1. Briefly describe the composition of optical fiber.
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: Including loss, dispersion, bandwidth, cut-off wavelength, mode field diameter, etc.
3. What are the causes of fiber attenuation? Answer: The attenuation of 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 caused by connectors and joints.
4. How is the fiber attenuation coefficient defined? Answer: It is defined by the attenuation (dB/km) of a uniform optical fiber per unit length in the steady state.
5. What is insertion loss? Answer: It refers to the attenuation caused by the insertion of optical components (such as connectors or couplers) into optical transmission lines.
6. What is the bandwidth of optical fiber related to? Answer: The bandwidth of optical fiber refers to the modulation frequency when the amplitude of optical power is reduced by 50% or 3dB from the amplitude of zero frequency in the transfer function of optical fiber.
The bandwidth of an optical fiber is approximately inversely proportional to its length, and the product of the bandwidth length is a constant.
7. What are the dispersions of optical fibers? Related to what? Answer: The dispersion of optical fiber refers to the broadening of the group delay in an optical fiber, including mode dispersion, material dispersion and structural dispersion.
Depends on the characteristics of the light source and optical fiber.
8. How to describe the dispersion characteristics of signals propagating in optical fibers? Answer: It can be described by three physical quantities: pulse broadening, fiber bandwidth, and fiber dispersion coefficient.
9. What is the cut-off wavelength? Answer: It refers to the shortest wavelength that can only transmit the fundamental mode in the optical fiber.
For single-mode fiber, its cut-off 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 light pulse to broaden during transmission in the optical fiber.
It affects the bit error rate, the length of the transmission distance, and the system rate.
11.What is backscatter method? Answer: Backscattering is a method of 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.
Use a spectrometer to observe the time curve of backscattering at the light emitter. From one end, not only can the length and attenuation of the connected uniform fiber be measured, but also local irregularities, breakpoints and Optical power loss caused by splices and connectors.
12. What is the testing principle of optical time domain reflectometer (OTDR)? What is the function? Answer: OTDR is based on the principle of backscattering and Fresnel reflection of light. It uses the backscattered light generated when light propagates in the optical fiber to obtain attenuation information. It can be used to measure fiber attenuation, joint loss, fiber fault point location and Understanding fiber loss along its lengthDistribution, etc., are indispensable tools in optical cable construction, maintenance and monitoring.
The main index parameters include: dynamic range, sensitivity, resolution, measurement time and blind zone, etc.
13.What is the blind area of OTDR? What impact will it have on testing? How to deal with blind spots in actual tests? Answer: The series of "blind spots" caused by the saturation of the OTDR receiving end caused by reflections from characteristic points such as movable connectors and mechanical joints are usually called blind spots.
The blind area in the optical fiber is divided into two types: event blind area and attenuation blind area: the reflection peak is caused by the intervention of the active connector, and the length distance from the starting point of the reflection peak to the receiver saturation peak is called It is the event blind zone; the reflection peak caused by the intervention of the active connector in the optical fiber, and the distance from the starting point of the reflection peak to the point where other events can be identified is called the attenuation blind zone.
For OTDR, the smaller the blind zone, the better.
The blind zone will increase as the width of the pulse broadening increases. Although increasing the pulse width increases the measurement length, it also increases the measurement blind zone. Therefore, when testing optical fibers, it is necessary to test the optical fiber attached to the OTDR. Narrow pulses are used for measurements at and adjacent event points, while wide pulses are used for measurements at the far end of the fiber.
Picture 14. Can OTDR measure different types of optical fibers? Answer: If you use a single-mode OTDR module to measure multi-mode optical fiber, or use a multi-mode OTDR module to measure single-mode optical fiber with a core diameter of 62.5mm, the measurement results of the fiber length will not be affected, but the measurement results such as fiber loss will not be affected. , optical connector loss, and return loss results are incorrect.
So, when measuring optical fiber, you must choose an OTDR that matches the optical fiber under test, so that you can get correct results for all performance indicators.
15. What does “1310nm” or “1550nm” in common light test instruments refer to? Answer: It refers to the wavelength of the optical signal.
The wavelength range used in optical fiber communication is in the near-infrared region, with wavelengths between 800nm and 1700nm.
