Pulse Compression Technique Research Articles

Air-coupled ultrasonic inspection of resin materials using single-probe vertical reflection method

Air-coupled ultrasonic inspection is a noncontact and convenient nondestructive testing method. However, its vertical reflection method using a single probe (i.e., pulse-echo mode) is known to be difficult to achieve, even though it is frequently used inspection mode in conventional contact or water immersion ultrasonic testing. This is because the reflectivity of ultrasonic waves at the boundary between air and the surface of inspection object is enormous, and faint signals received after transmitting into the object are difficult to detect. In this study, in order to achieve the air-coupled ultrasonic single-probe reflection method, transmitting chirp signals (waves with a linearly frequency modulation) with an appropriate window function and applying a pulse compression technique (a cross-correlation process between the received signal and a reference chirp signal) was examined, and its effectiveness was investigated through experiments for polystyrene specimens. Experimental results showed that the proposed method was effective in detecting faint reflection signals after propagating through the specimens, and that the difference in plate thickness could be detected by observing the change of the signal-receiving period of multiple reflection signals between the top and bottom surfaces of the specimens. These results demonstrate the feasibility of nondestructive testing using the air-coupled ultrasonic vertical reflection method and could show an important step toward its future practical applications.

Laser-induced damages to charge coupled devices with combined nanosecond/picosecond short-pulse lasers

The research on laser induced breakdown mechanism of charge coupled devices (CCDs) brings new insights into photoelectric countermeasures. So far combined laser irradiation has been proved to be a more effective measure to destroy CCD. Due to the limitation of short-pulse laser combination method, the mechanism of CCD damage caused by combined short-pulse laser remains unexplored. Here, the distribution of temperature and stress field during the interaction between a combined short-pulse laser and a CCD is analyzed. A nanosecond/picosecond combined short-pulse laser system based on Stimulated Brillouin Scattering (SBS) pulse compression technique is designed. The damage threshold (DT) and properties of CCD by combined laser irradiation are characterized. The results show that the complete DT of combined laser induced CCD breakdown is only 103 mJ/cm2, which is only 44% of that of picosecond laser. The main cause of combined short-pulse laser induced CCD breakdown is short circuit (SC) between silicon substrate and silicon electrode.

A Pre-Voiding Alarm System Using Wearable Ultrasound and Machine Learning Algorithms for Children With Nocturnal Enuresis.

Nocturnal enuresis is a bothersome condition that affects many children and their caregivers. Post-voiding systems is of little value in training a child into a correct voiding routing while existing pre-voiding systems suffer from several practical limitations, such as cumbersome hardware, assuming individual bladder shapes being universal, and being sensitive to sensor placement error. Methods: A low-voltage ultrasound system with machine learning has been developed in estimating bladder filling status. A custom-made flexible 1D transducer array has been excited by low-voltage coded pulses with a pulse compression technique for an enhanced signal-to-noise ratio. In order to minimize the negative influence of possible transducer misplacement, a multiple-position training strategy using machine learning has been adopted in this work. Three popular classification methods, KNN, SVM and sparse coding, have been utilized to classify the acquired different volumes ranging from 100 ml to 300 ml into two categories: low volume and high volume. The low-volume category requires no further action while the high-volume category triggers an alarm to alert the child and caregiver. Results: When the sensor placement is ideal, i.e., the position of the practical sensor placement is on spot with the trained position, the precision and recall of the classification using sparse coding are [Formula: see text] and [Formula: see text], respectively. Even if the transducer array is misplaced by up to 4.5 mm away from the ideal location, the proposed system is able to maintain high classification accuracy (precision [Formula: see text] and recall [Formula: see text]). Category: Early/Pre-Clinical Research Clinical and Translational Impact: The proposed ultrasound sensor system for nocturnal enuresis is of significant clinical and translational value as it addresses two major issues that limit the wide adoption of similar devices. Firstly, it offers enhanced safety as the entire system has been implemented in the lowvoltage domain. Secondly, the system features ample tolerance to sensor misplacement while maintaining high classification accuracy. These features combined provide a much more user-friendly environment for children and their caregivers than existing devices.

