An axial displacement measurement relying on the double-helix light beam

• Proposed a novel method to measure the axial displacement by using a double-helix (DH) beam.
• The axial displacement is measured by detecting the rotated angle of the connection line of the two lobes.
• Our system is immune to the energy fluctuations of the laser source.

A novel method of axial displacement measurement based on the double-helix (DH) beam is proposed in this paper. The DH point spread function (DH-PSF) with two prominent lobes which consist of optical beam modes superposing with each other in the Gauss–Laguerre (GL) modal plane has the characteristic that the two lobes can rotate like a binary star about the center of each other in the propagation of the light beam. We figure out the axial displacement with the variation of the rotated angle of the connection line of the two lobes. In this method, a CCD and a phase-only spatial light modulator (SLM) are used in a relatively reliable and simple experimental configuration, with which we can manage to realize high axial measuring resolution. One superb advantage is that the system is immune to the energy fluctuations of the laser source. As a result, the axial displacement measurement has a remarkable application prospect in modern engineering and scientific researches.