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1. Briefly describe the composition of the fiber.

A: The fiber consists of two basic parts: a core and a cladding, a coating made of a transparent optical material.


2. What are the basic parameters describing the transmission characteristics of optical fiber lines? A: Including loss, dispersion, bandwidth, cutoff wavelength, mode field diameter, etc.


3. What is the cause of the fiber attenuation?

A: Fiber attenuation refers to the reduction in optical power between two cross sections of an optical fiber, which is wavelength dependent. The main causes of attenuation are scattering, absorption, and light loss due to connectors and connectors.


4. How is the fiber attenuation coefficient defined?

A: Defined by the attenuation per unit length of a uniform fiber in the steady state (dB/km).


5. What is the insertion loss?

A: Refers to the attenuation caused by the insertion of optical components (such as plug connectors or couplers) in the optical transmission line.


6. What is the bandwidth of the fiber?

A: The bandwidth of an optical fiber refers to the modulation frequency at which the amplitude of the optical power is reduced by 50% or 3 dB over 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 length of the bandwidth is a constant.

7. What are the chromatic dispersions of optical fibers? What is it about?

A: The dispersion of an optical fiber refers to the broadening of the delay of a group within an optical fiber, including mode dispersion, material dispersion, and structural dispersion. It depends on the characteristics of both the light source and the fiber.


8. What is the dispersion characteristic of the signal propagating in the fiber?

A: It can be described by three physical quantities: pulse broadening, fiber bandwidth, and fiber dispersion coefficient.


9. What is the cutoff wavelength?

Answer: It refers to the shortest wavelength of the fiber that can only conduct the fundamental mode. For a single mode fiber, the cutoff wavelength must be shorter than the wavelength of the conducted light.


10. What effect does the dispersion of the fiber have on the performance of the fiber-optic communication system?

A: The dispersion of the fiber will cause the light pulse to broaden during transmission in the fiber. Affects the size of the bit error rate, the length of the transmission distance, and the size of the system.


11. What is backscattering?

A: Backscatter is a method of measuring attenuation along the length of a fiber. Most of the optical power in an optical fiber is forward propagating, but a small portion is backscattered toward the illuminator. The time curve of backscattering is observed by the spectroscope at the illuminator, and the length and attenuation of the connected uniform fiber can be measured from one end, and local irregularities, breakpoints and joints and connectors are detected. Optical power loss.


12. What is the test principle of the optical time domain reflectometer (OTDR)? What is the function?

A: OTDR is based on the principle of light backscattering and Fresnel reflection. The backscattered light generated by the propagation of light in the fiber is used to obtain the attenuation information, which can be used to measure fiber attenuation, joint loss, fiber fault location and Understanding the distribution of loss along the length of the fiber is an indispensable tool in the construction, maintenance, and monitoring of optical cables. Its main indicator parameters include dynamic range, sensitivity, resolution, measurement time and dead zone.


13. What is the blind spot of OTDR? What is the impact on the test? How to deal with blind spots in actual tests?

A: A series of “blind spots” caused by saturation of OTDR receiving ends caused by reflections of characteristic points such as movable connectors and mechanical joints are called blind spots. The blindness in the fiber is divided into two types: the event dead zone and the attenuation dead zone: the reflection peak caused by the intervention of the active connector, the length distance from the start point of the reflection peak to the receiver saturation peak is called the event dead zone;

The interventional activity connector causes a reflection peak, which is called the attenuation dead zone, from the starting point of the reflection peak to the distance between other event points. For OTDR, the smaller the blind zone, the better.

The dead zone increases with the width of the pulse broadening. Although increasing the pulse width increases the measurement length, it also increases the measurement dead zone. Therefore, when testing the fiber, the measurement of the fiber and adjacent event points of the OTDR accessory is performed. Use a narrow pulse and use a wide pulse when making measurements on the far end of the fiber.


14. Can OTDR measure different types of fiber?

A: If a single-mode OTDR module is used to measure multimode fiber, or a multimode OTDR module is used to measure a single mode fiber such as a core diameter of 62.5 mm, the fiber length measurement will not be affected, but such as fiber loss. The result of optical connector loss and return loss is incorrect. Therefore, when measuring the fiber, it is necessary to select the OTDR that matches the fiber to be measured for measurement, so that the performance results are correct.

