Fundamentals and Key Parameters of Optical Fiber Communication

1. What are the components of an optical fiber? An optical fiber consists of two basic parts: a transparent core made of optical material and a cladding/coating layer.

2. What basic parameters describe the transmission characteristics of an optical fiber link? The key parameters include loss, dispersion, bandwidth, cutoff wavelength, and mode field diameter.

3. What causes attenuation in optical fibers? Attenuation is the reduction of optical power between two cross‑sections of a fiber and is mainly caused by scattering, absorption, and losses from connectors or splices.

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

5. What is insertion loss? Insertion loss refers to the attenuation introduced when an optical component such as a connector or coupler is inserted into the transmission line.

6. What does fiber bandwidth depend on? Bandwidth is the modulation frequency at which the fiber’s transfer function drops by 3 dB (or 50 % amplitude). It is approximately inversely proportional to fiber length, and the bandwidth‑length product is a constant.

7. How many types of fiber dispersion exist and what do they depend on? There are modal, material, and waveguide dispersion, which depend on the light source and the fiber’s characteristics.

8. How are the dispersion characteristics of a signal in a fiber described? They can be described by pulse broadening, fiber bandwidth, and the dispersion coefficient.

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

10. How does dispersion affect fiber‑optic communication system performance? Dispersion causes pulse broadening, which degrades bit error rate, limits transmission distance, and reduces system speed.

11. What is the back‑scatter method? It is a technique that measures attenuation along a fiber by observing the back‑scattered light from a coupler at the transmitter end, allowing detection of length, loss, and local defects.

12. What is the principle and function of an OTDR (Optical Time‑Domain Reflectometer)? An OTDR uses back‑scatter and Fresnel reflection to obtain loss information, measuring fiber attenuation, splice loss, fault location, and loss distribution. Key parameters include dynamic range, sensitivity, resolution, measurement time, and dead zone.

13. What is the blind zone of an OTDR and how does it affect testing? Blind zones are regions where the OTDR receiver saturates due to strong reflections from events such as connectors. They are classified as event blind zones and attenuation blind zones; minimizing pulse width reduces blind zone length.

14. Can an OTDR measure different types of fiber? Yes, but the OTDR must match the fiber type (single‑mode or multi‑mode) to obtain accurate loss, splice, and return‑loss measurements.

15. What do the wavelengths “1310 nm” and “1550 nm” refer to? They denote the operating wavelengths used in fiber‑optic communication, with 850 nm as the short‑wave band and 1310 nm/1550 nm as the long‑wave band.

16. Which wavelength has the minimum dispersion and which has the minimum loss in commercial fibers? 1310 nm offers the minimum dispersion, while 1550 nm provides the minimum loss.

17. How are fibers classified by core refractive‑index profile? Fibers are either step‑index or graded‑index; step‑index fibers have narrower bandwidth and are suited for short‑distance links, while graded‑index fibers support higher bandwidth for longer distances.

18. How are fibers classified by transmission mode? They are single‑mode (core ~1‑10 µm, supports one mode, used for long‑distance high‑capacity links) or multi‑mode (core ~50‑60 µm, supports multiple modes, lower performance).

19. What is the significance of the numerical aperture (NA) of a step‑index fiber? NA indicates the light‑collecting ability of the fiber; a larger NA captures more light.

20. What is birefringence in single‑mode fiber? Birefringence is the difference in refractive index between two orthogonal polarization modes when the fiber is not perfectly circularly symmetric.

21. What are the common structures of optical cables? The main structures are twisted‑pair and armored (core‑sheath) designs.

22. What components make up an optical cable? An optical cable typically includes the fiber core, fiber gel, protective sheath, and PBT material.

23. What does cable armoring refer to? Armoring is the protective steel wire or tape used in special cables such as submarine cables, attached to the inner sheath.

24. What materials are used for cable sheathing? Sheaths are usually made of polyethylene (PE) or polyvinyl chloride (PVC).

