Satellite Pulses Research Articles

Elliptically polarized, nanosecond dual-pulse Raman laser with tunable pulse interval and pulse amplitude ratio

Dual-pulse solid state lasers with pulse interval of tens nanoseconds have been applied in laser-induced breakdown spectroscopy, laser drilling, medical treatment, and atmospheric monitoring. Here, a Yb:YAG/Cr4+:YAG/YVO4 passively Q-switched Raman laser has been developed for generating dual-pulse lasers with tunable pulse interval and elliptical polarization. Pulse interval and pulse amplitude ratio between peaks of satellite pulse and main pulse are controlled by adjusting the incident pump power (Pin). As Pin increases from 1.3 to 5.9 W, the pulse interval decreases from 63.1 to 36.3 ns, and the pulse amplitude ratio between peaks of satellite pulse and main pulse increases from 0.1 to 1.1 for dual-pulse Raman lasers. At Pin = 4.9 W, dual-pulse Raman laser with equal peak power of 1.7 kW and pulse interval of 40.2 ns is obtained. The pulse widths are 2.7 and 3.6 ns, and the pulse energy are 4.6 and 6.2 μJ for the main pulse and satellite pulse, respectively. The elliptical polarization with ellipticity tunable from 0.08 to 0.54 is obtained for dual-pulse passively Q-switched Raman laser as Pin increases. Such a nanosecond dual-pulse laser source has the potential to expand simple, compact portable and economical laser applications in various fields.

Ultrafast dissipative soliton generation in anomalous dispersion achieving high peak power beyond the limitation of cubic nonlinearity

The maximum peak power of ultrafast mode-locked lasers has been limited by cubic nonlinearity, which collapses the mode-locked pulses and consequently leads to noisy operation or satellite pulses. In this paper, we propose a concept to achieve mode-locked pulses with high peak power beyond the limitation of cubic nonlinearity with the help of dissipative resonance between quintic nonlinear phase shifts and anomalous group velocity dispersion. We first conducted a numerical study to investigate the existence of high peak power ultrafast dissipative solitons in a fiber cavity with anomalous group velocity dispersion (U-DSAD) and found four unique characteristics. We then built long cavity ultrafast thulium-doped fiber lasers and verified that the properties of the generated mode-locked pulses match well with the U-DSAD characteristics found in the numerical study. The best-performing laser generated a peak power of 330 kW and a maximum pulse energy of 80 nJ with a pulse duration of 249 fs at a repetition rate of 428 kHz. Such a high peak power exceeds that of any previous mode-locked pulses generated from a single-mode fiber laser without post-treatment. We anticipate that the means to overcome cubic nonlinearity presented in this paper can give insight in various optical fields dealing with nonlinearity to find solutions beyond the inherent limitations.

Study on satellite pulse characteristics of LD-end pumped sub-nanosecond Nd:YAG/Cr4+:YAG oscillator

Miniaturized sub-nanosecond laser is critical to the applications such as medical treatment, Lidar and manufacturing. In this paper, a high repetition rate hundred picosecond pulse oscillator based on Nd:YAG/Cr4+:YAG bonded crystal is demonstrated, and its output pulse characteristics are studied. It is found that the stability of amplitude and frequency of Q-switched pulse can be improved effectively by pulsedpumping. A satellite-free sub-nanosecond laser with a pulse width of 663 ps and an average power of 80 mW at the repetition rate of 1 kHz was developed, while the 50 μs pulse width of the pump was employed. Meanwhile, the near-diffraction limit output beam (M2 =1.03 ± 0.02) with pulse energy instability of less than 2 % was obtained. This study provides a reference for the development of highly stable satellite-pulse-free passive Q-switched lasers with a high repetition rate.

Design of Power Supply System for High Power SAR Satellite

Synthetic Aperture Radar (SAR) satellite power system has the characteristics of high power, pulse power supply and fast response speed. Based on the combination of domestic and foreign SAR satellite power supply structures, a SAR satellite power supply system scheme with a composite bus architecture is proposed in this paper. By adding battery discharge regulators (BDR) between the platform bus and the SAR load bus as a grid-connected controller, the SAR load bus can supply power to the platform load, and the solar array and battery pack can be used efficiently while achieving dual bus output. Aiming at the composite bus, this paper also proposes an efficient high-power filter technology. Finally, the ground test results verify the correctness of the scheme, which can meet the different power supply requirements of the SAR satellite stable and pulse load equipment.

