Optical orbital-angular-momentum-multiplexed data transmission under high scattering

Functionality Expansion of Guided Mode Radiation via On-Chip Metasurfaces

On-chip metasurfaces play a crucial role in bridging the guided mode and free-space light, enabling full control over the wavefront of scattered free-space light in an optimally compact manner. Recently, researchers have introduced various methods and on-chip metasurfaces to engineer the radiation of guided modes, but the total functions that a single metasurface can achieve are still relatively limited. In this work, we propose a novel on-chip metasurface design that can multiplex up to four distinct functions. We can efficiently control the polarization state, phase, angular momentum, and beam profile of the radiated waves by tailoring the geometry of V-shaped nanoantennas integrated on a slab waveguide. We demonstrate several innovative on-chip metasurfaces for switchable focusing/defocusing and for multifunctional generators of orbital angular momentum beams. Our on-chip metasurface design is expected to advance modern integrated photonics, offering applications in optical data storage, optical interconnection, augmented reality, and virtual reality.

Uniform intensity chiral optical field by multifocal synthesis

Chiral optical beams that carry orbital angular momentum (OAM) have a broad range of applications such as optical tweezers, chiral microstructure fabrication, and optical communications. However, some chiral optical beams have inhomogeneous intensity distribution that limits the application in these fields. In this Letter, two different types of chiral optical fields with uniform intensity and arbitrary length were proposed based on the amplitude encoding method and multifocal synthesis. The intensity distribution of the chiral optical fields is determined by the distance between the focal points that can greatly extend the modulation length of the chiral optical field. Moreover, since each focal point contains modulable amplitude and phase, an arbitrary interception of the optical field can be realized by selectively retaining a part of the focal points. By partitioning the chiral optical field and assigning different topological charges, the OAM space-division multiplexing and independent tunability of the topological charges can be realized. In addition, the composite multi-petal vortex array formed by combining two different chiral optical fields can greatly enhance the information capacity of the optical communications and may have potential applications in fields such as particle manipulation.

Tracking the transmission matrix of a moving fiber with the transmitted data

During data transmission, the dynamic change of a scattering medium will make the measured transmission matrix (TM) invalid, so it is necessary to repeatedly measure the TM to achieve a long-time data transmission, which requires stopping the data transmission process frequently to measure the TM and leads to a reduction in the communication capacity. To solve this problem, we propose a TM tracking method during data transmission. In the case of more than three discrete levels of phase modulation, this method can realize the calibration of the TM with the intensity pictures captured by the camera and the recovered data, so it does not require stopping the data transmission process to measure the TM and thus avoids the loss of communication capacity. We have proved the feasibility of this method through simulations and experiments and realized the continuous transmission of random data and image data through a moving fiber with high accuracy.

Construction of vector vortex beams on hybrid-order Poincaré sphere through highly scattering media

Vector vortex beams (VVBs) have attracted extensive attention due to their unique properties and their wide applications in fields such as optical manipulation and optical imaging. However, the wavefronts of the vector vortex beams are highly scrambled when they encounter highly scattering media (HSM), such as thick biological tissues, which greatly prevents the applications of VVBs behind HSM. To address this issue, we propose a scheme to construct VVBs of freewill position on the surface of hybrid-order Poincaré sphere (HyOPS) through HSM. With the measurement of two orthogonal scalar transmission matrices, the conjugated wavefronts for constructing orbital angular momentum beams with arbitrary topological charge in right and left circularly polarized states through HSM can be calculated, respectively. When an input wavefront superimposed by the two conjugated wavefronts with an appropriate ratio and phase delay, impinges on the HSM, the desired VVB can be created through HSM. To demonstrate the viability of our scheme, a series of VVBs on different locations of various HyOPSs have been reconstructed through a ZnO scattering layer experimentally. Furthermore, to characterize the polarization distribution of the generated beams, the polarization maps of these beams are derived by measuring the four Stokes parameters, which agree well with the theoretical distributions. This work will promote the applications of VVBs in highly scattering environments.

Ternary logic in the optical controlled-SWAP gate based on Laguerre-Gaussian modes of light

The need set by a computational industry to increase processing power, while simultaneously reducing the energy consumption of data centers, became a challenge for modern computational systems. In this work, we propose an optical communication solution, that could serve as a building block for future computing systems, due to its versatility. The solution arises from Landauer's principle and utilizes reversible logic, manifested as an optical logical gate with structured light, here represented as Laguerre-Gaussian modes. We introduced a phase-shift-based encoding technique and incorporated multi-valued logic in the form of a ternary numeral system to determine the similarity between two images through the free space communication protocol.

Fast measurement of coherence-orbital angular momentum matrices of random light beams using off-axis holography and coordinate transformation

We propose an effective protocol to measure the coherence-orbital angular momentum (COAM) matrix of an arbitrary partially coherent beam. The method is based on an off-axis holography scheme and the Cartesian-polar coordinate transformation, which enables to simultaneously deal with all the COAM matrix elements of interest. The working principle is presented and discussed in detail. A proof-of-principle experiment is carried out to reconstruct the COAM matrices of partially coherent beams with spatially uniform and non-uniform coherence states. We find an excellent agreement between the experimental results and the theoretical predictions. In addition, we show that the OAM spectrum of a partially coherent beam can also be directly acquired from the measured COAM matrix.

