Optical Amplifiers: EDFA, Raman, and SOA

Posted on Mar 1, 2025 in Physics

EDFAs use a pump laser (980 nm or 1480 nm) to bring electrons up to a higher energy level. Signal amplification is achieved by emitted photons of the same signal wavelength with the help of stimulated emission.

EDFA Parameters

• Gain: Ratio of the input power to the output power (in dB)
• Bandwidth: Range of wavelengths in which amplification is obtained
• Gain Saturation: Point at which the output power ceases to grow with respect to the increase in input power
• Noise Figure: Ratio of the SNR at the input to the SNR at the output (in dB)

• Photons at 1480 or 980 nm activate electrons into a metastable state.
• Electrons falling back emit light at 1550 nm, the low-loss wavelength region for silica optical fibers.

Working

When pumped with a 980 nm pump source:

• 980 nm pump photon interacts with an electron in the I_15/2 state.
• Electron absorbs energy from the 980 nm pump photon and is excited to a vibrational energy level at I_11/2.
• This is an unstable energy state, and the electron gets de-excited to a lower energy level I_13/2 with a half-life of around 1 µs.
• Energy given out by the electron is absorbed as lattice vibrations in the material lattice.
• I_11/2 is a meta-stable state, and the electron can stay a long period in this energy state before spontaneous emission. The half-life period of the electron is almost 11 ms.
• Spontaneous emission of the electron gives out radiation at about 1550 nm.
• When a signal photon in the C band encounters the electron, stimulated emission takes place.
• Emitting a coherent photon, exactly in the same phase, same direction, and having the same wavelength as that of the signal photon.
• If a population inversion exists, the signal gets amplified.

EDFA Technical Features

• Fiber length: 1-20m
• Self-regulating amplifiers: output power remains more or less constant even if the input power fluctuates significantly.
• Output power: 10-23 dBm
• Gain: 30 dB
• High power transfer efficiency from pump to signal power (> 50%).
• Wide spectral band amplification with relatively flat gain (>20 dB) useful for WDM applications.

EDFA Characteristics

• Large dynamic range.
• Low noise figure.
• Suitable for long-haul applications.
• Used in terrestrial and submarine links.

Raman Amplifier

• Pump excites the molecules to higher energy levels.
• A new wavelength (Stokes Wave) is generated upon de-excitation of these molecules.
• Stokes wave is having a frequency downshift of 13.2 THz.
• If a signal having the new frequency is entering the region having excited molecular levels, amplification of the signal occurs.
• The wave which irradiates the material is called the Pump wave.
• Pump excites the molecules to higher energy levels.
• New waves are generated upon de-excitation of these molecules. The waves with lower frequencies are called the Stokes waves and the waves with higher frequency are called the Anti-Stokes waves.
• The intensity of the Stokes waves is many orders of magnitude higher than the intensity of the anti-Stokes waves.
• Stokes wave has a frequency downshift of 13.2 THz.
• If a signal having this new frequency is entering the region having excited molecular levels, amplification of the signal occurs.
• Lower wavelength pump laser light travelling down an optical fiber along with the signal, scatters off atoms in the fiber, gives some energy to the atoms, and then continues its journey with the same wavelength as the signal.
• Thus, the signal has additional photons representing it and hence it is amplified.
• This new photon can now be joined by many more from the pump, which continue to be scattered as they travel down the fiber in a cascading process.

Working Principle

• SOAs amplify incident light through stimulated emission. The light travelling through the active region causes the electrons to lose energy in the form of photons and get back to the ground state. Those stimulated photons have the same wavelength as the optical signal, thus amplifying the optical signal.
• Similar to a laser cavity.
• Used as discrete amplifiers.
• Can be integrated into arrays of amplifying, switching, and gating devices.
• In order to get only the amplification function, it is necessary to protect the device against self-oscillations generating the laser effect.
• Accomplished by blocking cavity reflections using anti-reflection coating and the technique of angle cleaving the chip facets.