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The dependence of mode characteristics on built-in refractive-index difference is studied for V-channeled substrate inner stripe lasers by the introduction of large-optical-cavity structure. Temperature dependence of the far-field pattern indicates that the waveguiding mechanism in these devices can be clearly classified into index guiding and gain guiding. At high light outputs, index-guided devices oscillate on a single longitudinal mode and gain-guided devices oscillate on multimodes. In the index-guided lasers, however, the mode stabilization occurs at higher light outputs in the lasers with smaller built-in refractive-index difference.