Self-compression Of Pulses Research Articles

Compression and self-compression of frequency modulated spherical photon density waves in anisotropic scattering media

We report on the results of a simulation of the photon density waves with pulse amplitude modulation by a complex frequency modulated signal. The problem is considered for the optical properties typical for sea water with anisotropy factor values varying from 0.75 to 0.93 at source-detector distances up to 120 m. It is shown that multiple scattering in a medium does not prevent effective compression of a signal registered using matched detection. Two competing phenomena affecting the detected pulse duration and depending on the central frequency of the modulation signal are discussed. The effect of faster attenuation of high harmonics in a complexly modulated signal leads to the detected signal duration increase as a consequence of multiple scattering. On the other hand, anomalous dispersion of photon density waves in media with scattering anisotropy leads to the pulse self-compression. The simulation results presented in the paper demonstrate the prevalence of different phenomena depending on the central frequency of the modulation signal resulting in a pulse duration decrease or increase in different frequency ranges covering the band from 107 to 2⋅109 Hz. The effect of the phase function shape on the observed effect is also discussed.

Self-compression of laser pulses induced by asymmetric self-phase modulation aided by backward Raman scattering in periodic density-modulated plasma.

Here a mechanism for self-compression of laser pulses is presented, based on period density-modulated plasma. In this setup, two pump beams intersect at a small angle within the plasma. This interaction is facilitated by the ponderomotive ion mechanism, which causes a modulation in the density of plasma with long wavelengths and low amplitude. This modulation enhances the backward Raman scattering of the probe pulse. The trailing edge of the probe experiences greater energy loss, resulting in a steeper intensity gradient. This, in turn, induces an asymmetric self-phase modulation, which elevates the instantaneous frequency. It is notable that the laser in plasma exhibits opposite group velocity dispersion compared to traditional solid-state media. This unique property allows laser pulses to undergo dispersion compensation while broadening the spectrum, ultimately leading to self-compression. The 2D-PIC simulations demonstrate these phenomena, highlighting how period density-modulated plasma contributes to an asymmetric spectral distribution. The intricate interplay among self-phase modulation, group velocity, and backward Raman scattering results in the self-compressing of the laser pulse. Specifically, the pulses are compressed from their Fourier transform limit duration of 50 fs to a significantly reduced duration of 8 fs at plasma densities below 1/4 critical density, without the transverse self-focusing effects.

Single-step self-compression of ultrashort pulses below 20 fs by all-fiber supercontinuum generation

We present an ultrashort-pulsed laser source with time-widths as short as 20 fs at around 1.5 µm. For this purpose, the pulses have been spectrally broadened and temporarily compressed by means of supercontinuum generation in a highly nonlinear optical fiber. Inspired by numerical simulations, we have experimentally confirmed that the emission of a resonant dispersive wave is a key indicator for the optimum configuration to achieve maximum temporal compression. No dispersion control stage is needed, which results in the simplicity of the system. An extensive analysis has been made concerning the quality of the pulses obtained, particularly in terms of time-width (at half height and at 1/e height of the maximum) as well as their time-bandwidth product. The complex retrieval of the pulses shows that the smallest temporal width obtained experimentally is 17 fs. A comparison with values found in the literature also shows that our pulses are among the best that can be found with this technology.

Spatiotemporal reshaping of femtosecond laser pulses on interaction with gas sheath at ionization saturation intensity regime

Interaction of intense ultrashort laser pulse with gases generates a transient spatiotemporal electron density distribution via field ionization, which may lead to the spatiotemporal reshaping of the pulse, viz., its beam profile, pulse width, etc. Here, we present an experimental study on ultrashort laser pulse interaction with argon gas sheath in an ionization saturation intensity regime (∼1015–1017 W/cm2). The present investigation has been performed using a 6 mJ, 1 kHz, and 55−60 fs Ti:Sapphire laser pulse interaction with a ∼2.5 mm long argon sheath. After the laser gas interaction, the laser spatial profile exhibits a multi-ring structure around a central maximum spot. Laser gas interaction parameters, such as laser intensity, gas pressure, etc., affect the ring pattern significantly. Under optimum parameter conditions, the laser pulse has two rings in spatial profile, and the pulse width of the central spot is self-compressed to ∼35 fs. A theoretical calculation reveals that the laser beam’s spatiotemporal profile evolves as it propagates inside the gas sheath. The calculation also demonstrates that the gas ionization profile plays a crucial role in the spatiotemporal reshaping and self-compression of the laser beam. The calculation also shows that the generation of concentric ring patterns in the spatial profile is mainly due to the ionization of argon atoms into Ar+, Ar2+, and Ar3+ species in the interaction region. Such self-compressed laser pulses with concentric ring beam profiles may be useful for high-harmonic generation and shorter attosecond pulse trains.

