Basic Knowledge of Optical Fibers and Optical Cables

1. Briefly describe the composition of optical fiber.

Answer: Optical fiber consists of two basic parts: the core and cladding made of transparent optical materials, and the coating layer.

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

Answer: They include attenuation, dispersion, bandwidth, cutoff wavelength, mode field diameter, etc.

3. What are the causes of optical fiber attenuation?

Answer: Attenuation refers to the reduction of optical power between two cross-sections of a fiber and is wavelength-dependent. The main causes of attenuation are scattering, absorption, and optical losses caused by connectors and splices.

4. How is the optical fiber attenuation coefficient defined?

Answer: It is defined as the attenuation per unit length (dB/km) of a uniform fiber under steady-state conditions.

5. What is insertion loss?

Answer: Insertion loss refers to the attenuation caused by inserting optical components (such as connectors or couplers) into the optical transmission line.

6. What does optical fiber bandwidth depend on?

Answer: The bandwidth of optical fiber refers to the modulation frequency at which the optical power amplitude decreases by 50% (or 3 dB) relative to the zero-frequency amplitude in the fiber’s transfer function. The fiber bandwidth is approximately inversely proportional to its length; the bandwidth–length product is a constant.

7. How many types of optical fiber dispersion exist, and what does it depend on?

Answer: Fiber dispersion refers to the broadening of group delay within a fiber, including modal dispersion, material dispersion, and waveguide dispersion. It depends on both the light source and the characteristics of the fiber.

8. How is the dispersion characteristic of signal propagation in fiber described?

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

9. What is the cutoff wavelength?

Answer: It is the shortest wavelength at which only the fundamental mode can propagate in the fiber. For single-mode fiber, the cutoff wavelength must be shorter than the operating wavelength.

10. How does fiber dispersion affect the performance of optical communication systems?

Answer: Dispersion causes optical pulses to broaden during transmission, affecting the bit error rate, transmission distance, and system speed.

11. What is the backscattering method?

Answer: The backscattering method measures attenuation along the fiber. Most light propagates forward, but a small portion scatters backward. By observing the backscattered signal with a splitter at the source, one can measure fiber length, attenuation, local irregularities, break points, and losses from splices and connectors.

12. What is the principle and function of an OTDR?

Answer: OTDR (Optical Time-Domain Reflectometer) works on the principle of backscattering and Fresnel reflection. It uses backscattered light to obtain attenuation information. OTDR measures fiber attenuation, splice loss, fault location, and loss distribution along the fiber—essential for fiber installation, maintenance, and monitoring. Key parameters include dynamic range, sensitivity, resolution, measurement time, and dead zones.

13. What is the OTDR dead zone? Its impact and handling?

Answer: Dead zones occur when reflections from active connectors or mechanical splices saturate the OTDR receiver, creating regions where events cannot be resolved.

There are two types of dead zones:

• Event dead zone: Distance from the start of the reflection peak to the point where the OTDR receiver is saturated.
• Attenuation dead zone: Distance from the reflection peak start to the point where other events can be detected.

A smaller dead zone is better. Dead zones increase with pulse width. Narrow pulses are used for measuring nearby events; wide pulses are used for distant fiber measurements.

14. Can OTDR measure different types of fibers?

Answer: Using a single-mode OTDR on multimode fiber, or vice versa, may yield correct fiber length but incorrect fiber loss, splice loss, and return loss. Always match the OTDR to the fiber type.

15. What do “1310nm” or “1550nm” in fiber test instruments refer to?

Answer: They refer to the optical signal wavelength. Fiber communication typically uses wavelengths in the near-infrared range (800–1700 nm). Short wavelengths (850 nm) and long wavelengths (1310, 1550 nm) are commonly used.

16. Which wavelengths have minimum dispersion and minimum loss in commercial fibers?

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

17. How are fibers classified according to core refractive index profile?

Answer: Step-index fibers (narrow bandwidth, suitable for short-distance, small-capacity communication) and graded-index fibers (wider bandwidth, suitable for medium/large-capacity communication).

18. How are fibers classified according to transmitted modes?

Answer: Single-mode fibers (core ~1–10 μm, transmits only fundamental mode, suitable for long-distance high-capacity) and multimode fibers (core ~50–60 μm, transmits multiple modes, lower performance).

19. What does the numerical aperture (NA) of step-index fiber indicate?

Answer: NA indicates the fiber’s light-gathering ability. Larger NA means stronger light collection capability.

20. What is birefringence in single-mode fiber?

Answer: Two orthogonal polarization modes exist. When the fiber is not perfectly cylindrical, the difference in refractive indices of the two modes is the birefringence.

21. Common optical cable structures?

Answer: Layered stranding and skeleton structures.

22. Main components of an optical cable?

Answer: Core, optical grease, sheath materials, PBT, etc.

23. What is cable armoring?

Answer: Protective elements (usually steel wires or steel tape) used in special cables (e.g., submarine). Armoring is applied over the inner sheath.

24. Cable sheath materials?

Answer: Polyethylene (PE) or polyvinyl chloride (PVC), protecting the fiber from external impact.

