Single-pixel Detector Research Articles

Mask-based single-pixel tracking and imaging for moving objects.

Tracking and imaging for high-speed moving objects have a wide range of application prospects in many fields, such as transportation and security monitoring. In this paper, the chrome plated masks are designed to carry geometric moment and random binary encoding patterns, combined with single pixel detectors, to achieve real-time tracking and imaging of fast-moving object. By using the geometric moment principle to obtain the motion trajectory of the object, coding sub-patterns and corresponding detection signals are extracted at different positions to reconstruct the image of the object. Multiple optical paths are established to avoid the side effects of motion error, and a dedicated calibration approach is proposed to improve the accuracy of tracking. The feasibility of the method is demonstrated by simulations and experiments. The proposed scheme, which modulates light with static mask instead of spatial light modulator (SLM), improves the speed and spectral range meanwhile reduces the system cost.

High-fidelity and high-robustness free-space ghost transmission in complex media with coherent light source using physics-driven untrained neural network.

It is well recognized that it is challenging to realize high-fidelity and high-robustness ghost transmission through complex media in free space using coherent light source. In this paper, we report a new method to realize high-fidelity and high-robustness ghost transmission through complex media by generating random amplitude-only patterns as 2D information carriers using physics-driven untrained neural network (UNN). The random patterns are generated to encode analog signals (i.e., ghost) without any training datasets and labeled data, and are used as information carriers in a free-space optical channel. Coherent light source modulated by the random patterns propagates through complex media, and a single-pixel detector is utilized to collect light intensities at the receiving end. A series of optical experiments have been conducted to verify the proposed approach. Experimental results demonstrate that the proposed method can realize high-fidelity and high-robustness analog-signal (ghost) transmission in complex environments, e.g., around a corner, or dynamic and turbid water. The proposed approach using the designed physics-driven UNN could open an avenue for high-fidelity free-space ghost transmission through complex media.

Integrated Encapsulation and Implementation of a Linear-Mode APD Detector for Single-Pixel Imaging Lidar

Single-pixel imaging lidar is a novel technology that leverages single-pixel detectors without spatial resolution and spatial light modulators to capture images by reconstruction. This technique has potential imaging capability in non-visible wavelengths compared with surface array detectors. An avalanche photodiode (APD) is a device in which the internal photoelectric effect and the avalanche multiplication effect are exploited to detect and amplify optical signals. An encapsulated APD detector, with an APD device as the core, is the preferred photodetector for lidar due to its high quantum efficiency in the near-infrared waveband. However, research into APD detectors in China is still in the exploratory period, when most of the work focuses on theoretical analysis and experimental verification. This is a far cry from foreign research levels in key technologies, and the required near-infrared APD detectors with high sensitivity and low noise have to be imported at a high price. In this present study, an encapsulated APD detector was designed in a linear mode by integrating a bare APD tube, a bias power circuit, a temperature control circuit and a signal processing circuit, and the corresponding theoretical analysis, circuit design, circuit simulation and experimental tests were carried out. Then, the APD detector was applied in the single-pixel imaging lidar system. The study showed that the bias power circuit could provide the APD with an operating voltage of DC 1.6 V to 300 V and a ripple voltage of less than 4.2 mV. Not only that, the temperature control circuit quickly changed the operating state of the Thermo Electric Cooler (TEC) to stabilize the ambient temperature of the APD and maintain it at 25 ± 0.3 °C within 5 h. The signal processing circuit was designed with a multi-stage amplification cascade structure, effectively raising the gain of signal amplification. By comparison, the trial also suggested that the encapsulated APD detector and the commercial Licel detector had a good agreement on the scattered signal, such as a repetition rate and pulse width response under the same lidar environment. Therefore, target objects in real atmospheric environments could be imaged by applying the encapsulated APD detector to the near-infrared single-pixel imaging lidar system.

