Streak tube imaging lidar with kilohertz laser pulses and few-photons detection capability.

Lidar using active light illumination is capable of capturing depth and reflectivity information of target scenes. Among various technologies, streak tube imaging lidar (STIL) has garnered significant attention due to its high resolution and excellent precision. The echo signals of a STIL system using single laser pulse are often overwhelmed by noise in complex environments, making it difficult to discern the range of the target. By combining high-frequency laser pulses with the repetitive sweep circuit, the STIL system enables efficient detection of few-photons signal in weak-light environments. Additionally, we have developed a robust algorithm for estimating the depth and reflectivity images of targets. The results demonstrate that this lidar system achieves a depth resolution better than 0.5 mm and a ranging accuracy of 95 um. Furthermore, the imaging of natural scenes also validates the exceptional 3D imaging capability of this system.

Snapshot compressive imaging at 855 million frames per second for aluminium planar wire array Z-pinch.

This paper present a novel, integrated compressed ultrafast photography system for comprehensive measurement of the aluminium planar wire array Z-Pinch evolution process. The system incorporates a large array streak camera and embedded encoding to improve the signal-to-noise ratio. Based on the "QiangGuang-I" pulsed power facility, we recorded the complete continuous 2D implosion process of planar wire array Z-Pinch for the first time. Our results contribute valuable understanding of imploding plasma instabilities and offer direction for the optimization of Z-Pinch facilities.

Resolution enhancement via guided filtering for spatial-frequency multiplexing single-shot high-speed imaging.

The frequency recognition algorithm for multiple exposures (FRAME) is a progressive single-shot high-speed videography technique that employs the spatial-frequency multiplexing concept to provide high temporal and spatial resolution. However, the inherent crosstalk from the zero-frequency component to the carrier-frequency component leads to resolution degradation and artifacts. To improve recovered frames' quality, we propose a FRAME reconstruction method using guided filters for a removal of the zero-frequency component, which can minimize the artifacts while enhance spatial resolution. A total variation (TV) denoising operation is involved to remove artifacts further to achieve optimized performances. Simulations and experiments were conducted to demonstrate the robust and efficient post-processing capability of the proposed method. With a two-frame experimental system, the results of a USAF 1951 resolution target reveal a 1.8-fold improvement in spatial resolution from 16 lp/mm to 28.5 lp/mm. For complex dynamic scenarios, the wide field of high-speed fuel spray was shot and the proposed method can resolve two droplets with a 30 µm distance which outperforms the traditional method.

Design of multi-frequency point, high-isolation switches for micro-channel plate data acquisition.

In order to replace the phosphor screen of a proximity-gated x-ray framing camera with a readout circuit using a time-interleaved structure, this paper carries out the design of a high-isolation RF switch. In this paper, a Metal-Oxide-Semiconductor Field Effect Tube (MOSFET) switching circuit is designed to achieve high isolation and low insertion loss at 0.5-3 GHz. This solves the problem that the switching circuit cannot be turned off properly due to the parasitic capacitance of MOSFETs in the process of RF signal transmission, resulting in signal feedthrough. It also ensures that the input signal can be transmitted to the output intact when the switching circuit is turned on. High isolation is achieved by using parallel resonance to increase the voltage division and series resonance to leak the current. The switch achieves 76 dB isolation and 0.07 dB insertion loss at 1 GHz frequency. Isolation is increased by adding parallel branches near the 2 and 3 GHz frequency points, achieving greater than 33 dB isolation from 0.5 to 3 GHz.

Simulation study of an x-ray sub-picosecond resolution detection system based on time-domain amplification.

This study proposes what we believe to be a novel x-ray detection system that achieves a temporal resolution of 930 fs with photorefractive and four-wave mixing effects. The system comprises two parts: a signal-conversion system and signal-acquisition system. The signal-conversion system is based on the photorefractive effect, which converts x-ray evolution into the variation of infrared interference intensity. The signal-conversion sensor consists of ultra-fast response LT-GaAs and a high-resolution interference cavity, achieving a resolution of  767 fs. The signal-acquisition system consists of a time-domain amplification system based on four-wave mixing and a high-resolution signal-recording system with a resolution of 21 ps, providing a temporal resolution of 525 fs.

Theoretical study on time response of semiconductor photorefractive effects under subpicosecond ultra-fast X-rays.

