Basic knowledge of fiber optic cable

Optical fiber, optical cable
1. Briefly describe the composition of the optical fiber.
Answer: An optical fiber consists of two basic parts: a core and a cladding layer made of transparent optical materials, and a coating layer.

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

3. What are the reasons for fiber attenuation?
Answer: The attenuation of an optical fiber refers to the decrease of the 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: It is defined by the attenuation (dB/km) per unit length of a uniform fiber in the steady state.

5. What is the insertion loss?
Answer: 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 optical fiber related to?
Answer: 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 bandwidth length is a constant.

7. How many kinds of optical fiber dispersion? What is it related to?
Answer: The dispersion of an optical fiber refers to the broadening of the group delay within an optical fiber, including modal dispersion, material dispersion, and structural dispersion. Depends on the characteristics of both the light source and the optical fiber.

8. How to describe the dispersion characteristics of the signal propagating in the optical fiber?
Answer: 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 that can only transmit the fundamental mode in the optical fiber. For a single-mode fiber, its cut-off wavelength must be shorter than the wavelength of the transmitted light.

10. What effect will the dispersion of the optical fiber have on the performance of the optical fiber communication system?
Answer: The dispersion of the optical fiber will cause the light pulse to expand during the transmission process in the optical fiber. Affects the size of the bit error rate, the length of the transmission distance, and the size of the system rate.

11. What is the backscatter method?
Answer: 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 propagates in the forward direction, but a small part is scattered back toward the illuminator. Use a spectroscope to observe the time curve of backscatter at the illuminator. From one end, not only can the length and attenuation of the uniform optical fiber connected, but also local irregularities, breakpoints, and joints and connectors caused by it can be measured. Optical power loss.

12. What is the testing principle of optical time domain reflectometer (OTDR)? What's the function?
Answer: OTDR is made based on the principle of light backscatter and Fresnel reflection. It uses the backscattered light generated when light propagates in the optical fiber to obtain attenuation information. It can be used to measure optical fiber attenuation, connector loss, fiber fault location, and Understanding the loss distribution of optical fibers along the length is an indispensable tool in the construction, maintenance and monitoring of optical cables. Its main index parameters include: dynamic range, sensitivity, resolution, measurement time and blind zone, etc.

13. What is the dead zone of OTDR? What impact will it have on testing? How to deal with the blind area in the actual test?
Answer: A series of "blind spots" caused by the saturation of the OTDR receiving end caused by the reflection of characteristic points such as movable connectors and mechanical joints are usually called blind spots.
There are two types of blindness in optical fiber: event blind zone and attenuation blind zone: the reflection peak caused by the intervention of the movable connector, the length of the distance from the starting point of the reflection peak to the saturation peak of the receiver is called the event blind zone; The intervening movable connector causes the reflection peak, and the distance from the starting point of the reflection peak to the point where other events can be identified is called the attenuation dead zone.
For OTDR, the smaller the blind zone, the better. The blind area will increase with the increase of the pulse width. Although increasing the pulse width increases the measurement length, it also increases the measurement blind area. Therefore, when testing the optical fiber, the measurement of the optical fiber of the OTDR accessory and the adjacent event point Use a narrow pulse, and use a wide pulse when measuring the far end of the fiber.

14. Can OTDR measure different types of optical fibers?
Answer: If you use a single-mode OTDR module to measure a multimode fiber, or use a multimode OTDR module to measure a single-mode fiber with a core diameter of 62.5mm, the measurement result of the fiber length will not be affected, but the fiber loss will not be affected. The results of optical connector loss and return loss are incorrect. Therefore, when measuring optical fibers, an OTDR that matches the optical fiber under test must be selected for measurement, so that all performance indicators are correct.

15. What does "1310nm" or "1550nm" in common optical test instruments refer to?
Answer: 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 800nm and 1700nm. It is often divided into short-wavelength band and long-wavelength band, the former refers to 850nm wavelength, and the latter refers to 1310nm and 1550nm.

