Optical Fibre and Broadband Access
Optical Fibre and Broadband Access
This domain tests fibre-optic transmission and broadband access networks. Expect questions on total internal reflection, fibre types, attenuation windows, connectors and splices, OTDR testing, PON and FTTH.
Core rules
- Light is guided by total internal reflection when the core has a higher refractive index than the cladding and the angle exceeds the critical angle.
- Single-mode fibre has a small core and low dispersion over long distances; multimode has a larger core and is used over shorter distances.
- Common windows are 850 nm for multimode, and 1310 nm and 1550 nm for single-mode. Attenuation is lowest at 1550 nm.
- Connectors and splices add loss; contamination and poor cleaves cause high loss and reflections.
- A PON uses a passive splitter between the optical line terminal and optical network units. FTTH extends fibre to the home; DSL uses existing copper.
Worked example
A 10 km single-mode link has fibre loss 0.35 dB/km, two connectors at 0.5 dB each and two splices at 0.1 dB each. Fibre loss = 3.5 dB; connectors = 1.0 dB; splices = 0.2 dB; total = 4.7 dB. If the optical power budget is 20 dB, the remaining margin is 15.3 dB. A 1:8 splitter with 10.5 dB loss would still leave 4.8 dB, but additional connectors could consume that margin at the design stage.
Traps that cost marks
Swapping transmit and receive strands; looking into a live fibre; using multimode patch cords with single-mode equipment; ignoring splitter loss; and cleaning connectors with unsuitable materials.