CTE CTE Fiber Optics & Transmission Systems 1 — Questions and Answers
Question 1: What is the maximum attenuation typically specified for single-mode fiber at 1550 nm in modern DWDM systems?
- 10 dB/km
- 3.5 dB/km
- 0.2 dB/km (Correct answer)
- 1.0 dB/km
Correct answer: 0.2 dB/km
Modern single-mode fiber (ITU-T G.652) exhibits approximately 0.2 dB/km attenuation at the 1550 nm transmission window used by DWDM systems.
Question 2: What distinguishes single-mode fiber (SMF) from multimode fiber (MMF) in terms of core diameter?
- SMF has a 50 µm core; MMF has a 9 µm core
- SMF has a ~9 µm core; MMF has a 50–62.5 µm core (Correct answer)
- Both have identical core diameters but differ in cladding material
- SMF has a 100 µm core; MMF has a 62.5 µm core
Correct answer: SMF has a ~9 µm core; MMF has a 50–62.5 µm core
Single-mode fiber has a ~9 µm core that supports only one propagation mode, while multimode fiber has a 50 or 62.5 µm core supporting many modes.
Question 3: What type of dispersion in single-mode fiber causes different wavelength components of a pulse to travel at different speeds?
- Modal dispersion
- Polarization mode dispersion
- Chromatic dispersion (Correct answer)
- Intermodal dispersion
Correct answer: Chromatic dispersion
Chromatic dispersion (comprising material and waveguide dispersion) causes wavelengths to propagate at different group velocities, broadening the pulse over distance.
Question 4: In an OTDR trace, a sharp spike followed by a loss step typically indicates what type of event?
- A fiber bend exceeding the minimum bend radius
- A fusion splice with loss
- A mechanical connector or reflective splice (Correct answer)
- Fiber end-face damage
Correct answer: A mechanical connector or reflective splice
Mechanical connectors and air-gap splices cause Fresnel reflection (the spike) plus insertion loss (the step) on an OTDR trace.
Question 5: Which ITU-T recommendation defines Dense Wavelength Division Multiplexing (DWDM) channel spacing and frequency grid?
- ITU-T G.691
- ITU-T G.694.1 (Correct answer)
- ITU-T G.652
- ITU-T G.984
Correct answer: ITU-T G.694.1
ITU-T G.694.1 defines the DWDM frequency grid with nominal central frequencies and channel spacings of 12.5, 25, 50, or 100 GHz.
Question 6: What is the primary function of an Erbium-Doped Fiber Amplifier (EDFA) in a long-haul DWDM link?
- Regenerate the optical signal by converting it to electrical and back to optical
- Amplify multiple WDM channels simultaneously in the optical domain without O-E-O conversion (Correct answer)
- Compensate chromatic dispersion across the fiber span
- Monitor per-channel optical power for network management
Correct answer: Amplify multiple WDM channels simultaneously in the optical domain without O-E-O conversion
An EDFA boosts all WDM channels simultaneously in the C-band (1530–1565 nm) optically, avoiding costly per-channel electrical regeneration.
CTE CTE Fiber Optics & Transmission Systems 1
What is the maximum attenuation typically specified for single-mode fiber at 1550 nm in modern DWDM systems?