Dynamic gain equalizer for optical amplifiers

Abstract

An optical equalizer for use primarily with an erbium-doped fiber amplifier has an initial polarizer that convert the input beam to a predetermined polarization, followed by a series of dynamically-adjustable sinusoidal filters that provide attenuation as a sinusoidal function of beam wavelength. Each of the sinusoidal filters has a first liquid crystal cell adjustably rotating the polarization of the beam from the preceding polarizer. This is followed by a second optical element that retards the beam as a sinusoidal function of beam wavelength. For example, the second optical element can be a birefringent crystal that provided a fixed degree of retardance to the beam and a second liquid crystal cell that provides a variable degree of retardance, thereby allowing adjustment of the center frequency of the sinusoidal function. Finally, a third liquid crystal cell adjustably rotates the polarization of the beam. A final polarizer provides amplitude control of the beam based on the polarization rotations introduced by the first and third liquid crystal cells. A controller provides control signals to the liquid crystal cells of each sinusoidal filter so that their combined sinusoidal attenuation functions produce a desired equalization curve.