A theoretical model that can efficiently calculate the refractive index response of semiconductors under ultrafast X-ray radiation is established based on the photorefractive effect of semiconductors. The proposed model is used to interpret X-ray diagnostics experiments, and the results are in good agreement with experiments. In the proposed model, a rate equation model of free carrier density calculation is adopted with the X-ray absorption cross-sections calculated by atomic codes. The two-temperature model is used to describe the electron-lattice equilibration and the extended Drude model is applied to calculate the transient refractive index change. It is found that faster time response can be achieved for semiconductors with shorter carrier lifetime and sub-picosecond resolution can be obtained for InP and [Formula: see text]. The material response time is not sensitive to X-ray energy and the diagnostics can be used in the 1-10 keV energy range. This article is part of the theme issue 'Dynamic and transient processes in warm dense matter'.

Investigation of single-shot high-speed photography based on spatial frequency multiplexing.

The frequency recognition algorithm for multiple exposures (FRAME) is a spatial frequency multiplexing method that enables high-speed videography with high spatial resolution across a wide field of view and high temporal resolution up to femtoseconds. The criterion to design encoded illumination pulses is an essential factor that affects the sequence depth and reconstruction accuracy of FRAME but was not previously discussed. When the spatial frequency is exceeded, the fringes on digital imaging sensors can become distorted. To exploit the Fourier domain for FRAME with deep sequences and avoid fringe distortion, the maximum Fourier map for sequence arrangement was determined to be a diamond shape. The maximum axial frequency should be a quarter of the sampling frequency of digital imaging sensors. Based on this criterion, the performances of reconstructed frames were theoretically investigated by considering arrangement and filtering methods. To ensure optimal and uniform interframe quality, the frames near the zero frequency should be removed and optimized super-Gaussian filters should be employed. Experiments were conducted flexibly with a digital mirror device to generate illumination fringes. Following these suggestions, the movement of a water drip dropping on a water surface was captured with 20 and 38 frames with uniform interframe quality. The results prove the effectiveness of the proposed methods to improve the reconstruction accuracy and promote the development of FRAME with deep sequences.

A large-format streak tube for compressed ultrafast photography.

Streak cameras are powerful imaging instruments for studying ultrafast dynamics with the temporal resolution ranging from picosecond to attosecond. However, the confined detection area limits the information capacity of streak cameras, preventing them from fulfilling their potential in lidar, compressed ultrafast photography, etc. Here, we designed and manufactured a large-format streak tube with a large-size round-aperture gate, a spherical cathode, and a spherical screen, leading to an expanded detection area and a high spatial resolution. The simulation results show that the physical temporal resolution of the streak tube is better than 45 ps and the spatial resolutions are higher than 14 lp/mm in the whole area of 24 × 28 mm2 on the cathode. The experiments demonstrate the streak tube's application potential in weak light imaging benefiting from the imaging magnification of 0.79, a photocathode radiance sensitivity of 37 mA/W, a radiant emitting gain of 11.6 at the wavelength of 500 nm, and a dynamic range higher than 512:1. Most importantly, in the photocathode area of Φ35 mm, the static spatial resolutions at the center and the edge along the slit (R = 16 mm) reach 32 and 28 lp/mm, respectively, and are higher than 10 lp/mm in the whole area of 24 × 28 mm2 on the cathode, allowing for a considerable capacity for spatial information.

A Novel Reconstruction Algorithm with High Performance for Compressed Ultrafast Imaging.

Compressed ultrafast photography (CUP) is a type of two-dimensional (2D) imaging technique to observe ultrafast processes. Intelligence reconstruction methods that influence the imaging quality are an essential part of a CUP system. However, existing reconstruction algorithms mostly rely on image priors and complex parameter spaces. Therefore, it usually takes a lot of time to obtain acceptable reconstruction results, which limits the practical application of the CUP. In this paper, we proposed a novel reconstruction algorithm named PnP-FFDNet, which can provide a high quality and high efficiency compared to previous methods. First, we built a forward model of the CUP and three sub-optimization problems were obtained using the alternating direction multiplier method (ADMM), and the closed-form solution of the first sub-optimization problem was derived. Secondly, inspired by the PnP-ADMM framework, we used an advanced denoising algorithm based on a neural network named FFDNet to solve the second sub-optimization problem. On the real CUP data, PSNR and SSIM are improved by an average of 3 dB and 0.06, respectively, compared with traditional algorithms. Both on the benchmark dataset and on the real CUP data, the proposed method reduces the running time by an average of about 96% over state-of-the-art algorithms, and show comparable visual results, but in a much shorter running time.

Large area MCP-PMT design with good time performance.

A new large area photomultiplier tube based on the microchannel plates (MCP-PMT) with high collection efficiency (CE) and good time performance is proposed in this paper. A novel focusing system with two cylindrical and a conical barrels is designed to generate the accelerating and focusing electric field. A three-dimensional model is developed by CST Studio Suite to validate its feasibility. Finite Integral Technique and Monte Carlo method are combined to simulate the process. Results predict that CE of the novel MCP-PMT is expected to be 100%. TTS of the photoelectrons from the whole photocathode achieves 1.2 ns. Differ from other large area PMTs, it performs well in the geomagnetic field.

