Linear axicon surface testing by reversal double-pass interferometer with planar referencing

A straightforward interferometric method is proposed for testing a linear axicon surface by leveraging its inherent rotational symmetry. A planar referencing (PR) is placed at the center of the beam overlap region, referred to as the waist plane, to ensure ideal coincidence between the incident and reversal-reflected beams. This optical arrangement, termed the reversal double-pass configuration, is governed by a derived geometric relationship for PR positioning. The experimental implementation employs a modified Michelson interferometer integrated with a least-squares A-bucket phase retrieval algorithm, which enables robust phase extraction with arbitrary and unequal phase shifts, thus overcoming the limitations of the temporal phase-shifting techniques. The proposed method significantly simplifies alignment by reducing the required degrees of freedom (DoF) as the PR requires only three DoFs of adjustment. Surface testing of a commercial off-the-shelf linear axicon demonstrates strong agreement between the proposed interferometric method and those obtained from a commercial contact-type profiler (KLA Alpha-Step).