Non-orthogonal optical multiplexing empowered by deep learning

Orthogonality among channels is a canonical basis for optical multiplexing featured with division multiplexing, which substantially reduce the complexity of signal post-processing in demultiplexing. However, it inevitably imposes an upper limit of capacity for multiplexing. Herein, we report on non-orthogonal optical multiplexing over a multimode fiber (MMF) leveraged by a deep neural network, termed speckle light field retrieval network (SLRnet), where it can learn the complicated mapping relation between multiple non-orthogonal input light field encoded with information and their corresponding single intensity output. As a proof-of-principle experimental demonstration, it is shown that the SLRnet can effectively solve the ill-posed problem of non-orthogonal optical multiplexing over an MMF, where multiple non-orthogonal input signals mediated by the same polarization, wavelength and spatial position can be explicitly retrieved utilizing a single-shot speckle output with fidelity as high as ~ 98%. Our results resemble an important step for harnessing non-orthogonal channels for high capacity optical multiplexing.

High-fidelity multi-channel optical information transmission through scattering media

High-fidelity optical information transmission through strongly scattering media is challenging, but is crucial for the applications such as the free-space optical communication in a haze or fog. Binarizing optical information can somehow suppress the disruptions caused by light scattering. However, this method gives a compromised communication throughput. Here, we propose high-fidelity multiplexing anti-scattering transmission (MAST). MAST encodes multiple bits into a complex-valued pattern, loads the complex-valued pattern to an optical field through modulation, and finally employs a scattering matrix-assisted retrieval technique to reconstruct the original information from the speckle patterns. In our demonstration, we multiplexed three channels and MAST achieved a high-fidelity transmission of 3072 (= 1024× 3) bits data per transmission and average transmission error as small as 0.06%.

Wavefront shaping improves the transparency of the scattering media: a review

Significance

Wavefront shaping (WFS) can compensate for distortions by optimizing the wavefront of the input light or reversing the transmission matrix of the media. It is a promising field of research. A thorough understanding of principles and developments of WFS is important for optical research.

Aim

To provide insight into WFS for researchers who deal with scattering in biomedicine, imaging, and optical communication, our study summarizes the basic principles and methods of WFS and reviews recent progress.

Approach

The basic principles, methods of WFS, and the latest applications of WFS in focusing, imaging, and multimode fiber (MMF) endoscopy are described. The practical challenges and prospects of future development are also discussed.

Results

Data-driven learning-based methods are opening up new possibilities for WFS. High-resolution imaging through MMFs can support small-diameter endoscopy in the future.

Conclusion

The rapid development of WFS over the past decade has shown that the best solution is not to avoid scattering but to find ways to correct it or even use it. WFS with faster speed, more optical modes, and more modulation degrees of freedom will continue to drive exciting developments in various fields.

High-order orbital angular momentum mode-based phase shift-keying communication using phase difference modulation

Orbital angular momentum (OAM) mode offers a promising modulation dimension for high-order shift-keying (SK) communication due to its mode orthogonality. However, the expansion of modulation order through superposing OAM modes is constrained by the mode-field mismatch resulting from the rapidly increased divergence with mode orders. Herein, we address this problem by propose a phase-difference modulation strategy that breaks the limitation of modulation orders via introducing a phase-difference degree of freedom (DoF) beyond OAM modes. Phase-difference modulation exploits the sensitivity of mode interference to phase differences, thereby providing distinct tunable parameters. This enables the generation of a series of codable spatial modes with continuous variation within the same superposed OAM modes by manipulating the interference state. Due to the inherent independence between OAM mode and phase-difference DoF, the number of codable modes increases exponentially, which facilitates establishing ultra-high-order phase shift-keying by discretizing the continuous phase difference and establishing a one-to-one mapping between coding symbols and constructed modes. We show that a phase shift-keying communication link with a modulation order of up to 4 × 104 is achieved by employing only 3 OAM modes (+1, + 2 and +3), and the decode accuracy reaches 99.9%. Since the modulation order is exponentially correlated with the OAM modes and phase differences, the order can be greatly improved by further increasing the superimposed OAM modes, which may provide new insight for high-order OAM-based SK communication.

Multi-channel data transmission through a multimode fiber based on OAM phase encoding

Data transmission based on the transmission matrix method has realized the multiplexing of a large number of orbital angular momentum (OAM) modes under scattering, which encodes the data by modulating the amplitude of the OAM modes. However, this amplitude modulation (amplitude encoding) method has obvious cross talk when the number of output modes is small, resulting in a non-negligible bit error rate. Here, a multi-channel data transmission method based on OAM phase modulation (phase encoding) under scattering is proposed. This method can resist the multiple-scattering effect of multimode fibers and realize accurate data transmission with very few rows of camera pixels for output mode measurement, which is suitable for high-speed data transmission under scattering. Experimentally, we have achieved a bit error rate of less than 0.005% in the data transmission of a color image through a 60 m multimode fiber with only 2 rows of camera pixels for output mode measurement. Experiments also showed that the proposed method has a higher stability than amplitude encoding when the proportion of "1" or "0" in the code changes.

