The optical fiber consists of two basic parts: a core made of a transparent optical material, a cladding, and a coating layer.
1. Briefly describe the composition of the optical fiber.
A: The optical fiber consists of two basic parts: a core made of a transparent optical material, a cladding, and a coating.
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 causes the fiber attenuation?
A: The attenuation of an optical fiber refers to the reduction of optical power between two cross-sections of an optical fiber, which is related to the wavelength. The main causes of attenuation are scattering, absorption, and optical loss due to connectors and joints.
4. How is the fiber attenuation coefficient defined?
Answer: Defined by the attenuation (dB/km) per unit length of a uniform fiber in the steady state.
5. What is the insertion loss?
A: It refers to the attenuation caused by the insertion of optical components such as insertion connectors or couplers in optical transmission lines.
6. What is the bandwidth of the optical fiber?
A: The bandwidth of an optical fiber refers to the modulation frequency when the amplitude of the optical power is reduced by 50% or 3dB from the amplitude of the zero frequency in the transfer function of the optical fiber. The bandwidth of an optical fiber is approximately inversely proportional to its length, and the product of the length of the bandwidth is a constant.
7. There are several kinds of optical fiber dispersion? What is it about?
A: Dispersion of an optical fiber refers to the spread of the group delay within an optical fiber, including mode dispersion, material dispersion, and structural dispersion. Depends on the characteristics of both the light source and the fiber.
8. How does the dispersion characteristics of the signal propagating in the fiber be described?
Answer: It can be described by three physical quantities: the pulse width, the bandwidth of the fiber, and the dispersion coefficient of the fiber.
9. What is the cutoff wavelength?
A: It refers to the shortest wavelength in the fiber that can only transmit the fundamental mode. For single-mode fibers, the cutoff wavelength must be shorter than the wavelength of the transmitted light.
10. What is the effect of fiber dispersion on the performance of optical fiber communication systems?
A: The dispersion of the fiber will cause the light pulse to broaden during transmission in the fiber. Impact on the size of the bit error rate, and the length of the transmission distance, and the size of the system rate
11. What is backscatter?
A: Backscattering is a method of measuring attenuation along the length of an optical fiber. Most of the optical power in the fiber is forward-propagating, but there are very few back-scattered light emitters. Using the beam splitter to observe the time curve of backscatter at the emitter, not only can measure the length and attenuation of the uniform optical fiber from one end, but also can detect local irregularities, breakpoints, and joints and connectors. Optical power loss.
12. What is the test principle of the OTDR? What function?
A: The OTDR is based on the principle of light backscattering and Fresnel reflection. It uses the backscattered light generated when light travels through the fiber to obtain information on attenuation. It can be used to measure fiber attenuation, joint loss, and fault location of optical fibers. Understanding the loss distribution along the length of an optical fiber is an indispensable tool in the construction, maintenance, and monitoring of optical fiber cables. Its main index parameters include: dynamic range, sensitivity, resolution, measurement time and blind zone.
13. What is the blind spot of OTDR? How will it affect the test? How to deal with the blind area in the actual test?
A: A series of [blind spots" brought about by saturation of the OTDR receiver caused by the reflection of feature points such as active connectors, mechanical connectors, etc., are often referred to as dead zones.
The blind division in the optical fiber is the event dead zone and the attenuation blind zone: the reflection peak due to the intervention of the active connector, and the length distance from the start point of the reflection peak to the saturation peak of the receiver is called an event dead zone; in the optical fiber The connector of the intervention activity causes reflection peaks. The distance from the start point of the reflection peak to the point where other events can be identified is called the attenuation blind zone.
For OTDRs, the smaller the blind spot, the better. The blind zone will increase with the width of pulse broadening. Increasing the pulse width increases the measurement length, but it also increases the measurement blind zone. Therefore, when testing the optical fiber, measure the optical fiber and adjacent incidents of the OTDR accessory. To use a narrow pulse, use a wide pulse when measuring the far end of the fiber.
14. Can OTDR measure different types of fiber?
A: If you use a single-mode OTDR module to measure multimode fiber, or use a multimode OTDR module to measure a single-mode fiber such as a core diameter of 62.5mm, the measurement of fiber length will not be affected, but such as fiber loss The results of optical joint loss and return loss are incorrect. Therefore, when measuring an optical fiber, it is necessary to select an OTDR that matches the measured optical fiber for measurement, so as to obtain the correct result for each performance index.
