As a candidate for a laser structure which reduces spatial hole burning and provides narrower linewidths, we propose a coupled structure of phase shift DFBs with asymmetric cavity lengths. Mode properties of this asymmetric structure are analysed based on the transfer matrix method. It is found that its threshold gain difference between the fundamental mode and the next higher order mode is about 1.9 times larger than that of the conventional single phase-shift DFB structure, when the asymmetry parameter is optimally adjusted. The linewidth calculation taking into account the spatial hole burning effect predicts that, for a linewidth enhancement factor of α4, the minimum feasible linewidth is about 0.1 MHz, which is three times narrower than what can be achieved by the conventional single phase-shift structure.