Without referring to specific devices, a general discussion on the selectivity of the phase locked super modes imposed by the external cavity has been made for the broad stripe laser diode arrays (LDAs) locked by extended cavities (ECs). For a phase locked LDA based on the diffraction coupling between the nearest neighbor emitters, analytical solutions have been derived for the coupling matrix equation, including the finite reflection of the front facet of the LDA facing the EC. Even though the thresholds of the super modes of an LDA are affected by the residual reflection at the front facet, it can still be analytically proved that the favored mode of the LDA is either the lowest or highest order super mode. In consistent with the mode coupling theory, analysis reveals that between z t /(4n + 3) and z t /(4n+1), with z t being the Talbot distance and n=0, 1, 2 ... , there exist EC length ranges favoring the operation of the lowest order super mode, and between z t /(4n + 5) and z t /(4n + 3) favoring the highest order super mode.