiber (HCF).
Metal-coated optical fiber
Metal-coated optical fiber (Metal Coated Fiber) is an optical fiber coated with metal layers such as Ni, Cu, and Al on the surface of the optical fiber. There are also those that are coated with plastic outside the metal layer in order to improve heat resistance and allow for electrification and welding. It is one of the optical fibers that are resistant to harsh environments and can also be used as a component of electronic circuits. Early products were made by coating molten metal during the drawing process. Since the expansion coefficients of this method are too different between glass and metal, which will increase the micro-bending loss, the practical application rate is not high. Recently, due to the success of low-loss electrolytic coating methods on the surface of glass optical fibers, performance has been greatly improved.
Rare earth-doped optical fiber
In the core of the optical fiber, the optical fiber is doped with rare earth elements such as Er (Er), Nd (Nd), and Pr (Pr). In 1985, Payne and others from the University of Southampton in the UK first discovered that rare earth element-doped optical fiber (Rare Earth DoPed Fiber) has the phenomenon of laser oscillation and light amplification. As a result, the veil of optical amplification such as bait has been unveiled. The now practical 1.55pm EDFA uses bait-doped single-mode fiber and uses 1.47pm laser for excitation to obtain 1.55pm optical signal amplification. Additionally, doped fluoride fiber amplifiers (PDFA) are under development.
Raman fiber
The Raman effect means that when monochromatic light of frequency f is emitted into a substance, f±fR other than frequency f will appear in the scattered light. , scattered light with frequencies such as f±2fR, this phenomenon is called the Raman effect. Because it is produced by the energy exchange between the molecular motion and lattice motion of matter.
When a substance absorbs energy, the vibration number of light becomes smaller, and the scattered light is called a Stokes line. On the contrary, when energy is obtained from matter and the scattered light becomes larger in vibration number, it is called anti-Stokes line. Therefore, the deviation FR of the vibration number reflects the energy level and can show the inherent value in the substance. Optical fibers made from this nonlinear medium are called Raman fibers (RF: Raman Fiber). In order to confine light in a small fiber core and propagate it over long distances, the interaction effect between light and matter will occur, which can prevent the signal waveform from being distorted and achieve long-distance transmission. When the input light is enhanced, coherent induced scattered light is obtained. Equipment that uses induced Raman scattered light includes Raman fiber lasers, which can be used as power supplies for spectroscopic measurement and fiber dispersion testing. In addition, induced Raman scattering is being studied for use as an optical amplifier in long-distance optical fiber communications.
Eccentric optical fiber
The core of standard optical fiber is set in the center of the cladding, and the cross-sectional shapes of the core and cladding are concentric circles. However, due to different uses, the core position, core shape, and cladding shape are also 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 is a type of special-shaped optical fiber. Its core is set at an eccentric position off-center and close to the outer line of the cladding. Since the fiber core is close to the surface, part of the light field will overflow the cladding and propagate (this is called the Evanescent Wave). Using this phenomenon, the presence or absence of attached substances and changes in refractive index can be detected. Eccentric optical fibers (ECF) are mainly used as fiber optic sensors for detecting substances. Combined with the optical time domain reflectometer (OTDR) test method, it can also be used as a distributed sensor.
Light-emitting optical fiber
Uses optical fiber made of fluorescent substances. It is an optical fiber that produces part of the fluorescence when it is irradiated by radiation, ultraviolet and other light waves, and can be transmitted through the optical fiber closure. Luminescent fiber can be used to detect radiation and ultraviolet rays, perform wavelength conversion, or be used as a temperature sensor or chemical sensor. It is also called Scintillation Fiber in the detection of radiation. Luminescent optical fiber Plastic optical fiber is being developed from the perspective of fluorescent materials and doping.
Multi-core optical fiber
Usually an optical fiber is composed of a core area and a cladding area surrounding it. But multi-core fiber (Multi Core Fiber) has multiple cores in the same cladding area. Due to the proximity of the cores to each other, two functions are possible. One is that the fiber core spacing is large, that is, the structure does not produce optical coupling. This kind of optical fiber can increase the integration density per unit area of ??the transmission line. In optical communications, ribbon cables with multiple fiber cores can be made. In non-communication fields, as optical fiber image transmission bundles, tens of thousands of fiber cores can be made. The second is to bring the distance between the fiber cores closer, which can produce light wave coupling. Dual-core sensors or optical circuit devices are being developed using this principle.
