CTA Fiber Optics & Physical Layer Technologies 1 — Questions and Answers
Question 1: What is the primary difference between single-mode fiber (SMF) and multimode fiber (MMF)?
- SMF carries only analog signals; MMF carries digital signals
- SMF has a smaller core that supports one light path, enabling longer distances; MMF has a larger core supporting multiple light paths over shorter distances (Correct answer)
- SMF is always faster than MMF regardless of distance
- MMF is used for all long-haul backbone applications
Correct answer: SMF has a smaller core that supports one light path, enabling longer distances; MMF has a larger core supporting multiple light paths over shorter distances
Single-mode fiber's narrow core (~9 µm) allows only one propagation mode, minimizing dispersion and enabling distances of hundreds of kilometers, while multimode fiber's wider core (~50–62.5 µm) supports shorter enterprise runs.
Question 2: What phenomenon in fiber optic transmission causes different wavelengths of light to travel at slightly different speeds, limiting bandwidth over distance?
- Attenuation
- Chromatic dispersion (Correct answer)
- Modal noise
- Fresnel reflection
Correct answer: Chromatic dispersion
Chromatic dispersion occurs because different wavelengths travel at different speeds in glass, causing pulse spreading that limits high-speed transmission over long distances.
Question 3: In DWDM (Dense Wavelength Division Multiplexing), what is the standard channel spacing defined by the ITU-T grid?
- 10 nm
- 100 GHz (approximately 0.8 nm) (Correct answer)
- 1 nm
- 50 MHz
Correct answer: 100 GHz (approximately 0.8 nm)
The ITU-T G.694.1 standard defines DWDM channel spacing at 100 GHz (or 50 GHz for denser grids), allowing many wavelengths to be transmitted simultaneously on one fiber.
Question 4: What type of fiber connector is most commonly used in US data centers and enterprise networks for high-density single-mode applications?
- ST (Straight Tip)
- FC (Fiber Channel)
- LC (Lucent Connector) (Correct answer)
- MTRJ
Correct answer: LC (Lucent Connector)
The LC connector's small form factor makes it the dominant choice for high-density data center and enterprise applications with both single-mode and multimode fiber.
Question 5: What does an OTDR (Optical Time-Domain Reflectometer) measure in a fiber optic link?
- The total fiber bandwidth capacity
- The distance to faults, splices, and connectors by analyzing backscattered light pulses (Correct answer)
- The chromatic dispersion coefficient
- The number of wavelengths on a DWDM system
Correct answer: The distance to faults, splices, and connectors by analyzing backscattered light pulses
An OTDR sends light pulses into a fiber and analyzes the reflections and backscatter to locate and characterize faults, connectors, and splices along the link.
Question 6: What is the typical operating wavelength window for long-haul single-mode fiber transmission in DWDM systems?
- 850 nm window
- 1310 nm O-band only
- 1550 nm C-band (approximately 1530–1565 nm) (Correct answer)
- 1625 nm L-band exclusively
Correct answer: 1550 nm C-band (approximately 1530–1565 nm)
The C-band (around 1550 nm) provides the lowest attenuation in standard single-mode fiber and is the primary window for long-haul DWDM transmission.
CTA Fiber Optics & Physical Layer Technologies 1
What is the primary difference between single-mode fiber (SMF) and multimode fiber (MMF)?