Multiplexed vortex beam-based optical tweezers generated with spiral phase mask

The design and implementation of a multiplexed spiral phase mask in an experimental optical tweezers setup are presented. This diffractive optical element allows the generation of multiple concentric vortex beams with independent topological charges and without amplitude modulation. The generalization of the phase mask for multiple concentric vortices is also shown. The design for a phase mask of two multiplexed vortices with different topological charges is developed. We experimentally show the transfer of angular momentum to the optically trapped microparticles by enabling nearly independent orbiting dynamics around the optical axis within each vortex. The angular velocity of the confined particles versus the optical power in the focal region is also discussed for different combinations of topological charges.

Spatial multiplexing for robust optical vortex transmission with optical nonlinearity

Optical vortex beams, with phase singularity characterized by a topological charge (TC), introduces a new dimension for optical communication, quantum information, and optical light manipulation. However, the evaluation of TCs after beam propagation remains a substantial challenge, impeding practical applications. Here, we introduce vortices in lateral arrays (VOILA), a novel spatial multiplexing approach that enables simultaneous transmission of a lateral array of multiple vortices. Leveraging advanced learning techniques, VOILA effectively decodes TCs, even in the presence of strong optical nonlinearities simulated experimentally. Notably, our approach achieves substantial improvements in single-shot bandwidth, surpassing single-vortex scheme by several orders of magnitude. Furthermore, our system exhibits precise fractional TC recognition in both linear and nonlinear regimes, providing possibilities for high-bandwidth communication. The capabilities of VOILA promise transformative contributions to optical information processing and structured light research, with significant potential for advancements in diverse fields.

Generation of vector vortex wave modes in cylindrical waveguides

In this paper, we propose a method to generate Vector Vortex Modes (VVM) inside a metallic cylindrical waveguide at microwave frequencies and demonstrate the experimental validation of the concept. Vector vortex modes of EM waves can carry both spin and orbital angular momentum as they propagate within a tubular medium. The existence of such waves in tubular media can be beneficial to wireless communication in such structures. These waves can carry different orbital angular momentum and spin angular momentum, and therefore, they feature the ability to carry multiple orthogonal modes at the same frequency due to spatial structure of the phase and polarization. In essence, high data rate channels can be developed using such waves. In free space, Orbital Angular Momentum carrying vortex waves have beam divergence issues and a central field-minima, which makes these waves unfavorable for free space communication. But vector vortex mode waves in guided structures do not suffer from these drawbacks. This prospect of enhancement of communication spectrum in waveguides provides the background for the study of vortex wave in circular waveguides. In this work, new feed structures and a radial array of monopoles are designed to generate the VVM carrying waves inside the waveguide. The experimental findings on the distribution of the amplitude and phase of the electromagnetic fields inside the waveguide are presented and the relationship between the waveguide fundamental modes and VVMs are discussed for the first time. The paper also presents methods for varying the cutoff frequency of the VVMs by introducing dielectric materials in the waveguide.

Analysis of reconstruction quality for computer-generated holograms using a model free of circular-convolution error

Continuous complex-amplitude computer-generated holograms (CGHs) are converted to discrete amplitude-only or phase-only ones in practical applications to cater for the characteristics of spatial light modulators (SLMs). To describe the influence of the discretization correctly, a refined model that eliminates the circular-convolution error is proposed to emulate the propagation of the wavefront during the formation and reconstruction of a CGH. The effects of several significant factors, including quantized amplitude and phase, zero-padding rate, random phase, resolution, reconstruction distance, wavelength, pixel pitch, phase modulation deviation and pixel-to-pixel interaction, are discussed. Based on evaluations, the optimal quantization for both available and future SLM devices is suggested.

Complex structured beam direct generation by coherent superposition of a complete set of degenerate eigenmodes

Structured beams have played an important role in many fields due to their rich spatial characteristics. The microchip cavity with a large Fresnel number can directly generate structured beams with complex spatial intensity distribution, which provides convenience for further exploring the formation mechanism of structured beams and realizing low-cost applications. In this article, theoretical and experimental studies are carried out on complex structured beams directly generated by the microchip cavity. It is demonstrated that the complex beams generated by the microchip cavity can be expressed by the coherent superposition of whole transverse eigenmodes within the same order, thus forming the eigenmode spectrum. The mode component analysis of complex propagation-invariant structured beams can be realized by the degenerate eigenmode spectral analysis described in this article.

Super-resolution orbital angular momentum holography

Computer-generated holograms are crucial for a wide range of applications such as 3D displays, information encryption, data storage, and opto-electronic computing. Orbital angular momentum (OAM), as a new degree of freedom with infinite orthogonal states, has been employed to expand the hologram bandwidth. However, in order to reduce strong multiplexing crosstalk, OAM holography suffers from a fundamental sampling criterion that the image sampling distance should be no less than the diameter of largest addressable OAM mode, which severely hinders the increase in resolution and capacity. Here we establish a comprehensive model on multiplexing crosstalk in OAM holography, propose a pseudo incoherent approach that is almost crosstalk-free, and demonstrate an analogous coherent solution by temporal multiplexing, which dramatically eliminates the crosstalk and largely relaxes the constraint upon sampling condition of OAM holography, exhibiting a remarkable resolution enhancement by several times, far beyond the conventional resolution limit of OAM holography, as well as a large scaling of OAM multiplexing capacity at fixed resolution. Our method enables OAM-multiplexed holographic reconstruction with high quality, high resolution, and high capacity, offering an efficient and practical route towards the future high-performance holographic systems.

