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Phased-locked arrays of buried-ridge InP/InGaAsP diode lasers, emitting at 1.3 μm, were investigated both theoretically and experimentally. These arrays consist of index-guided buried-ridge lasers which are coupled via their evanescent optical fields. The field patterns and the modal gains of the array supermodes were calculated by using a simple waveguide model. The theoretical results show that buried-ridge arrays can be designed such that only the fundamental supermode is excited. In that case, the array lasers are coupled in-phase, which yields single-lobed, diffraction limited far-field patterns. The buried-ridge InP/InGaAsP arrays were grown by liquid phase epitaxy. These arrays exhibited single-lobed beams less than 4° in width up to more than twice the threshold current. Comparison of the measured field patterns and the calculated ones indicated that these arrays oscillated mainly in the fundamental supermode.