What are the types of optical fibers
single mode fiber
Single-mode fiber 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 TV and optical communication. Since the fiber core is very thin (about 10 μm) and the refractive index is distributed in a step shape, 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 the transmission frequency band is wider than that of mode fibers, but also the addition and cancellation of SMF's material dispersion and structural dispersion, and its synthetic characteristics just form the characteristics of zero dispersion, which makes the transmission frequency band wider. In SMF, there are many types due to differences in dopants and manufacturing methods. Depressed cladding fiber (DePr-essed Clad Fiber), its cladding forms a double structure, the cladding adjacent to the core has a lower refractive index than the outer cladding.
multimode fiber
Multimode fiber The fiber in which the optical fiber is multi-mode according to the working wavelength and its propagation possible mode is called a multimode fiber (MMF: MUlti ModeFiber). The fiber core diameter is 50 μm, and since the transmission mode can reach several hundred, the transmission bandwidth is mainly dominated by the mode dispersion compared with SMF. Historically used for short-distance transmission in cable TV and communication systems. Since the appearance of SMF optical fiber, it seems to form a historical product. But in fact, because MMF has a larger core diameter than SMF and is easy 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 communication. When MMF is classified according to the distribution of refractive index, there are two types: gradient (GI) type and step (SI) type. The refractive index of the GI type is the highest at the center of the fiber core, and gradually decreases along the cladding. Due to the time difference of each optical path during the reflection and progress of SI-type light waves in the optical fiber, the emitted light waves are distorted and the color shock is large. As a result, the transmission bandwidth is narrowed, and SI-type MMF is less used at present.
dispersion shifted fiber
When the operating wavelength of the 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 silica optical fibers, the transmission loss of the 1.55pm section is the smallest (about 0.2dB/km) from the raw material. Since the erbium-doped fiber amplifier (EDFA) that has been practical now works in the 1.55pm band, if zero dispersion can be achieved in this band, it will be more conducive to the long-distance transmission of the 1.55pm band. Therefore, by cleverly using the synthetic and offset characteristics of the dispersion of the quartz material in the fiber material and the dispersion of the fiber core structure, the original zero dispersion in the 1.3pm section can be shifted to the 1.55pm section 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 the long-distance transmission of optical communication, it is important, but not the only one, to have zero fiber dispersion. Other performances include low loss, easy splicing, small changes in characteristics during cabling or work (including bending, stretching and environmental changes). DSF takes these factors into consideration in the design.
Dispersion Flattened Fiber
Dispersion-shifted fiber (DSF) is a single-mode fiber designed to have zero dispersion in the 1.55pm band. However, Dispersion Flattened Fiber (DFF: Dispersion Flattened Fiber) refers to a fiber that can achieve very low dispersion in a wide band from 1.3pm to 1.55pm, and almost achieves zero dispersion. It is called DFF. Since the DFF needs to reduce the dispersion in the range of 1.3pm to 1.55pm. A complex design of the refractive index profile of the fiber is required. However, this fiber is very suitable for wavelength division multiplexing (WDM) lines. Because the process of DFF optical fiber is more complicated, the cost is more expensive. In the future, as production increases, prices will also decrease.
Dispersion Compensating Fiber
As for the trunk line system using single-mode optical fiber, most of them are formed by using optical fiber with zero dispersion in the 1.3pm band. However, the 1.55pm with the smallest loss now, due to the practical use of EDFA, it will be very beneficial if the 1.55pm wavelength can also work on the 1.3pm zero-dispersion optical fiber. 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 the dispersion is inserted in this optical fiber line, the dispersion of the entire optical line can be made zero. The optical fiber used for this purpose is called dispersion compensating fiber (DCF: DisPersion Compensation 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 line.
polarization maintaining fiber
The light waves propagating in the optical fiber have the properties of electromagnetic waves, so besides the basic single mode of light waves, there are actually two orthogonal modes of electromagnetic field (TE, TM) distribution. Usually, since the cross-sectional structure of the fiber is circularly symmetric, the propagation constants of the two polarization modes are equal, and the two polarized lights do not interfere with each other. The combination factor between two polarization modes is irregularly distributed on the optical axis. The dispersion caused by this change in polarized light is called polarization mode dispersion (PMD). For cable television, which mainly distributes images, the impact is not too great, but for some services that have special requirements for ultra-broadband in the future, such as:
① When heterodyne detection is used in coherent communication, when the polarization of light waves is required to be more stable;
② When the input and output characteristics of optical machinery and other equipment are required to be related to polarization;
③When making polarization maintaining optical couplers and polarizers or depolarizers, etc.;
④ Manufacture of optical fiber sensors using light interference, etc.,
In the case where the polarization is required to be kept constant, the fiber that has been improved to keep the polarization state constant is called a polarization maintaining fiber (PMF: Polarization Maintaining fiber), or it is called a fixed polarization fiber.
