Types and Applications of Optical Fiber

Ⅰ. Common interfaces of optical fiber

Optical fiber interfaces can be divided into: LC, FC, SC, ST, D4, DIN, MU, MT-R and other types according to the structure of the connector. Common interfaces are ST, LC, FC and SC.

1. Stab & Twisst

It is made of metal and has a snap-in interface, which is often used in optical fiber distribution frames.

2. Lucent Connector

It is made of plastic and is used to connect to the SFP optical module, and the interface can be stuck on the optical module. "LC" connectors are similar in shape to SC connectors, but smaller than SC connectors. "FC" joints are metal joints, generally used on the ODF side. Metal connectors can be plugged in more times than plastic. In the label indicating the pigtail connector, we can often see "FC/PC", "SC/PC" and so on.

3. Ferrule Connector

It was first used in storage area networks. The shell is made of metal, and the interface has threads, which can be fixed well with the optical module.

4. Square Connector

It is made of plastic and adopts push-pull connection, and the interface can be stuck on the optical module. It is commonly used in switches. "SC" fittings are standard square fittings. It adopts engineering plastics, which have the advantages of high temperature resistance and not easy to oxidize. The optical interface of the transmission equipment generally uses SC connectors.

Ⅱ. Types of optical fiber

1. Single-Mode Fiber (SMF)

When the geometric size of the fiber (mainly the core diameter) can be close to the wavelength of light, for example, the core diameter d1 is in the range of 5-10µm, the fiber only allows one mode (fundamental mode HE11) to propagate in it, and the rest of the higher-order modes are all cut off , such fibers are called single-mode fibers.

Since it has only one mode of propagation and avoids the problem of mode dispersion, single-mode fiber has an extremely wide bandwidth and is especially suitable for high-capacity fiber optic communications. Therefore, to realize single-mode transmission, we must make the parameters of the fiber meet certain conditions. It is calculated by the formula that, for the fiber with NA=0.12 to achieve single-mode transmission above λ=1.3µm, the radius of the fiber core should be ≤4.2µm, that is, the core diameter d1≤8.4µm. Since the core diameter of the single-mode optical fiber is very small, people put forward more stringent requirements for its manufacturing process.

2. Multimode Fiber (MMF)

When the geometric size of the fiber (mainly the core diameter d1) is much larger than the wavelength of light (about 1µm), there will be dozens or even hundreds of propagation modes in the fiber. Different propagation modes have different propagation speeds and phases, resulting in time delay and widening of optical pulses after long-distance transmission. This phenomenon is called modal dispersion (also known as intermodal dispersion) of optical fiber.

Modal dispersion will narrow the bandwidth of multimode fiber and reduce its transmission capacity, so multimode fiber is only suitable for small-capacity optical fiber communication. The refractive index distribution of multimode fiber is mostly parabolic distribution, that is, graded refractive index distribution. Its core diameter is about 50µm.

3. Special Optical Fiber

This is a general term for all kinds of optical fibers with special functions other than conventional communication optical fibers, including active optical fibers, energy transmission optical fibers, polarization maintaining optical fibers, etc. Special optical fibers have particularities in doping elements, working wavelengths, waveguide structures, optical properties, etc. The preparation process is complex and the technical requirements are high. It is more closely aligned with end user needs.

At present, the price of special optical fiber in China ranges from several thousand yuan to tens of thousands of yuan, and its downstream applications include aerospace, wind energy, fuel cells and other new industries. There is a large space for the localization of special optical fibers. Among them, the fiber optic gyroscope market has been occupied by domestic manufacturers, and the communication device market is still occupied by foreign manufacturers. It is predicted that the global special optical fiber market will reach $42.13 billion in 2025, with a CAGR of 18.1%. In 2024, the scale of my country's special optical fiber market will reach 18.74 billion yuan, with a CAGR of 13.9%.

4. Step-Index Fiber

The refractive index of this fiber is the same at the center and extremes of the core. The propagation of light is a bit uncontrolled, but all light eventually travels at the same speed.

