The recent developments of wavelength-division-multiplexed (WDM) systems suggest the possibility of implementing all optical networks. In these networks, the optical signals operating at different wavelengths would be added/dropped, or routed to various destinations without optical-to-electrical conversion. However, the dynamic control of optical channels could vary the optical powers incident on the erbium-doped fiber amplifiers (EDFA's) deployed along the transmission lines, which, in turn, cause the power transients of surviving channels due to the cross-gain modulation and impair the network's performance. To overcome this problem, several techniques have been proposed to control the gain of EDFA dynamically using (1) a compensating signal, (2) a feedback loop of ASE power, and (3) a pump power control.
In this thesis, we compare the performances of these techniques experimentally. The responses of EDFA's were measured while turning on and off seven out of eight WDM channels at 330 Hz. When an EDFA was used without these dynamic gain control techniques, the surviving channels suffered from a large power excursion (~ 6 dB) and power penalty (~3 dB @ 2.5 Gb/s). However, the power excursion was suppressed to be within 0.17 dB, 0.15~0.58 dB, 0.05 dB for a single amplifier when we controlled the EDFA's with a compensating signal, a feedback loop of ASE power, and a pump power control, respectively. The power excursions of the surviving channels measured after traversing through 5 EDFA's were about 2 dB when the EDFA's were controlled by using a compensating signal or a feedback loop of ASE power. The power penalties measured after transmitting one 2.5-Gb/s signal over 80 km and 400 km while turning on and off other seven channels were measured to be 0.2 dB and 0.9 dB, respectively.
We estimate the maximum number of EDFA's that can be used in concatenation in a transmission link by considering the control speed. In this estimation, we assumed that the maximum power excursion should be less than 1 dB and the power variations at steady state could be neglected. The result shows that the technique using a compensating signal could be used in a transmission link with up to 37 concatenated EDFA's by controlling only the first amplifier in a link. The technique using a feedback loop of ASE power could utilize up to 7 amplifier spans. These techniques would require flat EDFA gain bandwidth. In addition, more than one control channel may be needed in order to use these techniques in an optical network with a large number of channels. Thus, these control channels could reduce the usable gain bandwidth of EDFA's. The technique using a pump power control requires an individual gain compensation circuitry for every amplifier, although it could be implemented with inexpensive opto-electronic components.
We also demonstrate a dynamic gain and output power controlled EDFA by using a pump power control and an optical attenuator. This EDFA could accommodate the variation of signal power and number of multiplexed channels. The experimental results show that output power per channel (-3 dBm) was maintained even when six channels out of eight channels were dropped and the span loss was increased by 5 dB simultaneously.
