Acoustic Orbital Angular Momentum Research Articles

Phase saddle point transformation-enabled mode-adaptive detection of acoustic orbital angular momentum with high efficiency and low crosstalk

Phase saddle point transformation-enabled mode-adaptive detection of acoustic orbital angular momentum with high efficiency and low crosstalk

Evaluation of inner product-based demultiplexing of vortex-based underwater acoustic communications signals in realistic environments.

Acoustic vortex beams have garnered recent interest as an avenue to improve underwater acoustic communication bandwidth and speed. Design and deployment of these systems in ocean environments and at operational ranges has yet to be demonstrated due to the challenging and dynamic nature of the underwater acoustic environment. This manuscript presents methods to model the time series of vortex-based communication signals in ocean environments using ray tracing algorithms. The methods are used to assess the effects of Doppler, ocean turbulence, positional error, and range-dependent environmental parameters on the inner product demultiplexing of the communication signals encoded in the acoustic orbital angular momentum.

Acoustic Metasurfaces for Efficient Modulation and Demodulation of OAM Multiplexed Signals in Acoustic Communication

Abstract Orbital angular momentum (OAM) multiplexing has been suggested as a potential solution to increase the data capacity of acoustic communication systems. A method of modulation and demodulation of OAM multiplexed signals using acoustic metasurfaces is proposed in this manuscript. The amplitude and phase of independent acoustic orbital angular momentum (AOAM) multiplexed signal provide orthogonal degrees of freedom. By designing appropriate acoustic metasurfaces and combining them with binary differential phase coding techniques, we demonstrated that efficient modulation and fast demodulation of AOAM multiplexed signals can be achieved. Orthogonal acoustic spiral modes prevent mode coupling and crosstalk, enhancing acoustic multiplexing. The resonance-based structure provides signal frequency selectivity, boosting efficiency and simplifying terminal equipment for practical applications, which leads to the creation of high-capacity acoustic communication systems.

Multiplexing and demultiplexing acoustic orbital angular momentum beams in complex environments

The spiral sound waves carrying orbital angular momentum (OAM) are significantly more efficient in delivering information and energy than the ordinary plane waves. A major bottleneck in exploring the use of acoustic OAM is the excitation and detection of OAM in an arbitrary environment. Here we propose a scheme combining the multichannel least-mean-square (LMS) algorithm and an improved virtual rotating receiver (VRR) method to multiplex and demultiplex acoustic OAM beams in various complex environments. We experimentally demonstrate that the LMS-VRR scheme can excite and detect the OAM beams in indoor and outdoor real-world environments. We use the LMS algorithm to multiplex OAM beams with up to 40 channels at a circular microphone array by compensating the source signals with the equalization filters. On the other hand, the circular microphone array works as a VRR to demultiplex the OAM beams from the rotational Doppler effect. Additional preprocesses are taken to eliminate the plane wave pollution for the VRR method. The acoustic OAM beams have been multiplexed and demultiplexed over 20 m away from the sound sources on a shady lane. Our work has shown practical prospects in covert communication, and shall further unleash the application potential of acoustic OAM beams by removing the restrictions on exciting and detecting OAM in complex environments.

Robust orbital-angular-momentum-based underwater acoustic communication with dynamic modal decomposition method.

Recently, acoustic communication employing orbital angular momentum (OAM) opens another avenue for efficient data transmission in aquatic environments. Current topological charge (TC) detection of OAM beams relies on the orthogonality among different-order OAM beams. However, such strategy requires measurements of the complete azimuthal acoustic pressure, which inevitably reduces the efficiency and increases the bit error rate (BER). To address these challenges, this study proposes a modified dynamic modal decomposition (DMD) method by partially sampling the acoustic field for precise TC detection. Numerical simulations confirm the accuracy of this approach in extracting single or multiple TCs magnitudes within a partially sampled acoustic field. We theoretically compare the performance of the modified DMD approach with conventional orthogonal decoding method. Simulation results indicate that our modified DMD scheme exhibits lower BER under the same noise interference and is more robust to the array misalignment. This research introduces an efficient demodulation solution for acoustic OAM communication, offering potential benefits for simplifying receiver array design and enhancing long-distance underwater data transmission.

