In scientific and industrial research, three-dimensional (3D) imaging, or depth measurement, is a critical tool that provides detailed insight into surface properties. Confocal microscopy, known for its precision in surface measurements, plays a key role in this field. However, 3D imaging based on confocal microscopy is often challenged by significant data requirements and slow measurement speeds. In this paper, we present a novel self-supervised learning algorithm called SSL Depth that overcomes these challenges. Specifically, our method exploits the feature learning capabilities of neural networks while avoiding the need for labeled data sets typically associated with supervised learning approaches. Through practical demonstrations on a commercially available confocal microscope, we find that our method not only maintains higher quality, but also significantly reduces the frequency of the z-axis sampling required for 3D imaging. This reduction results in a remarkable 16× measurement speed, with the potential for further acceleration in the future. Our methodological advance enables highly efficient and accurate 3D surface reconstructions, thereby expanding the potential applications of confocal microscopy in various scientific and industrial fields.

Single-pixel imaging (SPI) captures two-dimensional images utilizing a sequence of modulation patterns and measurements recorded by a single-pixel detector. However, the sequential measurement of a scene is time-consuming, especially for high-spatial-resolution imaging. Furthermore, for spectral SPI, the enormous data storage and processing time requirements substantially diminish imaging efficiency. To reduce the required number of patterns, we propose a strategy by optimizing a Hadamard pattern sequence via Morton frequency domain scanning to enhance the quality of a reconstructed spectral cube at low sampling rates. Additionally, we expedite spectral cube reconstruction, eliminating the necessity for a large Hadamard matrix. We demonstrate the effectiveness of our approach through both simulation and experiment, achieving sub-Nyquist sampling of a three-dimensional spectral cube with a spatial resolution of 256 × 256 pixels and 181 spectral bands and a reduction in reconstruction time by four orders of magnitude. Consequently, our method offers an efficient solution for compressed spectral imaging.

Ghost imaging has been attracting more and more attention, which provides a way to obtain images of objects with only a single-pixel detector. Considering possible applications, it becomes urgent to clarify the sensitivity of ghost imaging. Due to the unique characteristics of single-pixel detectors, which collect photons without distributing them to multiple pixels, often outperforming array sensors, ghost imaging is believed to be more sensitive than conventional imaging. However, a systematic analysis on the sensitivity of ghost imaging is yet to be completed. In this paper, we present a method for quantitatively assessing the sensitivity of ghost imaging. A detailed comparison is provided between ghost imaging and conventional imaging, taking into account the particle nature of photons and the noise of detection. With the settings of the two imaging methods being the same to the most extent, the minimal required number of detected photons for images of a certain quality is considered. For the thermal source version, ghost imaging demonstrates enhanced sensitivity under practical situations, with noise considered. Employing an entangled source, ghost imaging surpasses conventional imaging techniques in terms of sensitivity obviously. In one word, ghost imaging promises higher sensitivity at low photon flux and noisy situations.

We proposed a hybrid imaging scheme to estimate a high-resolution absolute depth map from low photon counts. It leverages measurements of photon arrival times from a single-photon LiDAR and an intensity image from a conventional high-resolution camera. Using a tailored fusion algorithm, we jointly processed the raw measurements from both sensors and output a high-resolution absolute depth map. We scaled up the resolution by a factor of 10, achieving 1300 × 2611 pixels and extending ∼4.7 times the unambiguous range. These results demonstrated the superior capability of long-range high-resolution 3D imaging without range ambiguity.

High-sensitivity radio-frequency optically pumped magnetometers (RF-OPMs), working without cryogenic condition, play a critical role in magnetic field imaging (MFI) at low frequencies (e.g., 100 Hz to 1 MHz). We introduce the principle of operation and recent developments of RF-OPMs and focus on reviewing the MFI applications in magnetic induction tomography, ultralow-field magnetic resonance imaging, and magnetic particle imaging. For the applications of RF-OPMs, ranging from industrial monitoring to medical imaging and security screening, the unshielded and portable RF-OPMs (and RF-OPM array) techniques are still under the further development for detecting and scanning over the target object for accomplishing the final three-dimensional imaging, and thus extremely require the abilities of active compensation of the ambient magnetic field and sensor miniaturization in the future.

