Enhanced nonlinear effects in femtosecond laser-inscribed depressed cladding waveguides in sapphire crystals.

High-performance depressed cladding waveguides can be fabricated in crystals using ultrafast laser inscription. The investigation of nonlinear phenomena, which manifest during the transmission of high peak power femtosecond pulses within waveguides, holds significant importance for their practical integration into waveguide lasers and waveguide-based components, among other pioneering applications. In this study, the depressed cladding waveguides were successfully prepared in sapphire crystal by a femtosecond laser. The nonlinear phenomena occurring in this waveguide were investigated. The experimental results show that the nonlinearity in the depressed cladding waveguides is significantly enhanced compared with that of the bulk. This enhancement notably manifests through augmented nonlinear losses (NLs) and the third harmonic (TH) blueshift increase. Meanwhile, we theoretically investigate the influence of nonlinear effects on the TH, such as self-phase modulation (SPM), cross-phase modulation (XPM), and group delay. Our results reveal that the phase mismatch between the TH and the pump pulses is the main reason for the asymmetric broadening and blueshift of the TH spectrum. Our study reveals the unique nonlinear properties of the waveguides and lays the foundation for further relevant studies and applications of such waveguides.

Detection of infrared small target based on background subtraction local contrast measure and Gaussian structural similarity.

Infrared (IR) small target detection, especially in a complex background, continues to present challenges in the low false alarm rate and high robustness. In this paper, a background subtraction local contrast measure (BSLCM) and Gaussian structural similarity (GSS) integrated structural model is proposed to detect IR small target. In the proposed model, BSLCM is used to suppress the complex background and enhance the target. GSS calculation is conducted to eliminate the high-brightened background residual and noise further. To evaluate the performance of the proposed method, real IR sequences and seven state-of-the-art (SOTA) methods were adopted. The results demonstrated that the BSLCM can suppress all types of strong background clutter and enhance the true target effectively.

Generalized central slice theorem perspective on Fourier-transform spectral imaging at a sub-Nyquist sampling rate.

Fourier-transform spectral imaging captures frequency-resolved images with high spectral resolution, broad spectral range, high photon flux, and low stray light. In this technique, spectral information is resolved by taking Fourier transformation of the interference signals of two copies of the incident light at different time delays. The time delay should be scanned at a high sampling rate beyond the Nyquist limit to avoid aliasing, at the price of low measurement efficiency and stringent requirements on motion control for time delay scan. Here we propose, what we believe to be, a new perspective on Fourier-transform spectral imaging based on a generalized central slice theorem analogous to computerized tomography, using an angularly dispersive optics decouples measurements of the spectral envelope and the central frequency. Thus, as the central frequency is directly determined by the angular dispersion, the smooth spectral-spatial intensity envelope is reconstructed from interferograms measured at a sub-Nyquist time delay sampling rate. This perspective enables high-efficiency hyperspectral imaging and even spatiotemporal optical field characterization of femtosecond laser pulses without a loss of spectral and spatial resolutions.

Classification of Coronavirus Spike Proteins by Deep-Learning-Based Raman Spectroscopy and its Interpretative Analysis.

The outbreak of COVID-19 has spread worldwide, causing great damage to the global economy. Raman spectroscopy is expected to become a rapid and accurate method for the detection of coronavirus. A classification method of coronavirus spike proteins by Raman spectroscopy based on deep learning was implemented. A Raman spectra dataset of the spike proteins of five coronaviruses (including MERS-CoV, SARS-CoV, SARS-CoV-2, HCoVHKU1, and HCoV-OC43) was generated to establish the neural network model for classification. Even for rapidly acquired spectra with a low signal-to-noise ratio, the average accuracy exceeded 97%. An interpretive analysis of the classification results of the neural network was performed, which indicated that the differences in spectral characteristics captured by the neural network were consistent with the experimental analysis. The interpretative analysis method provided a valuable reference for identifying complex Raman spectra using deep-learning techniques. Our approach exhibited the potential to be applied in clinical practice to identify COVID-19 and other coronaviruses, and it can also be applied to other identification problems such as the identification of viruses or chemical agents, as well as in industrial areas such as oil and gas exploration.