16. In the current commercial optical fiber, what wavelength of light has the smallest dispersion? What wavelength of light has the least loss?
Answer: Light with a wavelength of 1310nm has the smallest dispersion, and light with a wavelength of 1550nm has the smallest loss.

17. According to the change of the refractive index of the fiber core, how to classify the fiber?
Answer: It can be divided into step fiber and graded fiber. Step fiber has a narrow bandwidth and is suitable for small-capacity short-distance communications; graded fiber has a wide bandwidth and is suitable for medium and large-capacity communications.

18. According to the different modes of light waves transmitted in the optical fiber, how to classify the optical fiber?
Answer: It can be divided into single-mode fiber and multi-mode fiber. The core diameter of a single-mode fiber is about 1-10μm. At a given working wavelength, only a single fundamental mode is transmitted, which is suitable for large-capacity long-distance communication systems. Multimode fiber can transmit light waves in multiple modes, and its core diameter is about 50-60μm, and its transmission performance is worse than that of single-mode fiber.
When transmitting the current differential protection of multiplexing protection, multi-mode optical fiber is 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 the step index fiber?
Answer: The numerical aperture (NA) indicates the light-receiving ability of the optical fiber. The larger the NA, the stronger the optical fiber's ability to collect light.

20. What is the birefringence of a single-mode fiber?
Answer: There are two orthogonal polarization modes in a single-mode fiber. When the fiber is not completely cylindrically symmetric, the two orthogonal polarization modes are not degenerate. The absolute value of the refractive index difference between the two orthogonal polarization modes is For birefringence.

21. What are the most common fiber optic cable structures?
Answer: There are two types: layer twist type and skeleton type.

22. What are the main components of optical cables?
Answer: It is mainly composed of: fiber core, optical fiber ointment, sheath material, PBT (polybutylene terephthalate) and other materials.

23. What is the armor of the optical cable?
Answer: Refers to the protective element (usually steel wire or steel belt) 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 the cable sheath?
Answer: The sheath or layer of the optical cable is usually composed of polyethylene (PE) and polyvinyl chloride (PVC) materials, and its function is to protect the cable core from external influences.

25. List the special optical cables used in power systems.
Answer: There are mainly three 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 function of ground wire and communication, effectively improving the utilization rate of power poles.
Wrap-type optical cable (GWWOP), where there are power transmission lines, this type of 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 poles, with a maximum span of up to 1000m.

26. What are the application structures of OPGW optical cables?
Answer: Mainly include: 1) The structure of plastic pipes + 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 (central stainless steel tube structure, stainless steel tube layered structure).

27. What are the main components of the stranded wire outside the core of the OPGW optical cable?
Answer: It is composed of AA wire (aluminum alloy wire) and AS wire (aluminum clad steel wire).

28. To choose the OPGW cable model, what are the technical conditions that should be met?
Answer: 1) Nominal tensile strength (RTS) (kN) of OPGW cable; 2) Number of fiber cores (SM) of OPGW cable; 3) Short-circuit current (kA); 4) Short-circuit time (s); 5) Temperature Range (℃).

29. How is the degree of bending of the optical cable restricted?
Answer: The bending radius of the fiber optic cable should not be less than 20 times the outer diameter of the fiber optic cable, and it should not be less than 30 times the outer diameter of the fiber optic cable during construction (non-stationary state).

30. What should be paid 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 fittings refer to the hardware used to install the optical cable, mainly including: strain clamps, suspension clamps, vibration absorbers, etc.

32. What are the two most basic performance parameters of optical fiber connectors?
Answer: Optical fiber connectors are commonly known as live connectors. For single-fiber connectors, the optical performance requirements are focused on the two most basic performance parameters of insertion loss and return loss.

33. How many types of optical fiber connectors are commonly used?
Answer: 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 fiber connectors and multi-mode fiber connectors; according to different structures, they can be divided into FC, SC, ST , D4, DIN, Biconic, 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 fiber optic connectors, SC fiber optic connectors, LC fiber optic connectors.

