Performance limitations of single- and two-stage EDFA's used as in-line devices in a multichannel hybrid AN-VSB/QAM video lightwave transmission system were studied. It was found that the two-stage EDFA has a much higher saturated NF (>5 dB) compared with the single-stage device when operating at the input signal levels (greater than or equal to 0 dBm) needed for analog transmission. In the 120-km SMF link with two identical in-line EDFA's, the best-achievable CNR of 50.3-dB was obtained using the single-stage devices, as compared to the CNR of 47.8-dB obtained using the two-stage devices under the same operating conditions. While the optimized two-stage EDFA design provides good performance characteristics as in-line amplifiers for long-haul digital transmission, it is shown experimentally and by simulations that the single-stage design with forward-pumping provides better performance than the two-stage design for in-line amplification in hybrid AM-VSB/QAM video lightwave trunking systems.
