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The tightly focused structured light field displays a non‐negligible component of the electric field in the direction of propagation, i.e., the longitudinal light field. The longitudinal light field results in various novel optical phenomena, such as transverse spin angular momentum and 3D topological light fields. However, it remains a challenge to generate customized longitudinal light fields. Here, based on inverse design theory, the longitudinal light field can be customized on demand under the tightly focusing condition, which allows to generate the longitudinal light field with controllable structured distribution for propagating waves or manipulate the complex structures for all the three components of the electric field. In experiment, an experimental system is built integrating the generation, customization, and detection of structured longitudinal light field, and the customization of the longitudinal light fields with various patterns is experimentally realized. The customized longitudinal light field structures may find applications such as information encoding, long‐distance light manipulation, and novel light–matter interactions. As an example, information encoding is realized by the structured longitudinal light fields.

Generation of nonlinear frequency combs in χ(3) optical microresonators has attracted tremendous research interest during the last decade. Recently, realization of the microcomb owing to χ(2) optical nonlinearity in the microresonator promises new breakthroughs and is a big scientific challenge. Moreover, it is of high scientific interest that the presence of both second- and third-order nonlinearities results in complex cavity dynamics. In particular, the role of χ(3) nonlinearity in the generation of the quadratic microcomb is still far from being well understood. Here, we demonstrate the interaction between the second- and third-order nonlinearity in the lithium niobate microresonator, which can provide a new way of phase matching to control the mode-locking condition and pulse number for the quadratic microcomb. Our results verify that the Kerr nonlinearity can benefit the quadratic microcomb. The principle can be further extended to other material platforms to provide more manipulation methods for comb generation based on χ(2) nonlinearity at mid-infrared.

Entanglement, as a crucial feature of quantum systems, is essential for various applications of quantum technologies. High-dimensional entanglement has the potential to encode arbitrary large amount of information and enhance robustness against eavesdropping and quantum cloning. The orbital angular momentum (OAM) entanglement can achieve the high-dimensional entanglement nearly for free stems due to its discrete and theoretically infinite-dimensional Hilbert space. A stringent limitation, however, is that the phase-matching condition limits the entanglement dimension because the coincidence rate decreases significantly for high-order modes. Here we demonstrate relatively flat high-dimensional OAM entanglement based on a spontaneous parametric down conversion (SPDC) from an ultrathin nonlinear lithium niobite crystal. The difference of coincidences between the different-order OAM modes significantly decreases. To further enhance the nonlinear process, this microscale SPDC source will provide a promising and integrated method to generate optimal high-dimensional OAM entanglement.

The generation of a quadratic soliton in a pulse-pumped microresonator has attracted significant interest in recent years. The strong second-order nonlinearity and high peak power of pumps offer a straightforward way to increase efficiency. In this case, the influence of the third-order nonlinearity effect becomes significant and cannot be ignored. In this paper, we study the quadratic soliton in a degenerate optical parametric oscillator driven synchronously by the pulse pump with third-order nonlinearity. Our simulations verify that the robustness of quadratic soliton generation is enhanced when the system experiences a perturbation from pump power, cavity detuning, and pump pulse width. These results represent a new way of manipulating frequency comb in resonant microphotonic structures.

We propose a novel scheme for designing and generating kaleidoscope-structured vector optical fields (KS-VOFs) by analogy with the principle of multiple mirror reflection in a kaleidoscope. For KS-VOFs with symmetric polarization states, we show the symmetry properties of the focal fields with various shapes for different applications. The redistributing symmetric local spin angular momentum (SAM) density indicates that the design method of the KS-VOFs plays a role as a catalyst to the redistribution process of polarization states and local SAM conversion in the tight focusing process. Meanwhile, the controllable transverse energy flow in the focal plane can be used to transport multiple absorptive particles and then to be fixed at certain locations. Our results may find applications in optical machining, trapping, and manipulation.

Polarizer, as an indispensable optical element, has been widely used in various optical systems, which splits a beam into two beams of orthogonal linear polarizations. Due to the appearance of the vector optical field (VOF) and its unique properties, the vector polarizer also meets the requirement in a wide variety of applications. Here we present and demonstrate the realization of dielectric broadband vector polarizer, being in fact a liquid-crystal-based space-variant uniaxial crystal (LC-SV-UAC), which is made by a wedged thin cell adhered with space-variant photo-aligned dichroic dye films and filled with the nematic liquid crystal with a thickness of tens of microns. The vector polarizer works based on the birefringent mechanism to spatially separate the orthogonally polarized ordinary and extraordinary beams. The vector polarizers via a LC-SV-UAC have the advantages of mass-production and easy fabrication of large-size and complex structures. In particular, the high-performance broadband vector polarizers we presented and fabricated can not only flexibly tailor the polarization structures and the intensity patterns of optical fields but also act as a high-efficiency generator of VOF, and a key element for realizing the VOF laser and for fabricating the novel photon states in the future.