Generalized spiral transformation for high-resolution sorting of vortex modes

We achieve high-resolution sorting of the orbital angular momentum (OAM) of light with two bespoke diffractive optical elements using the generalized spiral transformation. The experimental sorting finesse is 5.3, approximately two times better performance than what has been reported. These optical elements will be useful for optical communication based on OAM beams and can be easily extended to other fields that use conformal mapping.

65,536-ary orbital angular momentum-shift keying free-space optical communication based on few-shot learning

In an orbital angular momentum-shift keying free-space optical (OAM-SK FSO) communication system, precisely recognizing OAM superposed modes at the receiver site is crucial to improve the communication capacity. While deep learning (DL) provides an effective method for OAM demodulation, with the increase of OAM modes, the dimension explosion of OAM superstates results in unacceptable costs on training the DL model. Here, we demonstrate a few-shot-learning-based demodulator to achieve a 65,536-ary OAM-SK FSO communication system. By learning from only 256 classes of samples, the remaining 65,280 unseen classes can be predicted with an accuracy of more than 94%, which saves a large number of resources on data preparation and model training. Based on this demodulator, we first realize the single transmission of a color pixel and the single transmission of two gray scale pixels on the application of colorful-image-transmission in free space with an average error rate less than 0.023%. This work may provide a new, to the best of our knowledge, approach for big data capacity in optical communication systems.

Spatiotemporal Acoustic Communication by a Single Sensor via Rotational Doppler Effect

A longstanding pursuit in information communication is to increase transmission capacity and accuracy, with multiplexing technology playing as a promising solution. To overcome the challenges of limited spatial information density and systematic complexity in acoustic communication, here real-time spatiotemporal communication is proposed and experimentally demonstrated by a single sensor based on the rotational Doppler effect. The information carried in multiplexed orbital-angular-momentum (OAM) channels is transformed into the physical quantities of the temporal harmonic waveform and simultaneously detected by a single sensor. This single-sensor configuration is independent of the channel number and encoding scheme. The parallel transmission of complicated images is demonstrated by multiplexing eight OAM channels and achieving an extremely-low bit error rate (BER) exceeding 0.02%, owing to the intrinsic discrete frequency shift of the rotational Doppler effect. The immunity to inner-mode crosstalk and robustness to noise of the simple and low-cost communication paradigm offers promising potential to promote relevant fields.

Optical Encoding Model Based on Orbital Angular Momentum Powered by Machine Learning

Based on orbital angular momentum (OAM) properties of Laguerre-Gaussian beams LG(p,ℓ), a robust optical encoding model for efficient data transmission applications is designed. This paper presents an optical encoding model based on an intensity profile generated by a coherent superposition of two OAM-carrying Laguerre-Gaussian modes and a machine learning detection method. In the encoding process, the intensity profile for data encoding is generated based on the selection of p and ℓ indices, while the decoding process is performed using a support vector machine (SVM) algorithm. Two different decoding models based on an SVM algorithm are tested to verify the robustness of the optical encoding model, finding a BER =10-9 for 10.2 dB of signal-to-noise ratio in one of the SVM models.

Annular phase grating-assisted recording of an ultrahigh-order optical orbital angular momentum

Ultrahigh-order optical orbital angular momentum (OAM) states of the identification over ±270 orders are implemented by annular phase grating (APG) and Gaussian beams with different wavelengths. Particularly, the far-field diffraction intensity patterns feature the spiral stripes instead of Hermitian-Gaussian (HG)-like fringes. It's worth noting that the spiral stripes present uniform distribution, thus the order of OAM states can be intuitively acquired. More specifically, the OAM states can be confirmed from the total amount and rotating direction of the spiral stripes. Compared with traditional methods, the propose scheme contributes to the perfect-distributed and sharper spiral stripes. Moreover, it also makes an easier observation of the patterns in the CCD camera with limited imaging targets. In our experimental setup, the optical filter is removed and the APG parameters are not strictly required. Therefore, the propose optical transmission system is equipped with the advantages of efficiency, robustness and low cost, which paves a promising way for the communication capacity enhancement.

On-chip ultracompact multimode vortex beam emitter based on vertical modes

Free-space orbital angular momentum (OAM) communication is considered as one of the potential alternative on-chip optical interconnect solutions. The number of OAM modes determines the capacity of high-speed communication. However, existing integrated vortex beam emitters have a constraint relationship between the number of OAM modes and the emitter size, rendering it difficult to emit more OAM modes with a small-sized emitter. In view of the above, this study proposes an on-chip ultracompact multimode vortex beam emitter based on vertical modes, which permits more OAM modes without requiring an increase in the size of the emitter. Vertical modes in large-aspect-ratio waveguides are pointed out to enable multimode microrings with small radii because high-order vertical modes can maintain almost the same horizontal wave vector as that of the fundamental mode. Four-mode and five-mode vortex beam emitters with the same radius of 1.5 µm are designed and the effectiveness of these emitters is verified through simulation. Furthermore, a high-efficiency and low-crosstalk approach for high-order vertical mode coupling by varying the waveguide height is presented. This research not only promotes further integration of on-chip optical interconnection, but also provides a new strategy for optical waveguide mode selection in photonic integrated circuits design.

