## Special issue-Manipulation on optical vortex and its sensing application

2021, 50(9): 20210145. doi: 10.3788/IRLA20210145
[Abstract](362) [FullText HTML] (91) [PDF 1966KB](128)
Since Allen et al. have shown that laser beams with helical wavefront carry orbital angular momentums (OAMs), great advances have been achieved for manipulating beams’ OAMs, and contribute to lots of novel structured beams as optical phase and polarization vortices, laser beam lattices. Such structured fields can find applications in lots of domains including large-capacity data-transmission, remote detection, laser manufacture, high-resolution imaging. One of the important bases of above scenarios is diagnose the OAM spectrum. In the early stage, researchers concentrate more on the measurement of OAM distributions, and afterwards expanded gradually to the intensity proportion measurement of each OAM component, namely the orbital angular momentum spectrum. In this paper, the recent advances of OAM spectrum measurement for laser beams were systematically reviewed and summarized, covering approaches of OAM spectrum measurement based on diffraction, mode sorting and other novel methods.
2021, 50(9): 20200463. doi: 10.3788/IRLA20200463
[Abstract](298) [FullText HTML] (179) [PDF 2415KB](92)
Optical vortex is a new structured light field that carries orbital angular momentum and has a helical wave vibration surface. Since Allen et al. first proved in 1992 that a light field with a spiral phase factor had orbital angular momentum under paraxial conditions, optical vortex has received much attention because of its wide applications in the fields of optical manipulation, optical communication, optical measurement, and remote sensing. Especially in recent years, the application of optical vortex in the field of inertial measurement has attracted the research interest of many scholars. This article mainly involves three aspects: the research progress of optical vortex generation; the key applications of optical vortex in the field of inertial measurement, specifically the rotating Doppler effect and quantum gyros based on optical vortex. New requirements of inertial measurement on the generation of optical vortex were also discussed.
2021, 50(9): 20210408. doi: 10.3788/IRLA20210408
[Abstract](290) [FullText HTML] (101) [PDF 2801KB](49)
Vortex light has important applications in optical communication, quantum entanglement, new nonlinear optical effect, micro- and nano-mechanical processing, super-resolution imaging and optical tweezers, etc. The precondition of vortex light applications is the generation of high-quality vortex light. The method of spot defect mirror combined with solid-state laser technology has outstanding advantage in direct generation of high-order vortex light with high quality and high stability. At present, this method has mostly used in various two-mirror linear cavities for laser generation in continuous-wave mode. A spot defect mirror using an ultraviolet picosecond laser was fabricated. Then, an diode-end-pumped Nd:YVO4 laser with a V-shaped laser cavity configuration was arranged. Based on this laser, a 2.69 W vortex laser with topological charge of 3 was achieved. Slope efficiency of this 3rd vortex laser was about 23.6%. Further changing the cavity length and defect spot size, high-order vortex laser output with topological charges up to 11 and 13 was also achieved. This research clearly indicates that the spot defect mirror technology can directly generate high-order vortex laser in a structurally complicated laser resonator, which has provided reference for Q-switched and/or mode-locked high-order vortex laser generation.
2021, 50(9): 20210445. doi: 10.3788/IRLA20210445
[Abstract](433) [FullText HTML] (113) [PDF 2338KB](113)
Vortex light fields have attracted much attention, due to their nature of optical orbital angular momentum (OAM). The unique physical characteristics of OAM endow the vortex beam with an infinite degree of freedom in high-dimensional space, and at the same time, it also brings the unusual properties of interference, diffraction and transmission to the light field. OAM identification and detection technologies are critical to development of vortex beam from fundamental research towards practical applications. The paper focused on the geometric coordinate transformation of vortex beam, as one of important research directions in the domain of OAM detection, and reviewed the technology in detail in terms of its principles, features, research progresses and applications. Geometric coordinate transformation of vortex beam refered to transforming the whole geometric structure of vortex beam by a specialized modulation phase design, which could sort and detect OAM of vortex beam easily. Compared with traditional sorting and detecting technologies, the geometric coordinate transformation technology was favored with the advantages of passive device, no energy loss, compact structure and low cost. This unique method had become an efficient and powerful tool for the spatial separation and demultiplexing of vortex beams, and had provided a new platform for advanced applications such as classical or quantum density measurement, OAM multiplier and divider, classical optical communication and quantum entanglement, showing great potentials and broad development prospect.