Efficient single-pixel imaging encrypted transmission based on 3D Arnold transformation

Single-pixel imaging (SPI) can transform 2D or 3D image data into 1D light signals, which offers promising prospects for image compression and transmission. However, during data communication these light signals in public channels will easily draw the attention of eavesdroppers. Here, we introduce an efficient encryption method for SPI data transmission that uses the 3D Arnold transformation to directly disrupt 1D single-pixel light signals and utilizes the elliptic curve encryption algorithm for key transmission. This encryption scheme immediately employs Hadamard patterns to illuminate the scene and then utilizes the 3D Arnold transformation to permutate the 1D light signal of single-pixel detection. Then the transformation parameters serve as the secret key, while the security of key exchange is guaranteed by an elliptic curve-based key exchange mechanism. Compared with existing encryption schemes, both computer simulations and optical experiments have been conducted to demonstrate that the proposed technique not only enhances the security of encryption but also eliminates the need for complicated pattern scrambling rules. Additionally, this approach solves the problem of secure key transmission, thus ensuring the security of information and the quality of the decrypted images.

Single-pixel image reconstruction using coherent nonlinear optics.

We propose and experimentally demonstrate a novel, to the best of our knowledge, hybrid optoelectronic system that utilizes mode-selective frequency upconversion, single-pixel detection, and a deep neural network to achieve the reliable reconstruction of two-dimensional (2D) images from a noise-contaminated database of handwritten digits. Our system is designed to maximize the multi-scale structural similarity index measure (MS-SSIM) and minimize the mean absolute error (MAE) during the training process. Through extensive evaluation, we have observed that the reconstructed images exhibit high-quality results, with a peak signal-to-noise ratio (PSNR) reaching approximately 20 dB and a structural similarity index measure (SSIM) of around 0.85. These impressive metrics demonstrate the effectiveness and fidelity of our image reconstruction technique. The versatility of our approach allows its application in various fields, including Lidar, compressive imaging, volumetric reconstruction, and so on.

Mercury telluride colloidal quantum-dot focal plane array with planar p-n junctions enabled by in situ electric field–activated doping

Colloidal quantum dot (CQD)–based photodetectors are promising alternatives to bulk semiconductor-based detectors to be monolithically integrated with complementary metal-oxide semiconductor readout integrated circuits avoiding high-cost epitaxial growth methods and complicated flip-bonding processes. To date, photovoltaic (PV) single-pixel detectors have led to the best performance with background-limit infrared photodetection performance. However, the nonuniform and uncontrollable doping methods and complex device configuration restrict the focal plane array (FPA) imagers to operate in PV mode. Here, we propose a controllable in situ electric field–activated doping method to construct lateral p-n junctions in the short-wave infrared (SWIR) mercury telluride (HgTe) CQD–based photodetectors with a simple planar configuration. The planar p-n junction FPA imagers with 640 × 512 pixels (15-μm pixel pitch) are fabricated and exhibit substantially improved performance compared with photoconductor imagers before activation. High-resolution SWIR infrared imaging is demonstrated with great potential for various applications including semiconductor inspection, food safety, and chemical analysis.

High-throughput doubly-encoded single-pixel spectrometer with an extended aperture.

Infrared devices are increasingly used in industrial, medical, and environmental monitoring applications. Cost-effectiveness, robustness, and portability are characteristics that are highly sought after and they can be enabled by a dispersive spectrometer carrying a single-pixel detector. In this paper, we demonstrate a novel, high-throughput dispersive spectrometer that has its spectral resolution decoupled from its throughput. The proposed spectrometer implements a two-stage Hadamard transform encoding process that allows significantly more light into the system to enhance its signal-to-noise ratio. As a single-pixel detector is used to collect the spectral information, the proposed system can be easily implemented in other desired wavelengths. Furthermore, we develop a method to remove the need for uniform illumination at the entrance aperture by taking into consideration its spatial information during the reconstruction process, thereby increasing the ease of the design of devices required for in situ measurement.

