News & Solutions-40 basic knowledge of optical fiber/optical cable
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40 basic knowledge of optical fiber/optical cable
Release Date: 2022-05-10

1. Briefly describe the composition of optical fibers.

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

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

Including loss, dispersion, bandwidth, cutoff wavelength, mode field diameter, etc.

3. What is the cause of fiber attenuation?

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

4. How is the fiber attenuation coefficient defined?

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

5. What is insertion loss?

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

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

The bandwidth of the fiber refers to the modulation frequency when the amplitude of the optical power is 50% or 3dB lower than 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 bandwidth length is a constant.

7. How many kinds of dispersion are there in optical fiber? What is it about?

The dispersion of an optical fiber refers to the broadening of the group delay in an optical fiber, including modal 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 signals propagating in optical fibers

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 in the fiber that can only transmit the fundamental mode. For single-mode fibers, the cutoff wavelength must be shorter than the wavelength of the transmitted light.

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

The dispersion of the fiber will cause the optical pulse to spread during the transmission in the fiber. It 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 backscattering method?

Backscattering is a method of measuring attenuation along the length of an optical fiber. Most of the optical power in the fiber is forward propagating, but a small portion is backscattered towards the emitter. Using a spectroscope at the illuminator to observe the time curve of backscattering, from one end can not only measure the length and attenuation of the uniform optical fiber connected, but also measure local irregularities, breakpoints, and damage caused by joints and connectors. Optical power loss.

12. What is the test principle of Optical Time Domain Reflectometer (OTDR)? What is the function?

OTDR is made based on the principle of light backscattering and Fresnel reflection. It uses the backscattered light generated when light propagates in the fiber to obtain attenuation information. It can be used to measure fiber attenuation, splice loss, fiber fault location and understanding of fiber. The loss distribution along the length, etc., 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 dead zone.

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

Usually, a series of "blind spots" caused by the saturation of the OTDR receiving end caused by reflection from characteristic points such as movable connectors and mechanical joints are called blind zones.

The blind zone in the optical fiber is divided into two types: the event blind zone and the attenuation blind zone: the reflection peak caused by the intervention of the active connector, the length distance from the starting point of the reflection peak to the receiver saturation peak, is called the event blind zone; Intervention of the movable connector causes a reflection peak, and the distance from the onset of the reflection peak to the point at which other events can be identified is called the attenuation dead zone.

For OTDRs, the smaller the blind zone, the better. The blind area will increase with the increase of the width of the pulse broadening. 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 points Use narrow pulses and wide pulses when making measurements at the far end of the fiber.

14. Can an OTDR measure different types of fibers?

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 such as a 62.5mm core diameter, the fiber length measurement results will not be affected, but factors such as fiber loss, optical Splice loss, return loss results are incorrect. Therefore, when measuring the optical fiber, be sure to select the OTDR that matches the measured optical fiber for measurement, so as to obtain the correct results of all performance indicators.

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

Refers to the wavelength of an optical signal. The wavelength range used in 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 current commercial fibers, what wavelength of light has the smallest dispersion? What wavelength of light has the least loss?

Light at 1310nm wavelength has minimal dispersion and light at 1550nm wavelength has minimal loss.

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

It 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 gradient fiber has a wider bandwidth and is suitable for medium and large-capacity communication.

18. According to the different modes of light waves transmitted in the fiber, how are the fibers classified?

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

When transmitting the current differential protection of multiplexing protection, multi-mode 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 Numerical Aperture (NA) represents the light-collecting ability of the fiber. The larger the NA, the stronger the light-collecting ability of the fiber.

20. What is the birefringence of single mode fiber?

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, and the absolute value of the difference between the two orthogonal polarization modes is the double polarization. refraction.

21. What are the most common fiber optic cable structures?

There are two types of layer twist and skeleton.

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

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?

Refers to the protective element (usually steel wire or steel tape) used in special-purpose optical cables (such as submarine cables, etc.). The armor is attached to the inner jacket of the cable.

24. What material is used for the cable sheath?

The cable jacket or sheath 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 special optical cables used in power systems.

There are three main types of special cables:

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

Wrap-around fiber optic cable (GWWOP), which is wrapped or suspended from the ground wire where there is an existing transmission line.

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

26. What are the application structures of OPGW optical cable?

Mainly include: 1) structure of plastic pipe layer twist + aluminum pipe; 2) structure of central plastic pipe + aluminum pipe; 3) aluminum skeleton structure; 4) spiral aluminum pipe structure; 5) single-layer stainless steel pipe structure (central stainless steel pipe 6) Composite stainless steel tube structure (central stainless steel tube structure, stainless steel tube layered structure).

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

It is composed of AA wire (aluminum alloy wire) and AS wire (aluminum clad steel wire).

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

1) Nominal tensile strength (RTS) (kN) of OPGW cable; 2) Fiber core count (SM) of OPGW cable; 3) Short circuit current (kA); 4) Short circuit time (s); 5) Temperature range ( °C).

29. How is the bending degree of the optical cable limited?

The bending radius of the optical cable should not be 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-static state).

30. What should be paid attention to in ADSS optical cable project?

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?

Optical cable fittings refer to the hardware used to install optical cables, mainly including: tension clamps, suspension clamps, vibration isolators, etc.

32. There are two basic performance parameters for optical fiber connectors, what are they?

Optical fiber connectors are commonly known as live joints. For the optical performance requirements of single-fiber connectors, the focus is on the two most basic performance parameters, insertion loss and return loss.

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

According to different classification methods, fiber optic connectors can be divided into different types. According to different transmission media, they can be divided into single-mode fiber optic connectors and multi-mode fiber optic 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, and LC fiber optic connectors.

34. What is the insertion loss (or insertion loss) of the fiber optic connector?

Refers to the magnitude of the reduction in the effective power of the transmission line caused by the intervention of the connector. For the user, the smaller the value, the better. ITU-T stipulates that its value should not be greater than 0.5dB.

35. What is the most prominent difference between light emitting diodes and semiconductor lasers?

The light generated by the LED is incoherent light with a wide spectrum; the light generated by the laser is coherent light with a narrow spectrum.

36. What is the most obvious difference between the working characteristics of light emitting diodes (LEDs) and semiconductor lasers (LDs)?

LEDs do not have thresholds, while LDs have thresholds, and laser light is generated only when the injection current exceeds the threshold.

37. What are the two commonly used single longitudinal mode semiconductor lasers?

Both DFB lasers and DBR lasers are distributed feedback lasers, and their optical feedback is provided by distributed feedback Bragg gratings in the optical cavity.

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

There are mainly photodiodes (PIN tubes) and avalanche photodiodes (APDs).

39. What are the factors that cause the noise of the optical fiber communication system?

There are noise caused by unqualified extinction ratio, noise caused by random variation of light intensity, noise caused by time jitter, point noise and thermal noise of receiver, mode noise of fiber, noise caused by pulse broadening caused by dispersion, mode distribution of LD Noise, noise from the frequency chirp of the LD, and noise from reflections.

40. What effect will fiber nonlinearity have on transmission?

Non-linear effects can cause some additional losses and disturbances, degrading the performance of the system. WDM systems have high optical power and travel long distances along the fiber, thus producing nonlinear distortion. 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 energy of incident light and cause loss. It can be ignored when the input fiber power is small.

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