Nonlinear pulse compression technique based on in multi-pass plano-cancave cavity

<sec>Ultrafast femtosecond laser system with hundreds of microjoules of energy, operating at a repetition frequency of several kilohertz, has very important applications in many fields such as medicine, mid-infrared laser generation, industrial processing, and vibrational spectroscopy. The chirped pulse amplification technique provides a feasible path to obtain light sources with those parameters. However, the use of chirped pulse amplification increases the technical complexity and cost of the laser system. Recently, the proposal of a multi-pass cell (MPC) nonlinear pulse compression technique has enabled us to obtain high power ultrafast femtosecond pulses with reduced technical complexity and cost. The device requires only two concave mirrors and a nonlinear medium in between. In the past seven years, the multi-pass cell nonlinear pulse compression technique has made great progress, making it possible to obtain ultrashort pulses with average power of more than a few kW and peak power of tens to hundreds of TW.</sec><sec>In this work, we achieve nonlinear pulse compression of a 100-W picosecond laser by using an improved nonlinear pulse compression scheme that combines a hybrid of a plano-cancave multi-pass cell and multi-thin-plate. Using fused silica plates in plano-cancave cavity, the spectral bandwidth (FWHM) of input picosecond laser is broadened from 0.24 nm to 4.8 nm due to self-phase modulation effect, the pulse is compressed to 483 fs by dispersion compensation using grating pairs, which corresponds to a compression factor of 22, and the final output power of 44.2 W is obtained. Compared with traditional MPC, the plano-cancave cavity scheme we developed is a very promising solution for nonlinear compression due to its compactness, more stability and large compression ratio.</sec>

Improvements in the Compression Filter and Calibration Factor of the Progressive Pulse Compression Technique

Improvements in the Compression Filter and Calibration Factor of the Progressive Pulse Compression Technique

Sidelobes Reduction Technique for Binary Codes

Pulse compression techniques enable the radar designers to transmit long duration pulse to cater the need of high power at the transmitter and achieve the range resolution of short duration pulse at the radar receiver simultaneously. Due to easy generation and processing, binary codes are widely used as pulse compression codes in radars, sonars and spread-spectrum communication systems. But binary codes have high unwanted pulse compression sidelobes at the output of pulse compression receivers. In this paper, optimum binary codes reported in the literatures are overlaid on Discrete frequency sequences to reduce unwanted peaks sidelobes of optimum binary codes. The proposed approach not only reduced peak sidelobes of optimum binary codes but also increased the pulse compression ratio. In this paper, optimum discrete frequency sequences for overlaying with binary codes are optimized using the Modified Simulated Annealing Algorithm (MSAA). A Discrete Frequency Coded binary signal has a complex signal structure which will be difficult to detect and analyze by enemy Electronics Support Measures (ESM). In this paper, a unified adaptive radar model is also proposed to cater the needs of today's radar signal design requirements. The proposed model will help to improve range resolution, the detection capability of radar, reduce the peak sidelobes significantly, and also improve the ESM capability of radar systems.

Self‐Compression of High Energy Ultrashort Laser Pulses

AbstractNonlinear pulse compression techniques have been applied widely in the pursuit of ultra‐intense laser pulse with extremely high pulse energy (≈mJ) and extremely short pulse duration (≈sub‐10 fs). Although postcompression techniques using dispersive gratings or chirped mirrors have achieved 1 TW few‐cycle pulses, further increasing of the pulse intensity is limited by the damage threshold of dispersive optics, especially when self‐focusing and ionization in the propagation medium are considered. To overcome such limitations, ultrashort pulses self‐compression techniques without vulnerable dispersive optics have been explored in the past two decades. In this paper, the latest advances in these techniques will be reviewed with a discussion on the progress of various experimental approaches as well as their potential applications and roles in generating extremely intense optical fields for strong‐field physics research.