25. What special optical cables are used in power systems? The three main types are OPGW (optical ground‑wire), GWWOP (wrapped‑around‑wire optical cable), and ADSS (all‑dielectric self‑supporting) cable.

26. How many structural variants does OPGW have? Six variants: plastic‑tube‑wrapped + aluminum tube, central plastic tube + aluminum tube, aluminum‑core, helical aluminum tube, single‑layer stainless‑steel tube (center or wrapped), and composite stainless‑steel tube (center or wrapped).

27. What materials compose the OPGW conductor strands? They consist of AA (aluminum alloy) wire and AS (aluminum‑clad steel) wire.

28. What technical conditions must be met when selecting an OPGW model? Required parameters include rated tensile strength (kN), number of fiber cores, short‑circuit current (kA), short‑circuit duration (s), and operating temperature range (°C).

29. How is the bending radius of an optical cable limited? The minimum bending radius should be at least 20 times the cable outer diameter for static conditions and 30 times for non‑static handling.

30. What are the key technical points in ADSS cable projects? Mechanical design of the cable, determination of suspension points, and selection/installation of appropriate hardware.

31. What are the main types of cable hardware? Cable hardware includes tension clamps, suspension clamps, and vibration dampers.

32. What are the two basic performance parameters of fiber connectors? Insertion loss and return loss (echo loss) are the fundamental parameters.

33. What are the common categories of fiber connectors? Connectors are classified by mode (single‑mode or multi‑mode), by structure (FC, SC, ST, D4, DIN, Biconic, MU, LC, MT, etc.), and by polish type (FC/PC, FC/APC).

34. Identify the following items used in fiber‑optic systems: AFC, FC adapters; ST adapters; SC adapters; FC/APC and FC/PC connectors; SC connectors; ST connectors; LC patch cords; MU patch cords; single‑mode or multi‑mode patch cords.

35. What is connector insertion loss? It is the reduction in effective power caused by the connector; the ITU‑T recommends it not exceed 0.5 dB.

36. What is connector return loss (echo loss)? It measures the reflected power returning to the input; typical values should be better than 25 dB.

37. What is the main difference between LED and laser sources? LEDs emit incoherent, broadband light, whereas lasers emit coherent, narrow‑band light.

38. What is the most obvious operational difference between LEDs and LDs? LEDs have no threshold current, while laser diodes require a threshold current to generate laser light.

39. Which two types of single‑longitudinal‑mode semiconductor lasers are commonly used? Distributed‑feedback (DFB) lasers and distributed‑Bragg‑reflector (DBR) lasers.

40. What are the two main types of optical receivers? PIN photodiodes and avalanche photodiodes (APD).

41. What factors contribute to noise in fiber‑optic communication systems? Noise sources include extinction‑ratio degradation, intensity fluctuations, timing jitter, receiver shot and thermal noise, modal noise, dispersion‑induced pulse broadening, laser mode partition noise, laser frequency chirp, and reflections.

42. What are the main types of fibers used in modern transmission networks and their characteristics? G.652 (standard single‑mode, higher dispersion in C/L bands, requires dispersion compensation above 2.5 Gb/s), G.653 (dispersion‑shifted, near‑zero dispersion at 1550 nm, suitable for long‑haul but not DWDM), and G.655 (non‑zero dispersion‑shifted, low dispersion in C/L bands, mitigates four‑wave mixing, supports DWDM and high‑speed systems).

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

44. How does non‑linearity affect transmission? Non‑linear effects cause additional loss and interference, especially in high‑power, long‑distance WDM systems, resulting in Raman and Brillouin scattering losses.

45. What is a PON (Passive Optical Network)? A PON is a fiber‑based access network that uses passive components such as couplers and splitters to connect end users.

Additional Topics Covered: Causes of fiber attenuation (intrinsic, bending, micro‑bending, impurities, non‑uniformity, splicing), classifications of attenuation (intrinsic vs. extrinsic), absorption mechanisms (material absorption, impurity absorption, OH‑related loss), scattering loss (Rayleigh scattering and its wavelength dependence), waveguide scattering, and bend‑induced radiation loss.