Design of a compact off-axis freeform three-mirror system in a circular configuration.

A compact off-axis three-mirror system, especially with a wide field of view and small f-number, continues to present challenges in the optical design field. To design a compact off-axis three-mirror system based on a freeform surface, an optimization method with a circular configuration and four parameters is proposed. In the proposed method, the f-number and angles of mirrors are combined to optimize and achieve high-quality imaging, which means the modulation transfer function is close to the diffraction limit. To evaluate the performance of the proposed method, four design examples with different fields of view (4∘×4∘, 6∘×6∘) and f-numbers (2, 1.75) were created. The results showed that the compact off-axis three-mirror system based on a freeform surface can be designed with a wide field of view, small f-number, and high-quality imaging using the proposed method. Unlike other design methods, the surface parameters of mirrors are obtained with only one calculation and are close to the final optimization results, which saves both time and resources for optimization. The findings indicate that the method is accurate and effective for designing compact off-axis three-mirror systems with freeform surfaces.

Single-shot compressed optical field topography.

Femtosecond lasers are powerful in studying matter's ultrafast dynamics within femtosecond to attosecond time scales. Drawing a three-dimensional (3D) topological map of the optical field of a femtosecond laser pulse including its spatiotemporal amplitude and phase distributions, allows one to predict and understand the underlying physics of light interaction with matter, whose spatially resolved transient dielectric function experiences ultrafast evolution. However, such a task is technically challenging for two reasons: first, one has to capture in single-shot and squeeze the 3D information of an optical field profile into a two-dimensional (2D) detector; second, typical detectors are only sensitive to intensity or amplitude information rather than phase. Here we have demonstrated compressed optical field topography (COFT) drawing a 3D map for an ultrafast optical field in single-shot, by combining the coded aperture snapshot spectral imaging (CASSI) technique with a global 3D phase retrieval procedure. COFT can, in single-shot, fully characterize the spatiotemporal coupling of a femtosecond laser pulse, and live stream the light-speed propagation of an air plasma ionization front, unveiling its potential applications in ultrafast sciences.

Selective generation of narrow-band harmonics by a relativistic laser pulse interaction with a detuned plasma grating.

The spectral characteristics of high-order harmonics generated by the interaction of a linearly polarized relativistic laser pulse with a plasma grating target are investigated. Through particle-in-cell simulations and an analytical model, it is shown that a plasma grating target with periodic structure can select special harmonics with integer multiples of the grating frequency, and that low-order harmonics with frequencies being integer times of the laser frequency are generated nearly parallel to the target surface from a Fresnel zone plate target with an aperiodic structure. Spectral control of the harmonics can be achieved by introducing a correction factor ß to the radius formula of the Fresnel zone plate, which can create a slightly detuned plasma grating, and then only the narrow-band harmonics can be selected nearly parallel to the target surface. The center order of the narrow-band harmonics can be tuned by adjusting the correction factor ß, while the bandwidth of the harmonics can be selected by adjusting the other parameter λ_{f} of the detuned plasma grating.

Cooled infrared off-axis freeform three-mirror system design with convenience in testing and assembly.

A cooled infrared off-axis freeform three-mirror system, especially with a small f-number and 100% cold stop efficiency, continues to present challenges in design, testing, and assembly. A primary mirror and tertiary mirror integrated structural model is proposed to design a cooled infrared off-axis three-mirror system that is convenient to test and assemble. In the proposed model, the f-number is used to calculate the parameters of the mirrors, and a freeform surface is adopted to achieve high imaging quality, which means the spot diagram radius (RMS) is smaller than the Airy radius. To evaluate the performance of the proposed method, two design examples with different f-numbers (2, 1.4) were constructed. The results showed that the cooled infrared off-axis three-mirror system can be designed with a small f-number, 100% cold stop efficiency, and high convenience in testing and assembly.