Development of a large-field streak tube for underwater imaging lidar.

Streak tube imaging lidar (STIL) can obtain 4-D images of a target, and its performance is mainly determined by the streak tube sensor. To obtain a large field of view, we developed a streak tube with a photocathode length as large as 35.3 mm, which is larger than the commonly used ST-HDR (30 mm). At the same time, the temporal resolution and dynamic spatial resolution are 60 ps and 12 lp/mm, which are very suitable to obtain accurate target coordinates for 4-D imaging. In addition, the streak tube has a high detection sensitivity of 46 mA/W at 500 nm and, hence, prospects in remote imaging. To test the performance of the streak tube, an underwater STIL experiment was conducted. Echo signal processing was performed by means of a bandpass filter and a matched filter, and then the peak detection algorithm was used to reconstruct the image. The results indicate that a spatial resolution better than 9 mm is achieved in the limpid water with a depth of 20 m, and a range accuracy of 1 cm is achieved in the turbid water with a depth of 10 m. Such a performance suggests that the large-field streak tube is of great potential for underwater target imaging and other remote imaging applications.

Compressed fluorescence lifetime imaging via combined TV-based and deep priors.

Compressed fluorescence lifetime imaging (Compressed-FLIM) is a novel Snapshot compressive imaging (SCI) method for single-shot widefield FLIM. This approach has the advantages of high temporal resolution and deep frame sequences, allowing for the analysis of FLIM signals that follow complex decay models. However, the precision of Compressed-FLIM is limited by reconstruction algorithms. To improve the reconstruction accuracy of Compressed-FLIM in dealing with large-scale FLIM problem, we developed a more effective combined prior model 3DTGp V_net, based on the Plug and Play (PnP) framework. Extensive numerical simulations indicate the proposed method eliminates reconstruction artifacts caused by the Deep denoiser networks. Moreover, it improves the reconstructed accuracy by around 4dB (peak signal-to-noise ratio; PSNR) over the state-of-the-art TV+FFDNet in test data sets. We conducted the single-shot FLIM experiment with different Rhodamine reagents and the results show that in practice, the proposed algorithm has promising reconstruction performance and more negligible lifetime bias.

Optimization design of the large area Dynode-MCP-PMT.

The optimization work of a newly proposed 20-in. photomultiplier tube based on dynode and microchannel plates (Dynode-MCP-PMT) are conducted in this paper. Three-dimensional models are developed in CST STUDIO SUITE to systematically investigate the effects of the size and bias voltage of the two focusing electrodes, dynode and the glass envelop handle based on the Finite Integral Technique and Monte Carlo method. Results predict that the collection efficiency and the transit time spread of the optimized design are substantially improved which are 100% and 3.7 ns.

A High-Sensitivity Vacuum Diode Temperature Sensor Based on Barrier-Lowering Effect.

A new kind of temperature sensor based on a vacuum diode was proposed and numerically studied in this paper. This device operated under different electron emission mechanisms according to the electron density in the vacuum channel. The temperature determination ability of this device was only empowered when working in the electric-field-assisted thermionic emission regime (barrier-lowering effect). The simulated results indicated that the temperature-sensing range of this device was around 273 K-325 K with a supply current of 1 µA. To obtain a linear dependency of voltage on temperature, we designed a proportional-to-absolute-temperature (PTAT) circuit. The mathematic derivation of the PTAT voltage is presented in this study. The temperature-sensing sensitivity was calculated as 7.6 mV/K according to the measured I-U (current versus voltage) characteristic. The structure and principle of the device presented in this paper might provide an alternative method for the study of temperature sensors.

Histogram clustering for rapid time-domain fluorescence lifetime image analysis.

We propose a histogram clustering (HC) method to accelerate fluorescence lifetime imaging (FLIM) analysis in pixel-wise and global fitting modes. The proposed method's principle was demonstrated, and the combinations of HC with traditional FLIM analysis were explained. We assessed HC methods with both simulated and experimental datasets. The results reveal that HC not only increases analysis speed (up to 106 times) but also enhances lifetime estimation accuracy. Fast lifetime analysis strategies were suggested with execution times around or below 30 µs per histograms on MATLAB R2016a, 64-bit with the Intel Celeron CPU (2950M @ 2GHz).

Nanoscale Vacuum Diode Based on Thermionic Emission for High Temperature Operation.

