1.How are optical fibers combined?
Answer: Optical fiber consists of two basic parts: a core made of transparent optical materials and a cladding and coating layer.
2. What are the basic parameters that describe the transmission characteristics of optical fiber lines?
Answer: They include loss, dispersion, bandwidth, cutoff wavelength, mode field diameter, etc.
3. What are the causes of fiber attenuation?
Answer: Fiber attenuation refers to the reduction of optical power between two cross sections of a fiber, which is related to the wavelength. The main causes of attenuation are scattering, absorption, and optical loss caused by connectors and joints.
4. How is the optical fiber attenuation coefficient defined?
Answer: It is defined by the attenuation per unit length of a uniform optical fiber in a steady state (dB/km).
5. What is insertion loss?
Answer: It refers to the attenuation caused by inserting optical components (such as inserting connectors or couplers) into the optical transmission line.
6. What does the bandwidth of optical fiber relate to?
Answer: The bandwidth of optical fiber refers to the modulation frequency when the amplitude of optical power is reduced by 50% or 3dB compared with the amplitude of zero frequency in the transfer function of optical fiber. The bandwidth of optical fiber is approximately inversely proportional to its length, and the product of bandwidth and length is a constant.
7. How many types of optical fiber dispersion are there? What does it relate to?
Answer: The dispersion of optical fiber refers to the broadening of group delay in an optical fiber, including mode dispersion, material dispersion and structural dispersion. It depends on the characteristics of both the light source and the optical fiber.
8. How to describe the dispersion characteristics of signals propagating in optical fiber?
Answer: It can be described by three physical quantities: pulse broadening, bandwidth of optical fiber, and dispersion coefficient of optical fiber.
9. What is the cutoff wavelength?
Answer: It refers to the shortest wavelength that can only transmit the fundamental mode in the optical fiber. For single-mode optical fiber, its cutoff wavelength must be shorter than the wavelength of the transmitted light.
10. What impact will the dispersion of optical fiber have on the performance of optical fiber communication system?
Answer: The dispersion of optical fiber will cause the optical pulse to be broadened during transmission in the optical fiber, affecting the bit error rate, the transmission distance, and the system rate.
11. What is the backscattering method?
Answer: The backscattering method is a method for measuring attenuation along the length of an optical fiber. Most of the optical power in the optical fiber propagates forward, but a small part is backscattered toward the light emitter. Using a spectrometer at the light emitter to observe the time curve of backscattering, not only can the length and attenuation of the connected uniform optical fiber be measured from one end, but also the local irregularities, breakpoints and optical power loss caused by joints and connectors can be measured.
12. What is the testing principle of the optical time domain reflectometer (OTDR)? What are its functions?
Answer: OTDR is based on the principle of light backscattering and Fresnel reflection. It uses the backscattered light generated when light propagates in the optical fiber to obtain attenuation information. It can be used to measure optical fiber attenuation, joint loss, optical fiber fault point location, and understand the loss distribution along the length of the optical fiber. It is an indispensable tool in optical cable construction, maintenance and monitoring. Its main indicators include: dynamic range, sensitivity, resolution, measurement time and blind area.
13.What is the blind area of OTDR? What is the impact on the test? How to deal with the blind area in actual testing?
Answer: Usually, a series of "blind spots" caused by the saturation of the OTDR receiving end due to reflections generated by feature points such as active connectors and mechanical joints are called blind areas.
The blind areas in optical fibers are divided into event blind areas and attenuation blind areas: the length distance from the starting point of the reflection peak to the receiver saturation peak caused by the intervention of active connectors is called event blind areas; the distance from the starting point of the reflection peak to other identifiable event points caused by the intervention of active connectors in optical fibers is called attenuation blind areas.
For OTDR, the smaller the blind area, the better. The blind area will increase with the increase of the width of the pulse broadening. Although increasing the pulse width increases the measurement length, it also increases the measurement blind area. Therefore, when testing optical fibers, narrow pulses should be used to measure the optical fiber and adjacent event points of the OTDR accessories, while wide pulses should be used to measure the far end of the optical fiber.
14.Can OTDR measure different types of optical fibers?
A: If you use a single-mode OTDR module to measure a multimode fiber, or use a multimode OTDR module to measure a single-mode fiber with a core diameter of 62.5mm, the measurement result of the fiber length will not be affected, but the results of fiber loss, optical connector loss, and return loss will be incorrect. Therefore, when measuring optical fiber, you must choose an OTDR that matches the measured fiber to measure, so that you can get the correct results for all performance indicators.
15. What does "1310nm" or "1550nm" in common optical test instruments mean?
A: It refers to the wavelength of the optical signal. The wavelength range used in optical fiber communication is in the near-infrared region, with a wavelength between 800nm and 1700nm. It is often divided into short-wavelength bands and long-wavelength bands, the former refers to 850nm wavelength, and the latter refers to 1310nm and 1550nm.
16. In current commercial optical fibers, what wavelength of light has the smallest dispersion? What wavelength of light has the smallest loss?
Answer: Light with a wavelength of 1310nm has the smallest dispersion, and light with a wavelength of 1550nm has the smallest loss.
17. How are optical fibers classified according to the change in the refractive index of the optical fiber core?
Answer: They can be divided into step-index optical fibers and gradient-index optical fibers. Step-index optical fibers have a narrow bandwidth and are suitable for small-capacity short-distance communications; gradient-index optical fibers have a wide bandwidth and are suitable for medium- and large-capacity communications.
18. How are optical fibers classified according to the different light wave modes transmitted in optical fibers?
Answer: They can be divided into single-mode optical fibers and multi-mode optical fibers. The core diameter of single-mode optical fibers is approximately between 1 and 10μm. At a given working wavelength, only a single fundamental mode is transmitted, which is suitable for large-capacity and long-distance communication systems. Multi-mode optical fibers can transmit multiple modes of light waves, with a core diameter of approximately between 50 and 60μm, and their transmission performance is worse than that of single-mode optical fibers.
When transmitting the current differential protection of multiplexed protection, multi-mode optical fibers are often used between the optoelectronic conversion device installed in the communication room of the substation and the protection device installed in the main control room.
19. What is the significance of the numerical aperture (NA) of step-index optical fiber?
Answer: The numerical aperture (NA) indicates the light-collecting ability of the optical fiber. The larger the NA, the stronger the optical fiber's ability to collect light.
20. What is the birefringence of single-mode optical fiber?
Answer: There are two orthogonal polarization modes in a single-mode optical fiber. When the optical fiber is not completely cylindrically symmetrical, the two orthogonal polarization modes are not degenerate. The absolute value of the difference in the refractive index of the two orthogonal polarization modes is the birefringence.
21. What are the most common optical cable structures?
Answer: There are two types: layer-twisted type and skeleton type.