Investigation of Equivalence Between Non-Resonant Raman Excitation Spectroscopy and Conventional Raman Spectroscopy

As spontaneous Raman spectroscopy is demonstrated to be viable for many applications, it is still held back by its inherently low signal and typically extended acquisition time. While several techniques can overcome this by enhancing the scattered signal to quicken acquisition, a parallel field that quickens spectral acquisition using instrumentation has received less focus. In principle, Raman spectra could be obtained by tuning the excitation wavelength and measuring the Raman signal at a fixed wavelength using a single-pixel detector. Before exploring the potential of quicker acquisition, it is important to first study the equivalence of Raman Excitation Spectroscopy and conventional Raman Spectroscopy experimentally. We demonstrate the equivalence between the two techniques for biological and non-biological samples by measuring Raman Excitation Maps. Additionally, we explore the interpolation of low-resolution excitation spectra using excess information in the maps for higher resolution excitation spectra. This work, together with current progression in fast tunable lasers and highly sensitive single-pixel detectors, provides a promising prospect for dedicated Raman Excitation Spectroscopy instruments to be developed.

High-resolution self-corrected single-pixel imaging through dynamic and complex scattering media.

Imaging with single-pixel detectors becomes attractive in many applications where pixelated detectors are not available or cannot work. Based on a correlation between the probing patterns and the realizations, optical imaging with single-pixel detector offers an indirect way to recover a sample. It is well recognized that single-pixel optical imaging through dynamic and complex scattering media is challenging, and dynamic scaling factors lead to serious mismatches between the probing patterns and the realizations. In this paper, we report self-corrected imaging to realize high-resolution object reconstruction through dynamic and complex scattering media using a parallel detection with dual single-pixel detectors. The proposed method can supervise and self-correct dynamic scaling factors, and can implement high-resolution object reconstruction through dynamic and complex scattering media where conventional methods could not work. Spatial resolution of 44.19 µm is achieved which approaches diffraction limit (40.0 µm) in the designed optical setup. The achievable spatial resolution is dependent on pixel size of spatial light modulator. It is experimentally validated that the proposed method shows unprecedented robustness against complex scattering. The proposed self-corrected imaging provides a solution for ghost recovery, enabling high-resolution object reconstruction in complex scattering environments.

Sensing azimuthally symmetric objects by a single-pixel detector via COAM matrix

A coherence-orbital angular momentum (COAM) matrix describes the second-order spatial correlations among the pairs of the OAM states in a light beam at two radial positions. It provides a detailed information regarding the azimuthal features of the light beam itself or of the objects interacting with it. In this Letter, we reveal a close relationship existing between the COAM matrix's elements of a partially coherent light beam and the angular gratings that it passes through. It is demonstrated both in theory and in experiment that all the parameters of the angular gratings can be identified by measuring the COAM matrix elements. While the off-diagonal elements possess information about the angular shifts and the rotation speed of the angular grating, the diagonal elements account for the energy distribution among the OAM modes. The experimental results agree reasonably well with the theoretical analysis. One practically significant capability is that our technique offers the determination of angular shifts of azimuthally symmetric objects and/or their rotational speed, by means of a single-pixel detector.

Photon-noise: is a single-pixel camera better than point scanning? A signal-to-noise ratio analysis for Hadamard and Cosine positive modulation

In a single-pixel camera, an unknown object is sequentially illuminated by intensity patterns. The total reflected or transmitted intensity is summed in a single-pixel detector from which the object is computationally reconstructed. In the situation where the measurements are limited by photon-noise, it is questionable whether a single-pixel camera performs better or worse than simply scanning the object with a focused intensity spot—a modality known as point raster scanning and employed in many laser scanning systems. Here, we solve this general question and report that positive intensity modulation based on Hadamard or Cosine patterns does not necessarily improve the single-to-noise ratio (SNR) of single-pixel cameras, as compared to point raster scanning (RS). Instead, we show that the SNR is only improved on object pixels at least k times brighter than the object mean signal , where k is a constant that depends on the modulation scheme (modulation matrix, number of detectors, etc). The constant k is derived for several widespread cases and has important consequences on the choice of the optical deign. This fundamental property is demonstrated theoretically, numerically, and is experimentally confirmed in the spatial domain (widefield fluorescence imaging) and in the spectral domain (spontaneous Raman spectral measurements). Finally, we provide user-oriented guidelines that help decide when and how multiplexing under photon-noise should be used instead of point RS.

Mechanochemical Synthesis of High-Entropy Perovskite toward Highly Sensitive and Stable X-ray Flat-Panel Detectors.