Generation of ultrashort Rayleigh wave pulses using the combination of temporal and spatial pulse compression technique

Rayleigh waves generated by electromagnetic acoustic transducers (EMATs) have been widely used in nondestructive testing and evaluations. To improve the signal to noise ratio (SNR) and spatial resolution of Rayleigh waves simultaneously, the combination of temporal and spatial pulse compression (TSPC) technique is proposed. By analyzing the topology diagram of the typical pulse compression technique, a frequency modulated signal acts as the stretcher and a wavelength modulated meander line coil acts as the compressor. Experimental results indicate that the ultrasound fields of Rayleigh waves using the proposed TSPC technique show a spike shape in the spatial domain. Compared with the typical meander-line coil EMATs, the TSPC technique shows significant improvement of the generated Rayleigh waves. The SNR has increased by 12 dB while the main lobe width has decreased by about 98%.

Enhanced range-doppler algorithm for SAR image formation

A Synthetic Aperture Radar (SAR) system is a powerful source of information due to its capability to operate day and night and in all weather conditions. It is essential for military and civilian purposes. Pulse-focusing is employed by the SAR system for achieving both long-range detection and fine-range resolution. The Range-Doppler Algorithm (RDA) is the most popular pulse-focusing technique for creating images from coherent SAR data. Pulse compression techniques for SAR signal processing targets seek to improve map resolution, lower peak power, and increase the Signal-to-Noise Ratio (SNR) of the detected object. This paper demonstrates an enhanced RDA algorithm that uses a modified pulse compression technique based on mismatched filter optimization where Quadratic Constrained Quadratic Program (QCQP) is used. The performance evaluation of the proposed method is performed using visual assessment and the standard image quality metrics peak-side-lobe-ratio (PSLR), impulse-response-width (IRW), and integrated-side-lobe-ratio (ISLR). Simulated and real SAR raw data are used for performance evaluation. The experimental results demonstrate the proposed method’s robustness, efficiency, and reliability in providing a higher-quality SAR image than the traditional RDA algorithm.

Effective reduction of sidelobes in pulse compression radars using NLFM signal processing approaches

Pulse compression techniques are widely used in modern radar systems to enhance range resolution and detection capability. As signal design is one of the basic factors of an efficient radar system, the problem of designing a radar signal with good characteristics using pulse compression is addressed in this paper. A linear frequency modulated (LFM) waveform has been widely used for conventional radars. However, it has a higher sidelobe level which makes the detection of weak targets very difficult in presence of strong targets returns and as a result the problem of masking occurs. In order to get suppressed sidelobes of radar matched filter output as well as preserve the main lobe resolution and level to overcome the problem of masking, nonlinear frequency modulated (NLFM) signals are used in modern radars. In this paper, we will introduce two different approaches to design an optimized NLFM signal which is characterized by optimized sidelobe level (SLL). The first is the exponential piecewise linear function (EPWL) which is the modification of piecewise linear (PWL) functions relying on an exponential predistortioning function. The second is the odd-term polynomial approximation (OTPA) in which the generation of NLFM signal depends only on odd-powered terms of the polynomial function. Furthermore, the simulation results of autocorrelation functions (ACF) of the proposed signals show its superiority over the traditional LFM signals and significantly enhancement compared to the recent background work. The Doppler sensitivity of the designed signals has been evaluated, revealing that the first approach offers Doppler tolerance and is suitable for surveillance radar systems, while the second approach with a lower sidelobe level is used in applications such as Synthetic Aperture Radar (SAR). Finally, the ambiguity function of the designed signals has been measured to illustrate the effect of the Doppler effect relative to different velocities.