It is often divided into short wavelength band and long wavelength band, the former refers to the wavelength of 850nm, and the latter refers to 1310nm and 1550nm.
16. In currently commercial optical fibers, what wavelength of light has the smallest dispersion? What wavelength of light has the least loss? Answer: Light with a wavelength of 1310nm has minimum dispersion, and light with a wavelength of 1550nm has minimum loss.
17. How are optical fibers classified according to the changes in the refractive index of the optical fiber core? Answer: It can be divided into step fiber and gradient fiber.
The step optical fiber has a narrow bandwidth and is suitable for small-capacity and short-distance communications; the gradient optical fiber has a wider bandwidth and is suitable for medium and large-capacity communications.
18. According to the different modes of light waves transmitted in optical fibers, how are optical fibers classified?Answer: It can be divided into single-mode fiber and multi-mode fiber.
The core diameter of single-mode optical fiber is approximately between 1 and 10 μm. At a given operating wavelength, it only transmits a single fundamental mode and is suitable for large-capacity and long-distance communication systems.
Multi-mode optical fiber can transmit multiple modes of light waves, with a core diameter of approximately 50 to 60 μm, and its transmission performance is worse than that of single-mode optical fiber.
When transmitting current differential protection of multiplexing protection, multi-mode optical 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 step index fiber? Answer: 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 birefringence of single-mode fiber? Answer: There are two orthogonal polarization modes in single-mode fiber. When the 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 Is birefringence.
21. What are the most common optical cable structures? Answer: There are two types: layered type and skeleton type.
22. What does the optical cable mainly consist of? Answer: Mainly composed of: fiber core, fiber optic grease, sheath material, PBT (polybutylene terephthalate) and other materials.
23. What does the armoring of optical cables mean? Answer: It refers to the protective components (usually steel wire or steel strip) used in special-purpose optical cables (such as submarine optical cables, etc.).
The armor is attached to the inner sheath of the optical cable.
24. What material is used for optical cable sheath? Answer: The sheath or sheath of optical cables is usually made of polyethylene (PE) and polyvinyl chloride (PVC). Its function is to protect the cable core from external influences.
25. List the special optical cables used in power systems.
Answer: There are three main types of special optical cables: Ground wire composite optical cable (OPGW). The optical fiber is placed in the power line of the steel-clad aluminum strand structure.
The application of OPGW optical cable plays the dual functions of ground wire and communication, effectively improving the utilization rate of power towers.
Gunwound optical cable (GWWOP), where there are existing transmission lines, this optical cable is wound or suspended on the ground wire.
Self-supporting optical cable (ADSS) has strong tensile strength and can be hung directly between two power towers, with a maximum span of up to 1000m.
Picture 26. What are the application structures of OPGW optical cable? Answer: Mainly include: 1) The structure of plastic pipe layer + aluminum pipe; 2) The structure of central plastic pipe + aluminum pipe; 3) Aluminum skeleton structure; 4) Spiral aluminum pipe structure; 5) Single-layer stainless steel pipe structure (center Stainless steel tube structure, stainless steel tube layered structure); 6) Composite stainless steel tube structure (center stainless steel tube structure, stainless steel tube layered structure).
27.What does the stranded wire outside the OPGW optical cable core mainly consist of? Answer: Take AA wire (aluminum alloy wire) and AS wireMade of aluminum clad steel wire.
28. What are the technical conditions that should be met when choosing an OPGW optical cable model? Answer: 1) Nominal tensile strength (RTS) of OPGW optical cable (kN); 2) Number of fiber cores of OPGW optical cable (SM); 3) Short-circuit current (kA); 4) Short-circuit time (s); 5) Temperature Range (°C).
29. How is the bending degree of 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 not less than 30 times the outer diameter of the optical cable during construction (non-stationary state).
30. What should we pay attention to in the ADSS optical cable project? 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 optical cable fittings? Answer: Optical cable hardware refers 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 unions. 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 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 FC, SC, and ST. , D4, DIN, Biconic, MU, LC, MT and other types; according to the pin end face of the connector, it 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 optical fiber communication system, the following items are common, please indicate their names.
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 Line 35. What is PON (Passive Optical Network)? Answer: PON is an optical fiber loop optical network in the local user access network, based on passive optical components, such as couplers and optical splitters.