Self-compression of 4.9 µm pulses to sub-40 fs with 2 mJ energy in Zinc Sulfide

Nonlinear self-compression of few-cycle multi-mJ pulses at 4.9 µm in ZnS is presented. 80 fs input pulses are compressed to 37 fs with 2.1 mJ energy at a 1 kHz repetition rate. © 2024 The Author(s)

Optical solitons in hollow-core fibres

I review the historical observation and subsequent research on optical soliton dynamics in gas-filled hollow-core optical fibres. I include both large-core hollow capillary fibres, and hollow-core photonic-crystal or microstructured fibres with smaller cores, in particular photonic bandgap and antiresonant guiding fibres. I discuss how the optical guidance properties of these different fibre structures influence the soliton dynamics that can be obtained. The dynamics I consider include: soliton propagation at peak power levels ranging from the megawatt to terawatt level, and pulse energies from sub-microjoule to millijoule range; pulse self-compression, leading to sub-cycle and sub-femtosecond pulse duration; soliton self-frequency shifting due to both the Raman effect, and the influence of photoionisation and plasma formation; and resonant dispersive wave emission, leading to the generation of tuneable few-femtosecond pulses across the vacuum and deep ultraviolet, visible, and near-infrared spectral regions.

Spectral Self-Compression of Chirp-Free Pulses in Anomalously Dispersive Optical Fibers

In this work, we demonstrated, both experimentally and numerically, the existence of a chirp-free pulse propagation regime within a standard single-mode fiber. We found numerically that the pulse spectrum can undergo self-compression by more than a hundredfold, resulting in the creation of a spectrally limited pulse. In this regime, we experimentally achieved a 5× spectral self-compression, reducing the spectral width from 75 nm to 15 nm within the 1.3–1.5 μm spectral range. This was achieved with a standard telecommunication fiber with constant dispersion.

Self-compression of femtosecond laser pulses in ambient air through conical radiation.

We demonstrate self-compression of 98 fs near-infrared laser pulses down to 8.8 fs in ambient air, utilizing self-phase modulation in air and negative dispersion in the properties of a laser-induced plasma. The blueshifted pulses achieve self-compression through conical radiation, eliminating the need for additional dispersion compensation. The results highlight a simple and compact approach to generate sub-10 fs laser pulses without additional measures for time-resolved applications in ultrafast diagnostics and spectroscopy.

Effective soliton order approach for scaling of pulse self-compression in hollow-core fibers

Optical solitons can undergo extreme temporal and spectral reshaping at high pulse intensities. These dynamics can be utilized to extract few-cycle pulses in the negative dispersion region in the case of higher-order spatial modes in hollow fibers. One can achieve self-compression up to few-cycle duration in hollow capillary fibers without any external optical post-compression schemes using this technique. These intense few-cycle pulses are an essential tool for revealing the ultrafast dynamics in the femtosecond to attosecond timescale, where the tunability and scalability of few-cycle pulses are a necessity. Utilizing the soliton dynamics, we can scale the pulses up to multi-millijoule energies as well as terawatts of peak power. Scaling relations are relations that can be used to produce propagation dynamics that are invariant and essentially identical for multiple sets of input conditions. But, for the same input soliton order, the scaling relations derived under different dispersion conditions, such as different gas pressure, result in somewhat different scaling laws. This leads to an ambiguity in the compression factor and compression length for any particular soliton order N. It is thus necessary to find an accurate soliton order which can scale the self-compression dynamics over different dispersion conditions. We numerically simulate soliton self-compression in an argon gas-filled HCF across a wide range of dispersion conditions and present a set of efficient scaling laws for soliton dynamics. We introduce an effective soliton order Neff for explaining the behavior of the dynamics in systems with high third-order dispersion (TOD) such as the HCF. The proficiency of the new scaling laws over the earlier scaling laws in literature is elucidated in this work. Thus, an inclusive set of scaling laws for generating high-energy few-cycle pulses are defined which are critical for generating single and trains of attosecond pulses, as well as electron and ion acceleration strategies in intense laser pulses.