15. What does “1310nm” or “1550nm” refer to in common optical test instruments?

A: Refers to the wavelength of the optical signal. Optical fiber communication uses a wavelength range in the near-infrared region with a wavelength between 800 nm and 1700 nm. It is often divided into short-wavelength bands and long-wavelength bands, the former referring to the 850 nm wavelength and the latter to the 1310 nm and 1550 nm.

16. In current commercial fibers, what wavelength of light has the least dispersion? What wavelength of light has the smallest loss?

A: Light at 1310 nm has minimal dispersion, and light at 1550 nm has minimal loss.


17. According to the change of the refractive index of the fiber core, how to classify the fiber?

A: It can be divided into step fiber and graded fiber. The step fiber has a narrow bandwidth and is suitable for small-capacity short-distance communication. The tapered fiber has a wide bandwidth and is suitable for medium- and large-capacity communication.

18. How is the fiber classified according to the mode of the transmitted light wave in the fiber?

A: It can be divided into single mode fiber and multimode fiber. The single-mode fiber has a core diameter of about 1 to 10 μm, and transmits only a single fundamental mode at a given operating wavelength, which is suitable for a large-capacity long-distance communication system. Multimode fiber can transmit multiple modes of light waves with a core diameter of about 50-60 μm, and the transmission performance is worse than that of a single-mode fiber.


When transmitting the multiplex protection of the current differential protection, the multimode fiber is often used between the photoelectric 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 a step index fiber?

A: The value of the hole (NA) indicates the light-receiving ability of the fiber. The larger the NA, the stronger the ability of the fiber to collect light.


20. What is the birefringence of a single-mode fiber?

A: There are two orthogonal polarization modes in a single-mode fiber. When the fiber is not completely cylindrical, the two orthogonal polarization modes are not degenerate. The absolute value of the difference between the two orthogonal polarization modes is Birefringence


21. What are the most common cable constructions?

A: There are two types of strained and skeleton type.


22. What is the main component of the optical cable?

A: Mainly composed of: core, fiber ointment, sheath material, PBT (polybutylene terephthalate) and other materials.


23. What is the armor of the cable?

A: Refers to the protective components (usually steel wire or steel strip) used in special-purpose optical cables (such as submarine cables). The armor is attached to the inner sheath of the cable.

24. What materials are used for cable jacketing?

A: The cable jacket or sheath is usually made of polyethylene (PE) and polyvinyl chloride (PVC) materials to protect the core from external influences.


25. List the special fiber optic cables used in power systems.

A: There are three main types of optical cables: Ground wire composite optical cable (OPGW), the optical fiber is placed in the power line of the ladle aluminum stranded structure. The application of OPGW fiber optic cable plays the dual function of ground wire and communication, which effectively improves the utilization rate of power pole tower.


Wrap-around fiber optic cable (GWWOP), which is wrapped or suspended on the ground wire where existing transmission lines are present.


Self-supporting optical cable (ADSS) has a strong tensile capacity and can be hung directly between two power towers with a maximum span of 1000m.


26. What are the application structures of OPGW cable?

A: Mainly: 1) plastic tube layer twist + aluminum tube structure; 2) center plastic tube + aluminum tube structure; 3) aluminum skeleton structure; 4) spiral aluminum tube structure; 5) single layer stainless steel tube structure (center Stainless steel pipe structure, stainless steel pipe layer twisted structure); 6) Composite stainless steel pipe structure (central stainless steel pipe structure, stainless steel pipe layer twisted structure).


27. What is the main component of the stranded wire outside the OPGW cable core?

A: It consists of AA wire (aluminum alloy wire) and AS wire (aluminum steel wire).

28. What are the technical conditions to be selected for the OPGW cable model? Answer: 1) nominal tensile strength (RTS) (kn) of OPGW cable; 2) fiber core number (SM) of OPGW cable; 3) short circuit current (kA); 4) short circuit time (s); 5) temperature Range (°C).


29. How is the bending degree of the cable limited?

A: The bending radius of the cable should be no less than 20 times the outer diameter of the cable. During the construction (non-stationary state), it is not less than 30 times the outer diameter of the cable.


30. What should I pay attention to in the ADSS cable project?

A: There are three key technologies: mechanical design of the cable, determination of the suspension point and selection and installation of the matching hardware.


31. What are the main optical cable fittings?

A: Optical cable fittings refer to the hardware used to install optical cables, mainly including tensile clamps, suspension clamps, anti-vibrators, etc.

32. Fiber optic connectors have two basic performance parameters, what are they?

A: Fiber optic connectors are commonly known as live connectors. For the optical performance requirements 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 fiber optic connectors are commonly used?

