Spatially varying defocus map estimation from a single image based on spatial aliasing sampling method.

In current optical systems, defocus blur is inevitable due to the constrained depth of field. However, it is difficult to accurately identify the defocus amount at each pixel position as the point spread function changes spatially. In this paper, we introduce a histogram-invariant spatial aliasing sampling method for reconstructing all-in-focus images, which addresses the challenge of insufficient pixel-level annotated samples, and subsequently introduces a high-resolution network for estimating spatially varying defocus maps from a single image. The accuracy of the proposed method is evaluated on various synthetic and real data. The experimental results demonstrate that our proposed model outperforms state-of-the-art methods for defocus map estimation significantly.

Water-Based Coherent Detection of Broadband Terahertz Pulses.

Both solids and gases have been demonstrated as the materials for terahertz (THz) coherent detection. The gas-based coherent detection methods require a high-energy probe laser beam and the detection bandwidth is limited in the solid-based methods. Whether liquids can be used for THz detection and relax these problems has not yet been reported, which becomes a timely and interesting topic due to the recent observation of efficient THz wave generation in liquids. Here, we propose a THz coherent detection scheme based on liquid water. When a THz pulse and a fundamental laser beam are mixed on a free-flowing water film, a second harmonic (SH) beam is generated as the plasma is formed. Combining this THz-induced SH beam with a control SH beam, we successfully achieve the time-resolved waveform of the THz field with the frequency range of 0.1-18 THz. The required probe laser energy is as low as a few microjoules. The sensitivity of our scheme is 1 order of magnitude higher than that of the air-based method under comparable detection conditions. The scheme is sensitive to the THz polarization and the phase difference between the fundamental and control SH beams, which brings direct routes for optimization and polarization sensitive detection. Energy scaling and polarization properties of the THz-induced beam indicate that its generation can be attributed to a four-wave mixing process. This generation mechanism makes simple relationships among the probe laser, THz-induced SH, and THz field, favorable for robustness and flexibility of the detection device.

Nondestructive testing and 3D imaging of PE pipes using terahertz frequency-modulated continuous wave.

Polyethylene (PE) pipes are widely used as the main carrier for the transportation of natural gas, so nondestructive testing techniques for PE pipes are essential for the safety of natural gas transportation. In order to compensate for the shortcomings of conventional inspection methods, a terahertz (THz) three-dimensional imaging system for nondestructive inspection of PE pipes is designed. The system is based on frequency-modulated continuous-wave (FMCW) technology, with a THz source bandwidth of 0.225-0.330 THz and an output power of over 5 mW, which can achieve submillimeter spatial resolution in three dimensions. The system is used to scan PE pipes in three dimensions in a laboratory environment, and the results show that the system could achieve nondestructive testing and three-dimensional imaging of different defects in PE pipes. In addition, combined with the deep-learning-based THz transformer network, the intelligent identification of different defects is realized, and the accuracy rate can reach up to 88%. The above results provide technical guidance for the application of THz FMCW systems in the actual detection of PE pipes, and provide supplements and improvements for traditional detection methods.

MTF improvement for optical synthetic aperture system via mid-frequency compensation.

Optical synthetic aperture imaging system has grown out the quest for higher angular resolution in astronomy, which combines the radiation from several small sub-apertures to obtain a resolution equivalent to that of a single filled aperture. Due to the discrete distribution of the sub-apertures, pupil function is no longer a connected domain, which further leads to the attenuation or loss of the mid-frequency modulation transfer function (MTF). The mid-frequency MTF compensation is therefore a key focus. In this paper, a complete mid-frequency compensation algorithm is proposed, which can extract and fuse the frequency of different synthetic aperture systems and monolithic aperture systems according to their special MTF characteristics. The dimensions of the monolithic aperture and optical synthetic aperture system are derived, and the longest baseline of the monolithic aperture is much smaller than that of the optical synthetic aperture system. Then the separated spatial frequency information is extracted and synthesized according to the spatial frequency equivalence point. Finally, the full-frequency enhanced image is recovered by using improved Wiener-Helstrom filter, which adopts specific parameters based on different sub-aperture arrangements. The mid-frequency MTF of Golay-3 increases from 0.12 to 0.16 and that of Golay-6 increases from 0.06 to 0.18. Both the simulation and experiment prove that the proposed method not only realizes the spatial resolution determined by the longest baseline of the optical synthetic aperture system, but also successfully compensates its mid-frequency MTF.