Feature recognition of a 2D array vortex interferogram using a convolutional neural network

A vortex array has important applications in scenarios where multiple vortex elements with the same or different topological charges are required simultaneously. Therefore, the detection of the vortex array is vital. Here, the interferogram between the off-axis Walsh-phase plate and the vortex array is first obtained and then decoded through a convolution neural network (CNN), which can simultaneously determine the topological charge, chirality, and the initial angle. Both the theory and experiment prove that a CNN has a remarkable effect on the classification and detection of vortex arrays.

Data transmission under high scattering based on OAM-basis transmission matrix

Multiplexing of orbital angular momentum (OAM) channels is an important method to increase the optical communication capacity at present, but the multiple scattering and distortion of long-distance optical communication greatly limit its application. Here, a data transmission method based on an OAM-basis transmission matrix (TM) under high scattering is proposed. In this method, OAM modes are directly encoded by the OAM-basis TM, and the incident power spectral distribution of OAM modes can be directly acquired by the intensity profile of the speckle field on the camera. This method can realize the multiplexing of a large number of OAM channels and is easy to perform. Experimentally, we have achieved a maximum of 800 OAM modes multiplexed, and a bit error rate of 0.01% in the data transmission of color images.

Wavefront shaping: A versatile tool to conquer multiple scattering in multidisciplinary fields

Optical techniques offer a wide variety of applications as light-matter interactions provide extremely sensitive mechanisms to probe or treat target media. Most of these implementations rely on the usage of ballistic or quasi-ballistic photons to achieve high spatial resolution. However, the inherent scattering nature of light in biological tissues or tissue-like scattering media constitutes a critical obstacle that has restricted the penetration depth of non-scattered photons and hence limited the implementation of most optical techniques for wider applications. In addition, the components of an optical system are usually designed and manufactured for a fixed function or performance. Recent advances in wavefront shaping have demonstrated that scattering- or component-induced phase distortions can be compensated by optimizing the wavefront of the input light pattern through iteration or by conjugating the transmission matrix of the scattering medium. This offers unprecedented opportunities in many applications to achieve controllable optical delivery or detection at depths or dynamically configurable functionalities by using scattering media to substitute conventional optical components. In this article, the recent progress of wavefront shaping in multidisciplinary fields is reviewed, from optical focusing and imaging with scattering media, functionalized devices, modulation of mode coupling, and nonlinearity in multimode fiber to multimode fiber-based applications. Apart from insights into the underlying principles and recent advances in wavefront shaping implementations, practical limitations and roadmap for future development are discussed in depth. Looking back and looking forward, it is believed that wavefront shaping holds a bright future that will open new avenues for noninvasive or minimally invasive optical interactions and arbitrary control inside deep tissues. The high degree of freedom with multiple scattering will also provide unprecedented opportunities to develop novel optical devices based on a single scattering medium (generic or customized) that can outperform traditional optical components.

Direct binary search method for high-resolution holographic image projection

Complex-amplitude modulation of light fields with a digital micromirror device (DMD) has been widely used in holographic image projection. DMD is a binary-amplitude modulator, and its use for complex field modulation in a 4f configuration requires low-pass filtering. However, the reconstructed fields suffer from low resolution due to the limited bandwidth for the existing methods such as the Lee and superpixel methods. Here, we report a direct binary search (DBS) method to design high-resolution complex-amplitude holograms. The method is able to increase the spatial bandwidth up to twice that of the superpixel method. Numerical simulations and experiments are presented to demonstrate the method, which show that the errors are reduced by about 60% and 40% respectively for the test fields compared to the superpixel method. Furthermore, the measured efficiency of laser light can be improved by a maximum of 60%.

Data transmission with up to 100 orbital angular momentum modes via commercial multi-mode fiber and parallel neural networks

This work presents an artificial intelligence enhanced orbital angular momentum (OAM) data transmission system. This system enables encoded data retrieval from speckle patterns generated by an incident beam carrying different topological charges (TCs) at the distal end of a multi-mode fiber. An appropriately trained network is shown to support up to 100 different fractional TCs in parallel with TC intervals as small as 0.01, thus overcoming the problems with previous methods that only supported a few modes and could not use small TC intervals. Additionally, an approach using multiple parallel neural networks is proposed that can increase the system's channel capacity without increasing individual network complexity. When compared with a single network, multiple parallel networks can achieve the better performance with reduced training data requirements, which is beneficial in saving computational capacity while also expanding the network bandwidth. Finally, we demonstrate high-fidelity image transmission using a 16-bit system and four parallel 14-bit systems via OAM mode multiplexing through a 1-km-long commercial multi-mode fiber (MMF).

Meta-learning-based optical vector beam high-fidelity communication under high scattering

While spatial structured light based free space optical communication provides high-bandwidth communication with broad application prospect, severe signal distortion caused by optical scattering from ambient microparticles in the atmosphere can lead to data degradation. A deep-learning-based adaptive demodulator has been demonstrated to resolve the information encoded in the severely distorted channel, but the high generalization ability for different scattering always requires prohibitive costs on data preparation and reiterative training. Here, we demonstrate a meta-learning-based auto-encoder demodulator, which learns from prior theoretical knowledge, and then training with only three realistic samples per class can rectify and recognize transmission distortion. By employing such a demodulator to hybrid vector beams, high fidelity communication can be established, and data costs are reduced when faced with different scattering channels. In a proof-of-principle experiment, an image with 256 gray values is transmitted under severe scattering with an error ratio of less than 0.05%. Our work opens the door to high-fidelity optical communication in random media environments.