Observation of Spatiotemporal Self-Compression of Pulses in One-Dimensional Waveguide Arrays Fabricated by Laser Writing

The features of nonlinear propagation of high-intensity pulses in the short-wavelength infrared range in extended one-dimensional waveguide arrays with different spatial periods, formed in fused silica by laser writing, are studied. More than tenfold self-compression of femtosecond pulses up to a duration of several periods of the light field is experimentally observed.

Temporal Self-Compression and Self-Frequency Shift of Submicrojoule Pulses at a Repetition Rate of 8 MHz

We combine soliton dynamics in gas-filled hollow-core photonic crystal fibers with a state-of-the-art fiber laser to realize a turn-key system producing few-fs pulses at 8 MHz repetition rate at pump energies as low as 220 nJ. Furthermore, by exploiting the soliton self-frequency shift in a second hydrogen-filled hollow-core fiber, we efficiently generate pulses as short as 22 fs, continuously tunable from 1100 nm to 1474 nm.

High-quality 8-fold self-compression of ultrashort near-UV pulses in an Ar-filled ultrathin-walled photonic crystal fiber

We demonstrate generation of 7.6 fs near-UV pulses centered at 400 nm via 8-fold soliton-effect self-compression in an Ar-filled hollow-core kagomé-style photonic crystal fiber with ultrathin core walls. Analytical calculations of the effective compression length and soliton order permit adjustment of the experimental parameters, and numerical modeling of the nonlinear pulse dynamics in the fiber accurately predicts the spectrotemporal profiles of the self-compressed pulses. After compensation of phase distortion introduced by the optical elements along the beam path from the fiber to the diagnostics, 71% of the pulse energy was in the main temporal lobe, with peak powers in excess of 0.2 GW. The convenient setup opens up new opportunities for time-resolved studies in spectroscopy, chemistry, and materials science.

Average-power (4.13 W) 59 fs mid-infrared pulses from a fluoride fiber laser system

We report a high-average-power mid-infrared ultrafast laser system consisting of a fluoride fiber mode-locked oscillator and a nonlinear amplifier. A backward pumping scheme was used in the amplifier to simultaneously realize pulse amplification and self-compression. The input signal polarization was demonstrated to play an important role in the self-compression process. Through the optimization of input polarization, a 4.13 W average-power 59 fs pulse at 2.8 µm was achieved, with an estimated pulse energy of 42.2 nJ and a peak power of 715 kW. To the best of our knowledge, this is the highest average-power pulse with sub-100-fs duration generated from a mid-infrared fiber laser system to date.

Scaling of self-compression of near-IR femtosecond pulses in hollow-core fibers down to the single-cycle limit

We numerically investigate the scaling of self-compression processes with experimental parameters for near-infrared ultrashort pulses (30 fs) in gas-filled hollow-core fiber (HCF). These simulations over a wide-range of input pulse energies as well as filling gas pressures reveal a remarkable scaling of the self-compression process and dynamics. As a function of soliton order N, we identify the relation between the propagation distance after which self-compression in the HCF begins and the subsequent propagation length up to which the pulse remains maximally compressed; both these length scales decrease with an increase in N, the soliton order. Although previous investigations revealed pulse compression scaling laws which provide a good approximation for input pulse-widths ∼100 fs down to the limit where soliton fission begins to dominate the dynamics, these are not sufficiently accurate to describe the entire scaling dynamics. Instead, we identify a more generalized set of scaling laws by taking both third-order dispersion and the saturation of the compression factor due to soliton fission into account. These conclusions about scaling are robust: our simulations were carried out over a wide range of realistic input pulse energies and gas pressures as implemented in laboratories taking into account higher-order dispersive properties of the gaseous propagating medium. Therefore, given that these numerical investigations consider conditions typically applied in practice in laboratories, this work provides elegant design principles and guideposts relevant to realizing systems capable of achieving self-compression at substantially high pulse energies down to the few-cycle limit; they are of paramount importance in generating single as well as trains of attosecond pulses and acceleration strategies for electrons and ions in intense laser pulses.

Optimization of pulse self-compression in hollow capillary fibers using decreasing pressure gradients.

The improvement of techniques for the generation of near-infrared (NIR) few-cycle pulses is paving the way for new scenarios in time-resolved spectroscopy and the generation of ultrashort extreme-ultraviolet pulses through high-harmonic generation. In this work, we numerically study how to optimize the self-compression of NIR pulses using decreasing pressure gradients in hollow capillary fibers (HCFs). We identify a moderate nonlinear regime in which sub-cycle pulses are obtained with very good temporal quality from an input 30 fs pulse centered at a 800 nm wavelength and coupled as the fundamental mode of an argon-filled HCF fully evacuated at the output end. Surprisingly, we observe that there is a relatively broad region of parameters for which the optimum self-compression takes place, defined by a simple relation between the input pulse energy and the initial gas pressure.

