Networks - Amplification

Boost optical signals with EDFA amplifiers

The EDFA is positioned between optical isolators and pumped by 980 nm laser diodes from both directions. The erbium-doped core then behaves as a laser cavity, and amplifies 1550 nm light through stimulated emission.

As all optical fibres introduce optical loss regardless of wavelength, EDFAs can be used to counter the effects of optical loss. They can be placed at strategic positions in an optical network to compensate for previous losses incurred and extend the range of the optical network beyond that range which would otherwise be limited by attenuation. An erbium-doped fibre amplifier (EDFA) is a single mode optical fibre used to amplify C-band optical signals. The core of the fibre is doped with erbium atoms, which when optically pumped using 980nm or 1480 nm light, emit light in the 1550 nm wavelength band.

Optical signals passing through the EDFA and in the 1550 nm window stimulate the excited state Erbium atoms to emit photons at the same wavelength, and amplification results.

 

An EDFA in a pre-amplifier configuration amplifies signals just before arriving at the detector. A signal may also be boosted using an EDFA before entering a long length of fibre or another attenuating device.

Applications

EDFAs have a narrow wavelength working band. They operate specifically in the  C-band and are configured as a pre-amplifier, booster, or an inline amplifier.

  1. A pre-amplifier is normally positioned in front of a receiver. The function of a preamplifier is to amplify the signal after losses have taken place at previous stages of the link and ensure that  sufficient optical power reaches the receiver.
  2. A booster compensates for losses prior to entering a device with high optical attenuation.
  3. Inline EDFAs are used in ‘duplex’ applications where signals transmit through the same fibre in both directions.

Gain flatness

Gain is the increase in optical power upon amplification, the gain of an EDFA varies with wavelength. The range of gain variation across the band is referred to as “gain flatness”.

Large variations in gain flatness lead to large variations in signal amplification and signal strength between DWDM channels & can compromise signal integrity. Large variations in gain flatness are therefore not ideal and gain flattening may be required. Gain flattening reduces the variation in gain flatness across the amplification wavelength profile of the amplifier. Gain flattening is introduced to the EDFA using a gain flattening filter (GFF). This equalises the gain of the EDFA so that all C-band wavelengths have approximately equal power after amplification.

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