High-resolution optical orbital angular momentum sorter based on Archimedean spiral mapping

We propose a generalized spiral transformation scheme that is versatile to incorporate various types of spirals such as the Archimedean spiral and the Fermat spiral. Taking advantage of the equidistant feature, we choose the Archimedean spiral mapping and demonstrate its application in high-resolution optical orbital angular momentum (OAM) mode sorting. Experimental results show 90% efficiency and cross-talk of -8.78 dB that is sufficient to separate adjacent OAM modes. This generalized transformation scheme may also find various applications in optical transformation and can be easily extended to other fields related to conformal mapping.

Time-varying orbital angular momentum in tight focusing of ultrafast pulses

The orbital angular momentum (OAM) of light has important applications in a variety of fields, including optical communication, quantum information, super-resolution microscopic imaging, particle trapping, and others. However, the temporal properties of OAM in ultrafast pulses and in the evolution process of spin-orbit coupling has yet to be revealed. In this work, we theoretically studied the spatiotemporal property of time-varying OAM in the tightly focused field of ultrafast light pulses. The focusing of an incident light pulse composed of two time-delayed femtosecond sub-pulses with the same OAM but orthogonal spin states is investigated, and the ultrafast dynamicsa time delay of OAM variation during the focusing process driven by the spin-orbit coupling is visualized. Temporal properties of three typical examples, including formation, increase, and transformation of topological charge are investigated to reveal the non-uniform evolutions of phase singularities, local topological charges, self-torques, and time-varying OAM per photon. This work could deepen the understanding of spin-orbit coupling in time domain and promote many promising applications such as ultrafast OAM modulation, laser micromachining, high harmonic generation, and manipulation of molecules and nanostructures.

Observation of Magnetic Helicoidal Dichroism with Extreme Ultraviolet Light Vortices

We report on the experimental evidence of magnetic helicoidal dichroism, observed in the interaction of an extreme ultraviolet vortex beam carrying orbital angular momentum with a magnetic vortex. Numerical simulations based on classical electromagnetic theory show that this dichroism is based on the interference of light modes with different orbital angular momenta, which are populated after the interaction between light and the magnetic topology. This observation gives insight into the interplay between orbital angular momentum and magnetism and sets the framework for the development of new analytical tools to investigate ultrafast magnetization dynamics.

Broadband angular dispersion compensation for digital micromirror devices

In this Letter, we present a compact broadband angular dispersion compensation method for digital micromirror devices (DMDs) and ultrashort pulse lasers, which effectively extends the conventional single-wavelength compensation design to a wide wavelength range of 300 nm. First, a parametric model was developed for the dispersion compensation unit, consisting of a transmission grating and a 4f telescope sub-unit, to guide the selection of components and parameter optimization for broadband applications. In the experiments, we designed a single slit-based metrology system to measure and quantify the compensated angular dispersion of a Ti:sapphire femtosecond laser with a pulse width of 75 fs. The results indicate that our method can reduce the angular dispersion to 0.04°, i.e., pulse widening less than 20 fs, over a wavelength range of 750-1050 nm. To demonstrate this, the DMD system was used as a multi-wavelength beam shaper to reconstruct a wavefront that contains the "CUHK" pattern and the results confirmed its ability to provide effective broadband angular dispersion compensation. This means the DMD can be used in different applications that employ a broadband light source, e.g., wavelength tunable femtosecond laser, attosecond laser, supercontinuum laser, and multi-color LED.

Wavefront shaping for reconfigurable beam steering in lithium niobate multimode waveguide

Reconfigurable photonic devices are important constituents for future optical integrated circuits, where electro-optic manipulation of the light field in a lithium niobate (LN) waveguide is one of the promising solutions. Herein, we demonstrate a paradigm shift of the beam steering mechanism where reconfigurable beam steering is enabled by the wavefront shaping technology. Furthermore, this strategy is fully compatible with the electro-optic tuning mechanism of the LN multimode waveguide, where microstructured serrated array electrodes are employed to fine tune the output beam upon its reconfigurable output position. Our results provide new, to the best of our knowledge, insight for molding the flow of light in multimode waveguides and shed new light on beam steering photonic devices.

Adaptive demodulation by deep-learning-based identification of fractional orbital angular momentum modes with structural distortion due to atmospheric turbulence

Since the great success of optical communications utilizing orbital angular momentum (OAM), increasing the number of addressable spatial modes in the given physical resources has always been an important yet challenging problem. The recent improvement in measurement resolution through deep-learning techniques has demonstrated the possibility of high-capacity free-space optical communications based on fractional OAM modes. However, due to a tiny gap between adjacent modes, such systems are highly susceptible to external perturbations such as atmospheric turbulence (AT). Here, we propose an AT adaptive neural network (ATANN) and study high-resolution recognition of fractional OAM modes in the presence of turbulence. We perform simulations of fractional OAM beams propagating through a 1-km optical turbulence channel and analyze the effects of turbulence strength, OAM mode interval, and signal noise on the recognition performance of the ATANN. The recognition of multiplexed fractional modes is also investigated to demonstrate the feasibility of high-dimensional data transmission in the proposed deep-learning-based system. Our results show that the proposed model can predict transmitted modes with high accuracy and high resolution despite the collapse of structured fields due to AT and provide stable performance over a wide SNR range.