22. What are the main components of optical cables?
Answer: It is mainly composed of: fiber core, optical fiber grease, sheath material, PBT (polybutylene terephthalate) and other materials.
23. What does the armor of optical cables refer to?
Answer: It refers to the protective element (usually steel wire or steel belt) used in optical cables for special purposes (such as submarine optical cables, etc.). The armor is attached to the inner sheath of the optical cable.
24. What materials are used for the sheath of optical cables?
Answer: The sheath or sheath of optical cables is usually made of polyethylene (PE) and polyvinyl chloride (PVC) materials, and its function is to protect the cable core from external influences.
25. List the special optical cables used in power systems.
Answer: There are mainly three special optical cables:
Ground wire composite optical cable (OPGW), the optical fiber is placed in the power line of the steel-clad aluminum stranded structure. The application of OPGW optical cable has the dual functions of ground wire and communication, effectively improving the utilization rate of power poles and towers.
Wrapped optical cable (GWWOP), where there is an existing transmission line, this type of optical cable is wrapped or hung on the ground wire.
Self-supporting optical cable (ADSS) has strong tensile strength and can be directly hung between two power towers, with a maximum span of up to 1000m.
26. How many application structures are there for OPGW optical cable?
Answer: Mainly: 1) Plastic tube layer twisted + aluminum tube structure; 2) Central plastic tube + aluminum tube structure; 3) Aluminum skeleton structure; 4) Spiral aluminum tube structure; 5) Single-layer stainless steel tube structure (central stainless steel tube structure, stainless steel tube layer twisted structure); 6) Composite stainless steel tube structure (central stainless steel tube structure, stainless steel tube layer twisted structure).
27. What are the main components of the stranded wire outside the OPGW optical cable core?
Answer: It is composed of AA wire (aluminum alloy wire) and AS wire (aluminum clad steel wire).
28. What are the technical conditions required to select OPGW optical cable models?
Answer: 1) Nominal tensile strength (RTS) of OPGW cable (kN); 2) Number of fiber cores (SM) of OPGW cable; 3) Short-circuit current (kA); 4) Short-circuit time (s); 5) Temperature range (℃).
29. How is the bending degree of the optical cable limited?
Answer: The bending radius of the optical cable should be no less than 20 times the outer diameter of the optical cable, and no less than 30 times the outer diameter of the optical cable during construction (non-static state).
30. What should be paid attention to in ADSS optical cable engineering?
Answer: There are three key technologies: optical cable mechanical design, determination of suspension points, and selection and installation of supporting hardware.
31. What are the main types of optical cable fittings?
Answer: Optical cable fittings refer to the hardware used to install optical cables, mainly including: tension clamps, suspension clamps, vibration isolators, etc.
32. Optical fiber connectors have two most basic performance parameters, what are they?
Answer: Optical fiber connectors are commonly known as live joints. For the requirements of the optical performance of single-fiber connectors, the focus is on the two most basic performance parameters of insertion loss and return loss.
33. How many types of commonly used optical fiber connectors are there?
Answer: According to different classification methods, optical fiber connectors can be divided into different types. According to different transmission media, they can be divided into single-mode optical fiber connectors and multi-mode optical fiber connectors; according to different structures, they can be divided into various types such as FC, SC, ST, D4, DIN, Biconic, MU, LC, MT, etc.; according to the pin end face of the connector, they can be divided into FC, PC (UPC) and APC. Commonly used optical fiber connectors: FC/PC type optical fiber connector, SC type optical fiber connector, LC type optical fiber connector.
34. In the fiber optic communication system, the following items are commonly found. Please indicate their names.
AFC, FC adapter ST adapter SC adapter FC/APC, FC/PC connector SC connector ST connector LC patch cord MU patch cord Single-mode or multi-mode patch cord.
35. What is the insertion loss (or insertion loss) of the fiber optic connector?
Answer: It refers to the value of the reduction in effective power of the transmission line caused by the insertion of the connector. For users, the smaller the value, the better. ITU-T stipulates that its value should not exceed 0.5dB.
36. What is the return loss (or reflection attenuation, return loss, return loss) of the fiber optic connector?
Answer: It is a measure of the input power component reflected from the connector and returned along the input channel. Its typical value should be no less than 25dB.
37. What is the most prominent difference between the light emitted by light-emitting diodes and semiconductor lasers?
Answer: The light generated by a light-emitting diode is incoherent light with a wide spectrum; the light generated by a laser is coherent light with a very narrow spectrum.
38. What is the most obvious difference between the working characteristics of a light-emitting diode (LED) and a semiconductor laser (LD)?
Answer: LED has no threshold, while LD has a threshold. Laser will only be generated when the injected current exceeds the threshold.
39. What are the two commonly used single longitudinal mode semiconductor lasers?
Answer: DFB laser and DBR laser, both of which are distributed feedback lasers, and their optical feedback is provided by the distributed feedback Bragg grating in the optical cavity.
40. What are the two main types of optical receiving devices?
Answer: They are mainly photodiodes (PIN tubes) and avalanche photodiodes (APDs).
41. What are the factors that cause noise in optical fiber communication systems?
Answer: There are noise caused by unqualified extinction ratio, noise caused by random changes in light intensity, noise caused by time jitter, point noise and thermal noise of the receiver, mode noise of the optical fiber, noise caused by pulse broadening caused by dispersion, mode distribution noise of LD, noise caused by frequency chirp of LD, and noise caused by reflection.
42. What are the main optical fibers currently used for transmission network construction? What are their main features?
Answer: There are three main types, namely G.652 conventional single-mode optical fiber, G.653 dispersion-shifted single-mode optical fiber, and G.655 non-zero dispersion-shifted optical fiber.
G.652 single-mode fiber has a large dispersion in the C-band 1530-1565nm and L-band 1565-1625nm, generally 17-22psnm•km. When the system rate reaches 2.5Gbit/s or above, dispersion compensation is required. At 10Gbit/s, the system dispersion compensation cost is relatively high. It is the most commonly used fiber in the current transmission network.
The dispersion of G.653 dispersion-shifted fiber in the C-band and L-band is generally -1-3.5psnm•km, and it is zero dispersion at 1550nm. The system rate can reach 20Gbit/s and 40Gbit/s, making it the best fiber for single-wavelength ultra-long-distance transmission. However, due to its zero dispersion characteristics, nonlinear effects will occur when DWDM is used for capacity expansion, resulting in signal crosstalk and four-wave mixing FWM, so it is not suitable for DWDM.