Pulse self-compression down to the sub-cycle regime in hollow capillary fibers with decreasing pressure gradients

We theoretically demonstrate an enhancement in the generation of clean, near-infrared sub-cycle laser pulses by soliton self-compression in gas-filled hollow capillary fibers using decreasing pressure gradients. Furthermore, we identify the optimal input parameters for high quality compression and the main advantages of this promising technique which paves the way towards ultrafast vacuum experiments.

High-Sensitivity Gas Detection with Air-Lasing-Assisted Coherent Raman Spectroscopy

Remote or standoff detection of greenhouse gases, air pollutants, and biological agents with innovative ultrafast laser technology attracts growing interests in recent years. Hybrid femtosecond/picosecond coherent Raman spectroscopy is considered as one of the most versatile techniques due to its great advantages in terms of detection sensitivity and chemical specificity. However, the simultaneous requirement for the femtosecond pump and the picosecond probe increases the complexity of optical system. Herein, we demonstrate that air lasing naturally created inside a filament can serve as an ideal light source to probe Raman coherence excited by the femtosecond pump, producing coherent Raman signal with molecular vibrational signatures. The combination of pulse self-compression effect and air lasing action during filamentation improves Raman excitation efficiency and greatly simplifies the experimental setup. The air-lasing-assisted Raman spectroscopy was applied to quantitatively detect greenhouse gases mixed in air, and it was found that the minimum detectable concentrations of CO 2 and SF 6 can reach 0.1% and 0.03%, respectively. The ingenious designs, especially the optimization of pump-seed delay and the choice of perpendicular polarization, ensure a high detection sensitivity and signal stability. Moreover, it is demonstrated that this method can be used for simultaneously measuring CO 2 and SF 6 gases and distinguishing 12 CO 2 and 13 CO 2 . The developed scheme provides a new route for high-sensitivity standoff detection and combustion diagnosis.

Self-compression of mJ pulses to 10 fs in a hollow-core waveguide: effects of higher-order dispersion

Self-compression of an ultrashort pulse in a hollow-core waveguide filled with noble gas is a simple and promising approach to generate few and even single-cycle pulses. However, when the input pulse energy approaches to mJ level, ionization of the gas induces strong higher-order dispersion through multiple mechanisms, which makes the temporal compression process unstable and even fail. In this paper, we systematically study the effects of higher-order dispersion in the self-compression process of mJ few-cycle pulses. We found that the self-compression depends on the approaching routine of group delay dispersion and third-order dispersion optimization. There exists a steady and stable routine to maintain the pulse duration around the Fourier transform limit. Then, we successfully demonstrate stable and repeatable compression of 3 mJ pulses to 13.1 and 10.5 fs in a 2 cm hollow-core waveguide filled with Ar and Kr, respectively.

2-MW peak-power pulses from a dispersion-managed fluoride fiber amplifier at 2.8 µm.

We report on a scheme of pulse amplification and simultaneous self-compression in fluoride fiber for generating a high-peak-power pulse at 2.8-µm wavelength. We find dispersion management plays a key role for the amplification and self-compression process. Through dispersion management with a Ge rod, pulse amplification and simultaneous pulse self-compression were realized in the small anomalous dispersion region. A 2-MW peak-power pulse was achieved through amplification and self-compression in Er:ZBLAN fiber, with pulse energy of 101 nJ and pulse duration of 49 fs. To the best of our knowledge, this is the highest peak power obtained from fluoride fiber at 2.8 µm, and will benefit a series of applications.

Study of wavelength-dependent pulse self-compression for high intensity pulse propagation in gas-filled capillaries.

We theoretically investigate the wavelength-dependent pulse self-compression dynamics of intense femtosecond laser pulses in gas-filled capillaries. Simulations with λ = 1, 2, 3 and 4 µm using the multimode carrier-resolved unidirectional pulse propagation equation reveal pulse self-compression or pulse broadening depending on plasma and modal dispersion. Our study shows that the pulse at 1 µm exhibits better pulse self-compression compared with longer wavelengths due to smaller group velocity mismatch between fundamental and higher-order capillary modes.