15. What is "1310nm" or "1550nm" in common light test instruments?
A: It refers to the wavelength of the optical signal. The wavelength range used for optical fiber communication is in the near-infrared region, and the wavelength is between 800 nm and 1700 nm. It is often divided into short wavelength bands and long wavelength bands, the former refers to the 850nm wavelength, the latter refers to 1310nm and 1550nm.
16. In the current commercial fiber, what wavelength of light has the smallest dispersion? What wavelength of light has the least loss?
A: The 1310nm wavelength light has the smallest dispersion, and the 1550nm wavelength light has the smallest loss.
17. According to the fiber core refractive index changes, how to classify fiber?
A: It can be divided into step fiber and gradient fiber. Step fiber bandwidth is narrow, suitable for small-capacity short-distance communication; gradient fiber bandwidth is wider, suitable for medium and large-capacity communications
18. According to the different modes of transmission light wave in the optical fiber, how to classify the optical fiber?
A: It can be divided into single mode fiber and multimode fiber. Single-mode fiber core diameter of about 1 ~ 10μm, at a given working wavelength, only a single transmission mode, suitable for large-capacity long-distance communication systems. The multi-mode optical fiber can transmit light waves of multiple modes with a core diameter of about 50 to 60 μm, and the transmission performance is poorer than that of the single-mode optical fiber.
When transmitting multiplexed and protected current differential protection, multi-mode optical fiber is used between the photoelectric conversion device installed in the substation communication room 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 numerical aperture (NA) indicates the light receiving capability of the optical fiber. The larger the NA, the stronger the optical fiber collection ability.
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 symmetrical, the two orthogonal polarization modes are not degenerate. The absolute value of the difference between the refractive indices of two orthogonal polarizations is Birefringence.
21. What are the most common cable structures?
Answer: There are two kinds of layered and skeleton type.
22. What is the main component of optical fiber cable?
A: Mainly consists of: core, optical grease, sheath material, PBT (polybutylene terephthalate) and other materials.
23. What is fiber optic cable assembly?
A: It refers to the protective element (usually steel wire or steel tape) used in special-purpose cable (such as submarine cable). The armor is attached to the inner sheath of the cable.
24. What material is used for the cable sheath?
A: The cable sheathing or sheathing is usually composed of polyethylene (PE) and polyvinyl chloride (PVC) materials. Its role is to protect the core from external influences.
25. List special optical cables for use in power systems.
A: There are three main types of special optical cables:
The ground composite cable (OPGW), the optical fiber is placed in the power line of the aluminum stranded structure of the ladle. The application of OPGW fiber optic cable has played a dual function of ground wire and communication, effectively improving the utilization of the power pole tower.
Wrap-around optical cable (GWWOP), where the cable is wound or suspended on the ground, where it already exists.
Self-supporting cable (ADSS) has strong tensile strength and can be directly hung between two power towers with a maximum span of up to 1000m.
26. What are the application structures of the OPGW cable?
A: The main ones are: 1) the structure of the plastic pipe layer twisted + aluminum pipe; 2) the structure of the central plastic pipe + aluminum pipe; 3) the aluminum skeleton structure; 4) the spiral aluminum pipe structure; 5) the single layer stainless steel pipe structure (center Stainless steel pipe structure, stainless steel tube stranded structure) 6) Composite stainless steel pipe structure (center stainless steel pipe structure, stainless steel pipe layer twisted structure)
27. What are the main stranded wires outside the OPGW cable?
A: It consists of AA wire (aluminum alloy wire) and AS wire rod (aluminum steel wire).
28. What are the technical conditions for selecting an OPGW cable model?
Answer: 1) The nominal tensile strength (RTS) (kN) of the OPGW cable; 2) The number of optical fiber cores (SM) of the OPGW cable; 3) Short-circuit current (kA); 4) Short-circuit time (s); Range (°C).
29. How is the degree of cable bending limited?
Answer: The bending radius of the optical cable should not be less than 20 times of the outer diameter of the optical cable. During construction (non-stationary state), it should not be less than 30 times of the outer diameter of the optical cable.
30. What should be noted in the ADSS cable project?
A: There are three key technologies: the design of cable machinery, the determination of suspension points, and the selection and installation of supporting hardware.
31. What are the main cable hardware?
A: Optical cable hardware refers to the hardware used to install optical cables, including: strain-resistant clamps, suspension clamps, and vibration dampers.
