Optical Fiber Fundamentals and Transmission Principles
Optical Fiber Fundamentals and Transmission Principles
This domain tests how light travels in a fiber, the meaning of attenuation and dispersion, and basic link-budget arithmetic. The exam expects you to name the core, cladding and coating, distinguish single-mode from multimode fiber, and use dB and dBm correctly.
Core rules and concepts
- Light is guided by total internal reflection because the core has a slightly higher refractive index than the cladding.
- Single-mode fiber has a small core, commonly 9 µm, with 125 µm cladding; it carries 1310 nm and 1550 nm signals over long distances. Multimode fiber has a larger core, commonly 50 µm or 62.5 µm, and uses 850 nm or 1300 nm for shorter links.
- Typical attenuation for single-mode fiber is about 0.35 dB/km at 1310 nm and 0.20 dB/km at 1550 nm. Multimode loss is higher, roughly 3 dB/km at 850 nm.
- Loss in dB is added along a link. Power in dBm is an absolute level: 0 dBm = 1 mW, −10 dBm = 0.1 mW.
- Bending loss comes from tight macrobends; microbends come from poor cable handling. Dispersion spreads pulses and limits reach.
Worked example
A 3 km single-mode link at 1310 nm has fiber loss 0.35 dB/km, two connectors at 0.5 dB each and two splices at 0.1 dB each. Fiber loss = 3 × 0.35 = 1.05 dB. Connectors = 1.0 dB; splices = 0.2 dB. Total = 2.25 dB. If the transmitter launches 0 dBm, the receiver gets 0 − 2.25 = −2.25 dBm, before any margin.
Traps that cost marks
Do not add dBm values as if they were ordinary powers; convert to mW when multiplying. Do not confuse 9 µm core with 125 µm cladding. Do not treat a 0.2 dB/km figure as valid at 1310 nm. Do not ignore connector and splice losses in a budget.