Molecular dynamic investigation of ethanol-water mixture by terahertz-induced Kerr effect.

The terahertz Kerr effect (TKE) spectroscopy provides time-resolved measurement of low-frequency molecular motions of liquids. Here, the intense broadband terahertz (THz) pulses resonantly excite multiple molecular modes in pure ethanol and ethanol-water mixtures. For pure ethanol, the obtained unipolar TKE response contains the molecular relaxation information extending over tens of picoseconds, which originates from the coupling between the permanent molecular dipole moment of ethanol and the THz electric field. For ethanol-water mixtures with different molar proportions, the results observed on the sub-picosecond time scale can always be divided into the linear superposition of the TKE signals of pure ethanol and water. Under the observation time window over tens of picoseconds (after 1 picosecond), the relative molecular contribution of ethanol in the mixture changes nonlinearly with the increase of water molecules, implying the complex structural perturbation of ethanol hydrogen bond network in the mixture. This work provides a new perspective for further investigation on the hydrogen bond network structure and dynamics in aqueous amphiphilic solutions.

Anion-water hydrogen bond vibration revealed by the terahertz Kerr effect.

The microscopic mechanism for ionic influence on the hydrogen bond network of water has not been fully understood. Here we employ the terahertz Kerr effect (TKE) technique to map the intermolecular hydrogen bond dynamics in a series of aqueous halide solutions at the sub-picosecond scale. Compared with pure water, the significantly enhanced bipolar TKE response associated with polarization anisotropy in an ionic aqueous solution is successfully captured. We decompose the measured TKE response into different molecular motion modes and demonstrate that the obviously increasing positive polarity response is mainly due to the anion-water hydrogen bond vibration mode with the resonant THz electric field excitation. Our measurement results provide an experimental basis for further insight into the effects of ions on the structure and dynamics of a hydrogen bond in water.

Spectral polarization camera based on ghost imaging via sparsity constraints.

A spectral polarization camera based on ghost imaging via sparsity constraints (GISC) is presented. The proposed imager modulates three-dimensional spatial and spectral information of the target into two-dimensional speckle patterns using a spatial random phase modulator and then acquires the speckle patterns at four linear polarization channels through a polarized CCD. The experimental results verify the feasibility of the system structure and reconstruction algorithm. The GISC spectral polarization camera, which has a simple structure and achieves compressive sampling during the imaging acquisition process, provides a simple scheme for obtaining multi-dimensional information of the light field.

Optimized Golay-9 array configurations for mid-frequency compensation in optical sparse aperture systems.

Optical sparse aperture (OSA) imaging systems show great potential to generate higher resolution images than those of equivalent single filled aperture systems. However, due to the sparsity and dispersion of sparse aperture arrays, pupil function is no longer a connected domain, which further attenuates or loses the mid-frequency modulation transfer function (MTF), resulting in lower mid-frequency contrast and blurred images. Therefore, an improved traversal algorithm is proposed to optimize Golay-9 array configurations for compensating the mid-frequency MTF. Its structural parameters include diameters of sub-apertures, relative rotation angles between individual sub-apertures, and radius of concentric circles. Then, these parameters are traversed successively in order. Finally, the influences of the obtained optimized array configurations on the mid-frequency MTF are analyzed in detail, and the image performances are evaluated. The experimental results prove the contrast enhancement. Compared with a Golay-9 array at F=36.5%, the maximum MTF increases from 0.1503 to 0.307, and the mid-frequency MTF is boosted from 0.0565 to 0.0767. In addition, the peak signal to noise ratio of the degraded image is promoted from 19.75 dB to 20.63 dB. Both quantitative and qualitative evaluations demonstrate the validity of the proposed method.