High resolution imaging of acoustic orbital angular momentum wave based on orthogonal matching tracking

The orbital angular momentum (OAM) wave has shown great potential for improving radar imaging and underwater communication performance due to its helical wavefront phase and infinite orthogonal modes. However, there are currently no known applications of this technology in underwater imaging. In this paper, we employed acoustic OAM wave for underwater imaging and established transceiver signal models using the uniform circular array. We concurrently achieved two-dimensional imaging of azimuth and elevation angles, which differs from radar imaging. We proposed a matching process for the echo signal in the modal domain, the OAM wave beam image's sidelobe decreased by 7.9 dB in the elevation direction and 6.1 dB in the azimuth direction compared to the plane wave, with the mainlobe decreased by 0.2° in the elevation direction and 0.4° in the azimuth direction. Furthermore, this paper introduced OAM wave high-resolution image reconstruction based on the orthogonal matching pursuit (OMP) algorithm. Finally, we implemented broadband acoustic OAM wave for underwater imaging and introduced an image reconstruction method based on the modal domain OMP algorithm. Simulation results demonstrate that the use of OAM wave in underwater imaging is feasible, and the proposed scheme can achieve high-resolution imaging.

Acoustic vortex filter based on tunable metasurfaces

In this paper, we present an acoustic vortex filter (AVF) based on tunable metasurfaces, which can selectively filter the incident multiplexed vortices that carry different orbital angular momentum (OAM). Our metasurface-based AVF is composed of an upper acoustic metasurface (UAM) and a lower acoustic metasurface, of which the intrinsic topological charge (ITC) can be tuned by mechanically rotating the UAM along its central axis. Due to the critical order of the propagating vortex modes in waveguide, controlling the ITC of the AVF allows for the selective filtering of incoming multiplexed acoustic vortex beams based on the sound vortex diffraction in phase-gradient metasurface, which endows the vortex filter the capability that let the incident vortex of specific OAM pass through it. In the following demonstration, both in theory and experiment, we design the AVF and effectively filter the acoustic vortices with two opposite topological charges by simply altering the orientation angle of the UAM. Based on this, we further demonstrate its application in asymmetric acoustic wave transmission. Our work offers an approach to selectively filter the incident acoustic vortex, which improves the capability to control the acoustic OAM via metasurfaces.

Demultiplexing of 40 acoustic orbital angular momentum wavelength-division multiplexing (OAM-WDM) channels by an improved virtual rotating receiver method

Orbital angular momentum (OAM) multiplexing/demultiplexing technology is crucial in increasing the data transmission rate for acoustic communication. However, the existing acoustic OAM multiplexing/demultiplexing is still limited to eight channels, and its combination with other communication techniques has not been verified experimentally. Here, we experimentally demonstrate the demultiplexing of up to 40 data channels using OAM multiplexing combined with wavelength-division multiplexing (WDM). The proposed demultiplexing method is improved from the virtual rotating receiver method used to detect the rotational Doppler effect of the OAM waves by a static array of microphones. The improved method has overcome the challenges of insufficient response, cross-talk, and signal aliasing, which often hinder the existing demultiplexing methods in the low-frequency region. The proposed demultiplexing method can be used to quickly decode the massive information concealed in a large number of acoustic OAM-WDM channels. Our work also shows practical prospects in underwater communication applications, especially in long-range communication using acoustic waves at low frequencies.

Loss-induced multiple modal coalescences in an acoustic ring cavity

Chiral exceptional points refer to a class of non-Hermitian degeneracies where two counter-propagating whispering-gallery-like eigenmodes coalesce into one travelling eigenmode, which have been utilized to create orbital angular momentum in various wave systems. However, most studies in this field have so far focused on the modal coalescence for a specific azimuthal order. In this work, we investigate the eigenmodes evolution of an acoustic ring cavity under complex spatial modulation of refractive index along the azimuthal direction. We show that a careful engineering of the complex modulation, possibly realized with acoustic metamaterials, can lead to multiple modal coalescences to produce a series of travelling eigenmodes with different orbital angular momentum orders. These coalesced eigenmodes can be directly excited by a simple source at their resonant frequencies. Our work provides a straightforward approach to the generation and manipulation of acoustic orbital angular momentum.

Inverse‐Design Methodology for Generating Twisted Ultrasonic Motor in Free Space

A new level of freedom is made possible by orbital angular momentum (OAM), which can be used to create twisted ultrasonic motors for particle manipulation and control. For steady and free‐flowing particle management, it is essential to produce acoustic OAM beams of high order and high quality. However, this task is difficult due to the phased array's fundamental restriction of insufficient resolution or aperture. Herein, a generic approach is presented to inverse design that may be used to improve the OAM beam quality and produce twisted ultrasonic motors in free space. In comparison to the previous direct‐design method, up to a 14% improvement in amplitude uniformity and a 12 dB improvement in signal‐to‐noise ratio (SNR) of OAM modes are achieved. These notable benefits enable the ultrasonic motor to operate with a straightforward and compact system while enabling steady particle rotation in free space with an undersampled phased array, which is not possible with the conventional technique. This study paves the way for effective particle control, high‐fidelity communication, and other OAM‐based technologies by providing an inverse‐design framework for optimizing OAM.

