Wide-dynamic-range and channel-count-free thulium doped fiber amplifiers employing variable slope attenuation compensators designed for photonic networks

This paper presents the unique behavior of thulium doped fiber amplifiers (TDFAs) under circumstantial changes in the multi-wavelength photonic networking systems, namely in the span loss, the channel count, and the environmental temperature. Unlike EDFAs, TDFAs exhibit the dynamic gain-profile variation even when the gain-magnitude is fixed under the automatic gain control (AGC). In addition, it has been shown that the dynamic gain variation is less serious in the up-conversion pumping scheme. As a means to maintain the gain-profile of the up-conversion pumped TDFA, three techniques have been introduced and compared, namely: (i) the auxiliary pump around 1.56 mum, (ii) the mid-stage variable optical attenuator (VOA), and (iii) the mid-stage variable slope attenuation compensator (VASC). Employing the VASC, the output signal level has been successfully maintained within the error of 0.2 dB for the dynamic range of 8 dBp-p and the channel count from 4 to 32. Moreover, it has been shown that the temperature change also induces the gain tilt, which has been successfully compensated by the VASC with the level variation of less than 0.3 dB. In summary, the adjustable slope-compensating device will be useful to control the TDFAs under the various circumstantial changes. (C) 2003 Elsevier Science B.V. All rights reserved.