When a twin laser emits the mode fields in coherent form, a Y waveguide can be used to mix and focus the twin signals in the longitudinal axis of a fiber core. In best conditions of coupling, both modal fields are assumed to mix coherently in the waveguide junction. A general expression is derived for the coupling coefficient as a function of the longitudinal displacement between components and misalignments of vertical and angular errors. We show how the Y waveguide avoids the drop in coupling efficiency caused by the separation between main emitters and the longitudinal axis of the fiber, and calculate the coupling factors between each main signal and the signal transmitted through the furthest Y waveguide branch to estimate the order of magnitude of possible losses. The coupling efficiency between the TE fundamental mode field and one branch is compared with the expression obtained using the Gaussian approximation. The intensity transmitted by the whole system using a coupled twin-mode solution is analyzed, taking into account the general coupling coefficients between main stripe outputs and branches.
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