Imaging objects hidden behind turbid media is of great scientific importance and practical value, which has been drawing a lot of attention recently. However, most of the scattering imaging methods rely on a narrow linewidth of light, limiting their application. A mixture of the scattering light from various spectra blurs the detected speckle pattern, bringing difficulty in phase retrieval. Image reconstruction becomes much worse for dynamic objects due to short exposure times. We here investigate non-invasively recovering images of dynamic objects under white-light irradiation with the multi-frame OTF retrieval engine (MORE). By exploiting redundant information from multiple measurements, MORE recovers the phases of the optical-transfer-function (OTF) instead of recovering a single image of an object. Furthermore, we introduce the number of non-zero pixels (NNP) into MORE, which brings improvement on recovered images. An experimental proof is performed for dynamic objects at a frame rate of 20 Hz under white-light irradiation of more than 300 nm bandwidth.

We establish a quantum theory of computational ghost imaging and propose quantum projection imaging where object information can be reconstructed by quantum statistical correlation between a certain photon number of a bucket signal and digital micromirror device random patterns. The reconstructed image can be negative or positive, depending on the chosen photon number. In particular, the vacuum state (zero-number) projection produces a negative image with better visibility and contrast-to-noise ratio. The experimental results of quantum projection imaging agree well with theoretical simulations and show that, under the same measurement condition, vacuum projection imaging is superior to conventional and fast first-photon ghost imaging in low-light illumination.

Performance assessment of an imaging system is important for the optimization design with various technologies. The information-theoretic viewpoint based on communication theory or statistical inference theory can provide objective and operational measures on imaging performance. These approaches can be further developed by combining with the quantum statistical inference theory for optimizing imaging performance over measurements and analyze its quantum limits, which is demanded in order to improve an imaging system when the photon shot noise in the measurement is the dominant noise source. The aim of this review is to discuss and analyze the recent developments in this branch of quantum imaging.

Microscopes are indispensable tools in modern biology and medicine. With the development of microscopy, the signal-to-noise ratio of microscopes is now limited by the shot noise. Recently, quantum-enhanced microscopic imaging provides a feasible approach for improving the signal-to-noise ratio since it can beat the shot-noise limit by using quantum light. In this review, we first briefly introduce quantum states applied in quantum-enhanced microscopic imaging, and then we provide an overview of the principle and progress of quantum-enhanced stimulated Raman scattering microscopy, entangled two-photon microscopy, and quantum-enhanced differential interference contrast microscopy.

Since its first experimental demonstration, “ghost imaging” has attracted much attention, perhaps not only because of its interesting physics but also because of its attractive application. This review article discusses the physics and application of ghost imaging: (1) emphasizes the nonlocal two-photon interference nature of ghost imaging, including detailed analysis and calculations; (2) introduces three types of applications with unique advantages of ghost imaging, including a light detection and ranging device with imaging ability, namely, an Imaging Lidar or ILidar system; a turbulence-resistant, or turbulence-free, imaging technology; and a vibration-resistant X-ray microscope of high resolving capability. This article is prepared for a Special Issue of Chinese Optics Letters, intended for general audiences, especially young researchers and students who are interested in ghost imaging.

We developed a new single-layer atom chip with an additional U-shaped current-carrying structure. The new U-shaped microwire creates optimized magnetic field distribution, which increases the trapping volume of a magneto-optical trap (MOT) near the chip. Our approach allows one to localize more atoms, while a setup remains relatively simple (single-layer approach) and consumes low current (up to 10 A). The total number of trapped 87Rb atoms in our setup is 5 × 107.

Wide-field second-harmonic generation (SHG) was used to obtain the second-harmonic signal from the entire image area for rapid imaging, despite the fact that conventional Gaussian beam illumination has low energy utilization efficiency, which makes it easy to overexpose the intensity of the image center area. However, flat-top beam illumination has uniform spatial distribution, thereby improving the photon excitation efficiency in the entire image region and reducing laser damage and thermal effect. By combining flat-top beam illumination and wide-field SHG polarization measurement, we can calculate more myosin fibril symmetrical axis orientations through polarization analysis of 16 images at a fast imaging speed while expanding the field of view. More importantly, the application of a flat-top beam can further improve the capability of polarization measurement in SHG microscopy.