G.655 non-zero dispersion-shifted fiber: The dispersion of G.655 non-zero dispersion-shifted fiber in the C band is 1 to 6 psnm•km, and the dispersion in the L band is generally 6 to 10 psnm•km. The dispersion is small, avoiding the zero dispersion area, suppressing four-wave mixing FWM, and can be used for DWDM capacity expansion and opening high-speed systems. The new G.655 fiber can expand the effective area to 1.5 to 2 times that of ordinary optical fibers. The large effective area can reduce the power density and reduce the nonlinear effect of the optical fiber.
43. What is the nonlinearity of optical fiber?
Answer: It means that when the optical power of the fiber exceeds a certain value, the refractive index of the optical fiber will be nonlinearly related to the optical power, and Raman scattering and Brillouin scattering will be generated, causing the frequency of the incident light to change.
44. What effect will the nonlinearity of optical fiber have on transmission?
Answer: The nonlinear effect will cause some additional loss and interference, deteriorating the performance of the system. The optical power of the WDM system is large and is transmitted along a long distance along the optical fiber, so nonlinear distortion occurs. There are two types of nonlinear distortion: stimulated scattering and nonlinear refraction. Among them, stimulated scattering includes Raman scattering and Brillouin scattering. The above two types of scattering reduce the energy of the incident light, causing loss. It can be ignored when the input fiber power is small.
45. What is PON (Passive Optical Network)?
Answer: PON is a fiber-optic loop optical network in the local user access network, based on passive optical devices such as couplers and splitters.
Various causes of fiber optic attenuation
1. The main factors causing fiber attenuation are: intrinsic, bending, extrusion, impurities, unevenness and docking.
Intrinsic: It is the inherent loss of optical fiber, including: Rayleigh scattering, inherent absorption, etc.
Bending: When the optical fiber is bent, part of the light in the optical fiber will be lost due to scattering, causing loss.
Extrusion: Loss caused by a slight bend when the optical fiber is squeezed.
Impurities: Impurities in the optical fiber absorb and scatter the light propagating in the optical fiber, causing loss.
Unevenness: Loss caused by uneven refractive index of optical fiber material.
Docking: Loss caused when optical fibers are docked, such as: different axes (single-mode optical fiber coaxiality requirement is less than 0.8μm), the end face is not perpendicular to the axis, the end face is uneven, the docking core diameter does not match, and the quality of fusion is poor.
When light enters from one end of the optical fiber and exits from the other end, the intensity of the light will weaken. This means that after the optical signal propagates through the optical fiber, part of the light energy is attenuated. This shows that there are certain substances in the optical fiber or for some reason, blocking the passage of the optical signal. This is the transmission loss of the optical fiber. Only by reducing the loss of optical fiber can the optical signal pass smoothly.
2. Classification of optical fiber loss
Optical fiber loss can be roughly divided into the inherent loss of optical fiber and the additional loss caused by the use conditions after the optical fiber is made. The specific subdivisions are as follows:
Optical fiber loss can be divided into inherent loss and additional loss.
Inherent loss includes scattering loss, absorption loss and loss caused by imperfect optical fiber structure.
Additional loss includes microbend loss, bending loss and splicing loss.
Among them, additional loss is caused artificially during the laying of optical fiber. In practical applications, it is inevitable to connect optical fibers one by one, and optical fiber connection will cause loss. Microbending, squeezing and stretching of optical fibers will also cause loss. These are all losses caused by the use conditions of optical fiber. The main reason is that under these conditions, the transmission mode in the optical fiber core has changed. Additional loss can be avoided as much as possible. Below, we only discuss the inherent loss of optical fiber.
Among the inherent losses, scattering loss and absorption loss are determined by the characteristics of the optical fiber material itself, and the inherent loss caused at different working wavelengths is also different. It is extremely important to understand the mechanism of loss generation and quantitatively analyze the size of loss caused by various factors for the development of low-loss optical fiber and the rational use of optical fiber.
3. Absorption loss of materials
The materials used to make optical fibers can absorb light energy. After the particles in the optical fiber material absorb light energy, they vibrate and generate heat, and the energy is lost, thus generating absorption loss. We know that matter is composed of atoms and molecules, and atoms are composed of atomic nuclei and extranuclear electrons, and electrons revolve around the atomic nucleus in a certain orbit. This is just like the earth we live on and planets such as Venus and Mars revolve around the sun. Each electron has a certain energy and is in a certain orbit, or in other words, each orbit has a certain energy level.
The orbital energy level close to the nucleus is lower, and the orbital energy level farther from the nucleus is higher. The size of this energy level difference between orbits is called the energy level difference. When an electron transitions from a low energy level to a high energy level, it absorbs the energy of the corresponding energy level difference.
In an optical fiber, when an electron at a certain energy level is irradiated by light of a wavelength corresponding to the energy level difference, the electron in the low energy level orbit will transition to the orbit with a higher energy level. This electron absorbs light energy, resulting in light absorption loss.
Silicon dioxide (SiO2), the basic material for making optical fibers, absorbs light itself. One is called ultraviolet absorption and the other is called infrared absorption. At present, optical fiber communications generally only work in the wavelength range of 0.8 to 1.6 μm, so we only discuss the loss in this working range.
The absorption peak generated by electron transitions in quartz glass is around 0.1 to 0.2 μm wavelength in the ultraviolet region. As the wavelength increases, its absorption effect gradually decreases, but the affected area is very wide, up to wavelengths above 1 μm. However, ultraviolet absorption has little effect on quartz optical fibers working in the infrared region. For example, in the visible light region with a wavelength of 0.6 μm, ultraviolet absorption can reach 1 dB/km, and at a wavelength of 0.8 μm, it drops to 0.2 to 0.3 dB/km, and at a wavelength of 1.2 μm, it is only about 0.1 dB/km.
The infrared absorption loss of quartz optical fiber is caused by the molecular vibration of the infrared material. There are several vibration absorption peaks in the band above 2 μm.
Due to the influence of various doping elements in the optical fiber, it is impossible for quartz optical fiber to have a low loss window in the band above 2μm, and the theoretical limit loss at a wavelength of 1.85μm is ldB/km.
Through research, it was also found that there are some "destructive molecules" in quartz glass that are making trouble, mainly some harmful transition metal impurities, such as copper, iron, chromium, manganese, etc. These "bad guys" greedily absorb light energy under light irradiation, jump around, and cause light energy loss. Removing the "troublemakers" and chemically purifying the materials used to make optical fibers can greatly reduce the loss.
Another absorption source in quartz optical fiber is hydroxyl (OHˉ). According to the research of the period, people found that hydroxyl has three absorption peaks in the working band of optical fiber, which are 0.95μm, 1.24μm and 1.38μm, among which the absorption loss at the wavelength of 1.38μm is the most serious and has the greatest impact on the optical fiber. At a wavelength of 1.38μm, the absorption peak loss generated by the hydroxide content of only 0.0001 is as high as 33dB/km.
