Design of a FM-EDFA with Gain Correction for Few-Mode WDM Optical Networks

The need to increase the capacity of modern optical communication systems requires the development of new technologies. Optical systems based on modal division multiplexing (MDM) allow an increased aggregate capacity by transmitting signals in orthogonal modes. In long-range links, however, by using EDFAs, the initially equalized WDM signal is affected by the differential signal gain (DSG), by the differential modal gain (DMG) and by the noise figure degradation due to the addition of amplified spontaneous emission (ASE) to the amplified signal. This study presents a technique that minimizes DSG and spectral gain variations in EDFAs for few-mode optical systems. The technique is applied to an FM-EDFA that amplifies a signal composed of 3 modes x 32 wavelength channels (equally spaced between 1530 and 1561 nm). The maximum gain difference, considering a -20 dBm/channel signal power, is reduced from 4 dB to 0.6 dB. Also, considering an input power variation of 20 dB (-40 to -20 dBm/channel at 23 dB gain), the maximum gain difference is always lower than 2.5 dB.