Basic knowledge of optical fiber and optical cable

1. Briefly describe the composition of the fiber.

* The fiber consists of two basic parts: a core and a cladding, a coating made of a transparent optical material.

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

* Includes loss, dispersion, bandwidth, cutoff wavelength, mode field diameter, etc.

3. What is the cause of the fiber attenuation?

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

4. How is the fiber attenuation coefficient defined?

* Defined by the attenuation per unit length of a uniform fiber in the steady state (dB/km).

5. What is the insertion loss?

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

6. What is the bandwidth of the fiber?

* Fiber bandwidth refers to the modulation frequency at which the amplitude of the optical power is reduced by 50% or 3 dB over the amplitude of the zero frequency in the transfer function of the 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. What are the chromatic dispersions of optical fibers? What is it about?

*Dispersion of an optical fiber refers to the broadening of the delay of a group within an optical fiber, including mode dispersion, material dispersion, and structural dispersion. It depends on the characteristics of both the light source and the fiber.

8. What is the dispersion characteristic of the signal propagating in the fiber?

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

9. What is the cutoff wavelength?

* refers to the shortest wavelength of the fiber that can only conduct the fundamental mode. For a single mode fiber, the cutoff wavelength must be shorter than the wavelength of the conducted light.

10. What effect does the dispersion of the fiber have on the performance of the fiber-optic communication system?

* The dispersion of the fiber will cause the light pulse to broaden during transmission in the fiber. Affect the size of the bit error rate, the length of the transmission distance, and the size of the system.

11. What is backscattering?

*. Backscatter is a method of measuring attenuation along the length of a fiber. Most of the optical power in the fiber is forward propagating, but a small portion is backscattered towards the illuminator. The time curve of backscattering is observed by the spectroscope at the illuminator. From one end, not only the length and attenuation of the connected uniform fiber can be measured, but also local irregularities, breakpoints and joints and connectors are detected. Optical power loss.

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

*OTDR is based on the principle of light backscattering and Fresnel reflection. It uses the backscattered light generated by light propagation in the fiber to obtain the attenuation information, which can be used to measure fiber attenuation, joint loss, fiber fault location and understanding. The distribution of loss along the length of the fiber is an indispensable tool in the construction, maintenance and monitoring of the cable. Its main indicator parameters include: dynamic range, sensitivity, resolution, measurement time and dead zone.

13. What is the blind spot of OTDR? What is the impact on the test? How to deal with the blind spot in the actual test?

* A series of "blind spots" caused by saturation of OTDR receiving ends caused by reflections of characteristic points such as movable connectors and mechanical joints are usually referred to as dead zones. The blindness in the fiber is divided into two types: the event dead zone and the attenuation dead zone: the reflection peak caused by the intervention of the active connector, the length distance from the start point of the reflection peak to the receiver saturation peak is called the event dead zone; The interventional activity connector causes a reflection peak, which is called the attenuation dead zone, from the starting point of the reflection peak to the distance between other event points. For OTDR, the smaller the blind zone, the better. The dead zone increases with the width of the pulse broadening. Although increasing the pulse width increases the measurement length, it also increases the measurement dead zone. Therefore, when testing the fiber, the measurement of the fiber and adjacent event points of the OTDR accessory is performed. Use a narrow pulse and use a wide pulse when making measurements on the far end of the fiber.

14. Can OTDR measure different types of fiber?

* If a single-mode OTDR module is used to measure multimode fiber, or a multimode OTDR module is used to measure a single mode fiber such as a core diameter of 62.5 mm, the fiber length measurement will not be affected, but such as fiber loss, The result of optical connector loss and return loss is incorrect. Therefore, when measuring the fiber, it is necessary to select the OTDR that matches the fiber to be measured for measurement, so that the performance results are correct.

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

* refers to the wavelength of the optical signal. Optical fiber communication uses a wavelength range in the near-infrared region with a wavelength between 800 nm and 1700 nm. It is often divided into short-wavelength bands and long-wavelength bands, the former referring to the 850 nm wavelength and the latter to the 1310 nm and 1550 nm.

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

* 1310 nm wavelength light has minimal dispersion, and 1550 nm wavelength light has minimal loss.

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

* Can be divided into step fiber and graded fiber. The step fiber has a narrow bandwidth and is suitable for small-capacity short-distance communication. The tapered fiber has a wide bandwidth and is suitable for medium- and large-capacity communication.

18. How is the fiber classified according to the mode of the transmitted light wave in the fiber?

* Can be divided into single mode fiber and multimode fiber. The single-mode fiber has a core diameter of about 1 to 10 μm, and transmits only a single fundamental mode at a given operating wavelength, which is suitable for a large-capacity long-distance communication system. Multimode fiber can transmit multiple modes of light waves with a core diameter of about 50-60 μm, and the transmission performance is worse than that of a single-mode fiber. When transmitting the multiplex protection of the current differential protection, the multimode 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 a step index fiber?

