8. Optical Amplifiers

Once your fiber loss makes the signal too weak to detect, you have two options: regenerate (detect, retime, retransmit, called 3R) or amplify. Amplification is cheaper, simpler, and wavelength-blind, which lets it boost an entire WDM comb at once.

8.1 Erbium-doped fiber amplifier (EDFA)

The dominant optical amplifier. Core idea: a section of silica fiber doped with erbium ions (Er$^{3+}$) is pumped with a 980 or 1480 nm laser. The pump excites erbium ions into a higher energy level. A signal photon at 1530-1565 nm passing through stimulates the excited erbium to drop down, releasing a clone photon — stimulated emission, exactly the laser physics from Chapter 0, but now used as an amplifier instead of an oscillator.

   Energy levels of Er3+ in silica:
 
      4I_{11/2}  ─────                           pump excites here
                   │  fast non-radiative decay (femtoseconds)
                   ▼
      4I_{13/2}  ─────                           metastable, ~10 ms lifetime
                   │  signal photon stimulates emission to ground
                   ▼
      4I_{15/2}  ─────                           ground state

A typical EDFA architecture:

Key properties:

• Gain: 20 to 40 dB easily.
• Bandwidth: 1530-1565 nm (C-band), or 1565-1625 nm (L-band) with different pumping.
• Saturation: at high input power, gain compresses. Often used in saturated mode for power equalization.
• Noise figure: 3.5 to 5 dB. Each EDFA adds amplified spontaneous emission (ASE) noise, equivalent to roughly two photons per mode added by the amplifier.

EDFAs replaced electronic regenerators in long-haul systems in the 1990s. The historical impact is hard to overstate: a sub-sea cable went from "requires complex powered electronic regeneration every 50 km" to "requires a passive coil of doped fiber and a pump laser every 80 km." The cost per bit dropped by a factor of 10. Trans-Atlantic and trans-Pacific traffic costs collapsed; internet pricing followed.

Every long-haul submarine cable laid since 1995 uses EDFAs as repeaters. The pump lasers (typically 980 nm) are themselves chip-scale InGaAs/GaAs laser diodes powered by a few hundred milliwatts, fed by a high-voltage DC current carried along an armored conductor in the cable.

8.2 Semiconductor optical amplifiers (SOA)

A semiconductor optical amplifier is essentially a laser diode without the cavity mirrors, biased to provide gain to a passing optical signal. SOAs are much smaller than EDFAs (mm-scale chip vs. meters of fiber), have lower gain (15-25 dB), faster response (ns), and a significant nonlinearity that makes them less suited to multi-channel WDM amplification but useful for switching, wavelength conversion, and integrated photonic functions.

SOAs are increasingly used as booster amplifiers in compact optical transceivers and as gain stages in integrated photonic circuits.

8.3 Raman amplifiers

A Raman amplifier uses the fiber itself as the gain medium, exploiting stimulated Raman scattering: a high-power pump photon in the fiber transfers some of its energy to the silicon-oxygen vibrational modes of silica, leaving a "Stokes" photon shifted by about 13.2 THz (about 100 nm at 1550 nm). With the right pump wavelength, this provides distributed gain to a signal at the Stokes-shifted wavelength.

Two flavors:

• Distributed Raman amplifier (DRA): pump laser at the receive end of the fiber span. Gain happens along the last 20-30 km of the span, lifting the signal as it weakens. Better noise figure than a discrete EDFA at the same span end.
• Lumped Raman amplifier: a separate spool of dedicated fiber pumped to provide gain.

Raman amplifiers are common in modern long-haul terrestrial systems where every dB of OSNR matters.