Mid-wave infrared polarization detection of ship targets for small temperature difference conditions.

Ship detection under small temperature difference conditions is an important research direction for infrared (IR) detection of typical targets at present. To solve the problems of low contrast and difficult recognition of ship IR imaging due to small temperature differences, the degree of polarization (DOP) images is applied to the field of low-temperature aberration imaging based on the polarization principle. Meanwhile, the misalignment problem caused by the lens jitter in the polarization calculation is solved by the proposed mutual information iterative algorithm. We demonstrate improvement in the target/background local contrast of low-temperature aberration imaging by using the difference in polarization characteristics between the target and the background. The effectiveness of the method was verified by experiments. The results show that the contrast of DOP images combined with multi-angle polarization information is about 30 times higher indoors and three times higher outdoors than IR intensity images. Therefore, the IR polarization detection technique based on DOP images can effectively deal with the problem of low imaging contrast caused by small temperature differences.

Measuring fluence distribution of intense short laser based on the radiochromic effect.

This Letter demonstrates a novel, to the best of our knowledge, method to measure the fluence distribution of an intense short laser pulse based on the radiochromic effect. We discovered that an intense short laser pulse can induce the color reaction with a radiochromic film (RCF). Further, the net optical density of an irradiated RCF is proportional to the fluence of the incident laser pulse in a large range (${2 {-} 120}\;{{{\rm mJ}/{\rm cm}}^2}$). This method supports a large detection area up to near square-meter scale by splicing multi-pieces of RCFs (${8} \times 10\;{{\rm inch}^2}$ each). The spatial resolution reaches as high as 60 lines/mm. It offers a thin-film (${\sim}{100}\;{\unicode{x00B5}{\rm m}}$ thick), flexible, vacuum-compatible solution to intense short laser measurements, especially to laser facilities above petawatt, with beam sizes up to near square-meter scale, e.g., extreme light infrastructure.

Off-axis eight-pass neodymium glass laser amplifier with high efficiency and excellent energy stability.

A new relay-imaged off-axial eight-pass laser amplifier with several joules energy and 1 Hz repetition rate was demonstrated. The extraction efficiency and pulse-to-pulse energy stability were greatly improved. Under the single-pass small-signal gain of 3.6, a net gain of 900 and an effective extraction efficiency of 42.4% in the beam aperture were realized. Pulse-to-pulse energy stability of 0.83% (peak-valley) and 0.17% (root-mean-square) was achieved by the significant saturation of eight-pass amplification, which, to the best of our knowledge, is the best energy stability in several-joules-class amplifiers. The far-field quality was 2.52 times the diffraction limit, and the near-field modulation of the 90% beam aperture was 1.28. No parasitic oscillations or pencil beams were observed. Moreover, another key feature of the proposed amplifier was the ability to remarkably improve the pulse contrast with a unique design.

Beam alignment based on two-dimensional power spectral density of a near-field image.

Beam alignment is crucial to high-power laser facilities and is used to adjust the laser beams quickly and accurately to meet stringent requirements of pointing and centering. In this paper, a novel alignment method is presented, which employs data processing of the two-dimensional power spectral density (2D-PSD) for a near-field image and resolves the beam pointing error relative to the spatial filter pinhole directly. Combining this with a near-field fiducial mark, the operation of beam alignment is achieved. It is experimentally demonstrated that this scheme realizes a far-field alignment precision of approximately 3% of the pinhole size. This scheme adopts only one near-field camera to construct the alignment system, which provides a simple, efficient, and low-cost way to align lasers.

Multi-petawatt laser facility fully based on optical parametric chirped-pulse amplification.

We report on a multi-petawatt 3-cascaded all-optical parametric chirped-pulse amplification laser facility. The experimental results demonstrate that the maximum energy after the final amplifier and after the compressor is 168.7 J and 91.1 J, respectively. The pulse width (FWHM) is 18.6 fs in full width at half maximum after optimization of pulse compression. Therefore, 4.9 PW peak power has been achieved for the laser facility. To the best of our knowledge, this is the highest peak power reported so far for an all-optical parametric chirped-pulse amplification facility, and a compressed pulse shorter than 20 fs is achieved in a PW-class laser facility for the first time.

