DWDM

Chapter 4.4.4 - Optical Gain Equalizers

4.4.4 Optical Gain Equalizers

In DWDM transmission, optical gain equalization is required because the DWDM signals eventually become unequal in power. This is because the gain characteristic of amplifiers and filters is not flat, because optical cross-connects do not have the same loss characteristics for all channels, or because dispersion is not the same for all channels (see Fig. 4.23), and so on. In addition, as dropping and adding wavelengths takes place, the wavelengths added most likely will not have the same amplitude as those passing through. The end result is that all DWDM channels in the fiber do not arrive at the same optical strength at the receiver. Optical gain equalization improves the signal-to-noise ratio, and thus it enhances the performance of optical amplifiers and allows for longer fiber spans between amplifiers. Therefore, gain equalization is a key function in long-haul applications.

Optical gain equalizers monitor each wavelength channel and selectively make amplitude adjustments on each channel to flatten the optical power spectrum within a fraction of a decibel. They may be static or dynamic.

Static equalizers consist of filters with specific gain profile that counteract the gain variability of channels in the DWDM mix. Such static equalizers, although inexpensive, are applicable to networks that are not expected to (substantially) change.

As system and/or network scalability may alter the gain level, dynamic equalization is able to adjust to this changing environment. A dynamic gain equalizer is an opto-electronic feedback control sub-system that incorporates several components. Among them are an optical demultiplexer and multiplexer, power splitters, per channel variable optical attenuators (VOA), per channel optical power measuring mechanism, and a microprocessor which according to an algorithm performs per channel real-time gain management and VOA adjustments. Based on this, the DWDM signal is demultiplexed, each signal is monitored and adjusted for gain with the VOA, and then all signals are multiplexed again. Clearly, the key component in this method is the transfer function and accuracy of variable attenuators and how well they integrate with the mux/demux. Variable attenuators may be solid state or variable intensity filters; either technology requires voltage to control attenuation. Figure 4.25 illustrates a regenerator with dispersion compensation and gain equalization modules.

04_04_DWDM-26.jpg

Figure 4.25 A regenerator with dispersion compensation and gain equalization modules.

The salient characteristics of controllable or dynamic GEMs are:

  • attenuation range
  • attenuation resolution
  • voltage-attenuation linearity range
  • accuracy
  • drift over time
  • spectral range
  • spectral resolution and SR uniformity over spectral range
  • PDL and PDL uniformity over spectral range
  • PMD and PMD uniformity over spectral range
  • insertion loss and IL uniformity over spectral range
  • return loss
  • maximum input power
  • temperature sensitivity
  • power dissipation
  • physical size

These devices continue to evolve, improve and become more compact and accurate.

 

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