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A dynamic gain equalizer (DGE) is required to automatically compensate for gain variations related to fiber amplifiers. This paper proposes a DGE with thermooptic (TO) phase shifters based on a single-filtered-arm interferometer. A design procedure was newly developed for the control of the TO phase shifters, which could be used to accurately control the intended attenuation profile in one adjustment. In this procedure, the Taylor series method was used as an optimization algorithm, and thermal crosstalk was taken into account. From the point of view of thermal crosstalk, the behavior of phase shifters in an array and the effect of three-dimensional (3-D) trenches formed around the phase shifters were precisely investigated. A DGE module based on 1.5%-Δ planar lightwave circuits and the 3-D trench structure are demonstrated. The measured spectra were found to be in good agreement with arbitrarily designed profiles under open-loop control, even when there was some thermal crosstalk between phase shifters. The results indicate that the design procedure and the estimation of the thermal crosstalk are effective for accurate control of the DGE.