Optical Fibre and Transmission Systems
Optical fibre questions cover total internal reflection, fibre types, numerical aperture, attenuation, dispersion, connectors, splicing, OTDR testing and WDM. Expect dB calculations and wavelength-window matching.
Core rules
- Light is guided by total internal reflection when the core refractive index n1 exceeds cladding n2 and the angle exceeds the critical angle.
- Numerical aperture NA = √(n1² − n2²); it indicates the acceptance cone.
- Single-mode fibre has a small core (about 8–10 µm) and carries one mode; multimode has 50 or 62.5 µm cores and modal dispersion.
- Standard windows: 850 nm and 1300 nm for multimode; 1310 nm and 1550 nm for single-mode long haul.
- Attenuation in dB = 10 log10(Pin/Pout). Loss budget = fibre loss + splice loss + connector loss + margin.
- OTDR measures distance and loss by backscatter; fusion splicing gives lower loss than mechanical splices.
- WDM/DWDM multiplexes multiple wavelengths on one fibre.
Worked example
1 mW is launched and 0.1 mW emerges after 5 km. Loss = 10 log10(1/0.1) = 10 dB. Loss per km = 10/5 = 2 dB/km. If the link also has two 0.5 dB connectors and one 0.1 dB splice, total link loss = 10 + 1.0 + 0.1 = 11.1 dB, which must fit inside the transmitter-receiver power budget.
Traps that cost marks
- Confusing dB (ratio) with dBm (absolute power referred to 1 mW).
- Subtracting losses in dBm as if they were powers.
- Using 1550 nm for short multimode links.
- Forgetting bend loss and connector loss in the budget.
- Looking into a live fibre; laser safety rules always apply.
Exam approach
Convert powers to dBm or use the ratio formula, add all loss elements, then compare with the budget. State whether the link passes with margin.