Wavelength division multiplexing (WDM) is an attractive means for large capacity transmission systems and flexible optical networks. One of the key components for the WDM transmission system is a gain- flattened erbium-doped fiber amplifier (EDFA). This thesis proposes a simple and efficient method for designing gain-flattened EDFAs. Also demonstrated is a way to provide flat gain with constant per-channel output power in spite of abrupt variation of span loss and number of WDM channels.
Firstly, a simple method for determination of pump powers and the doped fiber lengths is proposed to design the gain-flattened EDFAs. While the conventional method uses time-consuming iteration to determine the two parameters, the proposed method uses the two linear relations between signal power and pump power, and between doped fiber length and flat gain in dB for a given erbium-doped fiber at a given value of the average inversion of erbium ions where the EDFA exhibits the maximum flat gain. By using the method, the flat gain error is made less than ±0.3dB for the per-channel input of -25 ~ -5 dBm and per-channel output of -5 ~ 5 dBm.
Secondly, an EDFA providing constant output power and flat gain regardless of abrupt variation of loss and number of channels is designed and fabricated by using the feed-forward control and a voltage-controlled attenuator (VCA). In the EDFA, the gain flatness control is achieved based on the linear relation between the pump power and the input power. In addition, the constant output power is obtained by controlling the attenuation of VCA depending on the span loss of the supervisory channel between amplifiers. The experimental result shows that per-channel output power is constant for fluctuation of the span loss of ±3dB in the single-stage EDFA and ±5dB in the two-stage EDFA.
Finally, the dynamic gain control scheme in an EDFA itself is demonstrated without use of a link control channel. Suppression of the surviving channel power excursions due to channel add/drop in the WDM optical network is obtained by maintaining the average inversion with maximum flat gain regardless of the number of WDM channels. The fast pump control circuit with the response time of 650 ns is used to instantaneously control the pump power by monitoring changes in the input power. The feasibility of eight-channel 10 Gbit/s WDM network is experimentally demonstrated using dynamic gain-controlled EDFAs that control the pump power with response time of 650 ns. One surviving channel is successfully transmitted over an 80 km dispersion-shifted fiber without additional power penalty when 7 out of 8 channels are added/dropped at 1 kHz.