5. Glass Optical Fiber

Glass optical fiber is a new type of optical fiber made of pure glass material. It is a glass optical fiber that transmits optical signals and is mainly used to transmit optical signals over long distances. Glass optical fiber has the advantages of low loss and broadband, and can be widely used in cable television and communication systems.

6. Graded-Index Fiber

In terms of refractive index, we see a large change in extreme refractive index from the center of the core to near the cladding. In the center, light travels the slowest because the index of refraction is highest here compared to other regions. However, fiber prices are slightly higher for graded variants than for ladder variants.

7. Plastic Optical Fiber

This is an acrylic or polycarbonate core with a resin cladding, usually made of silicone. They do have the ability to withstand environments with a lot of vibration and are more flexible and bendable than glass fiber optics. However, it is not suitable for harsh environments and will degrade over time. It is mainly used in decorative and lighting appliances. It is also used in medical devices that need to transmit narrow spectrum light.

8. DisPersion Compe-Nsation Fiber (DCF)

For trunk systems using single-mode optical fibers, most of them are formed using optical fibers with zero dispersion in the 1.3pm band. However, the smallest loss is 1.55pm. Due to the practicality of EDFA, it will be very beneficial if the 1.55pm wavelength can also work on the 1.3pm zero dispersion 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 fiber used for this purpose is called DCF. 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.

9. Dispersion Shifted Fiber (DSF)

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. Because the already practical erbium-doped fiber amplifier (EDFA) works in the 1.55pm band. If zero dispersion can also be achieved in this band, it will be more conducive to the long-distance transmission of the 1.55Pm band. Therefore, people cleverly use the synthetic and offset characteristics of the dispersion of the quartz material in the optical fiber material and the dispersion of the fiber core structure, so that the original zero dispersion at 1.3pm can be shifted to 1.55pm to form zero dispersion. Therefore, it is named DSF. 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, little change in characteristics during cabling or work (including bending, stretching and environmental changes). DSF takes these factors into consideration in the design.

10. Dispersion Flattened Fiber (DFF)

Dispersion flattened fiber is a single-mode fiber designed to have zero dispersion in the 1.55pm band. Dispersion flattened fiber can achieve very low dispersion in a wide band from 1.3pm to 1.55pm, and almost achieve zero dispersion. Since the DFF needs to reduce the dispersion in the range of 1.3pm to 1.55pm, we need to carry out complex design on 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 optical fiber is more complicated, the cost is more expensive. In the future, as the output increases, its price will also decrease.

11. 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. It is set on both lateral sides of the fiber core with a glass part with a large thermal expansion coefficient and a circular cross-section. During the high temperature fiber drawing process, these parts shrink. As a result, tension occurs in the y-direction of the fiber core, and at the same time, compressive stress appears in the x-direction, resulting in a photoelastic effect in the fiber material, and a difference in the refractive index between the x-direction and the y-direction. According to this principle, it can achieve the effect of keeping the polarization constant.

12. Fluorine Doped Fiber

Fluorine-doped fiber is one of the typical products of silica fiber. Usually, as a communication optical fiber in the 1.3μm wave domain, the dopant for controlling the core is germanium dioxide (GeO2), and the cladding is made of SiO2. However, the core of the fluorine-connected optical fiber mostly uses SiO2, but the cladding is doped with fluorine. Because the Rayleigh scattering loss is the light scattering phenomenon caused by the change of the refractive index. Therefore, it is desirable to have less dopant that forms a refractive index variation factor. The main function of fluorine is to reduce the refractive index of SIO2. Therefore, it is often used for cladding doping.

13. 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. They are also coated with plastic on the outside of the metal layer, the purpose is to improve heat resistance and allow for electrical and soldering. It is one of the anti-environment optical fibers and can be used as a component of electronic circuits. Early products were made by coating molten metal in a wire 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 success of low-loss electroless coating on the surface of glass optical fiber has greatly improved the performance.

14. Compound Fiber

Compound fiber is made of SiO2 raw material, which is properly mixed with oxides such as sodium oxide (Na2O), boron oxide (B2O3), potassium oxide (K2O), etc. to make multi-component glass optical fiber. Its characteristic is 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. It is mainly used in fiber optic endoscopes in the medical business.