A combined denoising method of empirical mode decomposition and singular spectrum analysis applied to Jason altimeter waveforms: A case of the Caspian Sea

During the satellite pulse propagation and reception, the altimeter waveform is inevitably affected by noise. To reduce the noise level in Jason altimeter waveforms, we used singular spectrum analysis (SSA), empirical mode decomposition (EMD), and the combination of SSA and EMD to obtain the denoised waveforms. The advantages of the combined method were verified and the accuracy of the mean sea surface height (MSSH) model was improved. Comparing the denoising effect of the three methods, the results show that the signal-to-noise ratio (SNR), correlation coefficient and root-mean-square error are effectively improved by the combination of SSA and EMD. The sea surface heights (SSHs) were remeasured with a 50% threshold retracker of denoised waveforms, and the MSSH model of the Caspian Sea with a grid of 1′×1′ was established from the retracked SSHs of Jason-1/2/3. Taking the mean value of the four models as a control, it is found that the model calculated by the combined denoising method has the highest accuracy. This indicates that using the combined denoising method to reduce the noise level is beneficial to improve the accuracy of the MSSH model.

Peak Shift of Coherent Edge Radiation Spectrum Depending on Radio Frequency Field Phase of Accelerator

Spectra of coherent edge radiation (CER) were observed at the S-band linac facility of Kyoto University Free Electron Laser. A local maximum was observed in the CER spectrum on-crest operation of the radio frequency (RF) field. As the phase of the RF field was shifted from the crest, the frequency of the maximum decreased, and the CER spectrum approached a spectrum of Gaussian-distributed electrons in a bunch. It was found that this strange spectrum can be explained by a model in which a satellite pulse exists around a main pulse in the electron bunch. Furthermore, it demonstrated that CER is an effective tool for monitoring the shape of the electron bunch.

Experimental revealing of fiber laser soliton build-up activated by shaking-soliton triplets

Optimization of self-starting in passively mode-locked optical soliton fiber laser resonators results from a complex transient dynamic involving soliton molecules, soliton-based complexes and supramolecular structures, with particle-like properties. By means of time-stretch dispersive Fourier transform based real-time spectroscopy, we identify a new route to the passive mode-locking of fiber laser solitons. A raised relaxation oscillation stage is followed by the generation of shaking-soliton molecular triplets. The relative phase between satellite and main pulses of shaking soliton triplets evolves chaotically, while the two satellite pulses are orthogonal to each other in their state of polarization. These results provide new perspectives into the soliton formation and the internal dynamics of soliton molecules with higher degrees of freedom.

Asymmetric interferometric frequency resolved optical gating for complete unambiguous ultrashort pulse characterisation

We propose a modification of the second-harmonic generation frequency resolved optical gating method (SHG-FROG) utilizing asymmetric spectral interferometry (aiFrog) for complete unambiguous reconstruction of ultrashort optical pulse intensity and phase. The large amount of information encoded in the aiFrog trace allows a straightforward calculation of the fundamental pulse spectrum and a direct reconstruction of the pulse envelope and phase. A simple and robust iterative algorithm with fast convergence is developed for pulse reconstruction from the aiFrog trace, taking advantage of large data redundancy and noise averaging. Due to the asymmetry of the setup, our method is free from the known challenges of a standard SHG-FROG (direction-of-time ambiguity, almost identical FROG traces for well separated pulses with different phases and for asymmetric well separated pulses). The method is highly sensitive to the weak satellite pulse of the main pulse, which can be used to track pre- or post-pulses even without full processing of the measured aiFrog trace.

Generation of Terawatt Attosecond Pulses from Relativistic Transition Radiation.

When a femtosecond duration and hundreds of kiloampere peak current electron beam traverses the vacuum and high-density plasma interface, a new process, that we call relativistic transition radiation (RTR), generates an intense ∼100 as pulse containing ∼1 terawatt power of coherent vacuum ultraviolet (VUV) radiation accompanied by several smaller femtosecond duration satellite pulses. This pulse inherits the radial polarization of the incident beam field and has a ring intensity distribution. This RTR is emitted when the beam density is comparable to the plasma density and the spot size much larger than the plasma skin depth. Physically, it arises from the return current or backward relativistic motion of electrons starting just inside the plasma that Doppler up shifts the emitted photons. The number of RTR pulses is determined by the number of groups of plasma electrons that originate at different depths within the first plasma wake period and emit coherently before phase mixing.

