Temporal Focusing Research Articles

Spectrally resolved point-spread-function engineering using a complex medium.

Propagation of an ultrashort pulse of light through strongly scattering media generates an intricate spatio-spectral speckle that can be described by means of the multi-spectral transmission matrix (MSTM). In conjunction with a spatial light modulator, the MSTM enables the manipulation of the pulse leaving the medium; in particular focusing it at any desired spatial position and/or time. Here, we demonstrate how to engineer the point-spread-function of the focused beam both spatially and spectrally, from the measured MSTM. It consists of numerically filtering the spatial content at each wavelength of the matrix prior to focusing. We experimentally report on the versatility of the technique through several examples, in particular as an alternative to simultaneous spatial and temporal focusing, with potential applications in multiphoton microscopy.

Study on Time Reversal Maximum Ratio Combining in Underwater Acoustic Communications

Time reversal (TR) can achieve temporal and spatial focusing by exploiting spatial diversity in complex underwater environments with significant multipath. This property makes TR useful for underwater acoustic (UWA) communications. Conventional TR is realized by performing equal gain combining (EGC) on the single element TR output signals of each element of the vertical receive array (VRA). However, in the actual environment, the signal-to-noise ratio (SNR) and the received noise power of each element are different, which leads to the reduction of the focusing gain. This paper proposes a time reversal maximum ratio combining (TR-MRC) method to process the received signals of the VRA, so that a higher output SNR can be obtained. The theoretical derivation of the TR-MRC weight coefficients indicates that the weight coefficients are only related to the input noise power of each element, and are not affected by the multipath structure. The correctness of the derivation is demonstrated with the experimental data of the long-range UWA communications conducted in the South China Sea. In addition, the experimental results illustrate that compared to the conventional TR, TR-MRC can provide better performance in terms of output SNR and bit error rate (BER) in UWA communications.

Holographic Imaging and Stimulation of Neural Circuits.

A critical neuroscience challenge is the need to optically image and manipulate neural activity with high spatiotemporal resolution over large brain volumes. The last three decades have seen the development of calcium imaging to record activity from neuronal populations, as well as optochemistry and optogenetics to optically manipulate neural activity. These methods are typically implemented with wide-field or laser-scanning microscopes. While the former approach has a good temporal resolution, it generally lacks spatial resolution or specificity, particularly in scattering tissues such as the nervous system; meanwhile, the latter approach, particularly when combined with two-photon excitation, has high spatial resolution and specificity but poor temporal resolution. As a new technique, holographic microscopy combines the advantages of both approaches. By projecting a holographic pattern on the brain through a spatial light modulator, the activity of specific groups of neurons in 3D brain volumes can be imaged or stimulated with high spatiotemporal resolution. In a combination of other techniques such as fast scanning or temporal focusing, this high spatiotemporal resolution can be further improved. Holographic microscopy enables all-optical interrogating of neural activity in 3D, a critical tool to dissect the function of neural circuits.

Temporal focusing multiphoton microscopy with optimized parallel multiline scanning for fast biotissue imaging.

.Significance: Line scanning-based temporal focusing multiphoton microscopy (TFMPM) has superior axial excitation confinement (AEC) compared to conventional widefield TFMPM, but the frame rate is limited due to the limitation of the single line-to-line scanning mechanism. The development of the multiline scanning-based TFMPM requires only eight multiline patterns for full-field uniform multiphoton excitation and it still maintains superior AEC.Aim: The optimized parallel multiline scanning TFMPM is developed, and the performance is verified with theoretical simulation. The system provides a sharp AEC equivalent to the line scanning-based TFMPM, but fewer scans are required.Approach: A digital micromirror device is integrated in the TFMPM system and generates the multiline pattern for excitation. Based on the result of single-line pattern with sharp AEC, we can further model the multiline pattern to find the best structure that has the highest duty cycle together with the best AEC performance.Results: The AEC is experimentally improved to from the of conventional TFMPM. The adopted multiline pattern is akin to a pulse-width-modulation pattern with a spatial period of four times the diffraction-limited line width. In other words, ideally only four spatial phase-shift scans are required to form a full two-dimensional image with superior AEC instead of image-size-dependent line-to-line scanning.Conclusions: We have demonstrated the developed parallel multiline scanning-based TFMPM has the multiline pattern for sharp AEC and the least scans required for full-field uniform excitation. In the experimental results, the temporal focusing-based multiphoton images of disordered biotissue of mouse skin with improved axial resolution due to the near-theoretical limit AEC are shown to clearly reduce background scattering.