In this study, we propose a superposed probabilistically shaped (PS) quadrature amplitude modulation (QAM) constellation scheme for multiple-input multiple-output visible light communication systems. PS QAM signals are generated from a nonlinear coding equation that converts uniformly distributed 8-level signals into PS 9- or 10-level signals, which are then mapped into PS 9QAM or 10QAM signals. Square-shaped 9QAM and trapezoid-shaped 10QAM constellations are introduced to maximize the minimum Euclidean distance (MED) of the superposed constellation. Finally, the PS 9QAM and 10QAM signals are superposed with the 4QAM signals in a flipped manner to obtain PS 36QAM or 40QAM signals at the receiver, respectively. To exploit the temporal correlation of the resulting signal from nonlinear coding, we developed a detection algorithm based on Viterbi decoding. Experimental results confirmed the superiority of the proposed schemes by achieving a higher MED and stronger ability to resist nonlinearity. Compared with the traditional scheme, the peak-to-peak voltage dynamic ranges of the superposed 36QAM and 40QAM constellation schemes were improved by 52% and 48%, respectively.

This Letter proposes a post-equalizer for underwater visible light communication (UVLC) systems that combines channel estimation and joint time-frequency analysis, named channel-estimation-based bandpass variable-order time-frequency network (CBV-TFNet). By utilizing a bandpass variable-order loss function with communication prior knowledge, CBV-TFNet enhances communication performance and training stability. It enables lightweight implementation and faster convergence through a channel estimation-based mask. The superior performance of the proposed equalization method over Volterra and deep neural network (DNN)-based methods has been studied experimentally. Using bit-power loading discrete multitone (DMT) modulation, the proposed method achieves a transmission bitrate of 4.956 Gbps through a 1.2 m underwater channel utilizing only 38.15% of real multiplication calculations compared to the DNN equalizer and achieving a bitrate gain of 440 Mbps and a significantly larger dynamic range over the LMS-Volterra equalizer. Results highlight CBV-TFNet’s potential for future post-equalization in UVLC systems.

We characterize the current crowding effect for microwave radiation on a chip surface based on a quantum wide-field microscope combining a wide-field reconstruction technique. A swept microwave signal with the power of 0–30 dBm is supplied to a dumbbell-shaped microstrip antenna, and the significant differences in microwave magnetic-field amplitudes attributed to the current crowding effect are experimentally observed in a 2.20 mm × 1.22 mm imaging area. The normalized microwave magnetic-field amplitude along the horizontal geometrical center of the image area further demonstrates the feasibility of the characterization of the current crowding effect. The experiments indicate the proposal can be qualified for the characterization of the anomalous area of the radio-frequency chip surface.

In three-dimensional imaging employing phase-shifting profilometry (PSP), the nonlinear response of projector and camera makes the fringe gray distribution non-sinusoidal, which further leads to phase error. Although the double 3-step phase-shifting method is simple and effective, it needs to add an additional set of fringe sequences, which reduces the measurement efficiency. To this end, this paper introduces a generic and flexible self-correction method for nonlinearity-induced phase error. First, according to the nonlinearity-induced phase error model, we introduce an additional wrapped phase with a phase difference of π/3. The error waveform of the two wrapped phases is opposite but not coincident. Then, we introduce an estimation algorithm for the additional wrapped phase offset. Finally, we fuse the two wrapped phases to correct the phase error. Experiments confirm that the root mean squared error of the proposed method is 64.1% lower than that of the traditional method and 13.3% lower than that of the Hilbert transform method. The proposed method does not require any additional fringes or hardware assistance and can be easily extended to 4-step or 5-step PSP.

The development of hybrid optics/microwave communication systems puts forward a new requirement for beam splitters to efficiently transmit microwave signals and simultaneously reflect optical signals. Owing to mechanical constraints, the physical thickness of beam splitters is of the order of tens of millimeters. The corresponding electrical thickness has the same order of magnitude as microwave wavelengths, and the resulting multi-beam interference effect significantly reduces the microwave transmittance, impacting the beam splitting quality. This study presents a new optics/microwave beam splitter based on the ability of the frequency selective surface (FSS) to shape the resonant curve. A beam splitter sample, whose physical thickness and substrate material are 20 mm and quartz glass, respectively, is designed, simulated, fabricated, and characterized to validate the feasibility of this strategy. The measured results show that the minimum microwave transmittance between 35 and 36.5 GHz with an incidence angle of 45° under TE polarization is 86.43%, and the mean value of the reflectance spectra from 450 to 900 nm and that from 7.7 to 10.5 μm both exceed 96%. This FSS-based optics/microwave beam splitter is expected to play a key role in hybrid optics/microwave communication systems.

