Rotational Doppler effect (RDE), as a counterpart of the conventional linear Doppler effect in the rotating frame, has attracted increasing attention in recent years on rotational object detection. In practical applications, it is common to see the condition that the beam is obstructed or not fully illuminated on the object, especially in long-rang detection conditions. For example, the fan blades are always wrapped inside the frame, the rotors of engines are arranged inside the machine, to name a few. How to realize the rotational speed measurement when the OV cannot fully illuminate the rotating target is a significant issue for the application of the RDE metrology. However, the principle of the RDE of fragmental optical vortex (FOV) has not been investigated yet. Here, we investigate the RDE of the fragmental optical vortex for the first time. Based on the orbital angular momentum (OAM) mode expansion technique, we establish the relationship between the OAM spectrum and the RDE frequency shift of FOV. The mechanism of the rotational speed extraction is analyzed and validated by the numerical simulation and experiments. Further, a dual Fourier transformation method is proposed to accurately obtain the rotational speed which successfully overcome the problem of the discrete distribution of the RDE signals. Our work may be useful for practical remote sensing based on the optical RDE metrology.
Vortex beam is a new structured light field with a spiral wavefront. Because of carrying orbital angular momentum (OAM) information, vortex beam has a higher degree of freedom of information modulation than traditional laser. It has great application potential in inertial measurement and radar imaging fields. In this paper, we firstly propose a method for two-dimensional imaging based on orbital angular momentum of vortex. Firstly, an imaging model of vortex is established based on linear frequency modulation signal. In the distance direction, pulse compression is achieved by dechirping (The target signal light and the local oscillation light are heterodyne mixed, and amplitude and phase information are obtained by detector).In the azimuth direction, vortex beams carrying different orbital angular momentum l (i.e. topological charge) are emitted, and the target azimuth information represented in frequency domain is obtained by Fourier transform using the approximate duality between the orbital angular momentum spectrum and azimuth variable. The simulation results show that this method can distinguish the point target well and isolate the noise to a certain extent, which lays a foundation for the application of optics vortex in radar target imaging.
We investigate the stability and excitations of two-dimensional circular-symmetry exciton-polariton condensates. The system is described by the circular-symmetry dissipative Gross-Pitaevskii (GP) equation based on the meanfield theory. The excitation properties around the steady-state is analytically calculated by means of the standard Bogoliubov-de Gennes (BdG) approach. We discuss the stability and modulational instability conditions which are determined by the parameters of the system and the parameters that characterize the Gaussian steady-state. We find that, the square of a parameter ¯ι plays a crucial role in characterizing the stability criterion and the excitaion spectrum.
Vortex beam rotational Doppler effect has been verified by experiments. But its mechanism interpretation is still being studied. Firstly, this paper introduces the vortex beam and its rotational Doppler effect, and then explains vortex beam rotational Doppler effect based on the Doppler effect and Sagnac effect. In 2014, professor Miles Padett proposed that the classical Doppler effect can be used to explain vortex beam rotational Doppler effect. This thesis gives its complete process by utilizing resolution of vectors on the spiral wave fronts. Based on the Sagnac effect, this paper uses the principle of independently propagating to deduce the formula of the superimposed vortex beam rotational Doppler effect. The mechanism derivation results agrees with the experimental results, which proves the accuracy and advantage of the mechanism derivation methods in this article. Moreover, this thesis also discovers the high order correction terms of the formula.
A hybrid plasmonic waveguide with an alternative structure is presented in this paper, which consists of a silver cylindrical nanowire coated with a low-index dielectric silica layer attached on a silicon-on-insulator (SOI) substrate with a rectangular ridge. The effects of the silver nanowire, silica-coated layer, silicon substrate layer, and the ridge’s width to thickness ratio on the modal properties have been numerically investigated based on the finite-difference time-domain method. The simulation results demonstrate that the introduction of the low-loss dielectric-coated layer and the ridge of SOI substrate can greatly increase the propagation distance while maintaining strong confinement of light. The propagation distance as long as 660 μm and mode field with subwavelength confinement can be simultaneously realized at telecommunication wavelength. The proposed waveguide consisting of the silica-coated silver nanowire and the SOI ridge substrate would be a beneficial complement to the expanding family of hybrid plasmonic structures.
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