Pulse Compression Research Articles

Acquiring pulse a priori information for chirp transform spectrometer systems through phase error measurements

Abstract With the rapid advancement in deep-space exploration, back-end systems such as Chirp Transform Spectrometers (CTS) must be equipped with larger bandwidths, higher frequency resolutions, and enhanced sensitivities. Phase errors in chirp signals distort pulse compression waveforms, significantly affecting CTS system performance. To correct the distorted pulses, we acquire a priori information about the pulse waveform by measuring the phase errors in the CTS system. This paper measures and analyzes phase errors in the expander and compressor signals of a 1GHz bandwidth CTS system. We discuss the impact of these phase errors on pulse waveforms, system frequency resolution, sensitivity, and other critical performance parameters. Additionally, we plot the in-band phase error curves for the system. By utilizing these phase error curves, we can determine the phase error for any input frequency, thereby providing valuable prior information for pulse compression waveform correction. This approach is crucial for guiding subsequent error optimization and compensation in the CTS system.

An Improved Detecting Algorithm of Moving Targets for Airborne Maritime Surveillance Radar.

The traditional method is capable of detecting and tracking stationary and slow-moving targets in a sea surface environment. However, the signal focusing capability of such a method could be greatly reduced especially for those variable-speed targets. To solve this problem, a novel tracking algorithm combining range envelope alignment and azimuth phase filtering is proposed. In this method, the motion of the airborne maritime surveillance radar platform is firstly compensated for target echoes. Secondly, range envelope alignment is performed to correct the unpredictable range migration of the target after pulse compression. The higher-order phase difference between the adjacent pulses is estimated and compensated. Ultimately, such pulse series are accumulated through azimuth Fourier transform. Traditional methods compensate only for platform motion, limiting their ability to handle variable-speed targets. The proposed algorithm addresses this limitation by compensating for both platform and target motion, significantly improving signal focusing and tracking accuracy. A detailed analysis shows that our algorithm can significantly increase the signal accumulating gain and improve the focusing effect. The simulation results are provided to demonstrate the effectiveness of the proposed algorithm.

Design of a 100-MeV compact VHEE beam line in Tsinghua University

A 100-MeV Compact Electron Accelerator design has been proposed for Very High Energy Electron (VHEE) radiotherapy research at Tsinghua University. The microwave source for this system is a 50 MW X-band klystron, paired with a pulse compressor featuring a correction cavity chain. During high-power test, the system achieved a flat-top power gain three times the input. The acceleration system consists of three main components: a backward traveling-wave buncher that bunches and accelerates electrons from a thermionic cathode gun to 8 MeV, followed by two 72-cell X-band constant-gradient traveling-wave accelerating structures, which further increase the electron energy to 100 MeV with a gradient of 80 MV/m. The total length of the system is 1.8 m, and its design is detailed in this paper.

Analysis of dispersion characteristics in helically corrugated coaxial waveguides: a theoretical and comparative study

Abstract Helically corrugated waveguides are valuable for constructing high-performance gyrotron devices and high-power pulse compressors. This paper establishes a theoretical model to analyze the dispersion characteristics of helically corrugated coaxial waveguides, where the dispersion equations for solving the propagation constants of coupled eigenmodes, as well as the detailed coupling coefficients for various mode combinations are provided. On this foundation, the dispersion characteristics of corrugated coaxial waveguides featuring single and dual helical surfaces were systematically analyzed and compared with those of helically corrugated cylindrical waveguides in the context of pulse compression application. The findings reveal that helically corrugated coaxial waveguides operating in TEM mode require fewer auxiliary components, exhibit a wider frequency range of monotonically decreasing group velocity, and have a larger dynamic range of group velocity changes; notably, the dispersion performance of the waveguides with dual corrugated conductors can even be manipulated by varying the initial phase difference between the inner and outer helical corrugations. These advantages contribute to a reduction in compressor’s length, an increase in compression ratio and energy efficiency, and an enhancement in applicability.

Development of a 300 kV/3kHz nanosecond pulse generator using semiconductor opening switches.

In this paper, we present the development of a nanosecond pulse generator utilizing semiconductor opening switches (SOS), designed to deliver high voltage and operate at a high repetitive frequency. The pulse generator comprises three main components: a primary charging unit, a magnetic pulse compression unit, and an SOS magnification unit. To ensure stable operation of the high-power charging unit at high repetitive frequencies, a rectifying resonant charging and energy recovery circuit are implemented, providing a 1kV charging voltage at a 3kHz repetition rate. The three-stage magnetic pulse compression is designed to reduce the pulse width from tens of microseconds to tens of nanoseconds, where self-demagnetization could be completed during repetitive frequency operation. To achieve an output voltage of 300kV, multiple SOS switches are employed in a series. The developed pulse generator achieves a final output of 300kV with a 3kHz repetitive frequency under a load of 2 kΩ. Furthermore, the effects of multiple factors on the output performance are characterized by both simulation and measurement for a comprehensive analysis.