Vacuum diodes, based on field emission mechanisms, demonstrate a superior performance in high-temperature operations compared to solid-state devices. However, when considering low operating voltage and continuous miniaturization, the cathode is usually made into a tip structure and the gap between cathode and anode is reduced to a nanoscale. This greatly increases the difficulty of preparation and makes it difficult to ensure fabrication consistency. Here, a metal-insulator-semiconductor (MIS) structural nanoscale vacuum diode, based on thermionic emission, was numerically studied. The results indicate that this device can operate at a stable level in a wide range of temperatures, at around 600 degrees Kelvin above 260 K at 0.2 V voltage bias. Moreover, unlike the conventional vacuum diodes working in field emission regime where the emission current is extremely sensitive to the gap-width between the cathode and the anode, the emission current of the proposed diode shows a weak correlation to the gap-width. These features make this diode a promising alternative to vacuum electronics for large-scale production and harsh environmental applications.

Theoretical investigations of a modified compressed ultrafast photography method suitable for single-shot fluorescence lifetime imaging.

A single-shot fluorescence lifetime imaging (FLIM) method based on the compressed ultrafast photography (CUP) is proposed, named space-restricted CUP (srCUP). srCUP is suitable for imaging objects moving slowly (<∼150/Mmm/s, M is the magnification of the objective lens) in the field of view with the intensity changing within nanoseconds in a measurement window around 10 ns. We used synthetic datasets to explore the performances of srCUP compared with CUP and TCUP (a variant of CUP). srCUP not only provides superior reconstruction performances, but its reconstruction speed is also twofold and threefold faster than CUP and TCUP, respectively. The lifetime determination performances were assessed by estimating lifetime components, amplitude- and intensity-weighted average lifetimes (τA and τI), with the reconstructed scenes using the least squares method based on a bi-exponential model. srCUP has the best accuracy and precision for lifetime determinations with a relative bias less than 7% and a coefficient of variation less than 7% for τA, and a relative bias less than 10% and a coefficient of variation less than 11% for τI.

Numerical study and improvement of the dynamic performance of dilation x-ray imager.

We present in this Note a numerical study on the dynamic performance of a Dilation X-ray Imager (DIXI). The DIXI including a photoelectron tube (PT) and a magnetic solenoid is modeled in 3D space. The initial parameters of the photoelectrons are sampled with a Monte Carlo code. The trajectories of the photoelectrons are calculated by using the particle-in-cell method, and the transit time spread (TTS) and temporal magnification are analyzed in detail. We have designed a PT with a double-microstrip structure and compared the performance of the double-microstrip PT with the traditional single-microstrip PT. The results show that the sensitivity of the TTS and the temporal magnification to the emission time of the photoelectrons can be significantly reduced by using the double-microstrip PT, resulting in an improvement of the time window. Therefore, the dynamic performance of the DIXI is improved.

Single-shot time-gated fluorescence lifetime imaging using three-frame images.

Qualitative and quantitative measurements of complex flows demand for fast single-shot fluorescence lifetime imaging (FLI) technology with high precision. A method, single-shot time-gated fluorescence lifetime imaging using three-frame images (TFI-TGFLI), is presented. To our knowledge, it is the first work to combine a three-gate rapid lifetime determination (RLD) scheme and a four-channel framing camera to achieve this goal. Different from previously proposed two-gate RLD schemes, TFI-TGFLI can provide a wider lifetime range 0.6 ~ 13ns with reasonable precision. The performances of the proposed approach have been examined by both Monte-Carlo simulations and toluene seeded gas mixing jet diagnosis experiments. The measured average lifetimes of the whole excited areas agree well with the results obtained by the streak camera, and they are 7.6ns (N2 = 7L/min; O2 < 0.1L/min) and 2.6ns (N2 = 19L/min; O2 = 1L/min) with the standard deviations of 1.7ns and 0.8ns among the lifetime image pixels, respectively. The concentration distributions of the quenchers and fluorescent species were further analyzed, and they are consistent with the experimental settings.

A compact large-format streak tube for imaging lidar.

The streak tubes with a large effective photocathode area, large effective phosphor screen area, and high photocathode radiant sensitivity are essential for improving the field of view, depth of field, and detectable range of the multiple-slit streak tube imaging lidar. In this paper, a high spatial resolution, large photocathode area, and compact meshless streak tube with a spherically curved cathode and screen is designed and tested. Its spatial resolution reaches 20 lp/mm over the entire Φ28 mm photocathode working area, and the simulated physical temporal resolution is better than 30 ps. The temporal distortion in our large-format streak tube, which is shown to be a non-negligible factor, has a minimum value as the radius of curvature of the photocathode varies. Furthermore, the photocathode radiant sensitivity and radiant power gain reach 41 mA/W and 18.4 at the wavelength of 550 nm, respectively. Most importantly, the external dimensions of our streak tube are no more than Φ60 mm × 110 mm.