The Decibel Budget
Every optical link has a power budget: the difference between the transmitter's launch power and the receiver's minimum detectable power (sensitivity). The total of all losses along the path — fiber attenuation, splice losses, connector losses, and safety margin — must fit within this budget. If losses exceed the budget, bits are lost. Link budgeting is the first step in any fiber network design.
Attenuation Mechanisms
Silica glass is remarkably transparent, but not perfectly so. Rayleigh scattering off nanoscale density fluctuations dominates at shorter wavelengths and falls as the inverse fourth power of wavelength. Infrared absorption by Si-O bonds rises at longer wavelengths. These two mechanisms create a loss valley near 1550 nm where attenuation drops to just 0.2 dB/km — the reason modern telecom lives in the C-band.
OTDR Trace Reading
An OTDR sends pulses into a fiber and records the backscattered light versus round-trip time. The resulting trace is a powerful diagnostic: the slope reveals attenuation per km, step drops indicate fusion splices, reflective spikes mark connectors or breaks, and the noise floor sets the measurement's dynamic range. This simulation generates a realistic OTDR trace from your link parameters.
Designing for Distance
As fiber length increases, loss accumulates linearly. Each splice adds a small but non-negligible step. Connector pairs at patch panels add 0.3-0.5 dB each. Safety margins of 3-6 dB account for aging, temperature, and future splices. The simulation lets you adjust each parameter and instantly see whether your design stays within budget or needs optical amplification.