15. Excentric Core 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. These fibers are called shaped fibers relative to standard fibers. Eccentric fiber 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 outer surface, part of the optical field will overflow the cladding and propagate (this is called Evanescent Wave). Taking advantage of this phenomenon, we can detect the presence or absence of attached substances as well as changes in the refractive index. Eccentric Fiber Optics (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 distribution sensor.

16. Infrared Optical Fiber

As the working wavelength of the quartz series optical fiber developed in the field of optical communication, although it is used in a short transmission distance, it can only be used for 2μm. For this reason, the optical fiber developed is called infrared optical fiber to work in the longer infrared wavelength field in the future. Infrared optical fiber is mainly used for light energy transmission, such as temperature measurement, thermal image transmission, laser scalpel medical treatment, thermal energy processing and so on.

17. Carbon Coated Fiber

A fiber coated with a carbon film on the surface of a silica fiber is called a 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).

18. Silica Fiber

Silica optical fiber is an optical fiber with silica (SiO2) as 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 fiber has the characteristics of low consumption and broadband. Now it has been widely used in cable television and communication systems. The advantage of quartz glass optical fibers is low loss. When the light wavelength is 1.0-1.7μm (around 1.4μm), its loss is only 1dB/km. It is lowest at 1.55μm at only 0.2dB/km.

Ⅲ. Optical fiber splicing

Fiber splicing includes cold connection and thermal connection.

1. Optical fiber cold connection

The thing used for this kind of cold connection is called a fiber optic cold connection. The optical fiber cold splicer is used when two pigtails are connected. The main part inside it is a precise V-groove. After the two pigtails are pulled out, the two pigtails are connected by cold joints. It is easier and faster to operate, saving time than welding with a fusion splicer.

There are generally two forms of cold splicing: the first is the on-site quick connector of the end; the second is the cold splicing of the optical fiber butt. With the rapid development of FTTH fiber to the home, people's demand for optical fiber cold connectors has also greatly increased. Fiber optic quick connectors and fiber optic cold connectors will play an irreplaceable role in FTTH access. The on-site termination technology of optical fiber quick connector just solves this problem. It is convenient and quick without fusion splicing, and the connection cost is low, and it can truly realize access anytime and anywhere.

2. Optical fiber thermal connection

Optical fiber fusion splicing technology mainly uses a fiber fusion machine to connect optical fibers to optical fibers or optical fibers to pigtails, and fuse the bare fibers and optical fiber pigtails in the optical cable together to form a whole. Pigtails, on the other hand, have a single fiber tip. Generally, community monitoring adopts thermal fusion to connect optical fibers.

3. The difference between cold connection and thermal connection

Thermal fusion requires the use of fusion splicers and fiber cutters to connect two optical fibers, and it does not require other auxiliary materials. Its advantages are stable quality and small splice loss (about 0.03 to 0.05). Its disadvantages are that the equipment cost is too high, the power storage capacity of the equipment is limited, and the field operation love is limited.

Cold connection does not require much equipment, only a fiber cutter is required. But each contact requires a quick connector. Its advantage is that it is easy to operate and suitable for field work. Its disadvantage is that the loss is too large, about 0.1 to 0.2dB per point. At present, there are few domestic manufacturers that can directly produce cold joints, and the cost is high. There is no room for choice in terms of business and technical services. Secondly, the matching liquid used in cold joints is less used, the time is short, and aging problems, It takes the test of time.

Ⅳ. The application of optical fiber

Fiber optics are used in a wide range of applications, including the following types:

1. Cable TV transmission: use PIN receiving technology to realize the transmission of TV signals.

2. SDH/SONET: such as submarine optical cables between major cities and ocean bottoms. It is used for high-speed, long-distance data transmission.

3. Fiber channel: It is used for the transmission of various storage devices such as storage devices and databases, as well as for the developing cloud computing service system.

4. GBE: used for Ethernet, including the current fiber-to-the-home (FTTH), to the building (FTTB), to the community, etc., mainly our home and office networks.

5. Other special-purpose transmissions: such as applications in fighter planes and ships.