Unscrambling OAM mode using digital phase-shifting in the Stokes fluctuations correlation

In this Letter, we propose and experimentally demonstrate a new, to the best of our knowledge, non-interferometric and highly stable technique to unscramble the incident orbital angular momentum (OAM) state and quantitatively measure the phase structure from the non-imaged random light. A new theoretical basis is developed and also verified by numerical simulation and experimental demonstration. We also quantitatively investigate the OAM modes of the incident light using orthogonal projection.

Designing optical fields in inhomogeneous media

Designing optical fields with predetermined properties in source-free inhomogeneous media has been a long-sought goal due to its potential utilization in many applications, such as optical trapping, micromachining, imaging, and data communications. Using ideas from the calculus of variations, we provide a general framework based on the Helmholtz equation to design optical fields with prechosen amplitude and phase inside an inhomogeneous medium. The generated field is guaranteed to be the closest physically possible rendition of the desired field. The developed analytical approach is then verified via different techniques, where the approach's validity is demonstrated by generating the desired optical fields in different inhomogeneous media.

Optical vortex switch based on multiplexed volume gratings with high diffraction efficiency

Systems of controllable orbital angular momentum (OAM) require more compact, higher conversion efficiency and more tolerable wavelength or polarization. We introduce an optical vortex switch based on a multiplexed volume grating (MVG). The MVG recorded in a piece of photo-thermo-refractive (PTR) glass exhibits high diffraction efficiency (DE, also known as conversion efficiency in transporting), sensitive angular selectivity, and polarization-insensitivity. The effects of the incident divergence angle and polarization on the DE and the far-field diffraction profiles are demonstrated and investigated. It turns out that the divergence angle of the probe beam can greatly affect the DE. The fluctuation of the DE caused by polarization variation is less than 1.59%. This switch can be potentially applied in vortex tweezers, optical communication, and high power systems.

Experimental generation of perfect optical vortices through strongly scattering media

Perfect optical vortices enable the unprecedented optical multiplexing utilizing orbital angular momentum of light, which, however, suffer from distortion when they propagate in inhomogeneous media. Herein, we report on the experimental demonstration of perfect optical vortice generation through strongly scattering media. The transmission-matrix-based point-spread-function engineering is applied to encode the targeted mask in the Fourier domain before focusing. We experimentally demonstrate the perfect optical vortice generation either through a multimode fiber or a ground glass, where the numerical results agree well with the measured one. Our results might facilitate the manipulation of orbital angular momentum of light through disordered scattering media and shed new light on the optical multiplexing utilizing perfect optical vortices.

Speckle filtering through nonlinear wave mixing

Light scattering by disordered media is a ubiquitous effect. After passing through them, the light acquires a random phase, masking or destroying associated information. Filtering this random phase is of paramount importance to many applications, such as sensing, imaging, and optical communication, to cite a few, and it is commonly achieved through computationally extensive post-processing using statistical correlation. In this work, we show that mixing noisy optical modes of various complexity in a second-order nonlinear medium can be used for efficient and straightforward filtering of a random wavefront under sum-frequency generation processes without utilizing correlation-based calculations.

3D intensity correlations in random fields created by vortex structured beams

We develop an analytical model for the 3D spatial coherence function of speckle fields generated by scattering of vortex and perfect optical vortex beams. The model is general and describes the spatial coherence along both the transversal and the longitudinal directions. We found that, on propagation, the 3D spatial coherence evolves differently for the different types of initially structured beams, which may affect their use in a variety of sensing applications.

Spiral spectrum of high-order elliptic Gaussian vortex beams in a non-Kolmogorov turbulent atmosphere

In a free space optical communication system based on vortex beams, the effects of spread and crosstalk caused by atmospheric turbulence should not be ignored. The orbital angular momentum (OAM) spectrum of the signal based on elliptic Gaussian beam (EGB) after propagation through non-Kolmogorov turbulent atmosphere are deduced, and a theoretical model of the spiral spectrum of EGB propagating through turbulent atmosphere is obtained. Numerically calculated OAM modes detection and crosstalk probability under different ellipticity parameters. The results show that the ellipticity parameter has a significant impact on the OAM spectral distribution of EGB and the transmission characteristics after turbulent atmosphere. The selection of appropriate ellipticity parameter can correspondingly reduce the degradation and crosstalk caused by turbulent atmosphere. We also compared a Laguerre-Gaussian beam (LGB) with EGB and pointed out the advantages and limitations of these two kinds of beams. The research results may be useful in the field of short distance optical communication and OAM-based multiplex communication.

Plasmonic fork-shaped hologram for vortex-beam generation and separation

We introduce a multifunctional compact device that integrates a polarization beam splitter and an orbital angular momentum generator based on a plasmonic nano-aperture assisted detour phase meta-hologram. The proposed metasurface, which combines a phase singularity characterized fork hologram and polarization featured Λ-shaped antenna, achieves vortex generation and spin-based vortex splitting in transmission mode. Experimental demonstrations are launched under a linearly polarized incident beam, with polarization tomography as the analysis method. We expect this work to have applications in chip-level beam shaping and high-capacity communication.

