Optical and Transmission Systems
This domain tests optical fibre transmission: attenuation, dispersion, optical sources and detectors, WDM, amplification, noise, receiver sensitivity, and digital transport hierarchies. Numerical power-budget questions are common.
Core rules
- Fibre attenuation is specified in dB/km. Standard single-mode fibre has low-loss windows near 1310 nm and 1550 nm; 1550 nm is preferred for long haul because loss is lower and EDFA gain is available.
- Chromatic dispersion broadens pulses; dispersion-shifted and dispersion-compensating fibre or digital compensation manage it. Polarization-mode dispersion is a statistical impairment.
- WDM/DWDM carries multiple wavelengths on one fibre. Channel spacing, filter passbands, and four-wave mixing constrain the design.
- Optical power budget: received power = transmitted power − fibre loss − connector/splice loss − margin. The result must exceed receiver sensitivity.
- OSNR degrades through amplifiers; BER depends on OSNR and receiver characteristics. Forward error correction relaxes the required OSNR.
- SDH/OTN provide multiplexing, framing, and protection; PONs use passive splitters and time-division multiple access in the downstream/upstream.
Worked example
A link transmits 0 dBm over 40 km of fibre with 0.35 dB/km loss, two connectors at 0.5 dB each, four splices at 0.1 dB each, and a 3 dB margin. Fibre loss = 14 dB; connectors = 1 dB; splices = 0.4 dB; total with margin = 18.4 dB. Received power = 0 − 18.4 = −18.4 dBm. If receiver sensitivity is −28 dBm, the remaining margin is 9.6 dB, so the link passes.
Traps
- Subtracting dBm from dBm instead of subtracting dB losses from an absolute power.
- Confusing attenuation with dispersion; dispersion limits reach even when power is sufficient.
- Forgetting connector and splice counts.
- Assuming amplifier gain improves OSNR without adding noise.
- Ignoring that WDM channel power must be managed per wavelength.