  The $J(\phi)$ relation in SFIFS, SNINS and SIS tunnel junctions is studied.
The method for analytical solution of linearized Usadel equations has been
developed and applied to these structures. It is shown that the Josephson
current across the structure has the sum of $\sin \phi $ and $\sin 2\phi $
components. Two different physical mechanisms are responsible for the sign of
$\sin 2\phi $. The first one is the depairing by current which contributes
positively to the $\sin 2\phi $ term, while the second one is the finite
transparency of SF or SN interfaces which provides the negative contribution.
In SFIFS junctions, where the first harmonic vanishes at 0 - $\pi$ transition,
the calculated second harmonic fully determines the $J(\phi)$ curve.
