Multi-pulse Processing Research Articles

Resolving Range Ambiguity Using Transmit Coding Modulation and Multi-Pulse Processing in FPC-PA Radar

Resolving Range Ambiguity Using Transmit Coding Modulation and Multi-Pulse Processing in FPC-PA Radar

Creation of NV centers over a millimeter-sized region by intense single-shot ultrashort laser irradiation

Recently, ultrashort laser processing has attracted attention for creating nitrogen-vacancy (NV) centers because this method can create single NV centers in spatially-controlled positions, which is an advantage for quantum information devices. On the other hand, creating high-density NV centers in a wide region is also important for quantum sensing because the sensitivity is directly enhanced by increasing the number of NV centers. A recent study demonstrated the creation of high-density NV centers by irradiating femtosecond laser pulses, but the created region was limited to micrometer size, and this technique required many laser pulses to avoid graphitization of diamond. Here, we demonstrate the creation of NV centers in a wide region using only an intense single femtosecond laser pulse irradiation. We irradiated a diamond sample with a femtosecond laser with a focal spot size of 41 µm and a laser fluence of up to 54 J/cm2, which is much higher than the typical graphitization threshold in multi-pulse processing. We found that single-pulse irradiation created NV centers without post-annealing for a laser fluence higher than 1.8 J/cm2, and the region containing NV centers expanded with increasing laser fluence. The diameter of the area was larger than the focal spot size and reached over 100 µm at a fluence of 54 J/cm2. Furthermore, we demonstrated the NV centers’ creation in a millimeter-sized region by a single-shot defocused laser pulse over 1100 µm with a fluence of 33 J/cm2. The demonstrated technique will bring interest in the fundamentals and applications of fabricating ultrahigh-sensitivity quantum sensors.

Joint Range-Doppler Estimation Based on Multipulse Processing of Composite Hyperbolic Frequency Modulated Waveforms

We herein investigate the joint estimation of range and Doppler based on multipulse processing of composite hyperbolic frequency-modulated (HFM) waveforms. The composite HFM waveform solves the range bias problem in the HFM waveform by using the mapping relationship between the radial velocity and the peak-to-peak interval in the outputs of matched filters. However, the result may be inaccurate in the presence of multiple false peaks. In this letter, we fit matched filter outputs into a hidden Markov model by utilizing the kinetic model of the target. We can subsequently solve the peak-pair selection problem and perform the joint range-Doppler estimation by using the Viterbi algorithm. The proposed method, by multipulse processing, lowers the signal-to-noise ratio (SNR) requirement, and exact detection of the target in each return pulse is no longer necessary. We prove the effectiveness of the proposed method via numerical simulations, and the simulation results demonstrate that this method outperforms existing methods in low SNR environments.

Ablation Threshold Measurement and Chemical Modification of UV Nanosecond Laser Micromachining of Polycrystalline Diamond

The effects of pulsed laser interaction with matter have become a center for research. The pulsed laser has become an indispensable manufacturing tool in the efficient machining of polycrystalline diamond (PCD) and other materials. Herein, the ablation threshold and chemical modification of PCD irradiated using ultraviolet (UV) nanosecond laser is scrutinized. Using a UV nanosecond laser source, a bulk of PCD is ablated in air, and multipulse processing is executed at a wavelength of 355 nm. The ablation threshold of PCD is reported to be ≈3.7326 J cm−2 based on the linear relationship between the laser pulse energy density and the crater diameter, with a Gaussian distribution of the laser intensity on the cross‐section. Furthermore, the sp3 structure transitioned to the sp2 graphite phase when the PCD is exposed to UV nanosecond laser irradiation. Consequently, this study provides a theoretical and experimental reference for UV nanosecond laser micromachining of PCD materials in the research and metal‐working industries.