The Raman random fiber laser (RRFL) is a typical complex physical system due to the intrinsic random feedback of the fiber, which causes complexity in the RRFL output. So far, the time-domain statistical attributes of the RRFL are still not fully characterized. In this paper, the temporal statistical properties of the RRFL are investigated comprehensively for the first time under the full bandwidth condition. First, the time-domain intensity statistical characteristics of the RRFL under the full bandwidth condition are theoretically studied: the results demonstrate that the intensity probability density function of the RRFL is related to the pump power and observing position and deviates inward from the exponential distribution, indicating that correlation exists between the different frequency components in the spectrum. Afterward, in the validation experiment, an elaborate structure is designed to realize a narrow-bandwidth 1053 nm RRFL, and its full bandwidth temporal intensity statistical features manifest an identical variation pattern to the simulation results. This work fills a vacancy in the study of RRFL temporal statistical features and rigorously reveals the different physical mechanisms between RRFL and amplified spontaneous emission light sources, providing instructions for the application of the RRFL.

We demonstrate an intracavity self-synchronized multi-color Q-switched fiber laser using a parallel-integrated fiber Bragg grating (PI-FBG), fabricated by a femtosecond laser with a point-by-point parallel inscription method. The multi-color Q-switched pulses can be always self-synchronized when the group delay differences between neighboring spectra range from -3.4 to 3.4 ps. The starting and evolution dynamics indicate that the saturable absorption effect of the carbon nanotube plays a dual role: synchronously triggering the startup of the pulse at successive colors by active Q-switching and spontaneously compensating to some extent the temporal walk-off of the multi-color pulses through the cross saturable absorption modulation. This work unveils the intracavity self-synchronization mechanism of the multi-color Q-switched pulses and also demonstrates the potential of PI-FBGs for the customizable generation of the synchronized multi-color pulse in a single cavity.

We have experimentally presented a watt-level noise-like (NL) pulse mode-locked all-polarization-maintaining (PM) fiber laser centered at ∼1995 nm, which can directly generate stable NL pulses with a maximum output power of ∼1.017 W and pulse energy of ∼0.61 µJ, representing the highest output power of mode-locked NL pulse at 2 µm from any fiber oscillators, to the best of our knowledge. The mode-locked NL pulse laser exhibits an excellent stability with a power fluctuation of ∼0.1% in 8 h of monitoring, and a signal-to-noise ratio of ∼83 dB at a fundamental frequency of ∼1.662 MHz. Moreover, the pulse envelope and coherence spike width of the NL pulse can be widely tuned from ∼4.5 ns to ∼16 ns, and ∼364 fs to ∼323 fs, respectively, with the enhancement of the pump power. Such an all-PM fiber oscillator is the ideal seed source for the implementation of a high-power NL pulse laser and has potential valuable applications in mid-infrared spectroscopy and industrial processing.

Photonic microwave harmonic down-converters (PMHDCs) based on self-oscillation optical frequency combs (OFCs) are interesting because of their broad bandwidth compared with plain optoelectronic oscillators. In this paper, a high-efficiency and flexible PMHDC is proposed and demonstrated. The properties of the OFC, such as the carrier-to-noise ratio (CNR), bandwidth and free spectral range (FSR), and the influence of optical injection, are investigated. The broadband OFC provides a frequency tunable and high-quality local oscillation (LO), which guarantees flexible down-conversion for the radio frequency (RF) signal. The sideband selective amplification (SSA) effect not only improves the conversion efficiency but also promotes single-sideband modulation. The conversion range can reach 100 GHz. The 12–40 GHz RF signal can be down-converted to intermediate frequency (IF) signals with a high conversion efficiency of 14.9 dB. The fixed 40-GHz RF signal is flexibly down-converted to an IF signal with the frequency from 55.4 to 2129.4 MHz. The phase noise of an IF signal at a frequency offset of 10 kHz is the same as that of the input RF signal. The PMHDC shows great performance and will find applications in radio-over-fiber (RoF) networks, electronic warfare receivers, avionics, and wireless communication systems.