Perovskites are attracting attention for optoelectronic devices. Despite their promise, the large-scale synthesis of perovskite materials with exact stoichiometry, especially high-entropy perovskites, has been a major challenge. Moreover, the difficulty in stoichiometry control also hinders the development of perovskite X-ray flat-panel detectors. Previous reports all employed simple MAPbI3 as the active layer, while the performance still falls short of optimized single-crystal-based single-pixel detectors. Herein, a scalable and universal strategy of a mechanochemical method is adopted to synthesize stoichiometric high-entropy perovskite powders with high quality and high quantity (>1kg per batch). By utilizing these stoichiometric perovskites, the first FA0.9 MA0.05 Cs0.05 Pb(I0.9 Br0.1 )3 -based X-ray flat-panel detector with low trap density and large mobility-lifetime product (7.5 × 10-3 cm2 V-1 ) is reported. The assembled panel detector exhibits close-to-single-crystal performance (high sensitivity of 2.1 × 104 µC Gyair -1 cm-2 andultralow detection limit of 1.25 nGyair s-1 ), high spatial resolution of 0.46 lp/pixel, as well as excellent thermal robustness under industrial standards. The high performance in the high-entropy perovskite-based X-ray FPDs has the potential to facilitate the development of new-generationX-ray-detection systems.

Microscopic single-pixel polarimetry for biological tissue

Single-pixel imaging (SPI) is a computational imaging modality that reconstructs images from information collected with a single-pixel detector. Due to the relatively large bandwidth and cheap price of single-pixel detectors over pixel-array detectors, SPI has been demonstrated with great success in many imaging scenarios. In this work, we further extended SPI to microscopic Mueller-matrix polarimetry, which is capable of providing polarization-sensitive properties of biological tissue with a millimeter-size field of view and micrometer-size resolution. As a proof of concept, we applied polarimetry on normal and cancerous esophagus tissues from patients and reconstructed their Mueller matrices. Effects of depolarization, retardation, and diattenuation were also examined by decomposing Mueller matrices. We envision that the developed computational technique can bring new possibilities for both SPI and Mueller-matrix polarimetry.

Efficient Fourier single-pixel imaging based on weighted sorting.

Fourier single-pixel imaging (FSI) has attracted increased attention in recent years with the advantages of a wide spectrum range and low cost. FSI reconstructs a scene by directly measuring the Fourier coefficients with a single-pixel detector. However, the existing sampling method is difficult to balance the noise suppression and image details within a limited number of measurements. Here we propose a new sampling strategy for FSI to solve this problem. Both the generality of the spectral distribution of natural images in the Fourier domain and the uniqueness of the spectral distribution of the target images in the Fourier domain are considered in the proposed method. These two distributions are summed with certain weights to determine the importance of the Fourier coefficients. Then these coefficients are sampled in order of decreasing importance. Both the simulations and experiments demonstrate that the proposed method can capture more key Fourier coefficients and retain more details with lower noise. The proposed method provides an efficient way for Fourier coefficient acquisition.

Comparison of Common Algorithms for Single-Pixel Imaging via Compressed Sensing.

Single-pixel imaging (SPI) uses a single-pixel detector instead of a detector array with a lot of pixels in traditional imaging techniques to realize two-dimensional or even multi-dimensional imaging. For SPI using compressed sensing, the target to be imaged is illuminated by a series of patterns with spatial resolution, and then the reflected or transmitted intensity is compressively sampled by the single-pixel detector to reconstruct the target image while breaking the limitation of the Nyquist sampling theorem. Recently, in the area of signal processing using compressed sensing, many measurement matrices as well as reconstruction algorithms have been proposed. It is necessary to explore the application of these methods in SPI. Therefore, this paper reviews the concept of compressive sensing SPI and summarizes the main measurement matrices and reconstruction algorithms in compressive sensing. Further, the performance of their applications in SPI through simulations and experiments is explored in detail, and then their advantages and disadvantages are summarized. Finally, the prospect of compressive sensing with SPI is discussed.