Нові технології лазерної поверхневої обробки

Subject and purpose. Currently, gas, solid-state, and fiber lasers are used to process materials in the aviation industry. For the thermal treatment of steels, gas CO2 lasers with a capacity of more than 1 kW used, which are reliable in operation but have high cost and low efficiency. There are no results on the use of low-power (up to 20 W) pulsed-mode lasers for surface hardening of steel products. The purpose of this work is to determine the modes of surface hardening of products from carbon and alloy steels using low-power solid-state pulsed YAG lasers. Methodology. For laser hardening, a 5 W solid-state YAG laser was used (diode pumping, radiation wavelength λ = 1,064 μm, pulse mode). The use of a nonlinear crystal made it possible to obtain UV radiation with λ = 0,355 μm (third harmonic). The following modes were investigated: processing with single pulses (duration 0,1...0,4 ms) and multi-pulse processing with short (30...70 ms) pulses. The scanning speed was 8...2 mm/s. The energy in the pulse was determined by the photoelectric method. Thermal hardening was performed on the following steels: У12, P6M5. The possibility of UV radiation hardening was evaluated on steel 20, 45, У12, and ШХ15. Findings. The optimum values of pulse duration for maximum hardness in laser hardening of the investigated steels. With multi-pulse treatment of steels, the pulse duration is shorter than with single-pulse treatment, the hardening intensity is higher, and the quality of the processed surface is better. Single-pulse and multi-pulse processing are accompanied by partial melting of the surface of steel products, which does not allow it to be used in cases where a high quality of the surface is required. Laser hardening of steel by ultraviolet radiation is not accompanied by melting. Conclusion. For surface hardening of products, where partial melting of the surface is possible, low-power lasers in pulse mode can be used. Laser hardening by ultraviolet radiation is a promising direction for thermal hardening of steels, which allows maintaining the original quality of the surface layer. Thermal hardening with low-power lasers can be effective for small-sized areas of the processed parts of the fuel equipment of aircraft engines, friction elements, and, especially, the tool is small.

Multipulse Processing Algorithm for Improving Mean Velocity Estimation in Weather Radar

In this article, we present a novel algorithm termed multipulse processing (MPP) for improving mean Doppler velocity estimation in weather radar applications. It can be used for both staggered pulse repetition time (PRT) and uniform-PRT sequences. Essentially, MPP consists of finding a particular zero of a functional composed of data autocorrelation estimates at multiple lags. To select the proper zero, an initial Doppler velocity estimate is required. Therefore, MPP can be considered as an estimation refinement stage. Its advantage lies in the fact that it uses the complete information contained in the radar signal autocorrelation. After a theoretical analysis, we compare the performance of MPP against other well-established methods of similar complexity and the Cramér–Rao lower bound, by means of Monte Carlo simulations using synthetic data. We show that the proposed estimator offers the lowest root-mean-square error (RMSE) at low signal-to-noise ratio (SNR) situations for a wide range of spectral widths. Finally, we evaluate the MPP algorithm performance using real data measured by the RMA Argentinian weather radar. The results of tests performed are consistent with those of Monte Carlo simulations and validate the proposed method.

Investigation of the ultrashort pulsed laser processing of zinc at 515 nm: Morphology, crystallography and ablation threshold

Bulk polycrystalline pure zinc is ablated in air using a picosecond laser source to perform single pulse and multi-pulse processing at a wavelength of 515 nm. The geometries and surface morphologies of the resulting craters are determined by confocal laser scanning microscopy and scanning electron microscopy. Further, the ablation threshold and its corresponding incubation coefficient is determined from the ablated volume for multiple laser pulses. Two different thermal ablation regimes are identified. The single pulse ablation thresholds are found to equal 0.1 J/cm2 and 0.68 J/cm2 respectively for the two regimes. It is found that the incubation coefficients are larger than unity, indicating material removal becomes energy expensive for multiple pulse laser irradiation. It is also found that an irradiated area undergoes laser induced preferred crystal orientation.

Simulation of an intracavity Q-switched optical parametric oscillator using rate equations

Based on rate equations we have refined the model of an intracavity Q-switched optical parametric oscillator (IOPO) by taking into account the influence of self-Raman stimulated scattering. The rate equations are solved and analysed in the plane-wave approximation for a Gaussian spatial beam distribution profile under various conditions. The spatial rate equations are also solved numerically for a beam with a field distribution of the TEM00 mode. The numerical analysis shows that stimulated self-Raman scattering leads to a decrease in the output signal power and an increase in the signal pulse width. This model allows one to predict that the output signal power can be increased with increasing ratio of Raman to fundamental photon lifetimes. This suggests a practical method for determining the self-Raman gain coefficient experimentally. In addition, a multi-pulse process for the signal beam of the IOPO can be regenerated by the presented model. However, in comparison with the previous research, the presented model is more accurate and allows one to design and optimise single-resonator IOPOs.The bibliography includes 25 references.

Heat accumulation during pulsed laser materials processing.