A: According to different classification methods, fiber optic connectors can be divided into different types. According to different transmission media, they can be divided into single-mode fiber connectors and multimode fiber connectors. According to different structures, they can be divided into FC, SC, and ST. , D4, DIN, MU, LC, MT and other types; according to the pin end face of the connector can be divided into FC, PC (UPC) and APC. Commonly used fiber optic connectors: FC/PC type fiber optic connectors, SC type fiber optic connectors, LC type fiber optic connectors.


34. In the fiber-optic communication system, the following items are common, please indicate their name. AFC, FC type adapter ST type adapter SC type adapter FC/APC, FC/PC type connector SC type connector ST type connector LC type jumper MU type jumper Single mode or multimode jumper.


35. What is the insertion loss (or insertion loss) of the fiber connector? A: It refers to the amount of effective power reduction of the transmission line caused by the intervention of the connector. For the user, the smaller the value, the better. The ITU-T specifies that its value should be no more than 0.5 dB.


36. What is the return loss (or reflection attenuation, return loss, return loss) of the fiber connector? A: A measure of the input power component reflected back from the connector and returned along the input channel, which should typically be no less than 25 dB.


37. What is the most prominent difference between the light emitted by a light-emitting diode and a semiconductor laser?

Answer: The light produced by the LED is incoherent and the spectrum is wide. The light produced by the laser is coherent and the spectrum is very narrow.


38. What is the most obvious difference in the operating characteristics of light-emitting diodes (LEDs) and semiconductor lasers (LDs)?

A: There is no threshold for the LED, and there is a threshold for the LD. Only when the injection current exceeds the threshold will the laser be generated.


39. Which two types of single longitudinal mode semiconductor lasers are commonly used?

A: DFB lasers and DBR lasers, both of which are distributed feedback lasers, whose optical feedback is provided by distributed feedback Bragg gratings in the optical cavity.


40. What are the two main types of light receiving devices?

A: There are mainly photodiodes (PIN tubes) and avalanche photodiodes (APDs).

41. What are the factors that cause noise in fiber-optic communication systems?

A: There is noise due to unqualified extinction ratio, noise with random variation of light intensity, noise caused by time jitter, point noise and thermal noise of the receiver, mode noise of the fiber, noise caused by dispersion of pulse dispersion, LD The mode distributes noise, the noise generated by the frequency of the LD, and the noise generated by the reflection.


42. What are the main optical fibers currently used for transmission network construction? What are its main features?

A: There are three main types, namely G.652 conventional single mode fiber, G.653 dispersion shifted single mode fiber and G.655 non-zero dispersion shifted fiber.

The G.652 single-mode fiber has a large dispersion in the C-band 1530~1565nm and the L-band 1565~1625nm, generally 17~22psnm•km. When the system rate reaches 2.5Gbit/s or more, the dispersion compensation needs to be performed at 10Gbit/s. When the system dispersion compensation cost is large, it is the most common type of fiber laid in the transmission network.

The G.653 dispersion-shifted fiber has a dispersion of -1 to 3.5 psnm•km in the C-band and L-band, zero dispersion at 1550 nm, and a system rate of 20 Gbit/s and 40 Gbit/s, which is a single-wavelength ultra-long-distance transmission. The best fiber. However, due to its zero dispersion characteristics, when DWDM is used for expansion, nonlinear effects occur, resulting in signal crosstalk, resulting in four-wave mixing FWM, so DWDM is not suitable.

G.655 non-zero dispersion-shifted fiber has a dispersion of 1 to 6 psnm•km in the C-band, 6 to 10 psnm•km in the L-band, and the dispersion is small, avoiding zero. The dispersion region suppresses the four-wave mixing FWM and can be used for DWDM expansion or high-speed systems. The new G.655 fiber can expand the effective area to 1.5 to 2 times that of a typical fiber. The large effective area can reduce the power density and reduce the nonlinear effect of the fiber.


43. What is the nonlinearity of the fiber?

Answer: It means that when the input optical power exceeds a certain value, the refractive index of the fiber will be nonlinearly related to the optical power, and Raman scattering and Brillouin scattering will be generated to change the frequency of the incident light.


44. What effect does fiber nonlinearity have on transmission?

A: Non-linear effects can cause some additional loss and interference, degrading the performance of the system. The WDM system has large optical power and transmits a long distance along the optical fiber, thus generating nonlinear distortion. Nonlinear distortion has both stimulated and nonlinear refraction. Among them, the stimulated scattering has Raman scattering and Brillouin scattering. The above two kinds of scattering reduce the incident light energy and cause loss, which can be ignored when the fiber input power is small.