Reconfigurable optical time delay array for 3D lidar scene projector.

3D lidar scene projector (LSP) plays an important role in the hardware-in-the-loop (HIL) simulation for autonomous driving system (ADS). It generates a simulated 3D lidar scene in laboratory by generating a 2D array of optical time delay signals. The reconfigurable optical time delay array (ROTDA) is crucial for LSP. However, current ROTDA solutions cannot support a LSP with a spatial resolution more than 10×10. In this paper, we proposed a novel ROTDA design based on the time slicing method. The optical signals with the same time delay but different spatial coordinates were treated as one time slice. Different time slices were superimposed into a composite image by a microlens-array-based imaging system to obtain a 3D lidar scene. And a spatial light modulator (SLM) was utilized to configure the time delay of each lidar scene pixel. We developed a ROTDA prototype with 64×64 pixels, each pixel can be reconfigured with up to 180 different time delays in one frame. The time delay resolution is 1 ns, the maximum time delay is 5000 s, and the 3D frame rate is 20Hz. The prototype can generate a continuous lidar scene with a distance span of 27 m, and can also generate up to 8 short scenes that are separated from each other along the lidar observation direction, each short scene covers a distance span of 3 m or 3.75 m. The design method proposed in this paper can also be applied to other occasions that demand a large number of time delay generators.

Ultrafast optical pulse polarization modulation based on the terahertz-induced Kerr effect in low-density polyethylene.

Controlling the polarization state of an optical pulse within a short gating time facilitates ultrafast all-optical data processing and recording. Using the innovative all-optical modulation method such as the transient terahertz Kerr effect (TKE), the polarization state of the optical pulse can be switched within the gating time on the sub-picosecond scale. In this work, we use high-frequency single-cycle terahertz (THz) pulses to excite the Kerr effects of materials and explore the potential to shorten the gating time of the polarization modulator. A low-density polyethylene (LDPE) material with good Kerr-related properties is proposed to improve the performance of the TKE-based modulator and the obtained ultrafast gating time (FWHM) can reach 86 fs. Experimental evidence for the thickness dependence of the Kerr response demonstrates that the errors caused by optical transmission factors in the LDPE medium can be ignored, and thus the ultrafast gating modulation is mainly limited by the duration of probe pulse. Compared with common TKE-based materials, we believe that the low-cost LDPE is a good candidate to achieve high-power TKE-based ultrafast pulse switching.

Enhancement of terahertz waves from two-color laser-field induced air plasma excited using a third-color femtosecond laser.

This study experimentally demonstrates and theoretically analyzes the enhancement of terahertz (THz) waves from two-color laser-field (consisting of a near-infrared femtosecond laser and its second-harmonic wave) induced air plasma using an additional 800 nm femtosecond laser. The experiments revealed that the additional 800 nm laser increased the THz energy up to 22 times. To understand the enhancement mechanism and reveal the maximum enhancement conditions, the effects of the 800 nm beam's polarization and energy variations of both beams on the THz amplification were studied. With the increase in the 800 nm pulse energy, the THz yield initially increases, and then decreases after reaching an inflection point. The THz increase rate continues to increase with the decrease in energy of the near-infrared two-color fields. The 800 nm beam could efficiently modulate the THz spectral energy distribution by increasing the high-frequency components, while decreasing the low-frequency components.

Image restoration for synthetic aperture systems with a non-blind deconvolution algorithm via a deep convolutional neural network.

Optical synthetic aperture imaging systems, which consist of in-phase circular sub-mirrors, can greatly improve the spatial resolution of a space telescope. Due to the sub-mirrors' dispersion and sparsity, the modulation transfer function is decreased significantly compared to a fully filled aperture system, which causes obvious blurring and loss of contrast in the collected image. Image restoration is the key to get the ideal clear image. In this paper, an appropriative non-blind deconvolution algorithm for image restoration of optical synthetic aperture systems is proposed. A synthetic aperture convolutional neural network (CNN) is trained as a denoiser prior to restoring the image. By improving the half-quadratic splitting algorithm, the image restoration process is divided into two subproblems: deconvolution and denoising. The CNN is able to remove noise in the gradient domain and the learned gradients are then used to guide the image deconvolution step. Compared with several conventional algorithms, scores of evaluation indexes of the proposed method are the highest. When the signal to noise ratio is 40 dB, the average peak signal to noise ratio is raised from 23.7 dB of the degraded images to 30.8 dB of the restored images. The structural similarity index of the results is increased from 0.78 to 0.93. Both quantitative and qualitative evaluations demonstrate that the proposed method is effective.