Where do these hydroxides come from? There are many sources of hydroxides. First, there is water and hydroxide compounds in the materials used to make optical fibers. These hydroxide compounds are not easy to be removed during the purification of raw materials, and finally remain in the optical fiber in the form of hydroxides; second, there is a small amount of water in the hydroxides used to make optical fibers; third, water is generated due to chemical reactions during the manufacturing process of optical fibers; fourth, water vapor is brought in by the entry of outside air. However, the current manufacturing process has developed to a fairly high level, and the hydroxide content has dropped to a sufficiently low level that its impact on optical fibers can be ignored.
4. Scattering loss
In the dark night, if you shine a flashlight into the sky, you can see a beam of light. People have also seen thick beams of light from searchlights in the night sky.
So why do we see these beams of light? This is because there are many tiny particles such as smoke and dust floating in the atmosphere. When light shines on these particles, it scatters and shoots in all directions. This phenomenon was first discovered by Rayleigh, so people named this scattering "Rayleigh scattering".
How does scattering occur? It turns out that the tiny particles such as molecules, atoms, and electrons that make up matter vibrate at certain inherent frequencies and can release light with a wavelength corresponding to the vibration frequency. The vibration frequency of a particle is determined by the size of the particle. The larger the particle, the lower the vibration frequency and the longer the wavelength of the light released; the smaller the particle, the higher the vibration frequency and the shorter the wavelength of the light released. This vibration frequency is called the inherent vibration frequency of the particle. However, this vibration is not generated by itself, it requires a certain amount of energy. Once a particle is irradiated with light of a certain wavelength, and the frequency of the irradiated light is the same as the inherent vibration frequency of the particle, it will cause resonance. The electrons in the particle begin to vibrate at this vibration frequency, resulting in the particle scattering light in all directions, and the energy of the incident light is absorbed and converted into the energy of the particle, and the particle re-emits the energy in the form of light energy. Therefore, for people observing from the outside, it seems that the light hits the particle and then flies out in all directions.
Rayleigh scattering also occurs in optical fibers, and the light loss caused by this is called Rayleigh scattering loss. Given the current level of optical fiber manufacturing technology, it can be said that Rayleigh scattering loss is unavoidable. However, since the magnitude of Rayleigh scattering loss is inversely proportional to the fourth power of the wavelength of light, the impact of Rayleigh scattering loss can be greatly reduced when the optical fiber operates in the long wavelength region.
5. Congenital deficiency, no one can help
The optical fiber structure is imperfect, such as bubbles, impurities, or uneven thickness in the optical fiber, especially the uneven core-cladding interface. When the light reaches these places, part of the light will be scattered in all directions, causing loss. This loss can be overcome by improving the optical fiber manufacturing process. Scattering causes light to be emitted in all directions, and part of the scattered light is reflected back in the opposite direction of the optical fiber propagation. This part of the scattered light can be received at the incident end of the optical fiber. The scattering of light causes part of the light energy to be lost, which is undesirable. However, this phenomenon can also be used by us, because if we analyze the strength of the received part of the light at the transmitting end, we can check the breakpoints, defects and loss of this optical fiber. In this way, through human ingenuity, bad things can be turned into good things.
Fiber loss In recent years, optical fiber communication has been widely used in many fields. An important issue in realizing optical fiber communication is to reduce the loss of optical fiber as much as possible. The so-called loss refers to the attenuation of optical fiber per unit length, and the unit is dB/km. The level of optical fiber loss directly affects the transmission distance or the distance between relay stations. Therefore, understanding and reducing optical fiber loss has great practical significance for optical fiber communications.
1. Absorption loss of optical fiber
This is caused by the absorption of light energy by optical fiber materials and impurities. They consume light energy in the form of heat energy in the optical fiber, which is an important loss in optical fiber loss. Absorption loss includes the following:
① Intrinsic absorption loss of material This is the loss caused by the inherent absorption of the material. It has two bands, one in the 8-12μm region of near-infrared. The intrinsic absorption of this band is due to vibration. The other intrinsic absorption band of the material is in the ultraviolet band. When the absorption is very strong, its tail will be dragged to the 0.7-1.1μm band.
②Absorption loss caused by dopants and impurity ions Optical fiber materials contain transition metals such as iron, copper, chromium, etc. They have their own absorption peaks and absorption bands and vary with their valence states. The optical fiber loss caused by the absorption of transition metal ions depends on their concentration. In addition, the presence of OH- also produces absorption loss. The basic absorption peak of OH- is near 2.7μm, and the absorption band is in the range of 0.5-1.0μm. For pure quartz optical fiber, the loss caused by impurities can be ignored.
③ Atomic defect absorption loss When the optical fiber material is heated or strongly radiated, it will be stimulated to produce atomic defects, resulting in absorption of light and loss, but in general this effect is very small.
2. Scattering loss of optical fiber
The scattering inside the optical fiber will reduce the transmission power and generate loss. The most important scattering is Rayleigh scattering, which is caused by the density and composition changes inside the optical fiber material.
During the heating process of the optical fiber material, due to thermal agitation, the compressibility of the atoms is uneven, the density of the material is uneven, and then the refractive index is uneven. This unevenness is fixed during the cooling process, and its size is smaller than the wavelength of the light wave. When light encounters these uneven materials that are smaller than the wavelength of the light wave and have random fluctuations during transmission, the transmission direction is changed, scattering occurs, and loss occurs. In addition, the uneven concentration of oxides contained in the optical fiber and uneven doping can also cause scattering and loss.
3. Waveguide scattering loss
This is the scattering caused by random distortion or roughness of the interface. In fact, it is the mode conversion or mode coupling caused by surface distortion or roughness. One mode will generate other transmission modes and radiation modes due to the fluctuation of the interface. Since the attenuation of various modes transmitted in the optical fiber is different, in the process of long-distance mode conversion, the mode with low attenuation becomes the mode with large attenuation. After continuous conversion and reverse conversion, although the loss of each mode will be balanced, the mode as a whole will produce additional loss, that is, additional loss is generated due to the conversion of the mode. This additional loss is the waveguide scattering loss. To reduce this loss, it is necessary to improve the optical fiber manufacturing process. For optical fibers that are well pulled or of high quality, this loss can basically be ignored.
4. Radiation loss caused by optical fiber bending
Optical fiber is soft and can be bent. However, after bending to a certain extent, although the optical fiber can guide light, it will change the transmission path of light. The conversion from transmission mode to radiation mode causes part of the light energy to penetrate into the cladding or pass through the cladding to become a radiation mode and leak out, thereby generating loss. When the bending radius is greater than 5 to 10 cm, the loss caused by bending can be ignored.
Source: Dongguan HX Fiber Technology Co., Ltd
1.How are optical fibers combined?
