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- 实验方案
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Multipass Spectral Broadening with Tens of Millijoule Pulse Energy
摘要： Nonlinear spectral broadening is widely used for applications like attosecond pulse generation, XUV sources, supercontinuum generation or ultrafast spectroscopy. In these cases the laser is typically spectrally broadened and compressed to reach pulse durations much shorter than originally available. For pulse energies in the millijoule regime, spectral broadening is routinely implemented using capillary optical fibers with remarkable results in terms of output pulse durations and beam quality. In addition to fiber-based approaches, the application of multipass cells for spectral broadening recently attracted increasing interest. These cells are capable of preserving a nearly Gaussian mode in the presence of nonlinear effects without a spatial filter or optical fiber, they are robust to input pointing, have a high throughput of more than 90% and can be used with high average powers and energies. In a multipass cell the nonlinear interaction with either gas or a thin solid provides a small amount of spectral broadening within each pass. Based on this concept we recently demonstrated output energies of almost 18 mJ by guiding near infrared pulses with a pulse duration of 1.3 ps and a repetition rate of 5 kHz through an argon filled multipass cell. We obtained a broadening factor of over 33 and could show compressibility down to 41 fs. The energy was limited by the breakdown intensity of argon and the damage threshold of the used optics. To increase the throughput energy, we developed a new mirror coating with a damage threshold measured to be about 50% higher than the coating used in [4]. The setup length was increased from 3m to 8m leading to about 170% bigger beam areas. Output energies of 39.5 mJ with a broadening factor over 38 (output bandwidth 85nm at 20 dB, output fourier limit 34.2 fs) are reached in first experiments by focusing a 1.3 ps pulse 43 times in 250 mBar of argon (cf. Fig. 1 and Fig. 2). The beam profile after propagation depicted as inset in Fig. 2 keeps its nearly Gaussian shape indicating a preserved beam quality. The throughput is well above 95% with an output spectrum typical for self-phase modulation. Ongoing optimization is expected to lead to even higher throughput energies. This laser source will combine pulse durations below 40 fs with high energies and kilohertz repetition rate and can be used as driver laser for an all-laser-driven Thomson X-ray source yielding a significant increase of photon flux. Moreover this technique is widely applicable to different wavelength ranges, input energies and input powers turning it into a promising scalable alternative to fiber based spectral broadening.
关键词： multipass cells,Nonlinear spectral broadening,pulse compression,high-energy lasers
更新于2025-09-11 14:15:04
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High power 4.2-cycle mid-infrared pulses from a self-compression optical parametric oscillator
摘要： We demonstrate the generation of few-cycle mid-infrared (MIR) pulses from a self-compressed optical parametric oscillator (OPO), synchronously pumped by a 105 fs Yb3+ doped fiber laser system. By using a ring cavity design, 4.2-cycle (45.3 fs) MIR pulses at a central wavelength of 3250 nm (with a 10-dB bandwidth of 648 nm) are obtained in a simple, practical and compact configuration. Detailed theoretical analysis and numerical simulation are performed to better understand the self-compression mechanism in such an OPO cavity. In addition, when pumped at 3.8 W, the OPO delivers idler output power up to 330 mW and the corresponding pump depletion is 74.7%.
关键词： Nonlinear optics,Pulse compression,Ultrafast lasers
更新于2025-09-11 14:15:04
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Three-Dimensional Terahertz Coded-Aperture Imaging Based on Geometric Measures
摘要： For synthetic aperture radars, it is dif?cult to achieve forward-looking and staring imaging with high resolution. Fortunately, terahertz coded-aperture imaging (TCAI), an advanced radar imaging technology, can solve this problem by producing various irradiation patterns with coded apertures. However, three-dimensional (3D) TCAI has two problems, including a heavy computational burden caused by a large-scale reference signal matrix, and poor resolving ability at low signal-to-noise ratios (SNRs). This paper proposes a 3D imaging method based on geometric measures (GMs), which can reduce the computational burden and achieve high-resolution imaging for low SNR targets. At extremely low SNRs, it is dif?cult to detect the range cells containing scattering information with an ordinary range pro?le. However, this dif?culty can be overcome through GMs, which can enhance the useful signal and restrain the noise. By extracting useful data from the range pro?le, target information in different imaging cells can be simultaneously reconstructed. Thus, the computational complexity is distinctly reduced when the 3D image is obtained by combining reconstructed 2D imaging results. Based on the conventional TCAI (C-TCAI) model, we deduce and build a GM-based TCAI (GM-TCAI) model. Compared with C-TCAI, the experimental results demonstrate that GM-TCAI achieves a more impressive performance with regards to imaging ability and ef?ciency. Furthermore, GM-TCAI can be widely applied in close-range imaging ?elds, for instance, medical diagnosis, nondestructive detection, security screening, etc.