Laser materials processing with ultra-short pulses allows very precise and high quality results with a minimum extent of the thermally affected zone. However, with increasing average laser power and repetition rates the so-called heat accumulation effect becomes a considerable issue. The following discussion presents a comprehensive analytical treatment of multi-pulse processing and reveals the basic mechanisms of heat accumulation and its consequence for the resulting processing quality. The theoretical findings can explain the experimental results achieved when drilling microholes in CrNi-steel and for cutting of CFRP. As a consequence of the presented considerations, an estimate for the maximum applicable average power for ultra-shorts pulsed laser materials processing for a given pulse repetition rate is derived.

Multiecho processing by an echolocating dolphin.

Bottlenose dolphins (Tursiops truncatus) use short, wideband pulses for echolocation. Individual waveforms have high-range resolution capability but are relatively insensitive to range rate. Signal-to-noise ratio (SNR) is not greatly improved by pulse compression because each waveform has small time-bandwidth product. The dolphin, however, often uses many pulses to interrogate a target, and could use multipulse processing to combine the resulting echoes. Multipulse processing could mitigate the small SNR improvement from pulse compression, and could greatly improve range-rate estimation, moving target indication, range tracking, and acoustic imaging. All these hypothetical capabilities depend upon the animal's ability to combine multiple echoes for detection and/or estimation. An experiment to test multiecho processing in a dolphin measured detection of a stationary target when the number N of available target echoes was increased, using synthetic echoes. The SNR required for detection decreased as the number of available echoes increased, as expected for multiecho processing. A receiver that sums binary-quantized data samples from multiple echoes closely models the N dependence of the SNR required by the dolphin. Such a receiver has distribution-tolerant (nonparametric) properties that make it robust in environments with nonstationary and/or non-Gaussian noise, such as the pulses created by snapping shrimp.

Comparison of pulsed magnetization processes for HTS bulk parts

With respect to the potential application as cryo-permanent magnets, melt-textured YBaCuO bulk parts were used to investigate various pulsed magnetization processes employing pulsed copper coils. Results for different multi-pulse processes including different peak fields, pulse durations, temperatures and geometries are compared. The strong current-carrying capabilities of HTS material, even for currents above the critical current Jc, result in strong shielding effects, in particular for short pulse durations. It turns out that the inhomogeneities present in all real materials play an important role for the initial magnetic flux penetration. To fully magnetize the bulk samples, the non-stationary conditions and geometrical effects require peak fields, which substantially exceed twice the saturation magnetization measured at the sample surface. Transient magnetizations as well as shapes and absolute values of trapped flux profiles are reported.

Pulsed magnetization processes for HTS bulk components

Magnetization experiments using pulsed Cu coils were carried out on YBCO rings employing multi-pulse processes with different peak fields and pulse durations. Trapped magnetic flux profiles were recorded, and the shapes and absolute values of the remanent magnetization are discussed in terms of the dynamics of such pulsed magnetization processes. While Bean's model with a constant current density J=J/sub c/ describes quasi-stationary processes well, in the pulsed magnetization the transient current density clearly exceeds J/sub c/ and leads to more complex flux flow effects.

Evolution of implanted carbon in silicon upon pulsed excimer laser annealing: epitaxial Si1−yCy alloy formation and SiC precipitation

Formation of epitaxial Si1−yCy alloy layers on monocrystalline silicon surfaces with y≈1 at% is reported. The preparation method was carbon ion implantation, followed by KrF excimer laser annealing in air. Results of Rutherford backscattering (RBS) measurements, secondary ion mass spectrometry (SIMS), Raman spectroscopy, infrared (IR) absorption analysis, and transmission electron microscopy (TEM) are compared. The evolution of the implanted carbon was affected by the implantation conditions, i.e., implantation energies, ion current, and implanted ion distribution (single-energy versus flat profile implantation), as well as by the laser annealing parameters. Up to approximately 1 at% carbon content, the dominant process was nonequilibrium trapping of carbon atoms in substitutional lattice sites upon fast resolidification, allowing perfect epitaxial recovery of the crystal lattice by laser annealing. Above this concentration, silicon carbide precipitates were formed, embedded into an otherwise well-ordered substitutional Si–C alloy matrix. Upon multipulse processing, the complex carbon redistribution processes were influenced by trapping of carbon in SiC precipitates, dissolution of SiC in the melt and segregation effects in the near-surface region.