Polarization-Resolved THz Imaging With Orthogonal Heterostructure Backward Diode Detectors

A fully integrated lens-coupled dual-polarization detector for imaging was designed, fabricated, and characterized in the terahertz (THz) region. Two zero-bias heterostructure backward diodes (HBDs) were monolithically integrated into a planar dual-polarization annular-slot antenna. An impedance matching network consisting of an interdigitated DC block and shorted stubs was implemented for maximum power transfer. The fabricated chip was then mounted on an extended hemispherical silicon lens for high antenna efficiency. The performance of the integrated detector has been characterized at 200 GHz. The detector module can simultaneously measure the two orthogonal linear polarized components of the incident THz waves, and hence obtain the polarization direction. On the basis of this polarimetric detection, polarization-resolved imaging of a Gaussian beam, as well as birefringent samples, was performed using the integrated detectors. The angular resolution of the polarization detection has been demonstrated to be as small as 3°. The single-pixel detector demonstrated here is amenable to array applications for THz focal-plane arrays that will have a significant impact on a wide range of remote sensing, though-barrier imaging, and detection/identification applications.

Feature ghost imaging for color identification.

On the basis of computational ghost imaging (CGI), we present a new imaging technique, feature ghost imaging (FGI), which can convert the color information into distinguishable edge features in retrieved grayscale images. With the edge features extracted by different order operators, FGI can obtain the shape and the color information of objects simultaneously in a single-round detection using one single-pixel detector. The feature distinction of rainbow colors is presented in numerical simulations and the verification of FGI's practical performance is conducted in experiments. Furnishing a new perspective to the imaging of colored objects, our FGI extends the function and the application fields of traditional CGI while sustaining the simplicity of the experimental setup.

High-fidelity ghost diffraction through complex scattering media using a modified Gerchberg-Saxton algorithm.

In this paper, we propose a modified Gerchberg-Saxton (GS) algorithm to generate random amplitude-only patterns as information carriers in ghost diffraction. With the generated random patterns, high-fidelity ghost diffraction through complex scattering media can be realized with a single-pixel detector. The modified GS algorithm adopts a support constraint in the image plane, which is divided into a target region and a support region. In the Fourier plane, amplitude of the Fourier spectrum is scaled to regulate the sum of the image function. A random amplitude-only pattern can be generated to encode a pixel of the data to be transmitted using the modified GS algorithm. Optical experiments are conducted to verify the proposed method in complex scattering environments, e.g., dynamic and turbid water with non-line-of-sight (NLOS). Experimental results demonstrate that the proposed ghost diffraction is of high fidelity and high robustness against complex scattering media. It is expected that an avenue could be opened up for ghost diffraction and transmission in complex media.

Time Coding-Based Single-Photon-Counting Lidar for Obtaining Spatial Location

This paper proposes a single-photon-counting lidar based on time coding that can obtain the target’s spatial location and measure the distance and azimuth angle in real time without needing a scanning device. Multiple optical fibers were used to introduce laser echo photons into a single-pixel single-photon detector. According to the deviation in the detection time of the echo photons passing through different optical fibers, multiple distances can be obtained simultaneously. Combining the measured distances with the fiber spacing allows the calculation of the distance, azimuth angle, and spatial coordinates of the target. This lidar has the advantages of high photon detection efficiency, short signal acquisition time, and low cost compared to array detectors.

Image-free active autofocusing with dual modulation and its application to Fourier single-pixel imaging.

Autofocusing is widely used in applications where sharp image acquisition or projection is needed. Here we report an active autofocusing method for sharp image projection. The method works with wide-field structured illumination and single-pixel detection. To find the focus position, the method illuminates the target object with a set of 3-step phase-shifting Fourier basis patterns repeatedly and collects the backscattered light by using a single-pixel detector through a grating. Dual modulation-dynamic modulation by the time-varying structured illumination and static modulation by the grating-embeds the depth information for the target object in the resulting single-pixel measurements. As such, the focus position can be determined by recovering the Fourier coefficients from the single-pixel measurements and searching for the coefficient with the maximum magnitude. High-speed spatial light modulation not only enables rapid autofocusing but also makes the method work even when the lens system is in continuous motion or the focal length of the lens is continuously adjusted. We experimentally validate the reported method in a self-built digital projector and demonstrate the application of the method in Fourier single-pixel imaging.