Fundamentals of Optical Fiber: Composition, Transmission Characteristics, and Common Issues

1. Composition of an optical fiber An optical fiber consists of two basic parts: a transparent core made of optical material and a cladding/coating layer surrounding it.

2. Basic transmission parameters of a fiber link Key parameters include loss, dispersion, bandwidth, cutoff wavelength, and mode field diameter.

3. Causes of fiber attenuation Attenuation is the reduction of optical power between two cross‑sections of a fiber and is mainly caused by scattering, absorption, and losses introduced by connectors or splices.

4. Definition of attenuation coefficient It is defined as the loss per unit length of a uniform fiber, expressed in dB/km.

5. Insertion loss Insertion loss refers to the attenuation introduced when an optical component (e.g., a connector or coupler) is inserted into the transmission line.

6. Bandwidth dependence Fiber bandwidth is the modulation frequency at which the power response drops by 3 dB; it is approximately inversely proportional to fiber length.

7. Types of dispersion Dispersion includes modal, material, and waveguide dispersion and depends on the light source and fiber characteristics.

8. Description of dispersion characteristics Signal dispersion in a fiber can be described by pulse broadening, bandwidth, and the dispersion coefficient.

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

10. Impact of dispersion on system performance Dispersion broadens pulses, affecting bit‑error rate, transmission distance, and system speed.

11. Back‑scatter method A technique that measures attenuation along a fiber by analyzing the back‑scattered light, allowing detection of loss points and faults.

12. OTDR testing principle and functions OTDR uses back‑scatter and Fresnel reflection to obtain loss information, measure fiber loss, splice loss, fault location, and loss distribution; key parameters include dynamic range, sensitivity, resolution, measurement time, and dead zone.

13. OTDR blind zones Blind zones (event and attenuation blind zones) occur when strong reflections saturate the receiver; they can be reduced by using narrower pulses for near‑end events and wider pulses for far‑end measurements.

14. OTDR suitability for different fiber types Using a single‑mode OTDR on multimode fiber or vice‑versa yields correct length measurements but inaccurate loss and splice values; the OTDR must match the fiber type.

15. Meaning of 1310 nm and 1550 nm These are common transmission windows in the near‑infrared band; 850 nm is the short‑wave band, while 1310 nm and 1550 nm are long‑wave bands.

16. Minimum dispersion and loss wavelengths 1310 nm offers minimum dispersion, while 1550 nm provides minimum loss.

17. Classification by core refractive‑index profile Step‑index fibers (narrow bandwidth) and graded‑index fibers (wider bandwidth) are distinguished by their index variation.

18. Classification by mode propagation Single‑mode fibers (core ~1‑10 µm) support one fundamental mode; multimode fibers (core ~50‑60 µm) support multiple modes.

19. Numerical aperture (NA) significance NA indicates the light‑collecting ability of a fiber; larger NA means higher acceptance angle.

20. Birefringence in single‑mode fiber When the fiber is not perfectly circular, two orthogonal polarization modes have slightly different refractive indices, resulting in birefringence.

21. Common cable structures Two main structures: twisted‑pair (layer‑twisted) and aerial‑type (skeleton) cables.

22. Main components of a fiber cable Core, fiber grease, protective sheath, and PBT material.

23. Cable armor Protective steel or steel‑tape armor used in special cables such as submarine cables.

24. Cable sheath materials Typically polyethylene (PE) or polyvinyl chloride (PVC) to protect the cable core.

25. Special cables used in power systems OPGW (optical‑ground‑wire), GWWOP (wrapped‑around cable), and ADSS (all‑dielectric self‑supporting) cables.

26. OPGW structural variants Six 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 OPGW armor wires AA (aluminum alloy) and AS (aluminum‑clad steel) wires.

28. Technical requirements for selecting OPGW Rated tensile strength, number of fiber cores, short‑circuit current, short‑circuit duration, and temperature range.

29. Cable bend radius limits Minimum static bend radius: ≥20 × cable outer diameter; during installation (dynamic): ≥30 × outer diameter.

30. ADSS installation considerations Key aspects: mechanical design of the cable, determination of suspension points, and selection/installation of supporting hardware.

31. Common fiber cable hardware (hardware accessories) Tension clamps, suspension clamps, and vibration dampers.

32. Fundamental connector performance parameters Insertion loss and return loss (back‑reflection loss).

33. Common connector types FC/PC, SC, LC, MU, ST, D4, DIN, Biconic, MT, etc., with both UPC (PC) and APC end‑faces.

34. Identification of common optical components AFC, FC adapters, ST adapters, SC adapters, FC/APC, FC/PC connectors, LC patch cords, MU patch cords, single‑mode/multimode patch cords.

35. Definition of insertion loss The reduction in effective power caused by a connector; ITU‑T recommends ≤0.5 dB.

36. Definition of return loss A measure of reflected power from a connector; typical requirement ≥25 dB.

37. Difference between LED and semiconductor laser LED emits incoherent, broadband light; lasers emit coherent, narrow‑band light.

38. Operational difference between LED and laser LEDs have no threshold current; lasers require a threshold current to generate coherent light.

39. Common single‑longitudinal‑mode lasers DFB (distributed feedback) and DBR (distributed Bragg reflector) lasers.

40. Main types of optical receivers PIN photodiodes and avalanche photodiodes (APD).

41. Sources of noise in optical communication systems Noise arises from extinction‑ratio imperfections, intensity fluctuations, jitter, receiver shot and thermal noise, modal noise, dispersion‑induced pulse broadening, LD modal partition noise, LD frequency chirp, and reflections.

42. Fiber types used in modern transport networks G.652 (standard single‑mode), G.653 (dispersion‑shifted), and G.655 (non‑zero dispersion‑shifted) fibers, each with distinct dispersion and loss characteristics.

43. Optical fiber non‑linearity When launched power exceeds a threshold, the refractive index becomes power‑dependent, leading to Raman and Brillouin scattering and frequency shifts.

44. Impact of non‑linearity on transmission Non‑linear effects cause additional loss and interference (e.g., Raman/Brillouin scattering, four‑wave mixing) that degrade system performance, especially in high‑power, long‑distance WDM links.

45. Definition of PON (Passive Optical Network) A fiber‑based access network that uses passive components such as splitters and couplers to deliver services to end users.

Additional Topics Covered Detailed discussion of fiber attenuation mechanisms (intrinsic loss, bending loss, micro‑bending, impurities, non‑uniformity, splicing), absorption loss origins (material absorption, impurity ions, OH⁻ groups), scattering loss (Rayleigh scattering, waveguide scattering, bend‑induced radiation loss), and methods to mitigate these losses.