Ultrasonic sensing system with thermophone and its applications

In recent decades, thermophone has been studied as a novel electro-acoustic transducer since it has notable features, such as thin, light weight, acoustic purity, and wide frequency range. Though power consumption and resulting temperature rise are considered fatal problem, they are drastically reduced by limiting operation within ultrasonic domain and adopting the pulse current drive. Ultrasonic sensing system consists of dedicated thermophone as transmitter and commercially available MEMS microphone as receiver is presented to demonstrate typical airborne ultrasonic application, such as range finder or obstacle detection. Applying pulse compression technique, few orders of improvement in distance resolution and effective noise rejection have been achieved. Details of the sensing system and applications cases as listed below will be disclosed. 1. Multiple miniature autonomous vehicles equipped the obstacle sensing system in order to verify the bats jamming avoidance behavior. 2. Bio-inspired active sonar system onboarded to the drone and demonstrated the obstacle avoidance navigation in a wild environment. 3. Small step identification with the obstacle sensing system for electric wheelchair and automatic guided vehicle (AGV). 4. Ultrasonic micro-meter level displacement measurement system for non-contact cardiac pulse monitoring.

Range resolution enhancement of SHAllow RADar (SHARAD) data via bandwidth extrapolation technique: Enabling new features detection and improving geophysical investigation

The SHAllow RADar (SHARAD) is a subsurface sounding radar provided by the Italian Space Agency (ASI) as a facility instrument for NASA's Mars Reconnaissance Orbiter (MRO) mission, the second longest-lived mission to orbit Mars after Mars Odyssey. Developed by an Italian-US Team, SHARAD was designed to investigate to depth of up to one kilometer in the Martian subsurface and to map dielectric discontinuities associated with compositional and/or structural changes. The radar mapping began in November 2006 and, after 16 years of operations, SHARAD has mapped most of the planet's surface (∼55%), contributing a data volume for the entire MRO mission that exceeds that of all the past Mars missions combined.Designed to complement the lower-frequency, relatively narrower bandwidth capability of the MARSIS sounding radar, SHARAD emits an 85μs, frequency-modulated (10 MHz) chirp at a central frequency of 20 MHz to increase the signal-to-noise ratio (SNR) and achieve a finer resolution compared to MARSIS. On the ground, echoes recorded by the radar are range compressed using conventional pulse compression techniques yielding a nominal range resolution of 15 m (vacuum, 5–8 m in typical Martian materials). Additionally, synthetic aperture processing is performed in along-track to increase the resolution up to 300–500 m.Throughout the entire mission duration, SHARAD provided a diverse and extensive range of imaging data. It was able to observe the internal structures of Planum Boreum and Planum Australe, two icy deposits that contain the North and South Polar Layered Deposits (NPLD-SPLD), thought to contain valuable information about the planet's past climate. The radar unveiled the massive deposits of buried CO2 ice below the southern residual ice cap (SRIC), which confirmed the influence of the planet obliquity and insolation in the accumulation of dry ice. Furthermore, it revealed the intricate lava flows structures in the Tharsis Volcanic region, suggesting that the planet experienced deposition by multiple volcanic episodes.However, radar resolution often poses a limitation for subsurface investigations in many regions. Only recently, processing techniques have focused on enhancing SHARAD radargrams vertical resolution by implementing the Bandwidth Extrapolation (BWE) technique. This approach has demonstrated an improvement of the vertical resolution while preserving the time-of-arrival (TOA) and amplitude of the detected echoes. The resulting super-resolved data products exhibit better separation between radar returns, thus improving the overall understanding of the investigated terrains. The obtained results demonstrate the value of implementing the BWE techniques in the data processing pipeline whenever radar sounders are involved.