Ray tracing of long-range underwater acoustic vortex wave propagation

The underwater acoustic communications environment is severely band-limited, which leads to a bottleneck in data transfer. Existing methods of data transfer in underwater acoustic communications applications typically rely primarily on conventional temporal and frequency modulation techniques and achieve bit rates peaking at approximately 40 kb/s. One method of easing the bottleneck and increasing the data rate is to explore further potential degrees of freedom which may be utilized. Acoustic orbital angular momentum (OAM) is a physical quantity that characterizes the rotation in a propagating helical pressure wavefront. The unique phase patterns of OAM carrying vortex waves form an orthogonal basis which may be useful as an additional degree of freedom in acoustics communications applications; however, the long-distance propagation of these waves is largely unstudied. By employing BELLHOP’s ray tracing algorithm, the dominant features of a propagating OAM carrying vortex wave are tracked over long ranges (to and beyond 1 km) under various environmental conditions. This provides essential guidance in the design of the sending and receiving arrays of high-speed underwater communications systems, which rely on multiplexing acoustic OAMs.

Chirality-selective transmission of acoustic orbital angular momentum through lossy metasurfaces

Wave fields with acoustic orbital angular momentum have drawn greatattention because of its wide range of applications such as acoustic communication and particle manipulation. We designed, fabricated, and experimentally validated a type of lossy metasurface which enables an extremely selective transmission of acoustic wave filed depending on the chirality of its orbital angular momentum. The metasurface is composed of a phase gradient metagrating (PGM) and microperforated panel (MPP). By harnessing the multiple internal reflection of PGM and the carefully engineered loss of MPP, the left-handed incident vortex has a near-zero power transmission while its right-handed counterpart has a 60% power transmission. Such an extremely selective transmission was experimentally validated by retrieving the full scattering matrix using a generalized twelve-microphone method, which shows a good agreement with numerical simulations.

Generating reconfigurable acoustic orbital angular momentum with double-layer acoustic metasurface

In this paper, a double-layer acoustic metasurface (DAM) composed of a fixed lower acoustic metasurface (LAM) and a rotatable upper acoustic metasurface (UAM) is proposed for the generation of mode-reconfigurable acoustic orbital angular momentum (OAM). The UAM and LAM are divided into multiple sections, in which the hybrid structures combining cascaded Helmholtz resonators and a straight pipe are adopted to achieve specific phase compensation. By rotating the UAM, the incident acoustic plane wave can be efficiently converted into the vortex acoustic waves of reconfigurable topological charges ranging from −5 to +5 with distinguishable purity. Furthermore, the influences of the parameters on the purity of the generated topological charges have been investigated and discussed, such as the distance between LAM and UAM, rotatable angle error, and operating frequency. With the capability of reconfigurable OAM modes, the proposed DAM can be used to efficiently increase capacity or to conveniently switch between different channels in underwater vortex acoustic communications.

Real-time visualisation and optimisation of acoustic waves carrying orbital angular momentum

Travelling waves, such as light and sound, can carry angular momentum. Orbital angular momentum (OAM) is one of the components which is determined by the helicity of the phase fronts. The helical waveform is characterised in terms of an integer l and an azimuthal phase term of exp(−ilθ), but for |l| > 1 the resulting high-order beam structure is unstable to perturbation. In this work, using Fourier transform profilometry and stroboscopic imaging techniques, we demonstrate the real-time visualisation of the OAM-carrying acoustic waveform by imaging the pressure imprint of the acoustic wave on a thin rubber sheet. Furthermore, based on the visualised waveform, we are able to optimise high-order (|l| > 1) OAM states by controlling the individual elements of the acoustic source. Beyond the study of acoustic OAM, the real-time monitoring and optimising methods could be a benefit to other applications requiring acoustic waveform shaping, such as acoustic communications, acoustic holograms, etc.

Experimental study of acoustic superradiance from a rotating absorber

The concept that classic waves reflected from a rotating absorbing cylinder will be amplified is analogously linked to the Penrose superradiance that may extract energy from a rotating black hole. The superradiance of acoustic waves carrying orbital angular momentum (OAM) from a sound absorber has been demonstrated in a recently published experiment using two rotating microphones. The experiment showed that the waves were amplified by up to 30% when the rotation rate of the absorber satisfied the Zel’dovich condition. Here, we proposed an experimental method to detect acoustic superradiance by static microphones. We demonstrated that the acoustic waves transmitted through a perforated absorber were amplified (by up to 1000%) even when the Zel’dovich condition was not satisfied. The experiment also showed that when the Doppler-shifted frequency of an observer was 0 Hz, the acoustic amplitude was very weak regardless of the rotation rate of the absorber. Our work was worthwhile not only in the proposed experimental method that was well-suited for observing the acoustic OAM wave but also in the high amplification achieved at a low rotation rate of the perforated absorber, showing a great prospect in practical applications such as amplifying the information-carrying OAM waves for high-speed acoustic communication.

