1. Describe the components of optical fibers.

A: Optical fiber consists of two basic parts: a core and cladding made of transparent optical materials, and a coating layer.

2. What are the basic parameters that describe the transmission characteristics of an optical fiber line?

A: Including loss, dispersion, bandwidth, cut-off wavelength, mode field diameter, etc.

3. What are the causes of optical fiber attenuation?

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

4. How is the fiber attenuation coefficient defined?

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

5. What is insertion loss?

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

6. What is the bandwidth related to the optical fiber?

A: The bandwidth of the fiber refers to the modulation frequency at which the amplitude of the optical power in the transfer function of the fiber is reduced by 50% or 3dB compared to the amplitude of the zero frequency. The bandwidth of an optical fiber is approximately inversely proportional to its length, and the product of the bandwidth length is a constant.

7. How many kinds of dispersion are there in optical fiber? With what?

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

8. How to describe the dispersion characteristics of signal propagation in optical fiber?

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

9. What is the cut-off wavelength?

A: refers to the shortest wavelength in the fiber that can only conduct the fundamental mode. For single-mode fibers, the cut-off wavelength must be shorter than the wavelength of the transmitted light.

10. What effect does the dispersion of optical fibers have on the performance of optical fiber communication systems?

A: The dispersion of the fiber will widen the light pulse as it travels through the fiber. It affects the size of the bit error rate, the length of the transmission distance, and the size of the system speed.

11. What is backscatter?

A: The backscatter method is a method of measuring attenuation along the length of an optical fiber. Most of the optical power in the fiber is propagated forward, but a small part is backscattered towards the luminizer. By observing the backscattering time curve at the luminizer, we can measure not only the length and attenuation of the connected uniform fiber, but also the local irregularity, breakpoint and optical power loss caused by the connectors and connectors.

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

A: OTDR is based on the backscattering and Fresnel reflection principles of light, using the backscattering light generated when the light propagates in the fiber to obtain attenuation information, can be used to measure fiber attenuation, connector loss, fiber fault location and understand the loss distribution along the length of the fiber, is an essential tool in the construction, maintenance and monitoring of optical cables. The main parameters include dynamic range, sensitivity, resolution, measurement time and blind area.

13. What is OTDR's blind spot? How does it affect testing? How to deal with the blind area in the actual test?

A: Usually a series of "blind spots" caused by the saturation of the OTDR receiving end caused by the reflection of feature points such as active connectors and mechanical joints are called blind spots.

The blind area in the optical fiber is divided into event blind area and attenuation blind area: the reflection peak caused by the intervention of the active connector, the length distance between the starting point of the reflection peak and the saturation peak of the receiver, is called the event blind area; The distance between the starting point of the reflection peak and other event points that can be identified is called the attenuation blind area.

For OTDR, the smaller the blind spot, the better. The blind area will increase with the increase of the width of the pulse broadening. Increasing the pulse width increases the measurement length, but it also increases the measurement blind area. Therefore, when testing the fiber, narrow pulses should be used for the measurement of the fiber of the OTDR attachment and the adjacent event points, and wide pulses should be used for the measurement of 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 a multi-mode fiber, or a multi-mode OTDR module is used to measure a single-mode fiber with a core diameter of 62.5mm, the measurement results of the fiber length will not be affected, but the results such as fiber loss, optical connector loss, and return loss are incorrect. Therefore, when measuring the optical fiber, it is necessary to choose the OTDR that matches the optical fiber to be measured, so as to obtain the correct results of various performance indicators.

15. What does "1310nm" or "1550nm" in common light test instruments refer to?

A: Refers to the wavelength of the light signal. The wavelength range used in optical fiber communication is in the near infrared region, the wavelength is between 800nm and 1700nm. It is often divided into short wavelength bands and long wavelength bands, the former refers to 850nm wavelength, the latter refers to 1310nm and 1550nm.

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

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

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

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

18. How are optical fibers classified according to the modes of light wave transmission in them?

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

Multimode 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 when transmitting the current differential protection with multiple protection protection.

19. What is the significance of numerical hole passage (NA) of step-index fiber?

A: The numerical hole length (NA) indicates the optical receiving capacity of the fiber. The larger the NA, the stronger the optical fiber collecting capacity.

20. What is birefringence of single-mode fiber?

A: There are two orthogonal polarization modes in single-mode fiber, when the fiber is not completely circular column symmetry, the two orthogonal polarization modes are not degenerate, and the absolute value of the difference of the mode refractive index of the two orthogonal polarization is birefringence.

21. What are the most common optical cable structures?

A: There are two kinds of stranding and skeleton.

22. What are the main components of optical cable?

A: It is mainly composed of fiber core, fiber optic ointment, sheath material, PBT (polybutylene terephthalate) and other materials.

23. What does the armor of optical cable mean?

A: refers to the protective elements (usually steel wire or steel strip) used in special purpose optical cables (such as submarine optical cables, etc.). The armouring is attached to the inner sheath of the cable.

24. What material is used for optical cable jacket?

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

25. List special optical cables used in power systems.

A: There are three special optical cables:

Ground composite optical cable (OPGW), the optical fiber is placed in the power line of the aluminum stranded structure of the ladle. The application of OPGW optical cable plays a dual function of ground wire and communication, and effectively improves the utilization rate of power tower.

Wound optical cable (GWWOP), which is wound or suspended on a ground wire where there is already a transmission line.

Self-supporting optical cable (ADSS), with strong tensile capacity, can be directly hung between two power towers, its maximum span up to 1000m.

26. How many kinds of OPGW cable application structure are there?

A: Mainly: 1) the structure of the plastic tube layer + aluminum tube; 2) The structure of the central plastic tube + aluminum tube; 3) Aluminum skeleton structure; 4) Spiral aluminum tube structure; 5) Single-layer stainless steel pipe structure (central stainless steel pipe structure, stainless steel pipe layer twist structure); 6) Composite stainless steel pipe structure (center stainless steel pipe structure, stainless steel pipe layer twist structure).

27. What is the main composition of the twisted wire outside the OPGW cable core?

A: AA wire (aluminum alloy wire) and AS wire (aluminum clad steel wire).

28. To choose OPGW cable model, what are the technical conditions that should be available?

A: 1) OPGW cable nominal tensile strength (RTS) (kN); 2) Number of fiber cores (SM) of OPGW cable; 3) Short circuit current (kA); 4) Short circuit time (s); 5) Temperature range (° C).

29. How is the degree of bending limited?

A: The bending radius of the cable should not be less than 20 times the outer diameter of the cable, and the construction process (non-static state) should not be less than 30 times the outer diameter of the cable.

30. What should we pay attention to in ADSS optical cable engineering?

A: 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 hardware?

A: Optical cable fittings refer to the hardware used to install optical cables, mainly: tension clamp, overhang clamp, anti-vibration device.

32. What are the two most basic performance parameters of optical fiber connectors?

A: Optical 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 of the intervention loss and return loss.

33. What types of fiber optic connectors are commonly used?

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

34. The following items are commonly used in optical fiber communication systems. Please indicate their names.

AFC, FC adapter ST adapter SC adapter FC/APC, FC/PC connector SC connector ST connector LC jumper MU jumper Single mode or multi-mode jumper

35. What is PON (Passive Optical Network)?

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