A telecommunications link is redesigned to run at 1600 nm rather than 1550 nm

PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Beams, resonators and guided-wave photonics · Beams, resonators and guided-wave photonics

What an Instrument Can See, Ch The optical constants of a medium

Problem

A telecommunications link is redesigned to run at 1600 nm rather than 1550 nm. As in a standard silica fibre, the attenuation is the sum of Rayleigh scattering, 0.8/λ4 dB km−10.8/\lambda^4\,\mathrm{dB\,km^{-1}}, and an infrared absorption tail, 7.81×1011 e−48.48/λ dB km−17.81\times10^{11}\,e^{-48.48/\lambda}\,\mathrm{dB\,km^{-1}}, with λ\lambda in micrometres in both formulas. The data sheet gives the dispersion parameter at 1600 nm as D=21 ps nm−1 km−1D = 21\,\mathrm{ps\,nm^{-1}\,km^{-1}}. The transmitter has a spectral width Δλ=0.15 nm\Delta\lambda = 0.15\,\mathrm{nm}, the receiver requires a link budget of 28 dB, and the system runs at 10 Gb/s, so the bit period is 100 ps. ((a)) Estimate the total attenuation at 1600 nm and state which of the two mechanisms dominates. ((b)) Estimate the loss-limited reach allowed by the 28 dB budget. ((c)) Estimate the dispersion-limited reach, taking the pulse spread D Δλ LD\,\Delta\lambda\,L equal to the bit period as the limiting condition. ((d)) State which mechanism actually sets the reach of this link.

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