Generation of Intense Sub-10 fs Pulses at 385nm* *Supported by the National Key Research and Development Program of China (Grant No. 2019YFA0307703), the Major Research Plan of NSF of China (Grant No. 91850201), and the National Natural Science Foundation of China (Grant Nos. 11974426, 11974425, U1830206 and 11604386).

We demonstrated the generation and characterization of 9.7 fs, 180 μJ pulses centered at 385 nm via the frequency doubling of few-cycle near-infrared pulses. Both moderate conversion efficiency (9.5%) and broad phase matching bandwidth (20 nm) were achieved by shaping the spectra of the fundamental pulses. The strong intensity dependence of second-order harmonic generation and well controlled material dispersion ensured the inexistence of satellite pulses, which was confirmed by the self-diffraction frequency resolved optical gating measurement.

Randomly Spaced Phase-Only Transmission Combs for Femtosecond Pulse Shaping

We present a new Randomized Multiple Independent Comb Shaping (RandoMICS) algorithm based on phase-only tailored transmission for ultrashort laser pulse replication. The benefit of this method is satellite-free generation of programmable laser pulse sequences. The result is achieved by creating a transmission function as a stochastic comb of disjoint segments of optical frequency continuum with numerically optimized segment width distribution. The algorithm is realized by generating a regular aperiodic comb and random permutations of its elements. Experimental demonstration is performed with an acousto-optic pulse shaper providing broadband multi-window transmission function with arbitrarily variable widths of the segments. Suppression of undesired satellite pulses by the factor of 8 is demonstrated as well as generating pulse replicas with extended usable delay range compared to phase-only pulse shaping with periodic transmission combs.

Noise effects and the impact of detector responses on the characterization of extreme ultraviolet attosecond pulses.

We employ numerical simulations to study the effects of noise on the reconstruction of the duration and satellite intensity ratio for transform-limited single and double pulses of 200 as duration. The forms of noise we implement are delay jitters between the attosecond pulse and the near-IR laser field, energy resolution of the photoelectron detector, and Poisson noise in streaking spectrograms with different count levels. We use the streaking method to characterize the pulse and the extended ptychographic iterative engine retrieval algorithm to reconstruct the pulse from the simulated streaking spectrogram. We found that, for practical purposes, when implementing a combination of all three mentioned noise contributions, the attosecond pulse duration will be overestimated when the photoelectron count level is low. Furthermore, the satellite pulse amplitude of the attosecond double pulse can be retrieved within 10% accuracy.

CW-to-pulse conversion using temporal Talbot array illuminators.

We report on the linear conversion of continuous-wave (CW) laser light to optical pulses using temporal Talbot array illuminators (TAIs) with fractional orders 1/q(q≤10), implemented by use of multilevel PM and dispersive propagation in a chirped fiber Bragg grating. The generated, sub-nanosecond optical pulse trains have repetition rates in the gigahertz range and show the presence of satellite pulses originated by the finite electrical modulation bandwidth (7.5GHz). Though this fact impacts the resulting extinction ratio, an experimental comparison with time and Fresnel lenses indicates that temporal TAIs represent compact systems with high light gathering efficiency (>87%) at moderate values of compression (q≤8), which can be tailored in repetition rate, gain, or width, through the fractional Talbot order for its use in pulse compression systems fed by CW light.

Attosecond pulse generation isolated with an asymmetric polarization gating

High harmonics generated using the polarization gating technique are simulated under the strong-field approximation for the cases of equal and unequal ratios between the amplitudes of the two counter-rotating pulses. The effect of the field asymmetry is observed on the high harmonic yield and cut-off in the frequency domain and on the attosecond pulse duration and satellite peak contrast in the time domain. The case where the strong pulse comes later is more favourable in terms of generating a higher cut-off and a shorter pulse duration. It is predicted that thin metal filters can reduce the impact of satellite pulses while simultaneously reducing the pulse duration.

Unstable and Multiple Pulsing Can Be Invisible to Ultrashort Pulse Measurement Techniques

Multiple pulsing occurs in most ultrashort-pulse laser systems when pumped at excessively high powers, and small fluctuations in pump power in certain regimes can cause unusual variations in the temporal separations of sub-pulses. Unfortunately, the ability of modern intensity-and-phase pulse measurement techniques to measure such unstable multi-pulsing has not been studied. Here we report calculations and simulations finding that allowing variations in just the relative phase of a satellite pulse causes the second pulse to completely disappear from a spectral interferometry for direct electric field reconstruction (SPIDER) measurement. We find that, although neither frequency-resolved optical gating (FROG) nor autocorrelation can determine the precise properties of satellite pulses due to the presence of instability, they always succeed in, at least, seeing the satellite pulses. Also, additional post-processing of the measured FROG trace can determine the correct approximate relative height of the satellite pulse and definitively indicate the presence of unstable multiple-pulsing.