We present a scheme for realizing femtosecond multi-filamentation with designable quantity and locations of filaments, based on the control of multi-focal spots formed by patterned optical fields (POFs) composed of multiple individual optical fields (IOFs). A computer-controlled spatial light modulator is used to engineer the POFs. In particular, we introduce a blazed phase grating in any IOF, which increases a degree of freedom, making the engineering of multi-focal spots becomes more flexible. We achieve experimentally the aim controlling femtosecond multi-filamentation in a K9 glass. Our scheme has great flexibility and convenience in controlling the multi-filamentation in quantity and locations of filaments and strength of interaction between filaments.

We present and demonstrate an approach for femtosecond laser processing by using patterned vector optical fields (PVOFs) composed of multiple individual vector optical fields. The PVOFs can be flexibly engineered due to the diversity of individual vector optical fields in spatial arrangement and distribution of states of polarization, and it is easily created with the aid of a spatial light modulator. The focused PVOFs will certainly result in various interference patterns, which are then used to fabricate multi-microholes with various patterns on silicon. The present approach can be expanded to fabricate three-dimensional microstructures based on two-photon polymerization [ABSTRACT FROM AUTHOR]

We proposed and demonstrated a room-temperature multiwavelength Yb-doped fiber ring laser with high stability based on a Mach-Zehnder interferometer by exploiting spatial hole burning and polarization hole burning effects, from which stable eight lasing wavelengths is obtained. The number of the lasing wavelengths (eight) is the most to our knowledge, and the wavelength spacing is 1.1 nm. Triple-, and dual-wavelength operation with the wavelength spacing of 1.1 nm or its integral multiples have been obtained by adjusting the intracavity polarization controllers. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 723–725, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23206 [ABSTRACT FROM AUTHOR]

High-dimensional (HD) entanglement provides a very promising way of transcending the limitations of the two-dimensional entanglement between qubits for increasing channel capacity in many quantum protocols. In the pursuit of capitalizing on the HD entangled states, one of the central issues is to unambiguously and comprehensively quantify and reconstruct them. The full quantum state tomography is a unique solution, but it is undesirable and even impractical because the measurements increase rapidly in d 4 for a bipartite d-dimensional quantum state. Here we present a very efficient and practical tomography method—asymptotical locking tomography (ALT), which can harvest full information of bipartite d-dimensional entangled states by very few measurements less than 2d 2 only. To showcase the validity and reasonableness of our ALT, we carry out the test with the two-photon spin-orbital angular momentum hyperentangled states in a four-dimensional subspace. Besides high-efficiency and practicality, our ALT is also universal and can be generalized into multipartite HD entanglement and other quantum systems. [ABSTRACT FROM AUTHOR]

In recent years, orbital angular momentum (OAM), as a new usable degree of freedom of photons, has been widely applied in both classical optics and quantum optics. For example, digital spiral imaging uses the OAM spectrum of the output beam from the object to restore the symmetry information of the object. However, the related experiments have been carried out in free space so far. Due to the poor anti-noise performance, limited transmission distance and other reasons, the practicability is seriously restricted. Here, we have carried out a digital spiral imaging experiment through a few-mode fiber, to achieve the identification of the symmetry of object by measuring the OAM spectrum of the output beam. In experiment, we have demonstrated the identification of the symmetry of amplitude-only and phase-only objects with the two-, three- and four-fold rotational symmetries. We also give the understanding of the physics. We believe that our work has greatly improved the practical application of digital spiral imaging in remote sensing. [ABSTRACT FROM AUTHOR]

Ghost imaging functions achieved by means of the spatial correlations between two photons is a new modality in imaging systems. With a small number of photons, ghost imaging is usually realized based on the position correlation of photon pairs produced from the spontaneous parametric down-conversion process. Here we demonstrate a way to realize multi-path ghost imaging by introducing an additional time correlation. Different delays of paths will induce the shift of the coincidence peak, which carries the information about objects. By choosing the suitable coincidence window, we obtain images of three objects simultaneously, with a visibility of 87.2%. This method provides insights and techniques into multi-parameter ghost imaging. It can be applied to other correlated imaging systems, for example, quantum spiral imaging. [ABSTRACT FROM AUTHOR]

We present a two-photon interference experiment in a modified Mach-Zehnder (MZ) interferometer in which two Hong–Ou–Mandel effects occur in tandem and construct superposed two-photon states. The signal photons pass both the arms of the MZ interferometer while the idler photons pass one arm only. Interestingly, the probability of the idler photons emerging from any output port still shows a sine oscillation with the two-photon phase difference and it can be characterized only by the indistinguishability of the two-photon amplitudes. We also observe a two-photon interference pattern with a period being equal to the wavelength of the parametric photons instead of the two-photon photonic de Broglie wavelength due to the presence of two-photon phase difference, in particular, with complementary probabilities of finding the two-photon pairs in two output ports. The abundant observations can facilitate a more comprehensive understanding of the two-photon interference. [ABSTRACT FROM AUTHOR]