Concepts of Time-frequency Structure and Pulse Compression of X-band Radiation Generated by an Oversized Coaxial Cherenkov Generator

Concepts of Time-frequency Structure and Pulse Compression of X-band Radiation Generated by an Oversized Coaxial Cherenkov Generator

X-ray pulse compressors in Laue diffraction geometry

We theoretically investigate the impact of nonlinear dispersion of crystals and multilayers used in Laue-type pulse compressors (LPCs) on chirped x-ray pulse compression, as well as the optimization method for the configuration of LPCs. We also study the application of LPCs to compress chirped x-ray free-electron laser pulses based on the parameters of LCLS-II-HE. The results show that the optimal thickness is half of the Pendellosung period, yielding the best compressor performance with minimal impact from the nonlinear dispersion. This work also provides a new idea for generating ultra-short two-color or multi-color x-ray pulses by considering deviations from the optimized thickness.

Intracavity coherent supercontinuum generation via high-order soliton dynamics in a dissipative soliton fiber laser

High-order solitons exhibit fascinating dynamics during their propagation in anomalous dispersion media. High-order soliton dynamics have been intensively exploited for extreme pulse compression and coherent ultra-broadband spectrum generation. Despite recent advances, most previous studies have been restricted to soliton propagation external to a laser cavity, leaving the intracavity generation and evolution of high-order solitons less explored. Here, we numerically demonstrate that by carefully optimizing the cavity parameters, intracavity coherent supercontinuum could be generated via high-order soliton dynamics in a dissipative soliton fiber laser. In particular, a positively chirped dissipative soliton is formed in normal dispersion gain fiber, which acts as a robust nonlinear attractor. After dechirping, a high (∼8.5) order soliton was formed in a 1.1 m highly nonlinear fiber, which subsequently experienced significant intracavity temporal self-compression and spectral broadening. The shortest pulse duration was ∼14 fs, corresponding to ∼2.7 optical cycles at 1551 nm, with the spectrum spanning over 600 nm at the −30 dB level. The generated supercontinuum maintains high coherence under quantum noise injection. In addition, the supercontinuum bandwidths were further optimized via a genetic algorithm, demonstrating the great promise of machine learning tools in optimizing complicated nonlinear systems. This work opens new possibilities for constructing compact and highly coherent supercontinuum light sources. Additionally, it provides valuable insights into the self-consistent evolution dynamics of high-order solitons within a fiber laser cavity.

Research on Wireless Passive Ultrasonic Thickness Measurement Technology Based on Pulse Compression Method.

Fixed-point thickness measurement is commonly used in corrosion detection within petrochemical enterprises, but it suffers from low detection efficiency for localized thinning, limitations regarding measurement locations, and high equipment costs due to insulation and cooling layers. To address these challenges, this paper introduces a wireless passive ultrasonic thickness measurement technique based on a pulse compression algorithm. The research methodology encompassed the development of mathematical and circuit models for single coil and wireless energy transmission, the proposal of a three-terminal wireless energy mutual coupling system, and the establishment of a finite element model simulating the ultrasonic body wave thickness measurement and wireless energy transmission system. An experimental setup was constructed to conduct thickness measurements on metal samples varying in thickness, shape, and material composition. The experimental findings revealed that the wireless ultrasonic echo signal, when processed using the pulse compression algorithm, achieved a thickness measurement accuracy approximately ten times superior to that of the untreated echo signal. This significant improvement in accuracy facilitates the high-density deployment of thickness measurement points in petrochemical applications.

Compression stability enhancement with low temperature-dependent SBS compressors at high repetition rates

The stimulated Brillouin scattering (SBS) pulse compression with stable compression efficiency in low temperature-dependent SBS compressors is demonstrated by suppressing the rising edge broadening under high repetition rates. Theoretical analysis reveals that the difficulty in achieving stable SBS compression is caused by the broadening of the phonon lifetime owing to thermal effect, and this undesirable fluctuation can be effectively suppressed by a low temperature-dependent medium. Reductions of 66.33% on Stokes pulse width broadening and 50% on rising time with repetition rate were achieved under 36.87% enhancement of kinematic viscosity stability. Benefiting from a stable gain coefficient, the maximum fluctuation of 3.51% in transient Stokes pulse energy was demonstrated at maximum pump power with stability enhancement of 16.28% on steady-state energy reflectivity.

