Erbium-doped fiber amplifier (EDFA)
An optical amplifier using erbium-doped silica fiber pumped at 980 nm or 1480 nm. The dominant amplifier technology in the telecom C and L bands.
An EDFA amplifies optical signals in the 1530–1610 nm range using stimulated emission in erbium-doped silica fiber. Pump light (typically 980 nm) excites Er ions to a metastable upper state; signal photons stimulate decay back to the ground state, producing amplified output at the signal wavelength.
Standard architecture:
| Component | Role |
|---|---|
| Pump laser | 980 nm or 1480 nm, 100 – 500 mW |
| WDM coupler | Combines pump and signal into the doped fiber |
| Er-doped silica fiber (EDF) | Gain medium, typically 5–30 m long |
| Isolators (input/output) | Block reflections that destabilize gain |
| Gain-flattening filter (in C-band amps) | Equalizes gain across the band |
| Output tap monitor | Power feedback |
Gain bandwidth covers the C-band (1530–1565 nm) and, with modified erbium doping or extended L-band fiber, the L-band (1565–1625 nm). C-band EDFAs are the workhorses of dense WDM telecom; L-band extensions added capacity in the late 1990s.
Typical performance:
| Parameter | Standard telecom EDFA |
|---|---|
| Small-signal gain | 20 – 35 dB |
| Saturation output power | +13 to +23 dBm |
| Noise figure | 4 – 6 dB |
| Pump efficiency | 10 – 30% |
| Gain ripple (with GFF) | 1 dB peak-to-peak |
| Polarization-dependent gain | 0.5 dB |
The EDFA's success in telecom comes from a fortunate coincidence: the Er emission band coincides with the silica-fiber low-loss window at 1550 nm, and the amplifier is polarization-insensitive, has high saturation power, and operates as a near-ideal small-signal amplifier with noise figure close to the quantum limit (3 dB).
EDFA gain saturates at the population-inversion-limit output power. Operating at saturation produces stable output (output power becomes nearly independent of input power) but limits per-channel power in WDM systems and introduces inter-channel crosstalk through gain dynamics. Most telecom EDFAs are operated in the gain-controlled regime via electronic feedback to maintain flat per-channel gain across the band.