Method of controlling a dynamic gain controller

#The present invention relates to the field of dynamic gain controllers/equalizers, and more particularly, to methods of controlling optical characteristics of individual pixelated elements in a dynamic gain equalizer having an array of controllable pixelated elements used in optical transmission systems.

#Optical transmission systems employ wavelength division multiplexing (WDM) to maximize use of a given band of wavelengths. The wavelength band is subdivided into multiple wavelengths or channels with each channel being independently modulated with a digital signal. Typically, the channels are multiplexed into an optical fiber and propagated along a transmission route between end systems.

#For transmission paths greater than a certain distance it is necessary to incorporate optical filters at points along the route to compensate for wavelength-dependent system gain and attenuation. Optical fibers and optical amplifiers do not propagate all wavelengths with equal gain and attenuation and in many applications it is necessary to equalize the net gain and attenuation between channels. This can be done utilizing a dynamic gain equalizer (DGE) or gain controller.

#Further features and advantages of the present invention will be described in the detailed description, taken in combination with the appended drawings, in which:

#FIG. 1 is a block diagram illustration of a representative configuration for an optical amplifier with dynamic gain equalization;

#FIG. 2 is a block diagram illustration of a representative DGE controller shown in FIG. 1;

#By way of background, the present invention is concerned with determining the performance of a dynamic gain equalizer in which input light is spectrally disbursed across a linear array of pixel elements (for example, liquid crystal cells) operating as variable attenuators and the reflected (or transmitted) spectrum is undisbursed and focused on an output fiber. In implementations using variable retarder liquid crystal cells, the beams incident on the liquid crystal pixels are in a fixed linear polarization. The liquid crystal pixels provide a variable rotation of the polarization of the reflected (or transmitted) light and only the component in the polarization of the incident beam couples to the output fiber to provide variable attenuation.

#Liquid crystal cells, using nematic liquid crystals, with the rubbing axes on the two plates parallel to each other, provide a variable retarder (or wave plate) functionality. With zero applied voltage the molecules orient themselves parallel to the rubbing axes providing a retarder with its principle axes parallel and perpendicular to the rubbing axes. As a voltage is applied between the electrodes the molecules rotate to align themselves along the field direction.

#In general, the present invention structures the control problem as a set of linear equations, which are used to efficiently and accurately determine an initial set of pixel settings, and can efficiently and accurately iterate them to determine the optimum pixel settings for a desired response function or change in the response function. The present invention, also provides a mechanism to use compensation pixels at the ends of the pixel array to compensate for the effects of the edges of the array where the compensation pixels are controlled in such a way as to minimize ripple in the responses of nearby active pixels.