Research of multi-pulses emission in passively Q-switched Nd3+:YAG microchip laser

Multi-pulses emission in the passively Q-switched microchip Nd3+:YAG/Cr4+:YAG laser was studied. The pulses include a main pulse and several satellite pulses. The width of the main pulse was 3.6ns under the pump power of 14.78W, corresponding to a pulse repetition rate of 7.89kHz. The spectrum of pulses varying in different pump power was analyzed. Multi-longitudinal-modes oscillation was observed. Experiment result shows multi-pulses emission is related with multi-longitudinal-modes oscillation.

Using irregularly spaced current peaks to generatean isolated attosecond X-ray pulse in free-electron lasers.

A method is proposed to generate an isolated attosecond X-ray pulse in free-electron lasers, using irregularly spaced current peaks induced in an electron beam through interaction with an intense short-pulse optical laser. In comparison with a similar scheme proposed in a previous paper, the irregular arrangement of current peaks significantly improves the contrast between the main and satellite pulses, enhances the attainable peak power and simplifies the accelerator layout. Three different methods are proposed for this purpose and achievable performances are computed under realistic conditions. Numerical simulations carried out with the best configuration show that an isolated 7.7 keV X-ray pulse with a peak power of 1.7 TW and pulse length of 70 as can be generated. In this particular example, the contrast is improved by two orders of magnitude and the peak power is enhanced by a factor of three, when compared with the previous scheme.

Comparison of high-order-harmonic generation on single-layer graphene flakes with armchair and zigzag types in an intense laser field

The high-order-harmonic generation (HHG) of graphene in an intense laser field is investigated using the strong-field approximation method. The initial wave function is presented by gaussian and gamess software. The molecular structure along the $x$ and $y$ axes represents different types of graphene: armchair and zigzag, respectively. The results show that the HHG intensity of the armchair type of graphene is two magnitudes higher than that of the zigzag type in the plateau area. The ionization yield and electron density distribution are also presented to further explain this difference. Finally, by superposing a properly selected range of harmonics, a main pulse with the duration of 91 and 99 attoseconds accompanied by weak satellite pulses will be generated for the case of armchair and zigzag graphene, respectively, and the corresponding intensity from armchair graphene is much higher than that from zigzag graphene.

Highly Accurate in Vivo Terahertz Spectroscopy of Healthy Skin: Variation of Refractive Index and Absorption Coefficient Along the Human Body

A method to reconstruct the terahertz (THz) refractive index and absorption coefficient of in vivo tissue using THz pulsed spectroscopy (TPS) has been proposed. The method utilizes a reference THz window to fix the sample of interest during the TPS reflection mode measurements. Satellite pulses caused by multiple THz-wave reflections in the reference window are taken into account to accurately solve the inverse problem. The stability of the proposed method in the presence of various factors, including digital noise in the TPS waveforms and fluctuations of the reference THz window position, has been accurately analyzed. The method has been implemented to study in vivo the THz refractive index and absorption coefficient of the human skin. The skin from three persons has been measured, and the results agree with the well-known data on healthy skin spectroscopy in general, except for several regions of the skin. Thus, for the elbow, the hand, the knee, and the heel the THz refractive index and absorption coefficient considerably differ from the average values. The observed results are of principle importance for further development of novel approaches to skin diagnosis based on THz technologies.

An impact of multiple wave reflections in a flat sample on material parameter reconstruction using THz pulsed spectroscopy

The present paper is dedicated to studying the accuracy of sample material parameter reconstruction using terahertz (THz) pulsed spectroscopy. The technique for characterizing the material parameters of thin flat samples allowing to take into account multiple wave reflections in a flat have been considered. While transmitting through the resonant sample, THz pulse undergoes multiple reflections, which result in satellite pulses in THz waveform. The accuracy of material parameter reconstruction strongly depends on the number of satellite pulses. We have analytically estimated an impact of satellite pulses on material parameter reconstruction by considering the local linearization of theoretical sample transfer function based on the model of quasi-Fabry-Perot-resonator.