Photobleaching Imprinting Enhanced Background Rejection in Line-Scanning Temporal Focusing Microscopy.

Compared with two-photon point-scanning microscopy, two-photon temporal focusing microscopy (2pTFM) provides a parallel high-speed imaging strategy with optical sectioning capability. Owing to out-of-focus fluorescence induced by scattering, 2pTFM suffers deteriorated signal-to-background ratio (SBR) for deep imaging in turbid tissue, Here, we utilized the photobleaching property of fluorophore to eliminate out-of-focus fluorescence. According to different decay rates in different focal depth, we extract the in-focus signals out of backgrounds through time-lapse images. We analyzed the theoretical foundations of photobleaching imprinting of the line-scanning temporal focusing microscopy, simulated implementation for background rejection, and demonstrated the contrast enhancement in MCF-10A human mammary epithelial cells and cleared Thy1-YFP mouse brains. More than 50% of total background light rejection was achieved, providing higher SBR images of the MCF-10A samples and mouse brains. The photobleaching imprinting method can be easily adapted to other fluorescence dyes or proteins, which may have application in studies involving relatively large and nontransparent organisms.

Optimization of pre-processing filter for time-reversal multi-user secure transmission systems based on artificial noise

Physical layer security is a way to realize the secure transmission of information by employing the characteristics of wireless channels. Thanks to its spatial and temporal focusing property, the time-reversal (TR) transmission can achieve effective secure communications even when the transmitter is only equipped with one antenna. This paper studies the design and optimization of the security scheme based on artificial noise (AN) with or without the eavesdropper's channel state information (ECSI) in a TR multi-user downlink multiple access system. The null-space AN is adopted when ECSI is unavailable, and the corresponding optimization problem is to minimize the signal power under each user's signal-to-interference-plus-noise ratio (SINR) target constraint, so that the power used to send AN is maximized and the interception rate of the eavesdropper is minimized. The problem is solved by transforming it into a semidefinite program (SDP) with semidefinite relaxation (SDR). The pre-processing filter and AN are jointly optimized to maximize the sum secrecy rate when ECSI is available. The problem is solved by transforming it into a convex program by using SDR and the first-order Taylor approximation. Simulation results show that the sum secrecy rate of the proposed scheme is significantly higher than that of the conventional TR transmission system with or without AN.

Dynamics and selective temporal focusing of a time truncated Airy pulse in varying dispersive media

We theoretically and numerically investigate the dynamics of a time truncated finite energy Airy pulse (FEAP) under longitudinally varying group velocity dispersion (GVD), which significantly influences the Airy dynamics. Realistic waveguide geometries are proposed that offer linear and oscillating GVD profiles through which one can efficiently control the ballistic Airy trajectory. We observe that a linear spatial variation of GVD may lead to a quasi-linear trajectory of the accelerating Airy pulse with modulated peak power. The impact of the periodic GVD on Airy dynamics is found to be interesting, where FEAP exhibits an oscillatory trajectory with a periodic modulation of peak power. Exploiting the transcendental relation between the modulation strength and period of GVD, we theoretically estimate an optimized length of the waveguide that delivers maximum power at the output. The effect of oscillatory third-order dispersion is dramatic for an Airy pulse, where it experiences successive singularities during its propagation. At singularity, the Airy pulse undergoes a temporal inversion after reaching a focal point. The rich dynamics of FEAP near the singularity point is theoretically investigated by solving the propagation equation. We provide detailed theoretical analyses to achieve selective temporal focusing of FEAP, which may be useful for practical applications. Theoretical results derived in this paper are verified numerically with satisfactory agreements.

Emerging micro-additive manufacturing technologies enabled by novel optical methods

This paper presents a comprehensive review of recent advances in micro-additive manufacturing enabled by novel optical methods with an emphasis on photopolymerization-based printing processes. Additive manufacturing, also known as three-dimensional (3D) printing, has become an important engineering solution to construct customized components or functional devices at low cost. As a green manufacturing technology, 3D printing has the advantages of high energy efficiency, low material consumption, and high precision. The rapid advancement of 3D printing technology has broadened its applications from laboratory research to industrial manufacturing. Generally, 3D objects to be printed are constructed digitally [e.g., via computer-aided design (CAD) programs] by connecting a 3D dot array, where a dot is defined as a voxel through mechanical, electrical, or optical means. The voxel size ranges from a few orders of magnitude of the wavelength of light to the sub-diffraction limit, achieved by material nonlinearity and precise power thresholding. In recent years, extensive research in optical additive manufacturing has led to various breakthroughs in quality, rate, and reproducibility. In this paper, we review various micro-3D printing techniques, including single-photon and two-photon processes, with a focus on innovative optical methods, e.g., ultrafast beam shaping, digital holography, and temporal focusing. We also review and compare recent technological advances in serial and parallel scanning systems from the perspectives of resolution, rate, and repeatability, where the strengths and weaknesses of different methods are discussed for both fundamental and industrial applications.

