Fiber attenuation & dispersion

Two enemies limit how far and how fast an optical fibre can carry data: attenuation, which makes the signal fainter, and dispersion, which makes it blurrier. Attenuation is light slowly being lost to absorption and scattering as it travels — measured in decibels per kilometre, it sets how far a pulse can go before a receiver can no longer tell light from dark. Dispersion is different: even if every photon arrives, a sharp pulse spreads out in time because different colours (or different bounce-paths) travel at slightly different speeds, smearing crisp '1's and '0's into each other.

Both are why long links need help. Attenuation is fought with optical amplifiers (like the erbium-doped fibre amplifier) that boost the whole beam without converting it back to electricity. Dispersion sets the real speed limit: push the bit rate up and the spreading pulses eventually overlap so badly that bits become unreadable — this is intersymbol interference. Engineers minimise it with single-mode fibre, narrow-linewidth lasers, and dispersion-compensating fibre, which is why the 1310 nm and 1550 nm wavelength windows (low loss, manageable dispersion) became the backbone of optical telecom.