The Design and Analysis of the Sidelobe Reduction Filter for Polyphase Frank Codes

Long-duration radar signals have a low-range resolution but have high-energy contents, whereas short-duration radar signals have a high-range resolution but have low-energy contents. The pulse compression technique allows the radar signal designers to achieve desired energy corresponding to long-duration signals and range resolution corresponding to short-duration signals simultaneously. The pulse compression technique is used in a high-range resolution radar to discriminate closely located targets in the same vicinity and it also improves radar parameters’ measurement accuracy. Polyphase codes are widely used as pulse compression signals in high-range resolution radar systems. The major disadvantage of pulse compression codes is unwanted correlation sidelobes at the output of the matched filter. This paper proposes the sidelobe reduction technique to reduce the Peak Sidelobes Level (PSL) of Frank polyphase codes. The weights of the proposed Sidelobes Reduction Filter (SRF) are optimized by using the Modified Genetic Algorithm (MGA). The proposed technique reduced autocorrelation peak sidelobe level significantly. Finally, the synthesized results of the proposed SRF are presented in this paper.

Application of Pulse Compression Technique in High-Temperature Carbon Steel Forgings Crack Detection with Angled SV-Wave EMATs.

In order to solve the difficulty in localization and poor signal-to-noise ratio (SNR) of the angled shear vertical wave (SV wave) electromagnetic acoustic transducer (EMAT) in cracks detection of high-temperature carbon steel forgings, a finite element (FE) model of the angled SV wave EMAT detection process was established, and the influence of specimen temperature on the EMAT excitation, propagation, and reception processes was analyzed. A high-temperature resistant angled SV wave EMAT was designed to detect carbon steel from 20 °C to 500 °C, and the influence law of the angled SV wave at different temperatures was analyzed. Then a circuit-field coupled FE model of angled SV wave EMAT in the carbon steel detection process based on the Barker code pulse compression technique was established, and the effects of the Barker code element length, impedance matching method, and matching component parameters on the pulse compression effect were analyzed. In addition, the noise suppression effect and the SNR of the crack-reflected wave in the tone-burst excitation method and the Barker code pulse compression technique were compared. The results show that the amplitude of the block-corner reflected wave decreases from 556 mV to 195 mV, and the SNR decreases from 34.9 dB to 23.5 dB when the specimen temperature increases from 20 °C to 500 °C. When the temperature is 500 °C, the SNR of the crack-reflected wave obtained by the Barker code pulse compression technique can be improved by 9.2 dB compared to the tone-burst excitation method with 16 synchronous averages. The study can provide technical and theoretical guidance for online crack detection for high-temperature carbon steel forgings.

Pulse Generation and Compression Techniques for Microwave Electronics and Ultrafast Systems

Pulse Generation and Compression Techniques for Microwave Electronics and Ultrafast Systems

A Method of Rayleigh Wave Combined With Coil Spatial Pulse Compression Technique for Crack Defects Detection

Rayleigh wave methods utilizing electromagnetic acoustic transducers (EMATs) are widely used to detect a crack defect on the metal surface due to the merits of high sensitivity and long detection distance. However, the current EMATs for Rayleigh wave has low energy conversion efficiency and usually produce a wide echo-wave packet, reducing the spatial resolution and signal-to-noise ratio of detection signals. To address the issues, the coil spatial pulse compression technique (CSPCT) is proposed to improve the physical structure of EMATs. First, a comprehensive analysis of the coil design is given to realize spatial phase encoding pulse compression. Second, the influence of the key parameters of EMATs on the pulse compression effect is studied. Finally, the crack detection experiment is conducted on an aluminum plate to verify the performance improvement of the designed EMAT. Compared to existing EMATs, our designed EMATs reduce the width of the main lobes and enhance the amplitude of the echo-wave signals. Results indicated that the spatial resolution was improved by up to 72% with an increase of 0.72 dB in signal-to-noise ratio. In comparison to the existing time-domain pulse compression technique, the CSPCT requires no complex hardware and software tools, which has the merits of low cost and fast response. This work expands the application boundary of EMAT systems.

