BDS Fiber Optic Systems in HFC 2 — Questions and Answers
Question 1: What does ONU stand for in fiber access networks?
- Optical Network Unit (Correct answer)
- Outbound Node Uplink
- Optical Noise Undertone
- Output Network Utility
Correct answer: Optical Network Unit
An Optical Network Unit (ONU) is a device at the subscriber end of a passive optical network (PON) that converts optical signals to electrical signals for delivery to the customer's devices.
Question 2: What is the typical wavelength used for downstream transmission in standard single-mode HFC fiber links?
- 850 nm
- 1310 nm
- 1550 nm (Correct answer)
- 1625 nm
Correct answer: 1550 nm
1550 nm is the standard downstream wavelength for HFC analog/digital RF-over-fiber transmission because fiber has its lowest attenuation (~0.2 dB/km) at this wavelength, enabling long spans.
Question 3: What is chromatic dispersion in fiber optics?
- Physical damage to fiber from bending
- The spreading of optical pulses over distance because different wavelengths travel at slightly different speeds in the fiber (Correct answer)
- Loss at fiber connectors due to end-face contamination
- Reflection of light back toward the transmitter
Correct answer: The spreading of optical pulses over distance because different wavelengths travel at slightly different speeds in the fiber
Chromatic dispersion causes different wavelength components of an optical signal to travel at different velocities, causing pulse broadening that limits bandwidth-distance product in fiber links.
Question 4: What is the difference between single-mode and multimode fiber?
- Single-mode carries one wavelength; multimode carries many wavelengths simultaneously
- Single-mode has a smaller core that allows only one propagation mode, enabling longer distances; multimode has a larger core supporting many modes but is limited to shorter distances (Correct answer)
- Single-mode is for upstream; multimode is for downstream in HFC
- Single-mode uses plastic; multimode uses glass
Correct answer: Single-mode has a smaller core that allows only one propagation mode, enabling longer distances; multimode has a larger core supporting many modes but is limited to shorter distances
Single-mode fiber (8–10 µm core) confines light to a single propagation mode, eliminating modal dispersion and enabling spans of tens of kilometers, while multimode fiber (50–62.5 µm core) supports shorter runs up to ~2 km.
Question 5: What is an EDFA and where is it used in HFC networks?
- Enhanced Digital Frequency Amplifier — used in headend signal processors
- Erbium-Doped Fiber Amplifier — used to optically amplify 1550 nm signals in long fiber spans (Correct answer)
- Electronic Distribution Fiber Attenuator — used at node input ports
- Extended DOCSIS Frequency Allocator — used in CMTS upstream cards
Correct answer: Erbium-Doped Fiber Amplifier — used to optically amplify 1550 nm signals in long fiber spans
An EDFA (Erbium-Doped Fiber Amplifier) amplifies optical signals at 1550 nm without converting to electrical form, enabling long-distance or wide-area fiber distribution without intermediate O/E/O conversion.
Question 6: What is optical return loss (ORL) and why is it important in HFC fiber links?
- The ratio of transmitted to received optical power
- The ratio of backward-reflected optical power to incident optical power, measured in dB — high ORL indicates poor connectors that can destabilize the laser (Correct answer)
- The efficiency of the optical node photodetector
- The attenuation of the fiber at 1310 nm compared to 1550 nm
Correct answer: The ratio of backward-reflected optical power to incident optical power, measured in dB — high ORL indicates poor connectors that can destabilize the laser
Optical return loss measures reflections back toward the transmitter; low ORL values (more reflection) can cause DFB laser instability and increased distortion in HFC analog transmission.
BDS Fiber Optic Systems in HFC 2
What does ONU stand for in fiber access networks?