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The analysis and design of two-section and multisection coupled-cavity lasers are treated in two parts. In this first part, the focus is on two-section laser design and control using threshold gains. In the second part numerical analysis of the transient behavior is given. The present treatment begins by using the poles of a linear transfer function for the coupled-cavity laser to obtain mode wavelengths and wavelength dependent threshold gains. A general wave scattering matrix describes the intercavity coupling, so that simple waveguide discontinuities or complex arrays of discontinuities can be modeled. Numerical examples are given. Design relationships obtained primarily from the scattering analysis, together with numerical examples, show the dependence and interdependence of the laser mode wavelengths and threshold gain minima on various parameters such as cavity and coupler lengths, indexes of refraction, and gains. These relationships show optimum or preferred coupling gap lengths and cavity length ratios for various design criteria. For example, for optimum mode and spurious rejection stability a short ( m) cavity can be coupled via a gap approximately an integer number of half wavelengths long to a second longer cavity ( m). Conversely, for maximum tunability (or sensitivity to input changes) together with good mode selectivity, two medium length cavities ( m) differing in length by a small amount ( m) should be chosen.