2021, 50(9): 20210283. doi: 10.3788/IRLA20210283
[Abstract](754) [FullText HTML] (349) [PDF 2155KB](179)
Optical vortex beams, carrying orbital angular momentum, possess tremendous advanced applications ranging from optical communication, micromanipulation and nonlinear optics to quantum information. Various methods have been proposed to generate optical vortices, such as spiral phase plate (SPP), mode converter and spatial light modulator (SLM). However, changing phase by accumulating propagation distance restrains the applications in integrated optics. Owing to the salient properties that metasurface is small-sized and easy to be integrated, it is expected to be an ideal optical vortices generator with excellent ability of regulating light field. The basic principles of generating vortex beams and recent progress in the use of metasurfaces were introduced in this article. First, methods of using dynamic phase, Pancharatnam-Berry (P-B) phase and hybrid phase to generate optical vortices were introduced. Subsequently, holography and encoding metasurfaces and multiplexing metasurfaces were reviewed to generate multiple vortices. Finally, based on the generation of optical vortices via metasurfaces, some potential applications and problems were briefly summarized and discussed.
2021, 50(9): 20210242. doi: 10.3788/IRLA20210242
[Abstract](186) [FullText HTML] (142) [PDF 3041KB](51)
Full Poincaré beams, as a more complicated class of novel structural beams, lead to impressive demonstrations recently in Free-Space Optical Communication (FSOC) due to the coupling of spin and orbital angular momentum in the cross section. However, a noteworthy limiting factor of FSOC system in this process is the varying atmospheric turbulence which results in beam expansion, drift, light intensity scintillation and other severe effects. Massive numerical simulations on turbulent atmospheric propagation of full Poincaré beams with C-point polarization, cylindrical vector beams with V-point polarization and homogeneous scalar polarized vortex beams through random phase screens were demonstrated. In this process, normalized correlation coefficient and mode purity were proposed to explore the robustness of full Poincaré beams on different locations of hybrid-order Poincaré sphere. The results show that full Poincaré beams with coordinates located in the southern hemisphere remain high robustness under weak and moderate turbulence (r0=0.5 m, 0.125 m) compared with cylindrical vector beams and scalar vortex beams with similar topological charges and intensity distribution. While the dominant area shrinks to 2σ∈[−5π/32, 0] (latitude coordinates) under strong turbulence (r0=0.056 m). These results will promote the selection of robust transmission media and the development of transmission quality in free-space communications with long-distance.
2021, 50(9): 20210451. doi: 10.3788/IRLA20210451
[Abstract](228) [FullText HTML] (116) [PDF 1413KB](57)
The vortex beam is a kind of spatially structured optical beam carrying orbital angular momentum, whose frequency shifts when it illuminates the surface of a rotating object. This phenomenon, known as the optical rotational Doppler effect (RDE), can be used to obtain the rotation frequency of a flat object by measuring the frequency shift. While the frequency shift is influenced by the incident condition, by revealing the influencing law of incident condition, the rotational frequency of the object can be measured directly. Firstly, a method of velocity projection was used to analyze the mechanism of optical RDE. Then, the rotational Doppler frequency shift distribution law at general incidence of vortex beam was obtained, and the theoretical method of extracting the rotational frequency was proposed. In the end, an experiment of RDE using a superimposed Laguerre-Gaussian beam with topological charge \begin{document}$l = \pm 18$\end{document} was set up, and rotational Doppler frequency shift spectrum at 4 incident conditions was obtained. The experimental spectrum and the theoretical result were combined, then rotational frequency of the object could be extracted with an error less than 1%.
2021, 50(9): 20210428. doi: 10.3788/IRLA20210428
[Abstract](314) [FullText HTML] (95) [PDF 1627KB](91)
The vortex beam can carry orbital angular momentum (OAM), which is associated with the spiral wavefront structure. The vortex beam has attracted a widespread attention due to its capability to significantly increase the transmission capacity and the spectral efficiency of the communication systems without the bandwidth increase. However, the turbulence in free space will cause the distortion on the spiral wavefront, resulting in the inter-mode crosstalk and the received power reduction, that degrade the communication systems’ performance. Adaptive optics is one of the effective methods to correct the distortion. The basic algorithms in adaptive optics for wavefront correction was reviewed, such as, Shack-Hartmann (SH), Stochastic-Parallel-Gradient-Descent (SPGD) and Gerchberg-Saxton (GS) algorithm, and the application of deep learning in wavefront correction was presented. At last, the authors' research work on the wavefront correction in underwater environment was introduced.