Hollow-core fiber
The fiber that is made hollow to form a cylindrical space for light transmission is called a hollow fiber. Hollow-core optical fiber is mainly used for energy transmission and can transmit X-ray, ultraviolet and far-infrared light energy. There are two types of hollow fiber structures: First, the glass is made into a cylindrical shape, and the core and cladding principles are the same as the step type. Utilizes the total reflection of light between air and glass. Because most of the light can propagate in the air without loss, it has the propagation function of a certain distance. The second is to make the reflectivity of the inner surface of the cylinder close to 1 to reduce reflection loss. In order to improve the reflectivity, a dielectric is installed inside the module to reduce the loss in the working wavelength range. For example, the loss of several dB/m can be achieved at a wavelength of 10.6pm.
Polymer optical fiber
According to material, there are inorganic optical fiber and polymer optical fiber. Currently, the former is widely used in industry. Inorganic optical fiber materials are divided into two categories: 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 content of transition metal ion impurities such as copper, iron, cobalt, nickel, manganese, chromium, and vanadium be less than 10 ppb. In addition, OH- ions are required to be 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, etc. This material is not yet widely available. Polymer optical fiber is an optical fiber made of transparent polymer, consisting of fiber core material and sheath material. The core material is fiber made from polymethyl methacrylate or polystyrene with high purity and high transmittance, and the outer layer is fluoropolymer or silicone polymer.
Polymer optical fiber has high optical loss. In 1982, Nippon Telegraph and Telegraph Company used deuterated methyl methacrylate polymerization as core material, and the optical loss rate was reduced to 20dB/km. However, the characteristics of polymer optical fiber are that it can produce optical fiber with large size and large numerical aperture, high light source coupling efficiency, good flexibility, micro-bending does not affect the light-guiding ability, easy arrangement and bonding, easy use, and low cost. However, the optical loss is large and it can only be used in short distances. Optical fibers with optical losses between 10 and 100dB/km can transmit hundreds of meters.
Polarization-maintaining fiber
Polarization-maintaining fiber: Polarization-maintaining fiber transmits polarized light and is widely used in various fields of the national economy such as aerospace, aviation, navigation, industrial manufacturing technology, and communications. In interferometric optical fiber sensors based on optical coherent detection, the use of polarization-maintaining optical fibers can ensure that the linear polarization direction remains unchanged and improve the coherent signal-to-noise ratio to achieve high-precision measurement of physical quantities. As a special optical fiber, polarization-maintaining fiber is mainly used in sensors such as fiber optic gyroscopes and fiber optic hydrophones, and fiber optic communication systems such as DWDM and EDFA. Since fiber optic gyroscopes and fiber optic hydrophones can be used for military inertial navigation and sonar, they are high-tech products, and polarization-maintaining optical fiber is their core component. Therefore, polarization-maintaining optical fiber has been included in the list of embargoes against China by Western developed countries. During the drawing process of polarization-maintaining optical fiber, the structural defects generated inside the optical fiber will cause the degradation of polarization-maintaining performance. That is, when linearly polarized light is transmitted along a characteristic axis of the optical fiber, part of the optical signal will be coupled into another perpendicular to it. Characteristic axis, ultimately resulting in a decrease in the polarization extinction ratio of the outgoing polarized light signal. This defect affects the birefringence effect in the fiber. In polarization-maintaining fiber, the stronger the birefringence effect and the shorter the wavelength, the better it is to maintain the polarization state of the transmitted light.
Applications and future development directions of polarization-maintaining optical fibers
Polarization-maintaining optical fibers 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 new photonic crystal fiber technology to manufacture new high-performance polarization-maintaining fibers Optical fiber;
(2) Develop temperature-adaptive polarization-maintaining optical fiber to adapt to the environmental requirements of aerospace and other fields;
(3) Develop various rare-earth-doped polarization-maintaining optical fiber, Meet the needs of optical amplifier and other device applications;
(4) Develop fluoride polarization-maintaining optical fiber to promote the development of fiber optical interference technology in the field of infrared astronomy technology;
(5) Low Attenuated polarization-maintaining optical fiber: With the continuous improvement of single-mode optical fiber technology, loss, material dispersion and waveguide dispersion are no longer the main factors affecting optical fiber communication. The polarization mode dispersion (PMD) of single-mode optical fiber has gradually become a factor limiting the quality of optical fiber communication. The most serious bottleneck is particularly prominent in high-speed optical fiber communication systems of 10 Gbit/s and above.
(6) Use Kerr effect and Faraday optical rotation effect to manufacture polarized light devices.
In addition, depending on the fiber head, there are: C-Lens. G-Lens. Green lens