Optical vortex was widely used for optical micromanipulation, biomedicine and optics quantum information processing. A major reason for this is that the phase of the vortex beam is distributed in a helical shape and the wave vector revolves around the vortex center. As a high-precision measurement approach, optical interferometry has the characteristics of non-contact, rapidness, full-field and three-dimensional (3D) imaging. In particular, phase-shift interferometry, as an important technology in optical interferometry, has prominent advantages in watermarking of 3D objects, pattern recognition and surface contouring.
Recently, a research team, led by Professor ZHU Jianqiang from Key Laboratory of High Power Laser and Physics, Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Science, have realized single-shot phase-shifting digital holographic imaging by recording the interference pattern between the vortex reference beam generated by Fermat-spiral sieves and the multi-copy images of the measured object. The relevant technical schemes were verified by experiment in the optical waveband.
This team have designed an amplitude-only Fermat-spiral sieves to produce the reference vortex wave and split the object wave into several parts by an equal-intensity splitting grating, and then verify the validity of the proposed single-exposure method. By recording an interference hologram, the measured object can be rapidly reconstructed using a phase-shift digital holographic algorithm in this method.
The experiment demonstrates that, for a vortex beam, the phase difference located at different positions corresponds to the azimuth angle of observation point which provides the foundation of phase-shifting interference background. In phase-shifting digital holography, the multi-copy images of the measured object are generated by the identical replication technology at designated positions in space in the grating system, and then interferes with vortex reference beam to form interference pattern. Several frames of phase-shifting hologram can be extracted from the single-exposure interference pattern for multi-step phase-shifting interference technique to reconstruct the measured object.
Their result shows, although the vortex beam has different phase in different position, the phase difference of sub-hologram calculation window is nearly equal to a constant for multi-copy images satisfying the angular distribution which is consistent with the traditional phase-shifting digital holography with fixed step size.
With advantages of operating in real time, reducing the effects of vibration, widely-used for phase measurement and separating the reconstructed image from unwanted terms, this kind of one-shot exposure technique provides a new idea for single exposure phase shift digital holography.
In addition, Fermat-spiral sieves as a kind of amplitude-type diffraction optical element is small in size, easy processing, light weight, and can be widely used in the measurement and imaging from coherent X-ray to terahertz band.

This research, entitled "Single-shot phase-shifting digital holography with amplitude-only Fermat-spiral sieves" has been published on January 15, 2019 at the Laser Physics Letters. This work was supported by the National Natural Science Foundation of China.

Fig. 1. Intensity distribution of the vortex beam and the interference pattern between the vortex and a plane wave

Fig. 2. Phase-shifting reconstruction result of resolution rest target

Article Website:
http://iopscience.iop.org/article/10.1088/1612-202X/aaf802

Contact:
Mr. Cao Yong
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS
Email: caoyong@siom.ac.cn