Temporal and Spatial Focusing in SPAD-Based Solid-State Pulsed Time-of-Flight Laser Range Imaging

The relation between signal and background noise strengths in single-photon avalanche diode (SPAD)-based pulsed time-of-flight 3-D range imaging is analyzed on the assumption that the SPAD detector is operating in the single photon detection mode. Several practical measurement cases using a 256-pixel solid-state pulsed time-of-flight (TOF) line profiler are presented and analyzed in the light of the resulting analysis. It is shown that in this case it is advantageous to concentrate the available optical average power in short, intensive pulses and to focus the optical energy in spatial terms. In 3-D range imaging, this could be achieved by using block-based illumination instead of the regularly used flood illumination. One modification of this approach could be a source that would illuminate the system FOV only in narrow laser stripes. It is shown that a 256-pixel SPAD-based pulsed TOF line profiler following these design principles can achieve a measurement range of 5–10 m to non-cooperative targets at a rate of ~10 lines/s under bright sunlight conditions using an average optical power of only 260 µW.

Exploiting dynamic sparsity for time reversal underwater acoustic communication under rapidly time varying channels

Underwater acoustic (UWA) communication encounters significant difficulties posed by the simultaneous presence of multipath and time variations. It has been recognized that, under the assumption that channel keeps static within the processing windows, time reversal (TR) processing approach is capable of mitigating multipath via the spatial–temporal focusing. However, for a rapidly time-varying UWA channel, the replica channel impulse response in time reversal processor need to be updated frequently to avoid performance degradation, which leads to huge computational complexity and significant overhead. Consider that after time reversal processing the resulting time-reversed channel response exhibits much more slow time variations compared to the original channel impulse response, previously Kalman filtering has been adopted to track the time-reversed channel response to alleviate the requirement of frequent channel update. Under the framework of dynamic compressed sensing (DCS), in this paper the time-reversed channel is formulated as a sparse set consisting of constant and slowly time-varying supports to derive the Kalman Filtered Compressed Sensing (KF-CS) channel estimation algorithm for time reversal communication. Based on the receiver structure by coupling the KF-CS estimator driven time reversal processor with a single channel decision-feedback equalizer (TR-DCS-DFE), shallow water experimental results with field data are provided to demonstrate the effectiveness of the proposed algorithm, compared to the classic time reversal communication scheme.

Subwavelength focusing of a spatio-temporal wave packet with transverse orbital angular momentum.

We report the method of producing a spatio-temporal (ST) wave packet carrying pure transverse orbital angular momentum (OAM) with subwavelength spatial sizes. Due to the lack of temporal focusing, an ST wave packet focused by a high numerical aperture (NA) objective lens experiences a "spatio-temporal astigmatism" effect similar to the focusing action of a cylindrical lens on the transverse profile of optical field. Thus an ST vortex with a spiral phase in the ST domain focused through a high NA objective will be distorted and lose the ST characteristic spiral phase pattern. With the understanding of such an ST astigmatism, the ST wave packet can be pre-conditioned such that an ST vortex carrying OAM with subwavelength transverse sizes can be obtained after strong focusing. This is the first revelation that a tightly focused ST vortex beam with transverse OAM can be realized, paving the way for potential applications including microscopy, optical trapping, laser machining, nonlinear light-matter interactions, and so on. The ST astigmatism effect also offers insights for the focusing and propagation studies of other types of ST wave packets.

Scanless two-photon excitation with temporal focusing.

Temporal focusing, with its ability to focus light in time, enables scanless illumination of large surface areas at the sample with micrometer axial confinement and robust propagation through scattering tissue. In conventional two-photon microscopy, widely used for the investigation of intact tissue in live animals, images are formed by point scanning of a spatially focused pulsed laser beam, resulting in limited temporal resolution of the excitation. Replacing point scanning with temporally focused widefield illumination removes this limitation and represents an important milestone in two-photon microscopy. Temporal focusing uses a diffusive or dispersive optical element placed in a plane conjugate to the objective focal plane to generate position-dependent temporal pulse broadening that enables axially confined multiphoton absorption, without the need for tight spatial focusing. Many techniques have benefitted from temporal focusing, including scanless imaging, super-resolution imaging, photolithography, uncaging of caged neurotransmitters and control of neuronal activity via optogenetics.