birefringent fiber
A birefringent fiber refers to a fiber that can transmit two intrinsic polarization modes that are orthogonal to each other in a single-mode fiber. The variation of the refractive index with the direction of polarization is called birefringence. It is also called PANDA fiber, that is, Polarization-maintai-ning AND Absorption-reducing fiber (Polarization-maintai-ning AND Absorption-reducing fiber). It is located on the two sides of the fiber core in the transverse direction, and the glass part with a large thermal expansion coefficient and a circular cross section is set. During fiber drawing at high temperatures, these portions shrink, resulting in tension in the y-direction of the core and compressive stress in the x-direction. As a result, the photoelastic effect appears in the fiber material, and the refractive index differs in the X direction and the Y direction. According to this principle, the effect of maintaining constant polarization is achieved. [3]
Anti-bad environment optical fiber
The usual working environment temperature of optical fiber for communication 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 work at lower or higher temperatures and in harsh environments that are subjected to high pressure or external forces and exposed to radiation are called Hard Condition Resistant Fibers. Generally, in order to mechanically protect the surface of the optical fiber, an additional layer of plastic is coated.
Hermetic Coated Fiber
In order to maintain the long-term stability of the mechanical strength and loss of the optical fiber, inorganic materials such as silicon carbide (SiC), titanium carbide (TiC), and carbon (C) are coated on the glass surface to prevent water and hydrogen from the outside. The manufactured optical fiber (HCF Hermetically Coated Fiber) was diffused. At present, it is common to use high-speed accumulation of carbon layers in the production process of chemical vapor deposition (CVD) to achieve sufficient sealing effect. This carbon-coated fiber (CCF) can effectively cut off the intrusion of the fiber and external hydrogen molecules. It is reported that it can be maintained 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 applied to systems that require high reliability in harsh environments, such as submarine optical cables.
carbon coated fiber
A fiber coated with a carbon film on the surface of a silica fiber is called a carbon coated fiber (CCF: Carbon Coated Fiber). The mechanism is to use the dense film layer of carbon to isolate the surface of the optical fiber from the outside world, so as to improve the mechanical fatigue loss of the optical fiber and the increase in the loss of hydrogen molecules. CCF is a type of hermetic coated fiber (HCF).
Metal Coated Optical Fiber
Metal coated fiber (Metal Coated Fiber) is an optical fiber coated with metal layers such as Ni, Cu, Al, etc. on the surface of the optical fiber. There are also those covered with plastic on the outside of the metal layer, the purpose is to improve heat resistance and allow for electrification and welding. It is one of the anti-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. Due to the large difference in expansion coefficient between glass and metal, this method will increase micro-bending loss, and the practical rate is not high. Recently, the performance has been greatly improved due to the success of low-loss electroless coating on the surface of glass optical fiber.
Eccentric Fiber
The core of the standard optical fiber is set at 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 position of the core, the shape of the core, and the shape of the cladding 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. Eccentric fiber (Excentric Core Fiber), which is a kind of special-shaped fiber. Its core is set at an eccentric position off-center and close to the outer line of the cladding. Since the core is close to the surface, part of the optical field will overflow the cladding and propagate (this is called Evanescent Wave). Utilizing this phenomenon, it is possible to detect the presence or absence of attached substances and changes in the refractive index. Eccentric fiber optics (ECF) are mainly used as fiber optic sensors for detecting substances. Combined with the test method of optical time domain reflectometer (OTDR), it can also be used as a distribution sensor.
Luminous Fiber
Optical fibers made of fluorescent substances are used. It is a part of the fluorescence generated when it is irradiated by light waves such as radiation and ultraviolet rays, and it can be transmitted through the optical fiber closure. Luminescent Fiber can be used to detect radiation and ultraviolet rays, and perform wavelength conversion, or as a temperature sensor, chemical sensor. It is also called Scintillation Fiber in the detection of radiation. Luminescent optical fibers are developing plastic optical fibers from the perspective of fluorescent materials and doping.
multi-core fiber
A typical optical fiber consists of a core region and a cladding region surrounding it. However, a multi-core fiber (Multi Core Fiber) has multiple cores in a common cladding region. Due to the mutual proximity of the cores, two functions are possible. One is that the distance between the fiber cores is large, that is, there is no structure for optocouplers. This optical fiber can increase the integration density per unit area of the transmission line. In optical communication, ribbon cables with multiple cores can be made, while in the non-communication field, tens of thousands of cores are used as optical fiber image bundles. The second is to make the distance between the fiber cores close to produce light wave coupling. Using this principle, dual-core sensors or optical circuit devices are being developed.
hollow fiber
The optical fiber is made hollow to form a cylindrical space, and the optical fiber used for light transmission is called a hollow fiber (Hollow Fiber). Hollow-core optical fibers are 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 the glass into a cylindrical shape, and the principle of the core and cladding is the same as that of the step type. Use the total reflection of light between air and glass to propagate. Because most of the light can be transmitted in the air without loss, it has a certain distance propagation function.
The second is to make the reflectivity of the inner surface of the cylinder close to 1 to reduce the reflection loss. In order to improve the reflectivity, there is a dielectric in the Jane to reduce the loss in the working wavelength band. For example, a loss of several dB/m at a wavelength of 10.6pm can be achieved.