Object-independent piston diagnosing approach for segmented optical mirrors via deep convolutional neural network.

Piston diagnosing approaches based on neural networks have shown great success, while a few methods are heavily dependent on the imaging target of the optical system. In addition, they are inevitably faced with the interference of submirrors. Therefore, a unique object-independent feature image is used to form an original kind of data set. Besides, an extremely deep image-based convolutional neural network (CNN) of 18 layers is constructed. Furthermore, 9600 images are generated as a data set for each submirror with a special measure of sensitive area extracting. The diversity of results among all the submirrors is also analyzed to ensure generalization ability. Finally, the average root mean square error of six submirrors between the real piston values and the predicted values is approximately 0.0622λ. Our approach has the following characteristics: (1) the data sets are object-independent and contain more effective details, which behave comparatively better in CNN training; (2) the complex network is deep enough and only a limited number of images are required; (3) the method can be applied to the piston diagnosing of segmented mirror to overcome the difficulty brought by the interference of submirrors. Our method does not require special hardware, and is fast to be used at any time, which may be widely applied in piston diagnosing of segmented mirrors.

Breadth-first piston diagnosing approach for segmented mirrors through supervised learning of multiple-wavelength images.

Piston diagnosing approaches for segmented mirrors via machine-learning have shown great success. However, they are inevitably challenged with 2π ambiguity, and the accuracy is usually influenced by the location and number of submirrors. A piston diagnosing approach for segmented mirrors, which employs the breadth-first search (BFS) algorithm and supervised learning strategies of multi-wavelength images, is investigated. An original kind of object-independent and normalized dataset is generated by the in-focal and defocused images at different wavelengths. Additionally, the segmented mirrors are divided into several sub-models of binary tree and are traversed through the BFS algorithm. Furthermore, two deep image-based convolutional neural networks are constructed for predicting the ranges and values of piston aberrations. Finally, simulations are performed, and the accuracy is independent of the location and number of submirrors. The Pearson correlation coefficients for test sets are above 0.99, and the average root mean square error of segmented mirrors is approximately 0.01λ. This technique allows the piston error between segmented mirrors to be measured without 2π ambiguity. Moreover, it can be used for data collected by a real setup. Furthermore, it can be applied to segmented mirrors with different numbers of submirrors based on the sub-model of a binary tree.

High-performance solution-processed colloidal quantum dots-based tandem broadband photodetectors with dielectric interlayer.

Recently, great attention has been paid to IV-VI colloidal quantum dots (CQDs) for their high photosensitivity, solution processability and low cost. Also, metal halide perovskites are very promising materials to realize the high-performance solution-processed visible-light photodetectors due to their cost-effective manufacturing, tunable absorption and photoluminescence in whole visible spectrum. In this paper, we present solution-processed CQDs-based tandem broadband photodetectors with low dark-current and high-sensitivity by inserting dielectric Polymethyl methacrylate (PMMA) interlayer between two sub-detectors. Our experimental data showed that the tandem broadband photodetector ITO/PEDOT:PSS/CsPbBr3:PbS0.4Se0.6/ZnO/PVK/CsPbBr3:PbS0.4Se0.6/ZnO/Au showed a maximum specific detectivity of 6.8 × 1013 Jones with a responsivity of 27 A W-1 under 57.8 µW cm-2 980 nm illumination. The device performance can be further enhanced by inserting a 50 nm dielectric PMMA layer between the two sub-photodetectors. As the result, the tandem photodetector ITO/PEDOT:PSS/CsPbBr3:PbS0.4Se0.6/ZnO/PMMA(50 nm)/PVK/CsPbBr3:PbS0.4Se0.6/ZnO/Au exhibits a maximum specific detectivity of 1.32 × 1014 Jones with a responsivity of 27.72 A W-1 under 57.8 µW cm-2 of 980 nm laser. Further, the physical mechanisms for the enhanced performance are discussed in detail.

