Abstract

The combination of erbium-doped fiber amplifiers (EDFAs) and wavelength-division multiplexing (WDM) offers a dramatic increase in the system capacity for ultra-long-distance transmission.¹ However, one key difficulty in implementing such a system is that the EDFA gain is not uniform with wavelength, especially when incorporating cascades of amplifiers.²³ Passive optical filtering for gain equalization can provide system performance improvement, but only for a static point-to- point link. This is because: (1) individual channel powers may vary significantly due to the dynamic nature of the WDM network (i.e., higher, lower, “OFF"), and (2) gain nonuniformity varies with dynamic input load. Because of its ability to function as a multiple-passband filter with independent control of each wavelength passband, the acousto-optic tunable filter (AOTF)⁴ with active feedback can be used to dynamically equalize the EDFA gain. We demonstrate experimentally the dynamic equalization of the nonuniform EDFA gain by using acoustic-optic tunable filters with active feedback. After 1000km transmission in a recirculating-loop experiment, the maximum power differential is reduced from 20 dB to 1 dB for three 2.5-Gbit/s WDM channels covering an 8-nm wavelength range. The active feedback is verified and ensures proper system performance when the power of any individual channel varies over time, as is likely in a dynamically-reconfigurable WDM network.

© 1996 Optical Society of America