No abstract

The rotational Doppler effect (RDE) of optical vortex beams provides an efficient way to measure the rotational frequency of objects based on rotational Doppler frequency shifts, while the frequency shift signals display a distinct broadening effect when the vortex beam is laterally misaligned with the center of rotation of a planar object. We use a modal decomposition method to reveal the broadening effect and obtain a linear fitting equation between the quantity of signals and lateral misalignments. In an experiment of RDE, the lateral misalignment is extracted from the quantity of signal peaks. The fitting equation is proved to be precise within the uncertainty of ± 0.17 m m ( ± 2.8 % of the vortex beam radius), and the center of rotation is located with an error less than 3.33% of the beam radius. Our work provides a new approach to locate the center of rotation of noncooperative objects, which may be valuable in mechanical manufacturing and optical noncontact metrology.

The capability to detect the rotational speed of non-cooperative targets in a long distance is a difficult problem to be solved. In recent years, vortex light provides a feasible solution for the measurement of rotational speed for its spiral phase and the orbital angular momentum. Laguerre-Gaussian (LG) mode, as the typical vortex beam, has been widely employed in rotational Doppler effect (RDE) experiments. Here, we show that the nonzero radial index LG beam not only has a specific physical meaning but also can enhance the light intensity and the amplitude of RDE frequency signal relative to a zero radial index LG beam. To this end, we theoretically analyze the reason of intensity enhancement of a nonzero radial index beam and verify the conclusion in a variable control experiment. Our study provides a new aspect of LG beams that can be considered in rotational speed detection based on RDE. It may produce an improvement of the detection range of rotating targets in practical applications.

A new, to the best of our knowledge, method for directly measuring the skew angle of a Poynting vector of optical vortices is reported in this Letter. We design an incomplete optical vortex phase to mimic the occlusion of actual objects on the light path. By capturing the intensity cross section of the incomplete vortex field, the energy flow can be observed directly; thus, the skew angle of the Poynting vector can be directly measured. In this Letter, we measure the skew angle of the Poynting vector with an error less than 3%. Further, the work in this Letter may provide a new way to sense the translational distance and measure the topological charge of the optical vortex.

The Doppler effect of motional polarization grating is studied for the first time to the best of our knowledge. Based on the optical properties of polarization grating, the Doppler effect principle of polarization grating is elucidated theoretically. A method to obtain the Doppler frequency shift based on beat frequency signal that is produced by superposition of order ± 1 diffraction beams of polarization grating is proposed, and a proof-of-concept experiment is conducted to measure the frequency signal of the motional polarization grating. The movement characteristics of polarization grating varying with time can be obtained after a short-time Fourier transformation of the light signal. The experimental results are in good agreement with the theoretical predication, which verifies the correctness of the theoretical analysis and achieves the measurement of linear motion velocity and acceleration of motional polarization grating with high accuracy. This study proposes a new idea for laser frequency shift and has potential significance for further development of optical heterodyne detection.