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.
A vector polarized beam undergoing a cyclic adiabatic transformation can obtain a Pancharatnam-Berry (PB) phase. This new geometric phase is proportional to light’s total angular momentum (TAM). In this work, experimental evidence is provided for the PB phase evolution on a hybrid-order Poincarésphere. A variety of hybrid-order Poincarésphere beams are directly generated by vortex half-wave plates using a non-interference method, while two TAM converters are used to ensure a closed cycle on the hybrid-order Poincarésphere. By rotating one of the converters, a significant change in PB phase can be observed. The PB phase is related to the topological charge and polarization state of the hybrid-order Poincarésphere bipolar beams and has implications in quantum information science as well as other physical systems such as electron vortex beams.
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.
The vortex beam with helical phase has great potential in detecting rotating targets. Offset, inclination and even the optical axis not in the same plane with the rotational axis often exist between the beam and the rotational axis when the non-cooperative rotating target is measured by vortex beam. To effectively measure rotational speed and eccentric distance (the distance of facula center and the center of rotation on the object being detected) of non-cooperative object, according to the scattering point model and rotational Doppler effect principle, this paper proposes a geometric projection method to study the detection problem about the incidence of the beam at arbitrary position relative to the object’s rotational axis, and analyzes the change of rotational Doppler frequency shift in the state of existing both offset and inclination, and obtains the rule of the extremum of the rotational Doppler frequency shift changing with the relative space position parameters. On this basis, the rotational speed and the eccentric distance are obtained when both eccentricity and inclination exist and the optical axis of vortex has no deflection. And a method for detecting the rotational speed and the eccentric distance of non-cooperative target is proposed. The conclusions obtained in this paper have some guiding significance for further application of rotational speed detection technology based on vortex beam.
The fringe model can directly explain the principle of dual beam differential Doppler effect. Superimposed state vortex light is a petal-like interference fringe superposed by positive and negative topological charge vortex beams. Based on this, the idea of fringe model is applied to the principle of rotational speed measurement based on the rotation Doppler. The rotation Doppler frequency shift based on the model was derived by means of spatial geometric transformation. According to the relative location between the optical axis of vortex and the object rotation axis, three typical incident conditions which are coincident, offset and tilt are considered. The results show that the frequency shift based on the fringe model is consistent with that of the traditional rotation Doppler model, which proves that the model is suitable for the interpretation of the principle of rotation Doppler velocity measurement. This study provides a more intuitive and concise theoretical explanation for the rotational Doppler effect, which has certain guiding significance.
Optical vortex is a kind of unique structure light with a helical phase distribution and contains orbital angular momentum. It has been found a myriad of important applications of optical vortex, such as micromanipulation, remote sensing and communication. In recent years, there are many researches on the particle manipulation based on the orbital angular momentum of optical vortex. Especially, there is a new kind of optical vortex which is so-called “perfect” vortex whose ring radius is independent of its topological charge has an outstanding performance on the micromanipulation. However, the exist experiments can only realize manipulation of particle and the rotation speed is difficult to obtain. In this paper, we proposed a new scheme which can manipulate the particle and simultaneously detect its rotation speed. We employ the superposition perfect vortex as the probe beam. It is worth to note that the topological charge of the perfect vortex we used has an opposite sign and different magnitude. Therefore, the probe beam possesses remaining orbital angular momentum and can generate the rotational Doppler effect. The rotational Doppler effect is the basics of rotation speed detection. A corresponding experiment has been conducted to verify our method. The results have confirmed the existence of orbital angular momentum of superposition optical vortex with different topological charge. Our scheme can realize manipulation and detection the rotation speed of the particle simultaneously. It is an important supplement on the micromanipulation and maybe useful in the optical gyroscope technique.
In order to study the method of optical vortex generation by cascaded spiral phase plates, theoretical analysis, simulations and experimental demonstration of this method are presented. Firstly, theory of optical vortex generation by cascaded spiral phase plates is analyzed. Secondly, an optical vortex generation setup is built, then two experimental groups of optical vortex generation is proposed and generation with single spiral phase plate is set as a control group. On this basis, correlation model is set up, then relative intensity and beam radius with propagation distance are simulated. Simulated graphs of relative intensity and optical radius with propagation distance are plotted and the related rate of change is calculated, initially proving the advantages of cascaded spiral phase plates. Finally, law of relative intensity and optical radius changing of optical vortex with topological charges 5 is studied based on experimental groups and control group, verifying the advantages of optical vortex generation, i.e., law of relative intensity and optical radius changing by cascaded spiral phase plates. By contrast of experimental results, the best configuration of cascaded spiral phase plates is settled.
The vortex light containing orbital angular momentum (OAM) has important application prospects in precision measurement, micro particle manipulation and basic physics. Because the Poynting vector of the vortex wave is not in line with the direction of the optical axis, more information is contained in the echo than the ordinary electromagnetic wave, so it has a unique advantage in the detection of unknown object. The wave propagation characteristics of the vortex beam are modeled and analyzed. Based on the Michelson interference principle, a new type of vortex light interference scheme is designed. The measurement scheme of the wavelength of the vortex light is proposed and the experimental verification is carried out. On this basis, a new method for detecting non-cooperative targets in space is proposed and analyzed theoretically. It provides a new way for measuring angular velocity of objects by vortex optics, and lays a good foundation for remote sensing of non-cooperative targets in actual demand in the future.
The visible supercontinuum (SC) sources has played an important role in biomedical applications. However, the small core size of photonic crystal fiber (PCF) restrict the development of high power SC by its small mode field area. In addition, the zero dispersion wavelength (ZDW) of the PCF with small core diameter is usually below 1 μm, which is far away from the 1.06 μm laser which is the most commonly used pump source. As the ZDW of PCF shifts away from the pump, the intensity of visible light decreases correspondingly. We promote a new technique to get an enhanced visible SC with high output power, which involves enhanced visible SC generation in a seven-core PCF pumped by a high power 1016 nm fiber laser. Muti-core PCFs offer a possibility of scaling up the mode field area to a large extent without remarkable change in dispersion properties, which show great potential in high power SC generation. Using a 1016 nm fiber laser as the pump makes the pump wavelength closer to the ZDW of PCF, which could raise the intensity of visible light. In this paper, we report an enhanced visible SC generation ranging from 400 nm to 2300 nm in a seven-core PCF pumped by a 1016 nm picosecond fiber laser. The visible light (400~800 nm) occupies 31% of the total SC power 24 W and the power of the visible light is about 7.4 W.
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