Design and optimization of the seed pulse generation scheme for precise beam synchronization in the Xingguang-III laser.

Xingguang-III is a large hybrid optical parametric amplification/chirped pulse amplification laser that outputs synchronized femtosecond, picosecond, and nanosecond beams with three different wavelengths, i.e., 800, 1053, and 527 nm, respectively. We present here an in-house front-end system design that generates the 1053 nm seed laser for the ps/ns beam directly from the 800 nm femtosecond laser, which enables the three beams to have intrinsic synchronization. The results show that a seed laser of 140 µJ energy and 1053 nm central wavelength with 34 nm spectral width (FWHM) is generated and the shot-to-shot timing jitter (peak-to-valley) between the fs and the ps beam is less than 1.32 ps.

First Investigation on the Radiation Field of the Spherical Hohlraum.

The first spherical hohlraum energetics experiment is accomplished on the SGIII-prototype laser facility. In the experiment, the radiation temperature is measured by using an array of flat-response x-ray detectors (FXRDs) through a laser entrance hole at four different angles. The radiation temperature and M-band fraction inside the hohlraum are determined by the shock wave technique. The experimental observations indicate that the radiation temperatures measured by the FXRDs depend on the observation angles and are related to the view field. According to the experimental results, the conversion efficiency of the vacuum spherical hohlraum is in the range from 60% to 80%. Although this conversion efficiency is less than the conversion efficiency of the near vacuum hohlraum on the National Ignition Facility, it is consistent with that of the cylindrical hohlraums used on the NOVA and the SGIII-prototype at the same energy scale.

Coherent combination of femtosecond pulses via non-collinear cross-correlation and far-field distribution.

We propose and demonstrate a method for active parallel coherent combinations of ultra-short laser pulses. The relative synchronization error, piston, and tilt between combined beams are controlled based on the non-collinear cross-correlation and the far-field distribution. In a proof-of-principle experiment, two 29.8 fs pulses are coherently combined with the combining efficiency as high as 99%. This study opens up a way to further scale the peak intensity and provides support for the next-generation ultra-intense laser systems.

Experimental research on the influences of smoothing by spectral dispersion on the Technical Integration Line.

We describe one-dimensional smoothing by spectral dispersion (SSD) in high fluence on the Technical Integration Line. The experimental results indicate that SSD did not influence the load capacity of the laser facility. The near- and far-field analysis prove that adopting SSD could smooth the high-frequency modulations in the near field and dramatically suppress 10 µm-100 µm spatial modulations in the far field. The focal spot contrast decreases from 2.58 to 0.80 after using SSD and adaptive optics. Adopting a 0.31 nm bandwidth frequency-modulated laser pulse and a 1200 l/mm dispersion grating, experimental results proved that 97% 3ω energy passed through the laser entrance hole using a 4 m focal length wedged lens and gold foil target with an 1100 µm hole.

Simulation analysis of the restraining effect of a spatial filter on a hot image.

Based on the restraining effect that spatial filtering has on the frequency spectrum of a beam, from the small-scale focusing theory of Bespalov and Talanov (B-T theory) we derive an expression for the pinhole diameter of the spatial filter corresponding to the fastest growing frequency. Then, compared with the theoretical pinhole diameter of the spatial filter, the restraining effect of the spatial filter on a hot image with different pinhole diameters is numerically investigated. The numerical results show that, if the pinhole diameter is larger than the theoretical one, the hot-image intensity will remain steady; once the pinhole diameter becomes smaller than the theoretical one, the hot-image intensity will begin to decrease. Moreover, as the pinhole diameter decreases, a more prominent restraining effect can be obtained. But reducing the diameter of the spatial filter would lead to greater beam energy loss. The parameters of the spatial filter must be chosen to guarantee that the scheme fulfills the demand for low beam energy loss and a satisfactory restraining effect simultaneously.