45. What is PON (passive optical network)?

A: PON is a fiber-optic loop optical network in a local user access network, based on passive optical components such as couplers and splitters.

Multiple reasons for fiber attenuation

1. The main factors causing fiber attenuation are: intrinsic, bending, extrusion, impurities, unevenness, and docking.


Intrinsic: is the inherent loss of fiber, including Rayleigh scattering, inherent absorption.

Bending: When the fiber is bent, the light in some of the fibers will be lost due to scattering, resulting in loss.


Extrusion: Loss caused by tiny bends in the fiber when it is squeezed. Impurities: Impurities in the fiber that absorb and scatter light propagating in the fiber.


Uneven: loss due to uneven refractive index of the fiber material.


Connecting: Loss caused when the fiber is docked, such as different axes (single mode fiber coaxiality requirement is less than 0.8μm), the end face is not perpendicular to the axis, the end face is not flat, the butt diameter is not matched, and the welding quality is poor. When light is incident from one end of the fiber and emitted from the other end, the intensity of the light is weakened. This means that after the optical signal propagates through the fiber, the light energy is attenuated. This means that some substances in the fiber or for some reason block the passage of the light signal. This is the transmission loss of the fiber. Only by reducing the fiber loss can the optical signal be unobstructed.

Classification of fiber loss

Fiber loss can be roughly divided into the inherent loss of the fiber and the additional loss caused by the usage conditions after the fiber is manufactured. The specific breakdown is as follows: Fiber loss can be divided into inherent loss and additional loss.


Intrinsic losses include scattering losses, absorption losses, and losses due to imperfect fiber structure. Additional losses include microbend loss, bend loss, and splice loss. Among them, the additional loss is artificially caused during the laying process of the optical fiber.


In practical applications, it is inevitable to connect the fibers one by one, and the fiber connection will cause loss. Micro-bending, squeezing, and tensile forces on the fiber can also cause losses. These are the losses caused by the conditions of use of the fiber. The main reason for this is that under these conditions, the transmission mode in the fiber core has changed. Additional losses can be avoided as much as possible.


Below, we only discuss the inherent loss of the fiber. In the intrinsic loss, the scattering loss and the absorption loss are determined by the characteristics of the fiber material itself, and the inherent losses caused by the different operating wavelengths are also different. It is extremely important to understand the mechanism of loss and quantitatively analyze the loss caused by various factors.


Absorption loss of materials

The material from which the fiber is made is capable of absorbing light energy. After the particles in the fiber material absorb the light energy, vibration and heat are generated, and the energy is lost, thus generating absorption loss. We know that matter is made up of atoms and molecules, and atoms are composed of nucleus and extranuclear electrons. The electrons rotate around the nucleus in a certain orbit. It’s like the earth we live in and the planets such as Venus and Mars are spinning around the sun. Every electron has a certain amount of energy, in a certain orbit, or each orbit has a certain energy level. The orbital energy level near the nucleus is lower, and the higher the orbital energy level from the nucleus is. The magnitude of this energy level difference between tracks is called the energy level difference. When electrons transition from a low energy level to a high energy level, they absorb the energy of the corresponding level difference.


In an optical fiber, when electrons of a certain energy level are irradiated with light of a wavelength corresponding to the energy level difference, electrons located in a low-level orbital state will transition to an orbit having a high energy level. This electron absorbs light energy and produces absorption loss of light. The basic material for the manufacture of optical fibers, silica (SiO2), absorbs light itself, one called UV absorption and the other called infrared absorption.


At present, optical fiber communication generally only works in the wavelength range of 0.8 to 1.6 μm, so we only discuss the loss of this working area. The absorption peak generated by the electronic transition in the quartz glass is about 0.1 to 0.2 μm in the ultraviolet region. As the wavelength increases, its absorption gradually decreases, but the area of ​​influence is wide, up to a wavelength of 1 μm or more. However, UV absorption has little effect on the quartz fiber operating in the infrared region. For example, in the visible light region of 0.6 μm wavelength, the 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 fiber is caused by the molecular vibration of the material in the infrared region. There are several vibration absorption peaks in the band of 2 μm or more.


Due to the influence of various doping elements in the fiber, it is impossible for the quartz fiber to have a low loss window in the band of 2 μm or more, and the theoretical limit loss at the wavelength of 1.85 μm is ldB/km.