Chirality-switchable acoustic vortex emission via non-Hermitian selective excitation at an exceptional point

Artificial structures provide an efficient method to generate acoustic vortices carrying orbital angular momentum (OAM) essential for applications ranging from object manipulation to acoustic communication. However, their flexibility in terms of chirality control has thus far been limited by the lack of reconfigurability and degrees of freedom like spin–orbit coupling. Here we show that this restriction can be lifted by controlling the individual on–off states of two coherent monopolar sources inside a passive parity-time-symmetric ring cavity at an exceptional point where the counter-propagating waves coalesce into one chiral eigenmode. One of the sources satisfies the chirality-reversal condition, generating a travelling wave field fully decoupled from and opposite to the chiral eigenmode, while the other source is phase-shifted such that the wave generated by the first source can be canceled out, and the remaining sound field circulates in the same direction as the chiral eigenmode. Such non-Hermitian selective excitation enables our experimental realization of acoustic vortex emission with switchable OAM but free of system reconfiguration. Our work offers opportunities for chiral sound manipulation as well as integrated and tunable acoustic OAM devices.

Acoustic angular momenta and their applications

Acoustic angular momentum is a physical quantity characterizing the rotation of pressure and local particle velocity fields. The acoustic angular momentum was first observed in vortex waves and was found to have many interesting physics including pseudospin based topological effects, spin-momentum locking, and trapping potential for contactless particle manipulations. In this presentation, we will introduce the categorization of acoustic angular momentum into spin and orbital angular momenta. While airborne sound is a longitudinal wave that was long thought to be spinless, we observed the acoustic spin in some special cases. The spin-momentum locking and spin induced torque for particle manipulation were experimentally demonstrated. In addition, the application of acoustic orbital angular momentum in high-speed communication will be discussed. These novel physical acoustic properties have many more applications ranging from underwater exploration to biomedical engineering.

Experimental and numerical study of the rotational Doppler shift in acoustic waves using static microphones

The rotational Doppler shift in acoustic waves carrying orbital angular momentum (OAM) has been applied in experiments to demonstrate the Zel'dovich amplification, which is analogously linked with the black hole radiance. However, rotating microphones used in the published experiments are difficult to assemble and intrinsically weak in the frequency response at frequencies around 0 Hz. To circumvent the limitations, this paper proposes an experimental method to study the rotational Doppler shift in acoustic OAM waves using static microphones. The Doppler shifted signals viewed by a rotating observer were extracted from the original acoustic signals at frequencies around 100 Hz, where the microphones had flat frequency response. The measured results not only demonstrated the extreme Doppler shift to a negative frequency but also showed that the acoustic amplitudes were very low when the Doppler shifted frequency was 0 Hz. This phenomenon was explained by showing in a time-domain numerical simulation that the wave was co-rotating with the observer. In addition, the characteristics of the simulated pressure contours of the rotating wave have been discussed. The proposed methods can further be used to study the physics related to the wave carrying OAM, such as the acoustic super-radiance.

Non-resonant waveguiding-spiralizer for generating acoustic orbital angular momentum with arbitrary number of channels

Acoustic orbital angular momentum (a-OAM) has important application values in several fields such as high-capacity acoustic communication or manipulation of objects. In this paper, a non-resonant waveguiding-spiralizer based on the spiral structure was proposed for generating a-OAM and achieved the wavefront rotation with any channels. Compared with active a-OAM generating devices, these waveguiding-spiral structures were compact enough that could be easily fabricated through three-dimensional (3D) printing technology. While compared with other passive a-OAM generating devices, the principle of wavefront rotation realized by the proposed device was geometrical aisles guiding, which belongs to the non-resonant structure. Therefore, it could effectively reduce the energy loss of sound wave, and avoid the narrow-band feature caused by the resonant structure. Most importantly, the working frequency could be adjusted to any working frequency bands by geometric scaling, and it could work in a variety of media or harsh conditions. In addition, the waveguiding-spiralizer could be designed for any number of channels to realize the multiplexing communication.

Generation of topologically diverse acoustic vortex beams with same divergence angle using discrete active helical arrays

In the process of multiplexing acoustic orbital angular momentum to realize underwater acoustic communication, the demodulation process is always affected by the larger divergence angle of higher-order acoustic vortex beams. To restrain the influences of this problem, proposed here are discrete active helical arrays with different heights and radii to generate topologically diverse underwater acoustic vortex beams, and the reasons for the different divergence angles of acoustic vortex beams with different orders are analyzed. In finite-element analysis and experiments, the same divergence angle of acoustic vortex beams with different orders is obtained, and an effective method is provided for emitting underwater acoustic vortex beams. The proposed design has potential applications in underwater acoustic communication.