Grating duty factor strongly affects the performance of gain-coupled (GC) distributed feedback (DFB) laser diodes with an absorptive grating. Through numerical analysis we have found an optimum value in the duty factor for their low threshold operation. The minimum threshold gain achievable at this optimum duty factor is found to be almost independent of the order of the grating. According to this prediction, we have fabricated GaAlAs/GaAs GC DFB lasers with a third order absorptive grating where the grating duty factor has been made close to the theoretical optimum value. In 200 mum-long devices with as-cleaved both facets, low CW threshold current of 25 mA, external efficiency of 0.5 mW/mA, and SMSR as high as 45 dB have been obtained, which is qualitatively consistent with the analysis. High yield of single mode oscillation seems the fruits of the gain coupling.