Answer: Optical fiber consists of two basic parts: a core made of transparent optical materials and a cladding and coating layer.
2. What are the basic parameters that describe the transmission characteristics of optical fiber lines?
Answer: They include loss, dispersion, bandwidth, cutoff wavelength, mode field diameter, etc.
3. What are the causes of fiber attenuation?
Answer: Fiber attenuation refers to the reduction of optical power between two cross sections of a fiber, which is related to the wavelength. The main causes of attenuation are scattering, absorption, and optical loss caused by connectors and joints.
4. How is the optical fiber attenuation coefficient defined?
Answer: It is defined by the attenuation per unit length of a uniform optical fiber in a steady state (dB/km).
5. What is insertion loss?
Answer: It refers to the attenuation caused by inserting optical components (such as inserting connectors or couplers) into the optical transmission line.
6. What does the bandwidth of optical fiber relate to?
Answer: The bandwidth of optical fiber refers to the modulation frequency when the amplitude of optical power is reduced by 50% or 3dB compared with the amplitude of zero frequency in the transfer function of optical fiber. The bandwidth of optical fiber is approximately inversely proportional to its length, and the product of bandwidth and length is a constant.
7. How many types of optical fiber dispersion are there? What does it relate to?
Answer: The dispersion of optical fiber refers to the broadening of group delay in an optical fiber, including mode dispersion, material dispersion and structural dispersion. It depends on the characteristics of both the light source and the optical fiber.
8. How to describe the dispersion characteristics of signals propagating in optical fiber?
Answer: It can be described by three physical quantities: pulse broadening, bandwidth of optical fiber, and dispersion coefficient of optical fiber.
9. What is the cutoff wavelength?
Answer: It refers to the shortest wavelength that can only transmit the fundamental mode in the optical fiber. For single-mode optical fiber, its cutoff wavelength must be shorter than the wavelength of the transmitted light.
10. What impact will the dispersion of optical fiber have on the performance of optical fiber communication system?
Answer: The dispersion of optical fiber will cause the optical pulse to be broadened during transmission in the optical fiber, affecting the bit error rate, the transmission distance, and the system rate.
11. What is the backscattering method?
Answer: The backscattering method is a method for measuring attenuation along the length of an optical fiber. Most of the optical power in the optical fiber propagates forward, but a small part is backscattered toward the light emitter. Using a spectrometer at the light emitter to observe the time curve of backscattering, not only can the length and attenuation of the connected uniform optical fiber be measured from one end, but also the local irregularities, breakpoints and optical power loss caused by joints and connectors can be measured.
12. What is the testing principle of the optical time domain reflectometer (OTDR)? What are its functions?
Answer: OTDR is based on the principle of light backscattering and Fresnel reflection. It uses the backscattered light generated when light propagates in the optical fiber to obtain attenuation information. It can be used to measure optical fiber attenuation, joint loss, optical fiber fault point location, and understand the loss distribution along the length of the optical fiber. It is an indispensable tool in optical cable construction, maintenance and monitoring. Its main indicators include: dynamic range, sensitivity, resolution, measurement time and blind area.
13.What is the blind area of OTDR? What is the impact on the test? How to deal with the blind area in actual testing?
Answer: Usually, a series of "blind spots" caused by the saturation of the OTDR receiving end due to reflections generated by feature points such as active connectors and mechanical joints are called blind areas.
The blind areas in optical fibers are divided into event blind areas and attenuation blind areas: the length distance from the starting point of the reflection peak to the receiver saturation peak caused by the intervention of active connectors is called event blind areas; the distance from the starting point of the reflection peak to other identifiable event points caused by the intervention of active connectors in optical fibers is called attenuation blind areas.
For OTDR, the smaller the blind area, the better. The blind area will increase with the increase of the width of the pulse broadening. Although increasing the pulse width increases the measurement length, it also increases the measurement blind area. Therefore, when testing optical fibers, narrow pulses should be used to measure the optical fiber and adjacent event points of the OTDR accessories, while wide pulses should be used to measure the far end of the optical fiber.
14.Can OTDR measure different types of optical fibers?
A: If you use a single-mode OTDR module to measure a multimode fiber, or use a multimode OTDR module to measure a single-mode fiber with a core diameter of 62.5mm, the measurement result of the fiber length will not be affected, but the results of fiber loss, optical connector loss, and return loss will be incorrect. Therefore, when measuring optical fiber, you must choose an OTDR that matches the measured fiber to measure, so that you can get the correct results for all performance indicators.
15. What does "1310nm" or "1550nm" in common optical test instruments mean?
A: It refers to the wavelength of the optical signal. The wavelength range used in optical fiber communication is in the near-infrared region, with a wavelength between 800nm and 1700nm. It is often divided into short-wavelength bands and long-wavelength bands, the former refers to 850nm wavelength, and the latter refers to 1310nm and 1550nm.
16. In current commercial optical fibers, what wavelength of light has the smallest dispersion? What wavelength of light has the smallest loss?
Answer: Light with a wavelength of 1310nm has the smallest dispersion, and light with a wavelength of 1550nm has the smallest loss.
17. How are optical fibers classified according to the change in the refractive index of the optical fiber core?
Answer: They can be divided into step-index optical fibers and gradient-index optical fibers. Step-index optical fibers have a narrow bandwidth and are suitable for small-capacity short-distance communications; gradient-index optical fibers have a wide bandwidth and are suitable for medium- and large-capacity communications.
18. How are optical fibers classified according to the different light wave modes transmitted in optical fibers?
Answer: They can be divided into single-mode optical fibers and multi-mode optical fibers. The core diameter of single-mode optical fibers is approximately between 1 and 10μm. At a given working wavelength, only a single fundamental mode is transmitted, which is suitable for large-capacity and long-distance communication systems. Multi-mode optical fibers can transmit multiple modes of light waves, with a core diameter of approximately between 50 and 60μm, and their transmission performance is worse than that of single-mode optical fibers.
When transmitting the current differential protection of multiplexed protection, multi-mode optical fibers are often used between the optoelectronic conversion device installed in the communication room of the substation and the protection device installed in the main control room.
19. What is the significance of the numerical aperture (NA) of step-index optical fiber?
Answer: The numerical aperture (NA) indicates the light-collecting ability of the optical fiber. The larger the NA, the stronger the optical fiber's ability to collect light.
20. What is the birefringence of single-mode optical fiber?
Answer: There are two orthogonal polarization modes in a single-mode optical fiber. When the optical fiber is not completely cylindrically symmetrical, the two orthogonal polarization modes are not degenerate. The absolute value of the difference in the refractive index of the two orthogonal polarization modes is the birefringence.
21. What are the most common optical cable structures?
Answer: There are two types: layer-twisted type and skeleton type.
