Abstract: An optical amplifier has a gain profile which is substantially flat and independent, over a wide range, of the pump power, power of the input signals, and the number of input signals. The amplifier utilizes an optical resonator having a gain medium whose gain broadening behaves inhomogeneously by pumping the gain medium at at least one wavelength in at least one absorption tail of the gain medium. The resonator is a ring resonator that preferably includes an erbium-doped fiber. Codopants may be added to the fiber to enhance the inhomogeneous broadening effect. A method of gain flattening introduces a pump signal into a gain medium. The pump signal has a wavelength in the tail of the absorption profile of the gain medium. A plurality of optical signals at different wavelengths are introduced into the gain medium. Stimulated emission within the gain medium clamps the gain of the gain medium.

Abstract: A new type of fiber filter is useable in optical communication systems. In particular, the fiber filter may be used to flatten the gain of erbium-doped fiber amplifiers (EDFAs). Such gain flattening is important for long-haul, dense (wavelength dependent multiplexed) WDM communication systems. The filter includes a periodic mechanical structure pressed against the side of a single-mode fiber to induce a wavelength-dependent loss in a signal propagating in the fiber core by coupling the signal to fiber cladding modes. The mechanical structure is a periodic comb of small ridges. Each ridge induces a local index change in the fiber via the photoelastic effect. For coupling to the right cladding modes, the period of the grating (and the comb) is in the range of few hundreds of microns. Thus, the grating is easy to fabricate with standard machining equipment.