Key Concepts and Frequently Asked Questions on Optical Fiber and Fiber‑Optic Communication

1. What are the basic components of an optical fiber?

Answer: An optical fiber consists of a transparent core and a cladding (coating) layer.

2. What are the basic transmission parameters of a fiber‑optic line?

Answer: Loss, dispersion, bandwidth, cut‑off wavelength, and mode field diameter.

3. What causes attenuation in optical fibers?

Answer: Scattering, absorption, and losses introduced by connectors or splices.

4. How is the attenuation coefficient defined?

Answer: It is defined as the loss per unit length of a uniform fiber, expressed in dB/km.

5. What is insertion loss?

Answer: The loss caused by inserting an optical component such as a connector or coupler.

6. What does fiber bandwidth depend on?

Answer: Bandwidth is inversely proportional to fiber length; the product of bandwidth and length is a constant.

7. What types of dispersion exist in fibers?

Answer: Modal, material, and waveguide dispersion, depending on the light source and fiber characteristics.

8. How is dispersion in a fiber described?

Answer: By pulse broadening, fiber bandwidth, and the dispersion coefficient.

9. What is cut‑off wavelength?

Answer: The shortest wavelength at which only the fundamental mode can propagate; for single‑mode fiber it must be shorter than the operating wavelength.

10. How does dispersion affect system performance?

Answer: It broadens pulses, increasing bit error rate, reducing transmission distance, and limiting system speed.

11. What is the principle of the back‑scatter method?

Answer: It measures attenuation along the fiber by analyzing the back‑scattered light from a splitter at the source.

12. What is the testing principle of an OTDR?

Answer: OTDR uses back‑scatter and Fresnel reflection to measure loss, splice loss, fault location, and loss distribution along the fiber.

13. What are the blind zones of an OTDR?

Answer: Event blind zones (saturation after a reflection peak) and attenuation blind zones (distance until the next identifiable event).

14. Can an OTDR measure different fiber types?

Answer: Yes, but the OTDR must match the fiber type; using a single‑mode OTDR on multimode fiber yields inaccurate loss measurements.

15. What do the wavelengths 1310 nm and 1550 nm represent?

Answer: They are common transmission windows in the near‑infrared band used for fiber‑optic communication.

16. Which wavelength has minimum dispersion and which has minimum loss?

Answer: 1310 nm has minimum dispersion; 1550 nm has minimum loss.

17. How are fibers classified by core refractive‑index profile?

Answer: Step‑index and graded‑index fibers.

18. How are fibers classified by mode propagation?

Answer: Single‑mode and multimode fibers.

19. What is the significance of the numerical aperture (NA) of step‑index fiber?

Answer: NA indicates the light‑collecting ability of the fiber; larger NA collects more light.

20. What is birefringence in single‑mode fiber?

Answer: It is the difference in refractive index between two orthogonal polarization modes caused by asymmetry.

21. Main structures of optical cable

Answer: Twisted‑pair and aerial (skeleton) structures.

22. Main components of an optical cable

Answer: Core, gel, protective sheath, and PBT material.

23. What does cable armoring refer to?

Answer: Protective steel or steel‑tape layers used in special cables such as submarine cables.

24. Materials used for cable sheathing

Answer: Polyethylene (PE) and polyvinyl chloride (PVC).

25. Special optical cables used in power systems

Answer: OPGW (optical‑ground‑wire), GWWOP (wrapped‑around), and ADSS (all‑dielectric‑self‑supporting) cables.

26. OPGW cable application structures

Answer: Six typical structures including plastic‑tube‑wrapped with aluminum, central plastic tube with aluminum, aluminum skeleton, helical aluminum, single‑layer stainless steel, and composite stainless‑steel structures.

27. Materials of the OPGW cable’s outer strands

Answer: Aluminum‑alloy (AA) and aluminum‑clad steel (AS) wires.

28. Technical conditions for selecting OPGW cable model

Answer: Rated tensile strength, number of fibers, short‑circuit current, short‑circuit duration, and temperature range.

29. How is cable bend radius limited?

Answer: Minimum static bend radius is 20× the cable outer diameter; during installation it should be at least 30×.

30. Key technical points in ADSS cable projects

Answer: Cable mechanical design, determination of suspension points, and selection/installation of accessories.

31. Main types of cable accessories

Answer: Tension clamps, suspension clamps, and vibration dampers.

32. Fundamental performance parameters of fiber connectors

Answer: Insertion loss and return loss (back‑reflection loss).

33. Common categories of fiber connectors

Answer: Single‑mode vs. multimode; connector styles such as FC, SC, LC, ST, MU, etc.; and polish types PC (UPC) and APC.

34. Identify the following items in a fiber‑optic system

Answer: AFC/FC adapters, ST adapters, SC adapters, FC/APC and FC/PC connectors, LC and MU jumpers, etc.

35. What is insertion loss (or connector loss)?

Answer: The reduction in optical power caused by a connector; typically should be ≤0.5 dB.

36. What is return loss (or back‑reflection loss)?

Answer: The ratio of reflected power to incident power; typically should be ≥25 dB.

37. Main difference between LED and semiconductor laser light

Answer: LED emits incoherent, broadband light; lasers emit coherent, narrow‑band light.

38. Operational difference between LED and laser diode

Answer: LEDs have no threshold current; lasers require a threshold current to emit.

39. Two common types of single‑mode semiconductor lasers

Answer: Distributed‑feedback (DFB) and distributed‑Bragg‑reflector (DBR) lasers.

40. Main types of optical receivers

Answer: PIN photodiodes and avalanche photodiodes (APD).

41. Sources of noise in fiber‑optic communication systems

Answer: Extinction‑ratio noise, intensity noise, jitter, shot noise, thermal noise, mode noise, dispersion‑induced noise, laser modal noise, frequency chirp, and reflection‑induced noise.

42. Main types of fibers used in transmission networks and their characteristics

Answer: G.652 (standard single‑mode, moderate dispersion), G.653 (dispersion‑shifted, near‑zero dispersion at 1550 nm), and G.655 (non‑zero dispersion‑shifted, low dispersion, larger effective area).

43. What is fiber non‑linearity?

Answer: Power‑dependent changes in refractive index causing Raman and Brillouin scattering, which shift the optical frequency.

44. Impact of non‑linearity on transmission

Answer: Introduces additional loss and distortion (e.g., Raman, Brillouin scattering) especially in high‑power, long‑distance WDM systems.

45. What is a PON (Passive Optical Network)?

Answer: A fiber‑to‑the‑home access network that uses passive components such as splitters and couplers.