An optical collimator for single mode fibers includes a fiber segment (S1) with mode expansion including an entry and an exit for a light beam. The entry of the fiber segment with mode expansion is on the side at which the core diameter is the smallest, and the exit is on the side at which the core diameter is the largest. The collimator further includes a fiber segment (S2) with expansion holding coupled to the exit, the core diameter of which is equal to or greater than the largest core diameter of the fiber segment with mode expansion, so that the diameter of the light beam guided for this segment is kept constant.