Fibonacci array-based focused acoustic camera for estimating multiple moving sound sources

Multiple moving sound sources’ aliasing problem occurs in many fields. The number and relative position of these sources provide critical information for further signal processing. However, estimating the number and relative position under aliasing of multiple moving sound sources is challenging. In this study, the Fibonacci sequence was applied to design a novel two-dimensional Fibonacci array, which has lower maximum sidelobe level than other typical array structures in a certain frequency scope. Conventional beamforming was utilized as an acoustic camera for stationary sound source localization by two-dimensional spatial scanning. Unlike the acoustic camera, a spatiotemporal map (STM) method was developed to estimate the number and relative position of multiple moving sound sources through spatial scanning and temporal focusing. STM is called focused acoustic camera because of the spatiotemporal information extraction of dynamic sources. A deconvolution approach called clean STM (CSTM) was created to estimate the spatiotemporal parameters for further signal processing and to overcome the influence of array pattern on STM. CSTM was combined with two-dimensional time-varying spatial filtering rearrangement and harmonic product spectrum-based sparse filtering enhancement. Consequently, a systematic technique solution was developed to be potentially applied in wayside acoustic bearing fault diagnosis systems. Simulation and experimental cases verified the effectiveness of the proposed method and the systematic technique solution.

Remote Spatio-Temporal Focusing Over Multimode Fiber Enabled by Single-Ended Channel Estimation

Multimode fiber (MMF) is attractive for a variety of applications requiring a large number of spatially overlapped modes including biophotonics and telecommunications. Superimposing these modes at the input by controlling the amplitude and phase of each individual mode can form arbitrarily structured light at the distal end of the MMF, which provides the possibility for ultra-thin fiber endoscopy, miniature multi-photon microscopy, and remote beam forming. Realistic deployment of this technology for example in a flexible endoscope would require a technique to continuously calculate the proper input modal superpositions over a large bandwidth as the fiber is perturbed without access to the distal end of the fiber. Here, we propose a single-ended channel estimation scheme which can measure broad-band MMF transfer matrix H at the transmitter employing reflective spatial pilots at the distal end of the MMF. Broadband multiple-input-multiple-output (MIMO) preprocessing is realized through electro-optic modulation for undoing mode scrambling, mitigating modal dispersion and achieving remote coherence control. We demonstrate spatial and temporal mode focusing at the receiver over an MMF guiding six spatial and polarization modes.

Achromatic reflected metalens for highly directional and long-distance acoustic probing

Simultaneous temporal and spatial focusing of a pulse is of significance for detection and imaging. Here, an achromatic reflected metalens is designed using hybrid resonance and anti-resonance. The theoretical result demonstrates that the anti-resonance provides an extra degree of freedom to control local phases of reflected waves, yielding an achromatic lens of thickness equal to one half of central wavelength. To overcome the shortcoming of traditional approach to design lenses (neglecting the intercell coupling), a boundary integral method is proposed to alleviate the focus deviation over a broadband. The achromatic feature of designed lens is then verified in the frequency range from 2800 to 5600 Hz by an experiment. Owing to a very weak frequency dependence of focal point and a high reflected focusing efficiency over a broadband, a highly directional and long-distance acoustic probing scheme (the mainlobe width about 80) is proposed with the aid of achromatic reflected metalens and being confirmed by another experiment, where a signal processing method using triple sensors separated by a subwavelength interval is adopted to eliminate the interferences between incident waves and reflected waves. Our result may find its application in a long-distance underwater acoustic probing.

Differential SAR Tomography Reconstruction Robust to Temporal Decorrelation Effects

Temporal decorrelation is one of the major problems in synthetic aperture radar (SAR) tomography (TomoSAR) of a natural environment that leads to blurring and spreading in focused image space. In the context of spatiotemporal focusing using the multi-temporal multi-baseline (MB) SAR data, a model-based differential TomoSAR is employed. Along this and with the aim of temporal decorrelation-robust focusing, a differential tomography framework based on generalized Capon estimator is investigated. The method can cope with temporal decorrelation of the distributed environment by spatiotemporal focusing with optimal bandwidth of the distributed signal. In addition, the method employs an additional parameter for coherence channel balancing in the model of generalized Capon that benefits from it in characterizing the spatiotemporal backscattering by mitigating the inconsistency between channels. The analysis is performed with a realistic simulation of temporal decorrelation in the presence of different decorrelation sources and taking into account the dependence on the vertical structure of the forested area. Effectiveness of the proposed framework has been assessed on both simulated and real data sets by evaluation and characterization of the canopy and under foliage ground in terms of deviation between the estimated covariance matrix and one of the generalized TomoSAR models.

