Gain Equalizer in Long-Haul WDM Transmission System

Takao NAITO  Naomasa SHIMOJOH  Takafumi TERAHARA  Terumi CHIKAMA  Masuo SUYAMA  

IEICE TRANSACTIONS on Electronics   Vol.E81-C   No.8   pp.1293-1300
Publication Date: 1998/08/25
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Issue on High-Capacity WDM/TDM Networks)
optical communication,  wavelength-division-multiplexing (WDM),  long-haul,  large-capacity,  gain-equalizer,  

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To expand signal wavelength bandwidth in long-haul, large-capacity WDM transmission systems, we investigated gain-equalizers (GEQs) for Erbium doped fiber amplifiers (EDFAs). We applied GEQs using Mach-Zehnder type filters with two different free-spectral-ranges (FSRs) to accurately compensate for the EDFAs gain-wavelength characteristics. The 1st GEQ with a longer FSR was the main GEQ to compensate for the overall gain-wavelength characteristics, and the 2nd GEQ with a shorter FSR was the secondary GEQ to compensate for the resultant gain undulation after the 1st GEQ. The 2nd GEQ had low maximum loss and long period of equalization-spacing compared to the 1st GEQ. We designed that the FSR for the 1st GEQ was twice the signal wavelength bandwidth, and the FSR for the 2nd GEQ was two thirds of the signal wavelength bandwidth. To compensate for the asymmetry in the EDFAs gain-wavelength characteristics, we designed that the 2nd GEQ minimum-loss wavelength was shorter than the 1st GEQ maximum-loss wavelength. Using a circulating loop with a 21-EDFA chain, we confirmed the signal wavelength bandwidth expanded by the above GEQs. We also investigated the trade-off relationship between the signal wavelength bandwidth and the optical signal-to-noise ratio, as the parameter of the number of the 1st GEQ inserted in the EDFAs chain. The achieved signal wavelength bandwidth after 10,000-km transmission was 12 nm. We successfully transmitted 170 Gbit/s (325. 332 Gbit/s) WDM signals over 9,879 km employing high alumina codoped EDFAs and Mach-Zehnder type filters with long FSRs.