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We analyze the wavelength-converting operation of directionally coupled semiconductor optical amplifiers (DCSOA's) with respect to their static and dynamic characteristics. A complete dynamic DCSOA model based on the modified time-dependent transfer-matrix method is developed. This model accurately considers longitudinal variations of carrier and photon densities, gain, refractive index, coupling coefficients, and lateral optical fields. The extinction ratio, modulation bandwidth, and frequency chirping of DCSOA-based wavelength converters are investigated. For most characteristics, the DCSOA scheme has advantages over cross-gain modulation. Furthermore, by optimizing the injection current into the DCSOA, even better performance can be attained.