41 W, 206 fs regenerative amplifier based on CPA-free amplification of femtosecond pulses

We present a low-cost method to mitigate the gain narrowing effects during regenerative amplification based on a high-repetition-rate, high-power continuously pumped Yb:YAG thin disk regenerative amplifier (RA). By inserting a 1-mm-thick SiO2 plate on a certain position in the RA, the narrowed bandwidth of the amplified spectrum of 3 nm was significantly broadened to a root-mean-square (RMS) bandwidth of 6.9 nm, which reveals the effectiveness of the method. At a repetition rate of 200 kHz, an output power of 41 W was obtained with a compressed pulse duration of 206 fs. The amplifier displayed stability with a root-mean-square power noise as low as 0.46%. The optical resonator remained nearly diffraction-limited beam quality (M2 < 1.35) until the maximum output power.

Robust design of mismatched filters in echo truncation for inter‐pulse waveform diversity radar

AbstractRange sidelobe modulation is a primary issue that forbids application of inter‐pulse and intra‐coherent processing interval (coherent processing interval) waveform diversity in moving target indication or moving target detection radars. Previous research has adopted mismatched filters (MMFs) to reconstruct identical compressed outputs based on an ideal model. In practice however, echo truncation is inevitable due to transceiver isolation, significantly degrading the effectiveness of MMFs. In this paper, the design of MMFs under an echo‐truncated model to achieve a robust design is considered. Specifically, the authors extract the dominant scattering profile from the close‐range echo and incorporate it into the MMF design model. To judiciously combine the robust design with the ideal one, a segmented pulse compression scheme is proposed. Numerical results indicate that the proposed robust processing scheme can effectively reduce clutter leakage in both truncated and intact scenarios.

Compression of high-power laser pulse leads to increase of electron acceleration efficiency

Compression of high-power laser pulse leads to increase of electron acceleration efficiency

Topologically-protected four-wave mixing enhanced by tailoring topological edge states

Laser pulse compression serves as a pivotal technique in nonlinear optics and light-matter interactions. Traditional methods, including Q-switching, mode-locking techniques, and chirped pulse amplification, are employed to generate high-intensity fields. However, these methods pose challenges when applied to topological lasers. In this study, we explore the pulse compression technique for topological edge states (TESs). Our findings reveal a significant correlation between the pulse width of TESs and their group velocity. Though factors such as nonlinear TES dispersion, disorders, and edge bending also affect pulse width, their impacts are comparatively less pronounced compared to that of group velocity. Through the customization of TES’s group velocity, we demonstrate a remarkable enhancement of four-wave mixing, showcasing it as a prime example of a nonlinear processing technique. Importantly, our approach promises seamless integration into topological laser systems, ensuring optimal performance with zero energy transfer loss and introducing a flexible dimension and robustness to topological nonlinear optical devices.

Investigating the transverse jitter in ultrashort electron beam generation with optical compression

Optical compression scheme is a highly potential candidate for the generation of ultrashort electron beam. This scheme utilizes a tightly focused radially polarized laser to compress the electron beam from the photocathode radio frequency (RF) gun. However, the unavoidable relative transverse jitter between the electron beam and the modulation laser leads to a performance degradation of the compressed pulse duration. To address this issue, we propose a method for correcting this transverse jitter through a specific design of optical system and an optimized decoupled solenoid. This method concentrates on the transverse jitter caused by the pointing instability of the laser. Its objective is to align both the transverse jitter direction and magnitude of the modulation laser with that of the electron beam. The simulation results demonstrate that the deleterious impact of transverse jitter can be effectively mitigated by this method, thereby ensuring the sustained generation of high-quality ultrashort electron beams.

Tunable nonlinear optical properties in polyaniline-multiwalled carbon nanotube (PANI-MWCNT) system probed under pulsed Nd:YAG laser