Fourth-harmonic generation of orbital angular momentum light with cascaded quasi-phase matching crystals

Orbital angular momentum (OAM) light, combined with the nonlinear process to expand the frequency range, has drawn increasing research interest in recent years. Here, we implement the first, to the best of our knowledge, experimental fourth-harmonic generation of OAM light with two cascaded quasi-phase-matching crystals. A Laguerre-Gaussian beam was transmitted through a duplet crystals system and frequency-doubled twice by two separate second-harmonic generation processes, which transduced the frequency of the OAM beam from telecom band to visible band and then to ultraviolet (UV) band. The topological charge of the OAM beam was increased substantially in the cascaded frequency conversion processes. In this experiment, we verify the OAM conservation by utilizing a specially designed interferometer, and the results correspond well with the numerical simulation. This work provides an effective method for the generation of UV OAM beams with high topological charges.

Propagation of structured light through tissue-mimicking phantoms

Optical interrogation of tissues is broadly considered in biomedical applications. Nevertheless, light scattering by tissue limits the resolution and accuracy achieved when investigating sub-surface tissue features. Light carrying optical angular momentum or complex polarization profiles, offers different propagation characteristics through scattering media compared to light with unstructured beam profiles. Here we discuss the behaviour of structured light scattered by tissue-mimicking phantoms. We study the spatial and the polarization profile of the scattered modes as a function of a range of optical parameters of the phantoms, with varying scattering and absorption coefficients and of different lengths. These results show the non-trivial trade-off between the advantages of structured light profiles and mode broadening, stimulating further investigations in this direction.

Image transmission with binary coding for free space optical communications in the presence of atmospheric turbulence

Understanding the influence of atmospheric turbulence on optical information transmission is important for free space optical communication. In this paper, the image transmission through a 1 km horizontal turbulent channel has been numerically investigated, and a simulation model including the process of image pixels encoding and decoding is given. The peak signal-to-noise ratio of the received image is evaluated, and the influences of the channel factors and detector noise are discussed in detail. The critical value of noise level and turbulence strength is given. Our results provide a simulation model for image transmission in a turbulent channel along with insight into the impacts of turbulence parameters and detector noise, which are useful for applications in optical communication.

Machine learning-aided classification of beams carrying orbital angular momentum propagated in highly turbid water

A set of laser beams carrying orbital angular momentum is designed with the objective of establishing an effective underwater communication link. Messages are constructed using unique Laguerre-Gauss beams, which can be combined to represent four bits of information. We report on the experimental results where the beams are transmitted through highly turbid water, reaching approximately 12 attenuation lengths. We measured the signal-to-noise ratio in each test scenario to provide characterization of the underwater environment. A convolutional neural network was developed to decode the received images with the objective of successfully classifying messages quickly. We demonstrate near-perfect classification in all scenarios, provided the training set includes some images taken under the same underwater conditions.

Orbital angular momentum density characteristics of tightly focused polarized Laguerre-Gaussian beam

The orbital angular moment (OAM) of light has been proved to be useful in plenty of applications. By transmitting the OAM of the focused light field to a particle, it will be orbited around the optical axis. Therefore, it is necessary to study the OAM distribution of the focused light field used to manipulate the particles. In this application, the widely used paraxial approximation is no longer sufficient due to the tightly focused beam. We employ the higher-order Poincaré sphere to represent the Laguerre-Gaussian (LG) beams with arbitrary polarization. Then the Rayleigh-Sommerfeld integral method and the q-parameter method are used to derive the analytical expression of the light field on the focal plane. Based on this, the OAM density expression of the tightly focused LG beam is derived. In the numerical simulation, we study and analyze the unique intensity distributions and OAM distributions of tightly focused linear polarized, radial polarized, and circular polarized LG beams. The results could be leveraged to further explore the applications of the polarized vortex beam.

Designing the phase and amplitude of scalar optical fields in three dimensions

The ability to generate any arbitrarily chosen optical field in a three-dimensional (3D) space, in the absence of any sources, without modifying the index of refraction, remains an elusive but much-desired capability with applications in various fields such as optical micromanipulation, imaging, and data communications, to name a few. In this work, we show analytically that it is possible to generate any desired scalar optical field with predefined amplitude and phase in 3D space, where the generated field is an exact duplicate of the desired field in case it is a solution of Helmholtz wave equation, or if the existence of such field is strictly forbidden, the generated field is the closest possible rendition of the desired field in amplitude and phase. The developed analytical approach is further supported via experimental demonstration of optical beams with exotic trajectories and can have a significant impact on the aforementioned application areas.

Distortion of a twisted beam passing through a plasma layer

In this paper, we explore what happens to the intensity profile, phase distribution, and centroid position of a vortex beam (VB) when it passes through a cold collision-less magnetized plasma layer. For this purpose, we utilize angular spectral expansion accompanied by a 4×4 matrix method to obtain the total transmission coefficient, intensity and phase profiles, and centroid shifts of VBs in the output plane. Based on numerical analyses, it is found that the evolution of transverse intensity as well as the distortion of the phase profile of transmitted VBs are greatly affected by variation of radial and angular mode numbers, external magnetic field, plasma number density, and incident angle. In addition, displacement of the VB centroid under variation of angular mode numbers is presented quantitatively. It is expected that the results of this study will give more insight into VB communication, radar probing, and plasma diagnostics.