22. What are the main components of optical cables?
Answer: It is mainly composed of: fiber core, optical fiber grease, sheath material, PBT (polybutylene terephthalate) and other materials.
23. What does the armor of optical cables refer to?
Answer: It refers to the protective element (usually steel wire or steel belt) used in optical cables for special purposes (such as submarine optical cables, etc.). The armor is attached to the inner sheath of the optical cable.
24. What materials are used for the sheath of optical cables?
Answer: The sheath or sheath of optical cables is usually made of polyethylene (PE) and polyvinyl chloride (PVC) materials, and its function is to protect the cable core from external influences.
25. List the special optical cables used in power systems.
Answer: There are mainly three special optical cables:
Ground wire composite optical cable (OPGW), the optical fiber is placed in the power line of the steel-clad aluminum stranded structure. The application of OPGW optical cable has the dual functions of ground wire and communication, effectively improving the utilization rate of power poles and towers.
Wrapped optical cable (GWWOP), where there is an existing transmission line, this type of optical cable is wrapped or hung on the ground wire.
Self-supporting optical cable (ADSS) has strong tensile strength and can be directly hung between two power towers, with a maximum span of up to 1000m.
26. How many application structures are there for OPGW optical cable?
Answer: Mainly: 1) Plastic tube layer twisted + aluminum tube structure; 2) Central plastic tube + aluminum tube structure; 3) Aluminum skeleton structure; 4) Spiral aluminum tube structure; 5) Single-layer stainless steel tube structure (central stainless steel tube structure, stainless steel tube layer twisted structure); 6) Composite stainless steel tube structure (central stainless steel tube structure, stainless steel tube layer twisted structure).
27. What are the main components of the stranded wire outside the OPGW optical cable core?
Answer: It is composed of AA wire (aluminum alloy wire) and AS wire (aluminum clad steel wire).
28. What are the technical conditions required to select OPGW optical cable models?
Answer: 1) Nominal tensile strength (RTS) of OPGW cable (kN); 2) Number of fiber cores (SM) of OPGW cable; 3) Short-circuit current (kA); 4) Short-circuit time (s); 5) Temperature range (℃).
29. How is the bending degree of the optical cable limited?
Answer: The bending radius of the optical cable should be no less than 20 times the outer diameter of the optical cable, and no less than 30 times the outer diameter of the optical cable during construction (non-static state).
30. What should be paid attention to in ADSS optical cable engineering?
Answer: There are three key technologies: optical cable mechanical design, determination of suspension points, and selection and installation of supporting hardware.
31. What are the main types of optical cable fittings?
Answer: Optical cable fittings refer to the hardware used to install optical cables, mainly including: tension clamps, suspension clamps, vibration isolators, etc.
32. Optical fiber connectors have two most basic performance parameters, what are they?
Answer: Optical fiber connectors are commonly known as live joints. For the requirements of the optical performance of single-fiber connectors, the focus is on the two most basic performance parameters of insertion loss and return loss.
33. How many types of commonly used optical fiber connectors are there?
Answer: According to different classification methods, optical fiber connectors can be divided into different types. According to different transmission media, they can be divided into single-mode optical fiber connectors and multi-mode optical fiber connectors; according to different structures, they can be divided into various types such as FC, SC, ST, D4, DIN, Biconic, MU, LC, MT, etc.; according to the pin end face of the connector, they can be divided into FC, PC (UPC) and APC. Commonly used optical fiber connectors: FC/PC type optical fiber connector, SC type optical fiber connector, LC type optical fiber connector.
34. In the fiber optic communication system, the following items are commonly found. Please indicate their names.
AFC, FC adapter ST adapter SC adapter FC/APC, FC/PC connector SC connector ST connector LC patch cord MU patch cord Single-mode or multi-mode patch cord.
35. What is the insertion loss (or insertion loss) of the fiber optic connector?
Answer: It refers to the value of the reduction in effective power of the transmission line caused by the insertion of the connector. For users, the smaller the value, the better. ITU-T stipulates that its value should not exceed 0.5dB.
36. What is the return loss (or reflection attenuation, return loss, return loss) of the fiber optic connector?
Answer: It is a measure of the input power component reflected from the connector and returned along the input channel. Its typical value should be no less than 25dB.
37. What is the most prominent difference between the light emitted by light-emitting diodes and semiconductor lasers?
Answer: The light generated by a light-emitting diode is incoherent light with a wide spectrum; the light generated by a laser is coherent light with a very narrow spectrum.
38. What is the most obvious difference between the working characteristics of a light-emitting diode (LED) and a semiconductor laser (LD)?
Answer: LED has no threshold, while LD has a threshold. Laser will only be generated when the injected current exceeds the threshold.
39. What are the two commonly used single longitudinal mode semiconductor lasers?
Answer: DFB laser and DBR laser, both of which are distributed feedback lasers, and their optical feedback is provided by the distributed feedback Bragg grating in the optical cavity.
40. What are the two main types of optical receiving devices?
Answer: They are mainly photodiodes (PIN tubes) and avalanche photodiodes (APDs).
41. What are the factors that cause noise in optical fiber communication systems?
Answer: There are noise caused by unqualified extinction ratio, noise caused by random changes in light intensity, noise caused by time jitter, point noise and thermal noise of the receiver, mode noise of the optical fiber, noise caused by pulse broadening caused by dispersion, mode distribution noise of LD, noise caused by frequency chirp of LD, and noise caused by reflection.
42. What are the main optical fibers currently used for transmission network construction? What are their main features?
Answer: There are three main types, namely G.652 conventional single-mode optical fiber, G.653 dispersion-shifted single-mode optical fiber, and G.655 non-zero dispersion-shifted optical fiber.
G.652 single-mode fiber has a large dispersion in the C-band 1530-1565nm and L-band 1565-1625nm, generally 17-22psnm•km. When the system rate reaches 2.5Gbit/s or above, dispersion compensation is required. At 10Gbit/s, the system dispersion compensation cost is relatively high. It is the most commonly used fiber in the current transmission network.
The dispersion of G.653 dispersion-shifted fiber in the C-band and L-band is generally -1-3.5psnm•km, and it is zero dispersion at 1550nm. The system rate can reach 20Gbit/s and 40Gbit/s, making it the best fiber for single-wavelength ultra-long-distance transmission. However, due to its zero dispersion characteristics, nonlinear effects will occur when DWDM is used for capacity expansion, resulting in signal crosstalk and four-wave mixing FWM, so it is not suitable for DWDM.
