1. What is the structure of the optical fiber?
2. The composition of the optical cable
3. What is the working wavelength of the optical fiber?
4. Minimum dispersion wavelength and minimum loss wavelength
5. What is the dispersion of optical fiber?
6. What is the cause of fiber attenuation?
7. What is the bandwidth of optical fiber related to?
8. What is insertion loss?
9. What is return loss?
10. What is the core size of the optical fiber?
11. What is the mode field diameter MFD
12. What is the numerical aperture NA?
13. What is the cutoff wavelength (Cutoff Wavelength)
14. What is Zero Dispersion Wavelength (Zero Dispersion Wavelength)
15. What is Dispersion Shifted Fiber (DSF)
16. What is non-zero dispersion fiber (NZDF)?
17. How to classify optical fiber
18. What are the main optical fibers currently used for transmission network construction?
19. What is single-mode fiber and multi-mode fiber?
20. OS1, OS2, OM1, OM2, OM3, OM4, OM5
Basic knowledge of fiber optic cable
1. What is the structure of the optical fiber?
The bare fiber of optical fiber is generally divided into three layers: core, cladding and coating.
Basic knowledge of fiber optic cable
The fiber core and cladding are composed of glass with different refractive indexes, with a high refractive index glass core (germanium-doped silica) in the center, and a low refractive index silica glass cladding (pure silica) in the middle. The light enters the optical fiber at a specific incident angle, and the total emission occurs between the optical fiber and the cladding (because the refractive index of the cladding is slightly lower than the core), so that it can propagate in the optical fiber. The main function of the coating is to protect the optical fiber from external damage, while increasing the flexibility of the optical fiber. As mentioned earlier, both the core and the cladding are made of glass and cannot be bent and fragile. The use of the coating layer protects and prolongs the life of the optical fiber.
2. The composition of the optical cable
The optical fiber is drawn from pure quartz with a special process into a glass tube thinner than a hair with a few dielectrics in the middle. Its texture is brittle and fragile, so an additional protective layer is needed. The outer layer of the optical fiber is combined with a plastic protective tube and a plastic sheath to form an optical cable.
Optical cables include optical fibers. Optical fibers are glass fibers in optical cables. Broadly speaking, optical fibers are optical cables, which are all transmission media. But in a strict sense, the two are different products. The difference between optical fiber and optical cable: optical fiber is a thin and soft medium that transmits light beams. Most optical fibers must be covered by several layers of protective structures before use, and the covered cables are called optical cables. Therefore, the optical fiber is the core part of the optical cable, and the optical fiber constitutes the optical cable through the protection of some components and the auxiliary protective layer.
3. What is the working wavelength of the optical fiber?
Light is defined by its wavelength. In optical fiber communication, the light used is light in the infrared region, where the wavelength of light is greater than that of visible light. In fiber optic communications, typical wavelengths are 800 to 1600nm, of which the most commonly used wavelengths are 850nm, 1310nm and 1550nm.
When selecting the transmission wavelength, the fiber loss and scattering are mainly considered. The goal is to transmit the most data to the farthest distance with the smallest fiber loss. The loss of signal strength in transmission is attenuation. The degree of attenuation is related to the length of the waveform. The longer the waveform, the smaller the attenuation. The light used in the optical fiber has longer wavelengths at 850, 1310, and 1550 nm, so the attenuation of the optical fiber is small, which also leads to less optical fiber loss. And these three wavelengths have almost zero absorption and are most suitable for transmission in optical fibers as available light sources.
4. Minimum dispersion wavelength and minimum loss wavelength
In the current commercial optical fiber, what wavelength of light has the smallest dispersion? What wavelength of light has the least loss? Light with a wavelength of 1310nm has the smallest dispersion, and light with a wavelength of 1550nm has the smallest loss.
5. What is the dispersion of optical fiber?
Fiber dispersion refers to the fact that the signal transmitted by the fiber is carried by different frequency components and different mode components, and the transmission speeds of different frequency components and different mode components are different, which leads to signal distortion.
Fiber dispersion is divided into material dispersion, waveguide dispersion and modal dispersion. The first two kinds of dispersion are caused by the signal not being a single frequency, and the latter kind of dispersion is caused by the signal being not a single mode. If the signal is not a single mode, it will cause mode dispersion. Single-mode fiber has only one fundamental mode, so there is only material dispersion and waveguide dispersion, and no modal dispersion. The multimode fiber has inter-mode dispersion. The dispersion of the optical fiber not only affects the transmission capacity of the optical fiber, but also limits the relay distance of the optical fiber communication system.