32. Fiber optic connectors have two basic performance parameters. What are they?
A: Fiber 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, the insertion loss and the return loss.
33. There are several types of commonly used fiber optic connectors?
A: According to different classification methods, optical fiber connectors can be divided into different types. According to the different transmission media, they can be divided into single-mode optical fiber connectors and multi-mode optical fiber connectors; according to the structure, they can be divided into FC, SC, and ST. , D4, DIN, Biconic, MU, LC, MT and other types; according to the connector pin face can be divided into FC, PC (UPC) and APC. Common fiber connector: FC / PC fiber connector, SC fiber connector, LC fiber connector.
34. In the optical fiber communication system, the following items are common, please indicate its name.
AFC, FC Adapters ST Adapters SC Adapters FC/APC, FC/PC Connectors SC Connectors ST Connectors LC Patch Cords MU Patch Cables Single or multi-mode patch cords.
35. What is the insertion loss (or insertion loss) of the fiber connector?
A: It is the magnitude of the reduction of the effective power of the transmission line due to the intervention of the connector. For the user, the smaller the value, the better. The ITU-T specifies that its value should not be greater than 0.5 dB.
36. What is the return loss of optical fiber connectors (or reflectance attenuation, return loss, return loss)?
A: A measure of the input power component that is reflected back from the connector and returned along the input channel. The typical value should not be less than 25 dB.
37. What is the most striking difference between light emitting diodes and semiconductor lasers?
A: The light produced by light emitting diodes is incoherent light, and the spectrum is broad; the light produced by lasers is coherent light, and the spectrum is very narrow.
38. What are the most obvious differences in the operating characteristics of light emitting diodes (LEDs) and semiconductor lasers (LDs)?
A: There is no threshold for LED, and there is a threshold for LD. Only the injected current exceeds the threshold value and the laser is generated.
39. Which two kinds 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 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 optical fiber communication systems?
Answer: There are noise due to unsatisfactory extinction ratio, noise with randomly varying light intensity, noise due to time jitter, point noise and thermal noise at the receiver, mode noise of the fiber, noise due to pulse broadening caused by dispersion, LD Mode-distributed noise, noise generated by the frequency of LD, and noise due to reflection
42. What are the main optical fibers currently used for transmission network construction? What are its main features?
Answer: 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.
G.652 single-mode fiber dispersion in the C-band 1530 ~ 1565nm and L-band 1565 ~ 1625nm, generally 17 ~ 22psnm? Km, when the system speed reaches 2.5Gbit/s or more, it needs to perform dispersion compensation. At 10Gbit/s, the dispersion compensation cost of the system is relatively large. It is currently the most common fiber in the transmission network.
Dispersion of G.653 dispersion shifted fiber in the C-band and L-band dispersion is generally -1 ~ 3.5psnm? Km, it is zero dispersion at 1550nm, the system speed can reach 20Gbit/s and 40Gbit/s, it is the best optical fiber of the single-wavelength ultra long distance transmission. However, due to its zero-dispersion characteristics, when DWDM is used for capacity expansion, nonlinear effects occur, resulting in signal crosstalk, resulting in four-wave mixing FWM. Therefore, it is not suitable for DWDM.
G.655 non-zero dispersion shifted fiber: Dispersion of G.655 non-zero dispersion shifted fiber in C band is 1~6psnm? Km, the dispersion in the L-band is generally 6 ~ 10psnm? With a small dispersion and avoiding the zero-dispersion zone, it suppresses the four-wave mixing FWM and can be used for DWDM expansion. It can also open high-speed systems. The new type of G.655 fiber can enlarge the effective area to 1.5 to 2 times that of the general 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?
A: When the fiber 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 occur, causing the incident light frequency to change.
44. What effect does fiber nonlinearity have on transmission?
A: Non-linear effects can cause some additional losses and *, deteriorating system performance. WDM systems have large optical power and travel long distances along the fiber, thus generating nonlinear distortion. There are two types of nonlinear distortion: stimulated scattering and nonlinear refraction. Among them, stimulated scattering has Raman scattering and Brillouin scattering. The above two kinds of scattering reduce the incident light energy and cause losses. It can be ignored when the input power is small.
45. What is PON (passive optical network)?
A: PON is a fiber loop optical network in the local user access network. It is based on passive optical components such as couplers and splitters.