关键词： three-dimensional (3D),geometric measures (GMs),pulse compression,coded-aperture imaging
更新于2025-09-10 09:29:36
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[IEEE 2018 17th International Conference on Ground Penetrating Radar (GPR) - Rapperswil, Switzerland (2018.6.18-2018.6.21)] 2018 17th International Conference on Ground Penetrating Radar (GPR) - An FPGA-based Flexible and MIMO-capable GPR System
摘要： Ground penetrating radar (GPR) has broad applications in non-destructive subsurface imaging. Most GPRs on the market are bistatic devices that illuminate the buried objects using analog pulses with simple Gaussian-like shapes. These GPRs suffer from drift in the scan results and have either a low-resolution or a low depth of scan, which limits their application. High resolution along with an increased depth of scan can be achieved by transmitting maximal length pseudorandom sequences (m-sequences) which enable pulse compression due to their near-ideal autocorrelation properties. In addition, improved object localization and reduced drift can be obtained with the spatial diversity offered by a MIMO transceiver. This paper discusses the design and implementation of a 8×8 MIMO-capable impulse-based GPR that transmits m-sequences generated on a low-cost FPGA platform, performs a quadrature transform on the received signal to reduce computation, and implements sub-sampling to sample the quadrature-converted signals using low-speed ADCs. Preliminary experimental results are also presented.
关键词： multistatic,MIMO,pulse compression,subsampling,m-sequence,GPR,Ground penetrating radar
更新于2025-09-09 09:28:46
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Wave Propagation Concepts for Near-Future Telecommunication Systems || Slow Electromagnetic Waves: Theory and New Applications
摘要： In this chapter, the usage of slow electromagnetic waves in several application domains is deeply discussed. Starting from an outline of the classical Cerenkov effect, various related topics are presented in detail, namely the generation of electromagnetic waves by the Cerenkov effect, the Cerenkov free-electron laser, pickup and kickers in accelerators, pulse compression in radar and linac, and compact components and waveguides in the microwave region.
关键词： propagation,slow electromagnetic waves,electron beam pickup and kicker,pulse compression
更新于2025-09-09 09:28:46
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[IEEE 2018 International Conference on Radar (RADAR) - Brisbane, Australia (2018.8.27-2018.8.31)] 2018 International Conference on Radar (RADAR) - Space- Time-Range Adaptive Processing for MIMO Radar Imaging
摘要： Multiple-input multiple-output (MIMO) radar has drawn considerable attention due to its superior performance over phased array radar. The performance is achieved through waveform diversity. In terms of MIMO radar space-time adaptive processing (STAP), it is often assumed that the received signals has already been separated by a bank of matched filters (MFs) or the covariance matrix of the waveforms is an identity matrix. We find that when the non-complete orthogonality of the waveforms is taken into consideration, MIMO radar STAP suffers performance degradation. Besides, inspired by the definition of waveform diversity, a space-time-range adaptive processing (STRAP) scheme is proposed based on the framework of adaptive pulse compression (APC) and the criterion of minimum variance distortionless response (MVDR) to overcome the influence of non-completely orthogonal waveforms by taking into account the interference of nearby range cells to estimate the covariance matrix. The proposed algorithms are assessed via simulation experiments in two scenarios in comparison with sequential processing and the results show the effectiveness of STRAP.
关键词： waveform separation,adaptive pulse compression,space-time-range adaptive processing,MIMO radar
更新于2025-09-04 15:30:14
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[IEEE 2018 25th IEEE International Conference on Image Processing (ICIP) - Athens, Greece (2018.10.7-2018.10.10)] 2018 25th IEEE International Conference on Image Processing (ICIP) - A Functional Framework for Ultrasound Imaging
摘要： Delay-And-Sum (DAS), the state-of-art in ultrasound imaging, is known to be sub-optimal, resulting in low resolution and contrast. Most proposed improvements involve ad-hoc re-weighting, or hit computational bottlenecks given real-time requirements. This paper takes a fresh perspective on the problem, leveraging a functional framework to obtain a regularized least-squares estimate of the tissue reflectivity function. An explicit solution is derived, which – for specific cases – can be efficiently implemented, making it suitable for real-time imaging. In our formulation, DAS appears as a back-projection without any optimal properties. We illustrate the framework through first a one-dimensional set-up, and then a two-dimensional extension with Synthetic Aperture Focusing Technique (SAFT). The one-dimensional simulations show a 77% resolution improvement with respect to DAS, which artificially limits the available bandwidth. On a standard performance-assessment phantom, simulations show that SAFT depth resolution can be improved by 71%.
关键词： Sampling Operator,Ultrasound,Array Signal Processing,Pulse Compression,Inverse Problem
更新于2025-09-04 15:30:14