Knowledge of optical fiber and cable engineering

1. Describe the components of optical fibers.

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

2. What are the basic parameters for describing the transmission characteristics of an optical fiber?

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 decrease in optical power between the two cross sections of an fiber and is wavelength dependent. The main causes of attenuation are scattering, absorption and optical loss due to connectors and joints.

4. How is the optical fiber attenuation coefficient defined?

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

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 of the optical fiber related to?

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

7. How many kinds of dispersion do optical fibers have? With what?

A: The dispersion of optical fiber refers to the broadening of the internal group delay of 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 to describe the dispersion characteristics of signal propagation in optical fiber?

Answer: 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 optical fiber can only conduct the fundamental mode of the shortest wavelength. For single-mode fibers, the cut-off wavelength must be shorter than the wavelength of the transmitted light.

10. What is the effect of optical fiber dispersion on the performance of optical fiber communication system?

A: The dispersion of the fiber will widen the optical 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 backscattering?

A: The backscatter method is a method of measuring attenuation along the length of an optical fiber. Most of the optical power in the optical fiber is propagated forward, but a small part of it is backscattered towards the luminizer. The backscattering time curve can be observed by using a splitter at the luminescence device. From one end, the length and attenuation of the homogeneous fiber can be measured, and the local irregularity, breakpoint and optical power loss caused by the joint and connector can be measured.

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

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

13. What is the dead zone of OTDR? What will be the impact on testing? How to deal with the blind area in the actual test?

A: Usually referred to as a series of "blind spots" caused by the saturation of OTDR receiving end caused by reflection from feature points such as movable connectors and mechanical joints.

There are two kinds of blind spots in optical fiber: event blind spots and attenuation blind spots. The length distance between the starting point of the reflection peak and the saturation peak of the receiver caused by the intervention of the active connector is called event blind spots. The distance between the starting point of the reflection peak and other event points that can be identified is called attenuation blind area.

For OTdrs, the smaller the dead zone, 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 pulse should be used for the measurement of the fiber attached to the OTDR and the adjacent event points, and wide pulse should be used for the measurement of the far end of the fiber.

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14. Can OTDR measure different types of optical fibers?

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 matching the measured optical fiber for measurement, so as to get the correct results of various performance indicators.

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

A: Refers to the wavelength of the optical signal. The wavelength range of optical fiber communication is in the near infrared region, between 800nm and 1700nm. It is often divided into short wavelength band and long wavelength band, 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 minimum loss?

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

17. How can optical fibers be classified according to the variation of refractive index of the fiber core?

A: It can be divided into step fiber and gradient fiber. Step fiber has narrow bandwidth and 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 communication.

18. How are optical fibers classified according to the modes of transmission of light waves 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 system. The multi-mode fiber can transmit multiple modes of light waves with a core diameter of 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 the numerical aperture (NA) of step index fiber?

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

20. What is birefringence of single-mode fiber?

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

21. What are the most common optical cable structures?

A: There are two types: stranding type and skeleton type.

22. What does optical cable mainly consist of?

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

23. What does armoured cable mean?

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

24. What kind of material is used for cable sheath?

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

25. List special optical cables used in power systems.

A: There are three special types of optical cable:

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

Wound optical cable (GWWOP), which is wound or suspended over ground wires where power lines already exist.

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

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26 OPGW cable application structure has several?

A: There are mainly: 1) the structure of plastic tube layer stranded + aluminum tube; 2) Structure of central plastic tube + aluminum tube; 3) aluminum skeleton structure; 4) spiral aluminum tube structure; 5) Single layer stainless steel pipe structure (center stainless steel pipe structure, stainless steel pipe layer stranded structure); 6) Composite stainless steel pipe structure (center stainless steel pipe structure, stainless steel pipe layer stranded structure).

27 OPGW cable core outside the stranded wire mainly composed of what?

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

28 to choose OPGW cable model, should have the technical conditions are?

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

29. How is the degree of bending limited?

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

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 the suspension point and the selection and installation of supporting hardware.

31. What are the main optical cable hardware?

A: Optical cable equipment refers to the installation of optical cable hardware, mainly include: tensioning clamp, overhanging clamp, anti-vibration device.

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

A: Optical fiber connectors are commonly known as live connectors. For the optical performance of the single fiber connector, the emphasis is on the two basic performance parameters of the intervention loss and return loss.

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

A: According to different classification methods, optical fiber connectors can be divided into different types, according to the 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 connectors are classified into FC, PC (UPC), and APC according to the pin faces. Common optical connectors are FC/PC optical connectors, SC optical connectors, and LC optical connectors.

34. Please indicate the names of the following items commonly found in optical fibre communication systems.

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 the fiber loop optical network in the local user access network, based on passive optical devices, such as couplers and splitters