We have analyzed a phased‐array semiconductor laser coupled to an external cavity containing a spatial filter. The results indicate that the external cavity‐spatial filter combination couples the natural supermodes of the array. Thus if a single supermode were to be emitted by the antireflection‐coated facet, the reflected field would couple into many supermodes of the same parity. By self‐consistently modeling the laser‐cavity configuration we show that substantial mode mixing occurs in practical cases and its effect is more pronounced when the coupling coefficient is small.