6. What is the cause of fiber attenuation?
The main causes of fiber attenuation are scattering, absorption, bending, squeezing, and optical power loss caused by connectors and fusion splices. The unit of attenuation is dB/km, the attenuation value of the optical signal power per kilometer of the optical fiber. Rayleigh scattering, inherent absorption, etc. are the inherent loss of the optical fiber. Some of the light in the optical fiber will be lost due to scattering when the optical fiber is bent, or loss caused by slight bending when the optical fiber is squeezed. At the same time, loss will occur when the fiber is butted, such as different axis, end face is not perpendicular to the axis, uneven end face, mismatched butt core diameter, and poor splicing quality.
7. What is the bandwidth of optical fiber related to?
The bandwidth of an optical fiber refers to the modulation frequency when the amplitude of the optical power is reduced by 50% or 3dB from the amplitude of the zero frequency in the transfer function of the optical fiber. The bandwidth of an optical fiber is approximately inversely proportional to its length, and the product of the bandwidth length is a constant.
8. What is insertion loss?
Insertion loss is Insertion Loss (usually referred to as IL), which mainly refers to the measurement of light loss between two fixed points in an optical fiber. It can be understood as the loss of optical power caused by the intervention of optical devices in the optical fiber link of the optical communication system, and the unit is dB.
Calculation formula: IL=-10 lg (Pout /Pin), Pout is the output optical power, and Pin is the input optical power.
The smaller the value of the insertion loss, the better the performance. For example, the insertion loss of 0.3dB is better than 0.5dB. Generally speaking, the attenuation difference between fusion splicing and manual connection (less than 0.1 dB) will be smaller than the connection between fiber optic connectors. Recommended maximum loss in dB for optical fiber cabling in data centers: maximum 15dB for LC multimode fiber connectors, maximum 15dB for LC single mode connectors, maximum 20dB for MPO/MTP multimode fiber connectors, and MPO/MTP single mode fiber connections The maximum is 30dB.
9. What is return loss?
When an optical fiber signal enters or leaves a certain optical component (such as an optical fiber connector), discontinuity and impedance mismatch will cause reflection or return. The power loss of the reflected or returned signal is the return loss, Return Loss ( Referred to as RL). Insertion loss is mainly to measure the resulting signal value when the optical link encounters loss, while return loss is the measurement of the reflected signal loss value when the optical link encounters component access.
Calculation formula: RL=-10 lg (P0/P1), P0 represents the reflected optical power, and P1 represents the input optical power.
The return loss value is expressed in dB, and is usually a negative value. Therefore, the larger the return loss value, the better. The typical specification range is -15 to -60 dB. According to industry standards, the return loss of Ultra PC polished fiber optic connectors should be greater than 50dB, and the return loss of bevel polishing is usually greater than 60dB. The PC type should be greater than 40dB. For multimode fibers, the typical RL value is between 20 and 40 dB.
10. What is the core size of the optical fiber?
The core size is the physical size of the fiber core. The core size of multimode fiber is between 7um and 3mm, the most common ones are 50um, 62.5um, 100um and 200um. The industry standard for data communication is now 50um and 62.5um multi-mode using quartz glass fiber. The typical core size of single-mode quartz glass fiber is 8.3um. For plastic optical fibers, the core size ranges from 0.25mm to 3mm, with 1mm being the most popular.
11. What is the mode field diameter?
Mode Field Diameter (MFD–Mode Field Diameter) is used to characterize the distribution of fundamental mode light in the core region of a single-mode fiber. Although most of the optical signal propagates inside the fiber core, it actually propagates through a slightly larger volume, including the inner edge of the fiber cladding. This effective area becomes the mode field diameter of the optical fiber. In telecommunication fibers operating above the cut-off value, the core diameter may be about 9 µm, and the MFD may be about 10.4 µm. Using very high NA fibers (about 0.2 or 0.3), the core diameter is only a few microns, and the MFD may be about 5 µm. For transmission fibers, the larger the mode field diameter, the better.
12. What is the numerical aperture NA?
The light-gathering ability of an optical fiber depends not only on the size of the fiber core, but also on its receiving angle. The acceptance angle is the range of angles within which light can enter the fiber and be trapped in the fiber core. Acceptance angle and numerical aperture are quality factors used to describe the angle associated with the propagation of light in an optical fiber. The sine of the half-angle of the receiving angle is called Numeric Aperture-NA (Numeric Aperture), NA=sinθ. Generally, for a 50um graded index multimode fiber, the numerical aperture is 0.20. For 62.5um graded index multimode fiber, the numerical aperture is 0.28.
13. What is the cutoff wavelength (Cutoff Wavelength)
The cut-off wavelength refers to a single-mode fiber that usually has a certain wavelength. When the transmitted light wavelength exceeds this wavelength, the fiber can only propagate one mode (fundamental mode). Below this wavelength, the fiber may propagate multiple modes. When the wavelength is greater than a certain value, a particular mode no longer exists, and this wavelength is called the cutoff wavelength of this mode.