Overcoming the barrier of nanoparticle production by femtosecond laser ablation in liquids using simultaneous spatial and temporal focusing

There exists an increasing demand of industrial-scale production of high-purity ligand-free nanoparticles due to the continuous development of biomedicine, catalysis, and energy applications. In this contribution, a simultaneous spatial and temporal focusing (SSTF) setup is first proposed for increasing nanoparticle productivity of the eco-friendly pulsed laser ablation in liquids (PLAL) technique. In spite of the fact that femtosecond pulses have proved to achieve higher ablation rates in air than picosecond pulses, in PLAL this is reversed due to the nonlinear energy losses in the liquid. However, thanks to the incorporation of SSTF, the energy delivered to the target is increased up to 70%, which leads to a nanoparticle production increase of a 2.4 factor. This breaks a barrier toward the employment of femtosecond lasers in high-efficiency PLAL.

Time-division-multiplexed observation bandwidth for ultrafast parametric spectro-temporal analyzer.

Parametric spectro-temporal analyzer (PASTA) has been demonstrated as a powerful tool for ultrafast spectrum measurement with superior frame rate and resolution. Compared with other time-stretch-based counterparts, the temporal focusing mechanism enlarges the initial condition and enables the observation of arbitrary waveform, especially the emission spectrum. However, due to the limited conversion bandwidth of the parametric mixing-based time-lens, the observation bandwidth of PASTA is constrained within the C (conventional) band, which hinders its practical applications. To overcome this constraint, both stokes and anti-stokes conversions of the parametric mixing process are leveraged, and the concept of time division multiplexing (TDM) is introduced to ensure their separability. Therefore, the TDM-based PASTA system successfully demultiplexes the C band and L (long) band spectra in two adjacent temporal frames. It is capable of reconstructing the wavelength-to-time sequence for arbitrary waveform over a record 58-nm observation bandwidth, which can be further improved by optimizing the filters and amplifiers. Meanwhile, both of these two bands achieve 20-pm resolution, 10-MHz frame rate, and -30-dBm sensitivity. Moreover, this TDM concept can also be applied to other parametric mixing-based temporal imaging systems to enlarge the working wavelength band, such as temporal magnification.

Out-of-band phase conjugation for auto-focusing and communications in the shallow ocean

Time-reversal methods in which received sound is inverted in the time domain and retransmitted back towards the source allows effective temporal and spatial auto-focusing in complex and dynamically changing environments such as the ocean. However, time-reversal typically involves significant time lapses between the receipt of a signal and its time-reversed retransmission. Given the continuously changing nature of the ocean significantly affecting higher frequencies, demonstrations of time-reversal systems were limited to low frequencies below 5 kHz. The closely related phase conjugation methods remove the time gap between signal receipt and its retransmission but are difficult to implement because of additional hardware needed to decouple the receiver and transmitter operating on the same frequency band. Moreover, phase conjugation preserves only the spatial focusing of time-reversal and loses the temporal focusing ability. In this presentation we will introduce the concept of out-of-band phase conjugation in which the phase conjugated sound is on a higher harmonic of the received sound thus effectively decoupling the transmitter and receiver. At the same time, out-of-band phase conjugation improves the resolution of the auto-focused phase conjugated sound. We will also discuss methods to mitigate the effects of imperfect temporal focusing.

Temporal Focusing Effects of Time-Reversal Frequency Diverse Array Antenna

Omnidirectional or the phased-array antenna is usually adopted in the time-reversal literature, but it cannot focus the power of the transmitted signals in both time- and space-domains. In this letter, we propose the hybridization of time reversal and frequency diverse array (FDA), which transmits the signal with a small frequency offset across the array to generate both angle- and range-dependent beampattern, for temporal focusing. The time-reversal FDA channel is modeled in multipath scenarios. For the given parameters with 12 transmit elements spaced by half-wavelength, carrier frequency 1 GHz, and frequency offset 1 kHz, the peak of time-reversal FDA focusing is 18.7% higher than that of a standard FDA. Both theoretic analysis and simulation results verify that, while maintaining FDA angular-time periodicity, the FDA range periodicity is eliminated and the energy is focused at the target position by the use of time-reversal processing.