The investigation of the nonlinear optical (NLO) properties of polyaniline (PANI) and polyaniline doped with multi-walled carbon nanotube (PANI-MWCNT) in both solution and film forms was conducted using the open-aperture z-scan approach. A Q-switched resonant Nd: YAG laser with a wavelength of 532nm and two different fluences was utilized in this study. Based on the z-scan experiments, it was observed that the NLO behaviour of PANI and PANI-MWCNT composites exhibited a transition from reverse saturable absorption (RSA) to saturable absorption (SA) when the samples changed from a liquid to a film state. Two photon absorption (TPA) and thermally induced nonlinear scattering are the main mechanisms behind the RSA behaviour of the materials in solution form. The reason for SA behaviour of these materials in film form is attributed as the ground-state free electron bleaching in the conduction band due to the increased laser intensity. Due to increased structural disorder and the defect states or trap levels created by the dopants in the PANI system, the NLO properties of the PANI-MWCNT composite in both solution and film forms have significantly improved compared to those of pure PANI. Urbach tail analysis and carbon cluster determination revealed the presence of defect states in the composites system. The composite between PANI and MWCNT was verified using Fourier transform infrared (FTIR) spectroscopy. The functional elements present in the composites are verified by X-ray photoelectron spectroscopy. The NLO parameters of the samples, viz., the nonlinear absorption coefficient (β), the imaginary part of the nonlinear, and susceptibility χi(3) the figure of merit (FOM) of PANI-MWCNT, exhibited a notable enhancement in their values over PANI. Moreover, the PANI-MWCNT film demonstrated superior SA performance and the PANI-MWCNT solution demonstrated better OL performance compared to that of the PANI film and solution respectively. Also the effects of dopant induced structural modifications and lattice disorder on the NLO behaviour of these materials are correlated with the obtained NLO parameters. The tunable NLO properties accomplished by controlling the structural phase and dopant-induced defects enhance the possibility of these nanostructures for applications in optical limiting, optical switching, optical modulation, optical pulse compression and laser pulse narrowing.

Backscatter Signal in Underwater Lidars with a Complexly Modulated Probe Beam

The results of statistical modeling of the backscatter signal in lidars when probing the water column with pulses with internal modulation by complex frequency modulated signals and their matched processing in the receiving path of the lidar are presented. The simulation results are compared with analytical calculations in the small-angle approximation. It is shown that the photons spread along their paths, associated with multiple scattering in the medium, does not prevent effective compression of the complex signal, and the small-angle approximation well describes the energy-carrying part of the signal backscattered by the water column. A comparison was made of the levels of backscatter signals when probing water with a short pulse and a complexly modulated pulse. It is shown that the use of complexly modulated illumination pulses makes it possible to reduce the power emitted by the source while maintaining the level of the backscattering signal in the lidar and its range resolution. Calculations of the levels of the signal backscattered by a localized diffusely reflecting object are performed and it is shown that at a delay value corresponding to the arrival time of ballistic photons, the compressed pulse is not distorted. At long delay times, a pulse tail is formed due to the spread of photons along their paths. An example of calculating the backscattering pulse in the presence of a non-reflecting object in the water is given.

Unraveling the mechanisms of organic contamination on gold pulse compression gratings: From cluster formation to stratified adsorption

Organic contamination on gold pulse compression gratings significantly hampers the performance of high-power laser systems under intense laser irradiation. Investigating the adsorption mechanisms of organic contaminants on microstructured gratings is essential for addressing contamination issues and mitigating damage. In this article, we determine the microstructured surface aggregation characteristics of volatile dibutyl phthalate (DBP), the stratified distribution pattern of amorphous DBP clusters, and the distribution points of molecules within microstructures using experiments and cross-scale simulations (molecular dynamics and quantum chemistry). Our analysis of the Reduced Density Gradient (RDG) and charge transfer reveals that adsorption between organic molecules and the gold substrate primarily stems from non-covalent van der Waals and electrostatic forces induced by ester functional groups. Based on this theoretical study, we propose the "molecule-substrate" and "bimolecular" adsorption modes of DBP to elucidate the adsorption mechanisms. Investigating organic compounds' distribution and adsorption mechanisms on optical component surfaces is fundamental to high-power laser-induced damage studies. These insights facilitate the efficient, non-destructive removal of organic contaminants from gold gratings, enhancing the energy output threshold of inertial confinement fusion devices.

High-power, frequency-doubled all-polarization-maintaining fiber laser system at 925 nm.

We demonstrate a high-power 925-nm pulsed laser system based on a frequency-doubled, all-polarization-maintaining (PM) fiber laser source operating at 1.8 µm. The seed is a figure-9 mode-locked oscillator, which incorporates a nonlinear amplifying loop mirror. After power scaling and pulse compression, the 1.8-µm laser source can provide femtosecond pulses with a repetition rate of 31.3 MHz and an output power of 2.24 W. Through frequency doubling in a nonlinear crystal, the 925-nm laser delivers a pulse duration of 503 fs and an output power of 818 mW, which is the highest power provided by all-PM fiber laser systems at this wavelength, as far as we know. Furthermore, this 925-nm all-PM fiber laser is employed as the excitation light source for two-photon microscopy (TPM) and second-harmonic generation (SHG) microscopy.

Sidelobe Suppression Method with Improved CLEAN Algorithm for Pulse Compression OTDR

Sidelobe Suppression Method with Improved CLEAN Algorithm for Pulse Compression OTDR