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 jumper MU jumper Single-mode or multi-mode jumper

35. What is the insertion loss (or insertion loss) of an optical fiber connector?
Answer: It refers to the amount of reduction in the effective power of the transmission line caused by the intervention of the connector. For users, the smaller the value, the better. ITU-T stipulates that its value should not be greater than 0.5dB.

36. What is the return loss of an optical fiber connector (or called reflection attenuation, return loss, return loss)?
Answer: It is a measure of the input power component reflected from the connector and returned along the input channel. The typical value should not be less than 25dB.

37. What is the most prominent difference between the light emitted by light-emitting diodes and semiconductor lasers?
Answer: The light produced by the light emitting diode is incoherent light with a wide frequency spectrum; the light produced by the laser is coherent light with a narrow frequency spectrum.

38. What is the most obvious difference between the operating characteristics of light emitting diodes (LED) and semiconductor lasers (LD)?
Answer: LED does not have a threshold, while LD has a threshold. Laser will only be generated when the injected current exceeds the threshold.

39. What are the two commonly used single longitudinal mode semiconductor lasers?
Answer: Both DFB lasers and DBR lasers are distributed feedback lasers, and their optical feedback is provided by the distributed feedback Bragg grating in the optical cavity.

40. What are the two main types of optical receiving devices?
Answer: There are mainly photodiodes (PIN tubes) and avalanche photodiodes (APD).

41. What are the factors that cause noise in optical fiber communication systems?
Answer: There are noise caused by unqualified extinction ratio, noise caused by random changes in light intensity, noise caused by time jitter, point noise and thermal noise of the receiver, mode noise of optical fiber, noise caused by pulse broadening caused by dispersion, and LD Mode distribution noise, the noise generated by the frequency chirp 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?
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 has a large dispersion in the C-band 1530~1565nm and L-band 1565~1625nm, generally 17~22psnm•km, when the system rate reaches 2.5Gbit/s or more, dispersion compensation is required, at 10Gbit/s Dispersion compensation cost of the system is relatively high, and it is the most common type of fiber laid in the transmission network at present.
The dispersion of G.653 dispersion-shifted fiber in C-band and L-band is generally -1～3.5psnm•km, with zero dispersion at 1550nm, and the system rate can reach 20Gbit/s and 40Gbit/s. It is single-wavelength ultra-long-distance transmission. The best fiber. However, due to its zero-dispersion characteristic, when DWDM is used for capacity expansion, nonlinear effects will occur, leading to signal crosstalk, resulting in four-wave mixing FWM, so DWDM is not suitable.
G.655 non-zero dispersion-shifted fiber: G.655 non-zero dispersion-shifted fiber has a dispersion of 1～6psnm•km in the C-band, and generally 6-10psnm•km in the L-band. The dispersion is small and avoids zero. The dispersion zone not only suppresses the four-wave mixing FWM, can be used for DWDM expansion, but also can open high-speed systems. The new G.655 fiber can expand the effective area to 1.5 to 2 times that of the ordinary fiber, and the large effective area can reduce the power density and reduce the nonlinear effect of the fiber.

43. What is the nonlinearity of optical fiber?
Answer: When the input optical power exceeds a certain value, the refractive index of the optical fiber will be nonlinearly related to the optical power, and Raman scattering and Brillouin scattering will occur, which will change the frequency of the incident light.

44. What is the effect of fiber nonlinearity on transmission?
Answer: Non-linear effects will cause some additional loss and interference, deteriorating the performance of the system. The WDM system has high optical power and transmits a long distance along the optical fiber, so nonlinear distortion is generated. There are two types of nonlinear distortion: stimulated scattering and nonlinear refraction. Among them, stimulated scattering includes Raman scattering and Brillouin scattering. The above two kinds of scattering reduce the incident light energy and cause loss. It can be ignored when the incoming fiber power is small.

45. 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 splitters.