2021, 50(9): 20210380. doi: 10.3788/IRLA20210380
[Abstract](209) [FullText HTML] (68) [PDF 1317KB](38)
The discovery of orbital angular momentum (OAM) opens up a new way for the study of optical tweezers. However, the size and shape of biological cells cannot be exactly the same, when the beam with OAM manipulates the particles. So, the uneven velocity of the particles will lead to uncontrollable spacing between the particles when it carries out operations such as rotation. To solve the problem, a cycloid beam with rich regulation modes was proposed by using an arbitrary curve shaping technique and adding curvature control parameters to the traditional cycloid formula. The OAM and gradient force of the cycloid beam was theoretically analyzed, and the possibility of solving the problem was theoretically analyzed. Finally, the start and stop of particles in the process of motion were realized in the experiment, and the three particles were successfully manipulated to rotate at the same distance. The experimental results show that the error of the distance variation of the three particles during the whole rotation process can be maintained at the nanometer level. The work paves the way for future applications of light to capture and manipulate a variety of particles in other fields, particularly in the biological sciences.
2021, 50(9): 20210447. doi: 10.3788/IRLA20210447
[Abstract](486) [FullText HTML] (186) [PDF 2153KB](127)
Polarization is an important property of light field. According to the spatial polarization distribution, optical field can be divided into the scalar field and the vector field. Since the vector field with inhomogeneous polarization distribution performs some interesting behaviors in light propagation, studying the vector beam is of either scientific significance or engineering applied importance. The hybrid vector beams, which contains all kinds of polarization state in the same wave front, has attracted a lot of interests since 2010. Due to the richer polarization states than the normal vector beam, the hybrid vector beam shows attractive prospect on the optical communication, optical manipulation and quantum communication fields. Here, the generation, focusing, propagating and nonlinear optics of the hybrid vector beam were summarized.
2021, 50(9): 20210450. doi: 10.3788/IRLA20210450
[Abstract](292) [FullText HTML] (88) [PDF 1323KB](44)
As an image processing technology, edge enhancement has important applications for studying the boundary information of objects. According to the correlation theory of thermal light ghost imaging, a phase object edge enhancement recognition system was established by combining the ghost imaging with the spiral phase contrast imaging technology. The phase object and the vortex filter were placed in the signal and idle optical path of the ghost imaging system non-locally. The results prove that by using the vortex filter with the fractional orbital angular momentum (OAM) from 0 to 1, the gradual edge enhancement of the phase object can be realized. The higher the OAM topological charge, the more obvious edge enhancement effect will be. Compared with the traditional spiral phase contrast imaging scheme, the limitation of the filter in the spatial frequency spectrum in the spectral plane is broken by the non-local spiral phase contrast imaging scheme. In addition, the coherence requirement of the active illumination light source is released, and the generalization ability of the spiral phase contrast imaging system is enhanced.
2021, 50(9): 20210616. doi: 10.3788/IRLA20210616
[Abstract](268) [FullText HTML] (120) [PDF 1472KB](41)
For traditional velocity detection method like superposition state vortex beam detection and vortex beam homodyne detection, the optical attenuation of the signal caused by long-distance transmission and light divergence would lead to that the detection system can not extract the signal accurately. However, the balanced detection based on vortex beam can solve this problem. But there is little analysis about accuracy and signal-to-noise ratio (SNR) of this detection system, which limits the engineering development of the detection system to a certain extent. Firstly, the homodyne detection was set as comparative item. By analyzing the accuracy change condition of balanced detection and homodyne detection based on vortex beam under different rotational velocity, it was verified that both of them can measure accurately. Secondly, it could be found that the balanced detection had significant advantage when signal light power was low by comparing the SNR of both with different signal light power. Finally, the relation of SNR of the balanced detection and local-oscillator power was revealed by analyzing the effect of SNR on different local-oscillator powers. And the cause of SNR changing with local-oscillator power was clarified.