The direct dependence of the longitudinal mode spectrum on active and passive waveguiding and stripewidth of metal‐cladded ridge‐waveguide‐stripe lasers is presented. Since the investigated lasers originate from the same epitaxial wafer, the influence of other laser parameters, such as layer characteristics and material quality, can be neglected. Experimental results agree well with a theory, based on the spontaneous emission coefficient as the dominant parameter, which is affected by the curvature of wave fronts.