We present an optical encryption approach based on the combination of fractal Fresnel lens (FFL) and fractional Fourier transform (FrFT). Our encryption approach is in fact a four-fold encryption scheme, including the random phase encoding produced by the Gerchberg–Saxton algorithm, a FFL, and two FrFTs. A FFL is composed of a Sierpinski carpet fractal plate and a Fresnel zone plate. In our encryption approach, the security is enhanced due to the more expandable key spaces and the use of FFL overcomes the alignment problem of the optical axis in optical system. Only using the perfectly matched parameters of the FFL and the FrFT, the plaintext can be recovered well. We present an image encryption algorithm that from the ciphertext we can get two original images by the FrFT with two different phase distribution keys, obtained by performing 100 iterations between the two plaintext and ciphertext, respectively. We test the sensitivity of our approach to various parameters such as the wavelength of light, the focal length of FFL, and the fractional orders of FrFT. Our approach can resist various attacks. [ABSTRACT FROM AUTHOR]

In traditional ghost imaging, the entangled photon pairs produced from the spontaneous parametric down conversion (SPDC) process are used. There is an intrinsic disadvantage that the utilization efficiency of the photon pairs is very low. Inasmuch as all the correlated photon pairs produced by the degenerate SPDC process can be used to record the image of an object, the ghost imaging scheme we present here has a higher utilization efficiency of the photon pairs. We also investigate the robustness of our experimental scheme. The experimental results show that, no matter whether the photon-pair source is two light cones or two beam-like spots, the clear image of the object can be obtained. The slight rotation of the nonlinear crystal has no influence on the imaging quality. Our experimental results also demonstrate that when the part of the photon-pair source in the signal path or the idler path is blocked by unwanted things, the clear ghost image of the object can still be recorded. [ABSTRACT FROM AUTHOR]

We investigate a novel spatial geometric phase of hybrid-polarized vector fields consisting of linear, elliptical and circular polarizations by Young’s two-slit interferometer instead of the widely used Mach–Zehnder interferometer. This spatial geometric phase can be manipulated by engineering the spatial configuration of hybrid polarizations, and is directly related to the topological charge, the local states of polarization and the rotational symmetry of hybrid-polarized vector optical fields. The unique feature of geometric phase has implications in quantum information science as well as other physical systems such as electron vortex beams. [ABSTRACT FROM AUTHOR]

We present a modified, more universal scheme for generating vector fields. Here we design in principle and experimentally generate a new kind of uniformly elliptically polarized vector field, which has the same ellipticity and sense of local elliptic polarization at any location and also has a flexibly designable distribution of orientation of the elliptic polarization. An introduced additional degree of freedom is used to flexibly change the ellipticity. In particular, the ellipticity and the orientation of polarization can be independently controlled by two parameters. This makes it easier to both control the spatial structure of the polarization and to engineer the focusing field. [ABSTRACT FROM AUTHOR]

We develop analytical formulae for describing the tight focusing field of the Laguerre–Gaussian vector field with space-variant distribution of states of polarization in the frame of the Rayleigh–Sommerfeld theory. The analytical formulae of three radial, azimuthal and longitudinal components are very beneficial in order to understand the tightly focused field. This result is of great importance to improving the design and engineering the tightly focused field, by using phase and polarization, for applications such as optical tweezers and optical manipulation. This method is also applicable to the homogeneously polarized optical fields and optical vortex fields. In particular, these analytical formulae can also describe the dynamic process of the tightly focused fields, for example, average photon trajectories. [ABSTRACT FROM AUTHOR]

A dual-wavelength narrow-linewidth laser system, with ultranarrow wavelength spacing, is proposed and demonstrated. The operation principle of the laser system is based on the pump-induced thermal effects of the Er-Yb-doped fiber distributed Bragg reflector (DBR) laser. We designed and constructed a common Er-Yb-doped DBR fiber laser with proper parameters. With the pump power of the DBR laser increasing, the Bragg wavelengths and the bandwidths of the two gratings will change unequally. When the laser is pumped with 45 mW, we obtain stable dual-wavelength lasing with a 3-dB bandwidth of ≈0.014 nm and a wavelength spacing of ≈0.055 nm. [ABSTRACT FROM AUTHOR]

We present experiments of line and parallel configurations of hybrid erbium-doped fiber amplifiers/fiber Raman amplifiers (EDFAs/FRAs). For the line structure, a gain flatness of ±1.15 dB over a range of 47 nm (1483 to 1530 nm) is obtained. In the parallel configuration, the gain flatness is below ±2 dB for 1487 to 1507 nm and 1520 to 1550 nm. Using a high-birefringence fiber loop mirror as a signal reflector, a noise figure is less than 6 dB in the gain bandwidth. Then performances of different hybrid configurations are compared. [ABSTRACT FROM AUTHOR]