G.655 non-zero dispersion-shifted fiber: The dispersion of G.655 non-zero dispersion-shifted fiber in the C band is 1 to 6 psnm•km, and the dispersion in the L band is generally 6 to 10 psnm•km. The dispersion is small, avoiding the zero dispersion area, suppressing four-wave mixing FWM, and can be used for DWDM capacity expansion and opening high-speed systems. The new G.655 fiber can expand the effective area to 1.5 to 2 times that of ordinary optical fibers. The large effective area can reduce the power density and reduce the nonlinear effect of the optical fiber.
43. What is the nonlinearity of optical fiber?
Answer: It means that when the optical power of the fiber exceeds a certain value, the refractive index of the optical fiber will be nonlinearly related to the optical power, and Raman scattering and Brillouin scattering will be generated, causing the frequency of the incident light to change.
44. What effect will the nonlinearity of optical fiber have on transmission?
Answer: The nonlinear effect will cause some additional loss and interference, deteriorating the performance of the system. The optical power of the WDM system is large and is transmitted along a long distance along the optical fiber, so nonlinear distortion occurs. There are two types of nonlinear distortion: stimulated scattering and nonlinear refraction. Among them, stimulated scattering includes Raman scattering and Brillouin scattering. The above two types of scattering reduce the energy of the incident light, causing loss. It can be ignored when the input fiber power is small.
45. What is PON (Passive Optical Network)?
Answer: PON is a fiber-optic loop optical network in the local user access network, based on passive optical devices such as couplers and splitters.
Various causes of fiber optic attenuation
1. The main factors causing fiber attenuation are: intrinsic, bending, extrusion, impurities, unevenness and docking.
Intrinsic: It is the inherent loss of optical fiber, including: Rayleigh scattering, inherent absorption, etc.
Bending: When the optical fiber is bent, part of the light in the optical fiber will be lost due to scattering, causing loss.
Extrusion: Loss caused by a slight bend when the optical fiber is squeezed.
Impurities: Impurities in the optical fiber absorb and scatter the light propagating in the optical fiber, causing loss.
Unevenness: Loss caused by uneven refractive index of optical fiber material.
Docking: Loss caused when optical fibers are docked, such as: different axes (single-mode optical fiber coaxiality requirement is less than 0.8μm), the end face is not perpendicular to the axis, the end face is uneven, the docking core diameter does not match, and the quality of fusion is poor.
When light enters from one end of the optical fiber and exits from the other end, the intensity of the light will weaken. This means that after the optical signal propagates through the optical fiber, part of the light energy is attenuated. This shows that there are certain substances in the optical fiber or for some reason, blocking the passage of the optical signal. This is the transmission loss of the optical fiber. Only by reducing the loss of optical fiber can the optical signal pass smoothly.
2. Classification of optical fiber loss
Optical fiber loss can be roughly divided into the inherent loss of optical fiber and the additional loss caused by the use conditions after the optical fiber is made. The specific subdivisions are as follows:
Optical fiber loss can be divided into inherent loss and additional loss.
Inherent loss includes scattering loss, absorption loss and loss caused by imperfect optical fiber structure.
Additional loss includes microbend loss, bending loss and splicing loss.
Among them, additional loss is caused artificially during the laying of optical fiber. In practical applications, it is inevitable to connect optical fibers one by one, and optical fiber connection will cause loss. Microbending, squeezing and stretching of optical fibers will also cause loss. These are all losses caused by the use conditions of optical fiber. The main reason is that under these conditions, the transmission mode in the optical fiber core has changed. Additional loss can be avoided as much as possible. Below, we only discuss the inherent loss of optical fiber.
Among the inherent losses, scattering loss and absorption loss are determined by the characteristics of the optical fiber material itself, and the inherent loss caused at different working wavelengths is also different. It is extremely important to understand the mechanism of loss generation and quantitatively analyze the size of loss caused by various factors for the development of low-loss optical fiber and the rational use of optical fiber.
3. Absorption loss of materials
The materials used to make optical fibers can absorb light energy. After the particles in the optical fiber material absorb light energy, they vibrate and generate heat, and the energy is lost, thus generating absorption loss. We know that matter is composed of atoms and molecules, and atoms are composed of atomic nuclei and extranuclear electrons, and electrons revolve around the atomic nucleus in a certain orbit. This is just like the earth we live on and planets such as Venus and Mars revolve around the sun. Each electron has a certain energy and is in a certain orbit, or in other words, each orbit has a certain energy level.
The orbital energy level close to the nucleus is lower, and the orbital energy level farther from the nucleus is higher. The size of this energy level difference between orbits is called the energy level difference. When an electron transitions from a low energy level to a high energy level, it absorbs the energy of the corresponding energy level difference.
In an optical fiber, when an electron at a certain energy level is irradiated by light of a wavelength corresponding to the energy level difference, the electron in the low energy level orbit will transition to the orbit with a higher energy level. This electron absorbs light energy, resulting in light absorption loss.
Silicon dioxide (SiO2), the basic material for making optical fibers, absorbs light itself. One is called ultraviolet absorption and the other is called infrared absorption. At present, optical fiber communications generally only work in the wavelength range of 0.8 to 1.6 μm, so we only discuss the loss in this working range.
The absorption peak generated by electron transitions in quartz glass is around 0.1 to 0.2 μm wavelength in the ultraviolet region. As the wavelength increases, its absorption effect gradually decreases, but the affected area is very wide, up to wavelengths above 1 μm. However, ultraviolet absorption has little effect on quartz optical fibers working in the infrared region. For example, in the visible light region with a wavelength of 0.6 μm, ultraviolet absorption can reach 1 dB/km, and at a wavelength of 0.8 μm, it drops to 0.2 to 0.3 dB/km, and at a wavelength of 1.2 μm, it is only about 0.1 dB/km.
The infrared absorption loss of quartz optical fiber is caused by the molecular vibration of the infrared material. There are several vibration absorption peaks in the band above 2 μm.
Due to the influence of various doping elements in the optical fiber, it is impossible for quartz optical fiber to have a low loss window in the band above 2μm, and the theoretical limit loss at a wavelength of 1.85μm is ldB/km.
Through research, it was also found that there are some "destructive molecules" in quartz glass that are making trouble, mainly some harmful transition metal impurities, such as copper, iron, chromium, manganese, etc. These "bad guys" greedily absorb light energy under light irradiation, jump around, and cause light energy loss. Removing the "troublemakers" and chemically purifying the materials used to make optical fibers can greatly reduce the loss.
Another absorption source in quartz optical fiber is hydroxyl (OHˉ). According to the research of the period, people found that hydroxyl has three absorption peaks in the working band of optical fiber, which are 0.95μm, 1.24μm and 1.38μm, among which the absorption loss at the wavelength of 1.38μm is the most serious and has the greatest impact on the optical fiber. At a wavelength of 1.38μm, the absorption peak loss generated by the hydroxide content of only 0.0001 is as high as 33dB/km.