14. What is Zero Dispersion Wavelength (Zero Dispersion Wavelength)
When the waveguide dispersion and the material dispersion cancel each other out at a certain wavelength, so that the total chromatic dispersion approaches zero, the wavelength is the zero-dispersion wavelength. The zero-dispersion wavelength of the conventional single-mode fiber is around 1310nm, and the lowest loss is around 1550nm. There is a higher positive dispersion value at 1550nm. Both G.652 fiber and G.654 fiber recommended by ITU-T belong to this type. The zero-dispersion wavelength is 1300～1324 nm, and the maximum dispersion D(λ)<3.5 ps/(nm•km). Dispersion slope S. ≤0.093/(nm²•km).
15. What is Dispersion Shifted Fiber (DSF)?
Dispersion-shifted fiber seeks to increase waveguide dispersion by changing the structural parameters and refractive index profile of the fiber, thereby shifting the zero-dispersion point from 1310nm to 1550nm, achieving the lowest attenuation and zero-dispersion wavelength at 1550nm. The working wavelength of this fiber is in the 1550nm region. It is very suitable for long-distance single-channel optical fiber communication systems.
16. What is non-zero dispersion fiber (NZDF)?
The dispersion of the fiber is not zero at the wavelength of 1550nm, so it is called a non-zero dispersion-shifted fiber. It has reasonable low dispersion in the 1550nm wavelength region, enough to support 10Gbit/s long-distance transmission without dispersion compensation, and its dispersion value maintains non-zero characteristics to suppress the influence of non-linear effects such as four-wave mixing and cross-phase modulation . This kind of optical fiber mainly uses dense wavelength division multiplexing transmission system.
17. How to classify optical fiber
For example, according to the mode of light, it can be divided into single-mode fiber and multi-mode fiber. Divided by refractive index: jumper fiber and graded fiber. According to the composition, it is divided into quartz optical fiber, fluorine-containing optical fiber and plastic optical fiber. According to the working wavelength, it is divided into short-wavelength fiber (typical wavelength is 850nm) and long-wavelength fiber (wavelength is 1310nm, 1550nm)
18. What are the main optical fibers currently used for transmission network construction?
Among them, according to ITU standards, optical fibers are divided into seven types: G651, G652, G653, G654, G655, G656, G657, among which G652 and G657 are commonly used.
Multimode fiber
G.651 fiber (multimode graded index fiber)
G.652 (dispersion non-shift single-mode fiber)
G.653 (dispersion shifted fiber)
G.654 (cut-off wavelength shifted fiber)
G.655 (non-zero dispersion shifted fiber)
G.656 (low slope non-zero dispersion shift fiber)
G.657 (bend-resistant fiber)
19. What is single-mode fiber and multi-mode fiber?
Single-mode fiber (Single Mode Fiber), light enters the fiber at a specific incident angle, and full emission occurs between the fiber and the cladding. When the diameter is small, only one direction of light is allowed to pass through, that is, a single-mode fiber; The central glass core of the mode fiber is very thin, the core diameter is generally 8.5 or 9.5 μm, and it works at 1310 and 1550 nm wavelengths.
Multimode fiber (MulTI Mode Fiber) is a fiber that allows multiple guided mode transmission. The core diameter of a multimode fiber is generally 50μm/62.5μm. Since the core diameter of a multimode fiber is relatively large, it can allow different modes of light to be transmitted on one fiber. The standard wavelengths of multimode are 850nm and 1300nm respectively. There is also a new multimode fiber standard called WBMMF (Wideband Multimode Fiber), which uses wavelengths between 850nm and 953nm.
Both single-mode fiber and multi-mode fiber have a cladding diameter of 125μm.
20.OS1, OS2, OM1, OM2, OM3, OM4, OM5
OS1 and OS2 are both single-mode optical fibers. OS1: the ordinary single-mode optical fiber used earlier; OS2: the ordinary optical fiber in use now, the low-water peak optical fiber. Generally speaking, OM1 is conventional 62.5/125um; OM2 is conventional 50/125um; OM3 is a 50um core diameter multimode fiber optimized by 850nm laser. In 10Gb/s Ethernet using 850nm VCSEL, the fiber transmission distance can reach 300m; OM4 is an upgraded version of OM3. OM4 multimode fiber optimizes the differential mode delay (DMD) generated by OM3 multimode fiber during high-speed transmission, so the transmission distance is greatly improved, and the fiber transmission distance can reach 550m; OM5 fiber jumper The cable is a new standard for fiber optic patch cords defined by TIA and IEC. The fiber diameter is 50/125μm. Compared with OM3 and OM4 fiber optic patch cords, OM5 fiber optic patch cords can be used for higher bandwidth applications. The bandwidth and maximum distance for different levels of transmission are different.
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