Through research, it has also been found that some “destructive molecules” in quartz glass are in trouble, mainly some harmful transition metal impurities such as copper, iron, chromium and manganese. These “bad guys” greedily absorb light energy under the illumination of light, causing the loss of light energy. By eliminating the “chaotic molecules” and performing chemical purification on the materials used to make the optical fibers, the loss can be greatly reduced.


Another source of absorption in quartz fibers is the study of the hydroxide (OHˉ) phase. It has been found that hydroxide has three absorption peaks in the fiber working band, which are 0.95 μm, 1.24 μm and 1.38 μm, respectively, of which 1.38 μm wavelength The absorption loss is the most serious, and the impact on the fiber is also the biggest. At a wavelength of 1.38 μm, the absorption peak loss of the hydroxide of only 0.0001 is as high as 33 dB/km. Where did these hydroxides come from? There are many sources of hydroxide. First, there are water and hydroxide in the material used to make the fiber. These hydroxides are not easily removed during the purification of the raw materials, and finally remain in the form of hydroxide in the form of hydroxide; The oxyhydrogen produced by the optical fiber contains a small amount of water; the third is the formation of water due to a chemical reaction in the manufacturing process of the optical fiber; and the fourth is the introduction of external air to bring water vapor. However, the current manufacturing process has reached a fairly high level, and the hydroxide content has been reduced to a sufficiently low level, and its impact on the fiber is negligible.

Scattering loss

How is scattering generated? The tiny particles of molecules, atoms, electrons, and the like that originally constitute the substance vibrate at some natural frequencies, and can emit light having 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 released light; the smaller the particle, the higher the vibration frequency, and the shorter the wavelength of the released light. This vibration frequency is called the natural vibration frequency of the particles. But this vibration is not self-generated, it requires a certain amount of energy. Once the particles are irradiated with light having a certain wavelength, and the frequency of the irradiated light is the same as the natural vibration frequency of the particles, resonance is caused. The electrons in the particle start to vibrate at the vibration frequency. As a result, the particle scatters 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-extracts the energy in the form of light energy.


Therefore, for those who observe outside, it seems that after the light hits the particle, it flies out in all directions.


There is also Rayleigh scattering in the fiber, and the resulting optical loss is called Rayleigh scattering loss. In view of the current level of fiber manufacturing technology, it can be said that Rayleigh scattering loss is unavoidable. However, since the Rayleigh scattering loss is inversely proportional to the fourth power of the light wavelength, the effect of Rayleigh scattering loss can be greatly reduced when the fiber operates in the long wavelength region. 

Optical fiber absorption loss

This is caused by the absorption of light energy by fiber materials and impurities. They consume light energy in the form of heat energy in the fiber, which is an important loss in fiber loss. The absorption loss includes the following:


1. Intrinsic Absorption Loss This is the loss due to the inherent absorption of the material. It has two frequency bands, one in the near-infrared region of 8 to 12 μm, and the intrinsic absorption of this band is due to vibration. The intrinsic absorption band of another substance is in the ultraviolet range. When the absorption is strong, its tail will be dragged into the 0.7-1.1 μm band.


2. Absorption losses due to dopants and impurity ions Fiber materials contain transition metals such as iron, copper, chromium, etc., which have their own absorption peaks and absorption bands and vary with their valence states. The loss of fiber caused by the absorption of transition metal ions depends on their concentration. In addition, absorption of OH- also causes absorption loss, the basic absorption peak of OH- is around 2.7 μm, and the absorption band is in the range of 0.5 to 1.0 μm. For pure silica fibers, the effects of impurities caused by impurities can be ignored.


3. Atomic Defect Absorption Loss Due to the heat or strong radiation, the fiber material is excited to generate atomic defects, which cause absorption of light and loss, but in general, this effect is small.


Optical fiber scattering loss

The scattering inside the fiber reduces the power transmitted and generates losses. The most important of the scattering is Rayleigh scattering, which is caused by changes in density and composition inside the fiber material. During the heating process of the optical fiber material, due to the thermal turbulence, the compressibility of the atoms is not uniform, the density of the material is not uniform, and the refractive index is not uniform.


This unevenness is fixed during the cooling process and its size is smaller than the wavelength of the light wave. When the light encounters these light-wavelengths, which are smaller than the wavelength of the light wave, and the uneven material with random fluctuations, the transmission direction is changed, scattering occurs, and loss is caused. In addition, uneven concentration of oxides contained in the optical fiber and uneven doping may cause scattering and loss.