*The value of the hole (NA) indicates the light-receiving ability of the fiber. The larger the NA, the stronger the ability of the fiber to collect light.

20. What is the birefringence of a single-mode fiber?

* There are two orthogonal polarization modes in a single-mode fiber. When the fiber is not completely cylindrical, the two orthogonal polarization modes are not degenerate. The absolute value of the difference between the two orthogonal polarization modes is Birefringence.

21. What are the most common cable constructions?

* There are two types of layered and skeleton type.

22. What is the main component of the optical cable?

* Mainly composed of: core, fiber ointment, sheath material, PBT (polybutylene terephthalate) and other materials.

23. What is the armor of the cable?

* refers to the protective element (usually steel wire or steel strip) used in special-purpose optical cables (such as submarine cables). The armor is attached to the inner sheath of the cable.

24. What materials are used for cable jacketing?

* Cable jacket or sheath is usually made of polyethylene (PE) and polyvinyl chloride (PVC) materials to protect the core from external influences.

25. List the special fiber optic cables used in power systems.

* There are mainly three special optical cables: ground wire composite optical cable (OPGW), and the optical fiber is placed in the power line of the ladle aluminum twisted structure. The application of OPGW fiber optic cable plays the dual function of ground wire and communication, effectively improving the utilization rate of power tower. Wrap-around fiber optic cable (GWWOP), which is wrapped or suspended on the ground wire where existing transmission lines are present. Self-supporting optical cable (ADSS) has a strong tensile capacity and can be hung directly between two power towers with a maximum span of 1000m.

26. What are the application structures of OPGW cable?

*Rotary aluminum tube structure; 5) Single-layer stainless steel tube structure (central stainless steel tube structure, stainless steel tube layer twisted structure); 6) Composite stainless steel tube structure (central stainless steel tube structure, stainless steel tube layer twisted structure).

27. What is the main component of the stranded wire outside the OPGW cable core?

*Composed of AA wire (aluminum alloy wire) and AS wire (aluminum steel wire).

28. What are the technical conditions to be selected for the OPGW cable model?

*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 (°C).

29. How is the bending of the cable limited?

* The bending radius of the cable should be no less than 20 times the outer diameter of the cable. During the construction (non-stationary state), it is not less than 30 times the outer diameter of the cable.

30. What should I pay attention to in the ADSS cable project?

* There are three key technologies: mechanical design of the cable, determination of the suspension point and selection and installation of the matching hardware.

31. What are the main types of optical cable fittings?

* Optical cable fittings refer to the hardware used to install optical cables, mainly including: tensile clamps, suspension clamps, anti-vibrators, etc.

32. Fiber optic connectors have two basic performance parameters, what are they?

* Fiber optic 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 insertion loss and return loss.

33. How many types of fiber optic connectors are commonly used?

*According to different classification methods, optical fiber connectors can be divided into different types, which can be divided into single-mode fiber connectors and multi-mode fiber connectors according to different transmission media; 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 type fiber optic connectors, SC type fiber optic connectors, LC type fiber optic connectors.

34. In the fiber-optic communication system, the following items are common, please indicate their name. 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 35. What is fiber connection Intrusion loss (or insertion loss)?

* refers to the amount of effective power reduction of the transmission line caused by the intervention of the connector. For the user, the smaller the value, the better. The ITU-T specifies that its value should be no more than 0.5 dB.

36. What is the return loss (or reflection attenuation, return loss, return loss) of the fiber connector?

* is a measure of the input power component reflected back from the connector and returned along the input channel, which should typically be no less than 25 dB.

37. What is the most prominent difference between the light emitted by a light-emitting diode and a semiconductor laser?

* The light produced by the LED is incoherent and the spectrum is wide; the light produced by the laser is coherent and the spectrum is very narrow.

38. What is the most obvious difference in the operating characteristics of light-emitting diodes (LEDs) and semiconductor lasers (LDs)?

*The LED has no threshold, and the LD has a threshold. Only when the injection current exceeds the threshold will the laser be generated.

39. Which two types of single longitudinal mode semiconductor lasers are commonly used?

*DFB lasers and DBR lasers, both of which are distributed feedback lasers, whose optical feedback is provided by distributed feedback Bragg gratings in the optical cavity.

40. What are the two main types of light receiving devices?

*Mainly have photodiodes (PIN tubes) and avalanche photodiodes (APD).

41. What are the factors that cause noise in fiber-optic communication systems?

* There are noise due to unqualified extinction ratio, noise with random variation of light intensity, noise caused by time jitter, point noise and thermal noise of the receiver, mode noise of the fiber, noise caused by dispersion of the dispersion due to dispersion, mode of LD The noise is distributed, the noise generated by the frequency 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?

* 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.