Amplifying Light with Light
The erbium-doped fiber amplifier is arguably the most important invention in telecommunications since the laser itself. By doping a short length of silica fiber with erbium ions and pumping it with a semiconductor laser, engineers created an all-optical amplifier operating right at the 1550 nm minimum-loss wavelength of telecom fiber. EDFAs amplify all wavelength channels simultaneously, making modern WDM networks possible.
Population Inversion and Gain
Erbium ions in glass form a three-level laser system. A 980 nm pump excites ions from the ground state to a short-lived upper level, which rapidly relaxes to a metastable level with a lifetime of about 10 ms. Signal photons at 1530-1565 nm stimulate these excited ions to emit coherent copies — the gain mechanism. The gain depends on pump power, fiber length, erbium concentration, and signal wavelength.
Noise Figure and ASE
Not all photons emitted by excited erbium ions are stimulated by the signal. Some are emitted spontaneously in random directions and wavelengths. Those that happen to fall within the guided mode become amplified spontaneous emission (ASE) — broadband noise that degrades signal quality. The noise figure, defined as the input SNR divided by the output SNR, has a quantum limit of 3 dB. This simulation computes the noise figure from the population inversion parameter.
Interactive Gain Spectrum
This visualization shows the EDFA gain spectrum across the C-band, with your selected signal wavelength highlighted. Adjust pump power to see gain increase and saturate; change signal wavelength to explore the non-flat gain profile; vary EDF length to find the optimum between gain and ASE. The energy level diagram animates the pump-absorb-emit cycle in real time.