Simple, stable and efficient nonlinear pulse compression through cascaded filamentation in air

Abstract Nonlinear compression has become an obligatory technique along with the development of ultrafast lasers in generating ultrashort pulses with narrow pulse widths and high peak power. In particular, techniques of nonlinear compression have experienced a rapid progress as ytterbium (Yb)-doped lasers with pulse widths in the range from hundreds of femtoseconds to a few picoseconds have become mainstream laser tools for both scientific and industrial applications. Here, we report a simple and stable nonlinear pulse compression technique with high efficiency through cascaded filamentation in air followed by dispersion compensation. Pulses at a center wavelength of 1040 nm with millijoule pulse energy and 160 fs pulse width from a high-power Yb:CaAlGdO4 regenerative amplifier are compressed to 32 fs, with only 2.4% loss from the filamentation process. The compressed pulse has a stable output power with a root-mean-square variation of 0.2% over 1 hour.

A novel power-combination method using a time-reversal pulse-compression technique

The electromagnetic time-reversal (TR) technique has the characteristics of spatiotemporal focusing in a time-reversal cavity (TRC), which can be used for pulse compression, thus forming an electromagnetic pulse with high peak power. A time-reversed pulse-compression method in a single channel has high pulse compression gain. However, single channel pulse compression can only generate limited gain. This paper proposes a novel TR power-combination method in a multichannel TRC to obtain higher peak power based on TR pulse-compression theory. First, the TR power-combination model is given, and the crosstalk properties of the associated channel and the influence of the reversal performance are studied. Then, the power-combination performances for the TR pulse compression, such as combined signal to noise ratio (SNR) and combined compression gain, are analyzed by numerical simulation and experimental methods. The results show that the proposed method has obvious advantages over pulse-compression methods using a single channel cavity, and is more convenient for power combination.

Thermodynamic and nonlinear optical analysis of solid-state multipass cells for compression of picosecond pulses in the 2−μm range

Multipass cells (MPCs) recently emerged as a new femtosecond pulse compression technique for lasers with high pulse energy. While most of the work focused on the compression of near-infrared laser systems so far, we address the case of holmium-based gain materials, which offer an enormous potential in terms of pulse energy, yet can only host a very narrow bandwidth in the 2-$\ensuremath{\mu}\text{m}$ wavelength range. Compression into the few-cycle regime, therefore, requires at least a hundredfold spectral broadening. Specifically, we investigate the utility of four different solid-state materials in the MPC, namely, fused silica, sapphire, YAG, and diamond. Spectral broadening dynamics is numerically investigated using the $2D+1$-unidirectional pulse propagation equation. To this end, we put particular emphasis on the thermal properties of the nonlinear optical material, which turn out as a critical issue above 2-$\ensuremath{\mu}\text{m}$ wavelength. Solving the heat equation for each of the materials, we then estimate the maximum temperature at beam center. These considerations show that outside the near infrared, thermodynamic parameters of nonlinear optical materials become at least equally important as their optical properties. Among the four materials under test, in particular, diamond stands out as it combines highly favorable thermal properties with a large optical nonlinearity. Finally, a concomitant slow degradation of spatial coherence is monitored. Our findings provide an effective guideline for the design of high pulse-energy compressors at wavelengths of $2\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\text{m}$ and beyond.

Improved spectral processing for a multi-mode pulse compression Ka–Ku-band cloud radar system

Abstract. Cloud radars are widely used in observing clouds and precipitation. However, the raw data products of cloud radars are usually affected by multiple factors, which may lead to misinterpretation of cloud and precipitation processes. In this study, we present a Doppler-spectra-based data processing framework to improve the data quality of a multi-mode pulse-compressed Ka–Ku radar system. Firstly, non-meteorological signal close to the ground was identified with enhanced Doppler spectral ratios between different observing modes. Then, for the Doppler spectrum affected by the range sidelobe due to the implementation of the pulse compression technique, the characteristics of the probability density distribution of the spectral power were used to identify the sidelobe artifacts. Finally, the Doppler spectra observations from different modes were merged via the shift-then-average approach. The new radar moment products were generated based on the merged Doppler spectrum data. The presented spectral processing framework was applied to radar observations of a stratiform precipitation event, and the quantitative evaluation shows good performance of clutter or sidelobe suppression and spectral merging.