25. Special cables in power systems?

Answer:

• OPGW: Fiber placed in steel-aluminum power line; combines grounding and communication functions.
• GWWOP (wrapped cable): Suspended on existing lines.
• ADSS: Self-supporting, high tensile, span up to 1000 m.

26. OPGW cable application structures?

Answer: 1) Plastic tube stranding + aluminum tube; 2) central plastic tube + aluminum tube; 3) aluminum skeleton; 4) spiral aluminum tube; 5) single-layer stainless steel tube (central or stranded); 6) composite stainless steel tube.

27. OPGW cable stranded materials outside core?

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

28. Technical requirements for selecting OPGW cable?

Answer: 1) Rated tensile strength (RTS, kN); 2) Fiber core count (SM); 3) Short-circuit current (kA); 4) Duration (s); 5) Temperature range (℃).

29. Cable bending limitations?

Answer: Bending radius ≥ 20× cable diameter; during installation (dynamic), ≥ 30× cable diameter.

30. Key points in ADSS cable engineering?

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

31. Main optical cable accessories?

Answer: Hardware for cable installation, e.g., tension clamps, suspension clamps, vibration dampers.

32. Two basic performance parameters of optical fiber connectors?

Answer: Insertion loss and return loss.

33. Common fiber connectors?

Answer: By type: single-mode vs multimode; structure: FC, SC, ST, D4, DIN, Biconic, MU, LC, MT; end-face: FC, PC (UPC), APC. Common: FC/PC, SC, LC.

34. Common items in fiber communication systems:

Answer: AFC, FC adapter, ST adapter, SC adapter, FC/APC & FC/PC connectors, SC, ST, LC, MU patch cords, single-mode/multimode patch cords.

35. What is connector insertion loss?

Answer: Loss caused by connector insertion; smaller is better. ITU-T specifies ≤0.5 dB.

36. What is connector return loss?

Answer: Measure of power reflected back through the connector; typical ≥25 dB.

37. Main difference between LED and semiconductor laser?

Answer: LED emits incoherent light with broad spectrum; laser emits coherent light with narrow spectrum.

38. Key difference in operating characteristics of LED and LD?

Answer: LED has no threshold; LD requires threshold current to emit laser light.

39. Common single longitudinal-mode lasers?

Answer: DFB (Distributed Feedback) and DBR (Distributed Bragg Reflector) lasers.

40. Main optical receivers?

Answer: PIN photodiode and avalanche photodiode (APD).

41. Noise sources in fiber communication systems?

Answer: Extinction ratio, optical intensity fluctuation, timing jitter, receiver dark & thermal noise, mode noise, dispersion-induced pulse broadening, laser mode partition noise, frequency chirp, reflections.

42. Common fibers in transmission networks and features?

Answer: G.652 (standard SM), G.653 (dispersion-shifted SM), G.655 (non-zero dispersion-shifted).

• G.652: high dispersion in C/L bands, requires compensation >2.5 Gbit/s, widely deployed.
• G.653: zero dispersion at 1550 nm, suitable for single-wavelength ultra-long transmission, not ideal for DWDM.
• G.655: small dispersion, avoids zero-dispersion, supports DWDM, large effective area reduces nonlinearity.

43. What is fiber nonlinearity?

Answer: At high input power, refractive index becomes power-dependent, causing Raman & Brillouin scattering and frequency shifts.

44. Impact of nonlinearity on transmission?

Answer: Causes extra loss and interference, degrading system performance; significant in high-power WDM systems over long distances.

45. What is PON?

Answer: Passive Optical Network—fiber loop access network for local users using passive optical devices like couplers and splitters.

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Causes of Optical Fiber Attenuation

1. Main factors: intrinsic loss, bending, squeezing, impurities, non-uniformity, splicing.

• Intrinsic: Rayleigh scattering, inherent absorption.
• Bending: microbends cause scattering loss.
• Squeezing: stress-induced microbends.
• Impurities: absorption and scattering.
• Non-uniformity: refractive index irregularity.
• Splicing: axial misalignment, end-face angle, core mismatch, poor fusion.

1. Classification of fiber loss: intrinsic loss (scattering, absorption, structural imperfection) and additional loss (microbending, bending, splicing). Additional loss is mainly avoidable.
2. Material absorption loss:

• Atoms absorb photon energy, causing electron transitions, vibrations, and heat.
• SiO₂ has UV and IR absorption; OH⁻ causes absorption peaks at 0.95, 1.24, 1.38 μm.

1. Scattering loss:

• Rayleigh scattering due to microscopic density and composition variations; inversely proportional to wavelength⁴.

1. Structural imperfections:

• Bubbles, impurities, non-uniform core-cladding interface cause scattering.

1. Bending-induced radiation loss:

• Curvature converts guided modes to radiative modes; negligible for bending radius >5–10 cm.

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Fiber Color Code and Arrangement

Fusion splicing color sequence:
Blue, Orange, Green, Brown, Gray, White, Red, Black, Yellow, Violet, Pink, Aqua

Cable bundle arrangement:

1. Multi-fiber cables in bundles: Green first, White second, etc., Red last.
2. Individual tubes bound by colored strings in the above color sequence.
3. Each bundle or tube contains up to 12 fibers in the same color sequence.