This paper reports a photonics-assisted millimeter-wave (mm-wave) joint radar jamming and secure communication system constructed through a photonic upconversion technique. In the experiments, a 30 GHz constant envelope linear frequency-modulated orthogonal frequency division modulation (CE-LFM-OFDM) signal with an instantaneous bandwidth of 1 GHz is synthesized by encoding 1 GBaud encrypted 16-quadrature amplitude modulation (16-QAM) OFDM signal. The velocity deception jamming is achieved with a spurious suppression ratio over 30 dB. Furthermore, we efficiently execute range deception jamming with a time shift of 10 ns. Simultaneously, the encrypted 16-QAM OFDM signal is successfully transmitted over a 1.2 m wireless link, with a data rate of 4 Gbit/s.

In this work, we propose a novel approach that combines a bidirectional deep neural network (BDNN) with a multifunctional metasurface absorber (MMA) for inverse design, which can effectively address the challenge of on-demand customization for absorbers. The inverse design of absorption peak frequencies can be achieved from 0.5 to 10 terahertz (THz), covering the quasi-entire THz band. Based on this, the BDNN is extended to broadband absorption, and the inverse design yields an MMA at the desired frequency. This work provides a broadly applicable approach to the custom design of multifunctional devices that can facilitate the evaluation and design of metasurfaces in electromagnetic absorption.

In the field of long-wave infrared (LWIR) thermal imaging, vital for applications such as military surveillance and medical diagnostics, metalenses show immense potential for compact, lightweight, and low-power optical systems. However, to date, the development of LWIR broadband achromatic metalenses with dynamic tunable focus, which are suitable for both coaxial and off-axis applications, remains a large unexplored area. Herein, we have developed an extensive database of broadband achromatic all-As2Se3 microstructure units for the LWIR range. Utilizing this database with the particle swarm optimization (PSO) algorithm, we have designed and demonstrated LWIR broadband achromatic metalenses capable of coaxial and off-axis focusing with three dynamic tunable states. This research may have potential applications for the design of compact, high-performance optical devices, including those with extreme depth-of-field and wide-angle imaging capabilities.

Cholesteric liquid crystal (CLC) has been widely used in flat optical elements due to the Pancharatnam–Berry (PB) phase modulation. In order to achieve PB phase modulation for both circular polarizations, it is natural to come up with stacking CLCs with opposite chirality. Here, various optical properties of diverse CLC stacking structures are systematically investigated by numerical calculations. With the thickness of the CLC sublayers becoming smaller, the reflection bandgap splits into three main parts, and the rotatory dispersion gradually becomes negligible. Vector beams provide a more intuitive verification. These results provide theoretical guidance for future studies on stacked chiral anisotropic media.

Using an identical monolithic InGaN/GaN light emitting diode (LED) array as the sensing module and a well-designed data processing module, we demonstrate a small-size concentration sensing prototype. Overlap between the emission and the response spectra of the InGaN/GaN LED makes each pair of LEDs in the arrayed chip form a sensing channel. The changes in liquid concentration can be transformed into variation of photocurrent. The system’s sensing properties are further optimized by varying the position, number of receivers, and packaging reflectors. With methyl orange as a tracer agent, the sensing system’s resolution is 0.286 µmol/L with a linear measurement region below 40 µmol/L.

Coherent optical emission of free induction decay (FID) excited by the multiple-photon resonance has been recently observed in nitrogen ions produced by intense femtosecond laser pulses. Here we report that orbit angular momentum (OAM) of the pump laser pulses can be transferred to the much longer pulsed free induction decay emission with the OAM conserved. It was found that when the pump laser frequency approaches the fraction of the 391.4 nm emission (ν0/5), an FID emission is emitted whereas its topological charge was identified to be 5 times of the pump laser. Due to the reduced laser intensity with a vortex pump compared to the conventional Gaussian beam, the resonant pump laser wavelength presents a redshift, and the FID signal is significantly weaker.