Edge detection for optical synthetic apertures based on conditional generative adversarial networks.

Detecting the interference fringes of the optical synthetic aperture is the core in preventing misalignments of the sub-mirrors in piston, tip, and tilt. These fringes are characterized as follows: (1) the edge information of sub-mirrors is accompanied by complex shapes and large gaps; and (2) the traditional edge detection algorithms have different optimal thresholds under different interference fringes, and they may lose boundary information. To address these problems, a novel method for detecting the edge of synthetic aperture fringe images is proposed. Because conditional generative adversarial networks avoid the difficulty of designing the loss function for specific tasks, they are suitable for our project. We trained over 8000 images based on real images and simulated images. Experiments prove that the proposed method can reduce the false detection rate to 0.2, compared with 0.56 by Canny algorithm. This method can also directly detect the fringe edge of the optical synthetic aperture systems, which are accompanied by varied shapes and a growing number of sub-mirrors. When the input images lose boundary information, the traditional algorithm does not restore the boundary, but the proposed method makes a decision globally, and thus it guesses and then fills the boundary. The maximum error of the generated boundary and the actual boundary is two pixels.

Increasing aperture and depth of field simultaneously with wavefront coding technology.

We present a method that simultaneously increases the aperture and depth of field (DOF). A novel, to the best of our knowledge, method called lens-combined wavefront coding (WFC) is proposed for optical design. By rationally balancing rather than minimizing the aberrations, the DOF enhances instead of reduces with expansion of the aperture size. Two optical systems by traditional design and lens-combined WFC are designed to demonstrate our concept. Experiments are conducted using the manufactured lenses to show the extension of the DOF both in the laboratory and the real scene. A spatially invariant deconvolution algorithm is exploited to further suppress the aberrations regarding the field of view. The results show that the aperture and the DOF can be successfully enhanced at the same time.

Rotating asymmetrical phase mask method for improving signal-to-noise ratio in wavefront coding systems.

We present a method that combines an asymmetrical phase mask (PM) and its rotated PM to potentially improve the signal-to-noise ratio in wavefront coding systems. The property of the rotated PM is analyzed. A complementary promotional synthetic optical transfer function is generated as the deconvolution filter for image recovery. The image artifacts are suppressed through processing in the frequency domain. Simulation results provide a quantitative analysis that is based on a cubic PM. Experimental results show that our method can improve image quality over a wide range of defocus.

Composite multiscale entropy analysis of reflective terahertz signals for biological tissues.

We demonstrate a composite multiscale entropy (CMSE) method of terahertz (THz) signal complexity analysis to distinguish different biological tissues. The THz signals reflected from fresh porcine skin and muscle tissues were measured and analyzed. The statistically significant difference and separation of the two tissues based on several parameters were analyzed and compared for THz spectroscopy and imaging, which verified the better performance of the CMSE method and further enhancement of the contrast among THz signals that interact with different tissues. This process provides a better analysis and discrimination method for THz spectroscopy and imaging in biomedical applications.

Diode-based microbolometer with performance enhanced by broadband metamaterial absorber.

This Letter reports a microbolometer integrated with a broadband metamaterial absorber (MMA) to enhance its performance, which contains series-connected silicon diodes as the temperature sensor. The broadband MMA is readily integrated into the device by introducing an array of different-sized square resonators on the silicon nitride structural layer, while the widened titanium interconnecting wires between individual diodes serve as the ground plane. In a comparative experiment, the broadband MMA was demonstrated to be superior to the ordinary silicon nitride absorber in a broad spectra range, especially in a long-wavelength IR regime, which directly leads to an increase in IR responsivity by 60%. More importantly, this enhancement in responsivity was achieved with no sacrifice of the response time due to the negligible thermal mass of the introduced resonator array.