CTE CTE Fiber Optics & Transmission Systems 2 — Questions and Answers
Question 1: What does the term 'dispersion-shifted fiber' (DSF, ITU-T G.653) refer to?
- Fiber designed to shift the zero-dispersion wavelength from 1310 nm to 1550 nm (Correct answer)
- Fiber with enhanced bend resistance for aerial deployment
- Fiber with reduced attenuation below 0.15 dB/km
- Fiber designed for multimode operation at 1550 nm
Correct answer: Fiber designed to shift the zero-dispersion wavelength from 1310 nm to 1550 nm
DSF moves the zero-dispersion wavelength to 1550 nm to minimize dispersion at the lowest-attenuation window, though it suffers from four-wave mixing in DWDM use.
Question 2: In a passive optical network (PON), what does the optical splitter enable?
- It amplifies the signal from the OLT before distribution to ONTs
- It allows a single feeder fiber from the OLT to serve multiple ONTs without active electronics (Correct answer)
- It performs wavelength routing in the distribution network
- It provides fault isolation between ONT branches
Correct answer: It allows a single feeder fiber from the OLT to serve multiple ONTs without active electronics
The passive optical splitter divides (and combines) light between the OLT's feeder fiber and multiple ONT drop fibers using no active electronics, reducing cost and maintenance.
Question 3: What is the purpose of dispersion compensation fiber (DCF) used in long-haul fiber links?
- To reduce fiber attenuation over repeater spans
- To introduce negative dispersion that cancels accumulated positive chromatic dispersion (Correct answer)
- To convert single-mode operation to multimode for short-reach segments
- To amplify weak signals using stimulated Raman scattering
Correct answer: To introduce negative dispersion that cancels accumulated positive chromatic dispersion
DCF has large negative dispersion coefficient (~−80 to −100 ps/nm·km), intentionally added to counteract positive chromatic dispersion accumulated over standard SMF spans.
Question 4: Which optical connector type is most commonly specified in US carrier outside plant (OSP) single-mode fiber installations due to its push-pull latching and low insertion loss?
- ST connector
- SC connector (Correct answer)
- FC/PC connector
- MPO/MTP connector
Correct answer: SC connector
The SC (Subscriber Connector) with push-pull coupling is widely used in US carrier OSP for its low insertion loss (<0.3 dB), repeatability, and compact form factor.
Question 5: In SONET/SDH terminology, what does the STS-1 signal rate of 51.84 Mbps represent?
- The base rate of the SONET hierarchy from which higher rates (STS-3, STS-12…) are derived by multiplexing (Correct answer)
- The maximum payload capacity after overhead removal
- The DS3 signal rate including framing bits
- The minimum line rate for metro Ethernet over SONET
Correct answer: The base rate of the SONET hierarchy from which higher rates (STS-3, STS-12…) are derived by multiplexing
STS-1 at 51.84 Mbps is the fundamental SONET building block; higher rates (OC-3 = 155.52 Mbps, OC-48 = 2.488 Gbps, etc.) are integer multiples of this base.
Question 6: What is the typical optical power budget (loss budget) consideration when designing a 1000BASE-LX Gigabit Ethernet link over single-mode fiber?
- Maximum 3 dB loss for distances up to 300 m
- Maximum 6 dB loss for distances up to 5 km on SMF (Correct answer)
- Maximum 22 dB loss for distances up to 70 km on SMF
- No power budget limit — fiber length is only limited by dispersion
Correct answer: Maximum 6 dB loss for distances up to 5 km on SMF
1000BASE-LX on single-mode fiber supports up to 5 km with a maximum allowable channel insertion loss of 6 dB including connectors and splices.
CTE CTE Fiber Optics & Transmission Systems 2
What does the term 'dispersion-shifted fiber' (DSF, ITU-T G.653) refer to?