Where do these hydroxides come from? There are many sources of hydroxides. First, there is water and hydroxide compounds in the materials used to make optical fibers. These hydroxide compounds are not easy to be removed during the purification of raw materials, and finally remain in the optical fiber in the form of hydroxides; second, there is a small amount of water in the hydroxides used to make optical fibers; third, water is generated due to chemical reactions during the manufacturing process of optical fibers; fourth, water vapor is brought in by the entry of outside air. However, the current manufacturing process has developed to a fairly high level, and the hydroxide content has dropped to a sufficiently low level that its impact on optical fibers can be ignored.
4. Scattering loss
In the dark night, if you shine a flashlight into the sky, you can see a beam of light. People have also seen thick beams of light from searchlights in the night sky.
So why do we see these beams of light? This is because there are many tiny particles such as smoke and dust floating in the atmosphere. When light shines on these particles, it scatters and shoots in all directions. This phenomenon was first discovered by Rayleigh, so people named this scattering "Rayleigh scattering".
How does scattering occur? It turns out that the tiny particles such as molecules, atoms, and electrons that make up matter vibrate at certain inherent frequencies and can release light with a wavelength corresponding to the vibration frequency. The vibration frequency of a particle is determined by the size of the particle. The larger the particle, the lower the vibration frequency and the longer the wavelength of the light released; the smaller the particle, the higher the vibration frequency and the shorter the wavelength of the light released. This vibration frequency is called the inherent vibration frequency of the particle. However, this vibration is not generated by itself, it requires a certain amount of energy. Once a particle is irradiated with light of a certain wavelength, and the frequency of the irradiated light is the same as the inherent vibration frequency of the particle, it will cause resonance. The electrons in the particle begin to vibrate at this vibration frequency, resulting in the particle scattering light in all directions, and the energy of the incident light is absorbed and converted into the energy of the particle, and the particle re-emits the energy in the form of light energy. Therefore, for people observing from the outside, it seems that the light hits the particle and then flies out in all directions.
Rayleigh scattering also occurs in optical fibers, and the light loss caused by this is called Rayleigh scattering loss. Given the current level of optical fiber manufacturing technology, it can be said that Rayleigh scattering loss is unavoidable. However, since the magnitude of Rayleigh scattering loss is inversely proportional to the fourth power of the wavelength of light, the impact of Rayleigh scattering loss can be greatly reduced when the optical fiber operates in the long wavelength region.
5. Congenital deficiency, no one can help
The optical fiber structure is imperfect, such as bubbles, impurities, or uneven thickness in the optical fiber, especially the uneven core-cladding interface. When the light reaches these places, part of the light will be scattered in all directions, causing loss. This loss can be overcome by improving the optical fiber manufacturing process. Scattering causes light to be emitted in all directions, and part of the scattered light is reflected back in the opposite direction of the optical fiber propagation. This part of the scattered light can be received at the incident end of the optical fiber. The scattering of light causes part of the light energy to be lost, which is undesirable. However, this phenomenon can also be used by us, because if we analyze the strength of the received part of the light at the transmitting end, we can check the breakpoints, defects and loss of this optical fiber. In this way, through human ingenuity, bad things can be turned into good things.
Fiber loss In recent years, optical fiber communication has been widely used in many fields. An important issue in realizing optical fiber communication is to reduce the loss of optical fiber as much as possible. The so-called loss refers to the attenuation of optical fiber per unit length, and the unit is dB/km. The level of optical fiber loss directly affects the transmission distance or the distance between relay stations. Therefore, understanding and reducing optical fiber loss has great practical significance for optical fiber communications.
1. Absorption loss of optical fiber
This is caused by the absorption of light energy by optical fiber materials and impurities. They consume light energy in the form of heat energy in the optical fiber, which is an important loss in optical fiber loss. Absorption loss includes the following:
① Intrinsic absorption loss of material This is the loss caused by the inherent absorption of the material. It has two bands, one in the 8-12μm region of near-infrared. The intrinsic absorption of this band is due to vibration. The other intrinsic absorption band of the material is in the ultraviolet band. When the absorption is very strong, its tail will be dragged to the 0.7-1.1μm band.
②Absorption loss caused by dopants and impurity ions Optical fiber materials contain transition metals such as iron, copper, chromium, etc. They have their own absorption peaks and absorption bands and vary with their valence states. The optical fiber loss caused by the absorption of transition metal ions depends on their concentration. In addition, the presence of OH- also produces absorption loss. The basic absorption peak of OH- is near 2.7μm, and the absorption band is in the range of 0.5-1.0μm. For pure quartz optical fiber, the loss caused by impurities can be ignored.
③ Atomic defect absorption loss When the optical fiber material is heated or strongly radiated, it will be stimulated to produce atomic defects, resulting in absorption of light and loss, but in general this effect is very small.
2. Scattering loss of optical fiber
The scattering inside the optical fiber will reduce the transmission power and generate loss. The most important scattering is Rayleigh scattering, which is caused by the density and composition changes inside the optical fiber material.
During the heating process of the optical fiber material, due to thermal agitation, the compressibility of the atoms is uneven, the density of the material is uneven, and then the refractive index is uneven. This unevenness is fixed during the cooling process, and its size is smaller than the wavelength of the light wave. When light encounters these uneven materials that are smaller than the wavelength of the light wave and have random fluctuations during transmission, the transmission direction is changed, scattering occurs, and loss occurs. In addition, the uneven concentration of oxides contained in the optical fiber and uneven doping can also cause scattering and loss.
3. Waveguide scattering loss
This is the scattering caused by random distortion or roughness of the interface. In fact, it is the mode conversion or mode coupling caused by surface distortion or roughness. One mode will generate other transmission modes and radiation modes due to the fluctuation of the interface. Since the attenuation of various modes transmitted in the optical fiber is different, in the process of long-distance mode conversion, the mode with low attenuation becomes the mode with large attenuation. After continuous conversion and reverse conversion, although the loss of each mode will be balanced, the mode as a whole will produce additional loss, that is, additional loss is generated due to the conversion of the mode. This additional loss is the waveguide scattering loss. To reduce this loss, it is necessary to improve the optical fiber manufacturing process. For optical fibers that are well pulled or of high quality, this loss can basically be ignored.
4. Radiation loss caused by optical fiber bending
Optical fiber is soft and can be bent. However, after bending to a certain extent, although the optical fiber can guide light, it will change the transmission path of light. The conversion from transmission mode to radiation mode causes part of the light energy to penetrate into the cladding or pass through the cladding to become a radiation mode and leak out, thereby generating loss. When the bending radius is greater than 5 to 10 cm, the loss caused by bending can be ignored.
Source: Dongguan HX Fiber Technology Co., Ltd
