Erbium-doped fiber amplifiers (EDFAs) have been intensively studied for wavelength-division-multiplexing (WDM) communication systems. The stabilization of the amplifier gain against the input signal-power variation is one of the key issues for WDM networks. The gain-stabilizing technique using a passive optical feedback device, such as a bandpass filter or a fiber Bragg grating, has been reported. The gain medium, in conjunction with the passive feedback device acting as a cavity mirror, can provide a lasing light for gain saturation. Therefore, the gain profile of the EDFA in a WDM system is stable against the input signals added or dropped. This method is simple and effective for gain stabilization. However, because the lasing light propagates in the same direction as the input signal, the input signal cannot be close to the lasing wavelength determined by the passive feedback component. Furthermore, the additional lasing light would degrade the EDF noise figure, because it depletes the population inversion during the lasing mechanism. We experimentally demonstrate a simple gain-stabilizing technique for an EDFA in WDM systems. A backward-injected Fabry-Perot laser diode (FP-LD) is adopted as an optical feedback device instead of a passive optical filter for a gain-clamped EDFA. By using this proposed scheme, we not only can stabilize the gain value but also can dynamically control the gain profile by adjusting the bias current of the FP-LD. To solve the problem of noise figure degradation, we propose a configuration of a preamplification stage with a short erbium-doped fiber (EDF). This preamplification EDF locates at the front of the entire gain-clamped EDFA system, and the backward-injected FP-LD cannot affect the gain of this preamplification stage. Consequently, the dynamic gain-clamped profile is retrieved and the degradation of the noise figure is improved.
展开▼
