A model of a semiconductor optical amplifier (SOA) has been developed. Using this model we simulated the distortions of a quadrature-modulated (QAM) signal propagating through a fiber-optic line with SOAs and determined the baudrates and powers which guarantee a non-critical number of errors during transmission.

We measured and simulated the spectrally resolved angular distributions of 0.3–1-THz emission from the two-color filament with its plasma length (∼40 mm) exceeding the dephasing length (∼25 mm) between the fundamental (740 nm) and the second harmonic (370 nm) pulses in air. We show that only the forwardly propagating on-axis terahertz (THz) radiation is sensitive to the variation of the phase offset φ between fundamental and second harmonics, while the ring-like THz beam carrying ≳80% of the overall THz yield is independent of φ. Utilization of the THz ring allows one to omit the tedious adjustment of the frequency-doubling crystal position in the experiment.

Subject of study. The study focuses on a fiber-optic communication line incorporating semiconductor optical amplifiers, specifically spontaneous emission and the nonlinear effects in the amplifiers that lead to digital signal transmission errors. Aim of study. The aim of this study was to develop a model of a fiber-optic communication line with integrated semiconductor optical amplifiers, taking into account distortion sources for both weak and strong signals, namely, spontaneous emission and nonlinear effects in the active medium of the amplifiers. Method. The study was conducted using numerical simulations. The model accounted for the propagation of radiation along a 100-km optical fiber span, during which extensive dispersive spreading and absorption occur. The average power of the laser radiation after this span determined the amplifier's quasi-stationary operating mode, including the power of spontaneous radiation in it. In this operating mode, the transformation of a quadrature-modulated signal within the semiconductor amplifier was calculated. Main results. A numerical model of a fiber-optic communication line with integrated semiconductor optical amplifiers has been developed. This model has been used to analyze information loss in a quadrature-modulated signal; this information loss is attributed to gain dispersion, gain saturation, and spontaneous emission in the active medium of the amplifiers. When the laser pulse duration is 12 ps or longer, the amplifier's input power dynamic range exceeds 20 dBm (from -37.5 dBm to -17.5 dBm). Within this range, compensation for amplifier-induced distortion is not required. Practical significance. The results obtained in this study can be used to increase data transmission rates in fiber-optic communication lines over distances ranging from hundreds to thousands of kilometers. (c) 2025 Optica Publishing Group

At the selected frequencies from 0.3 to 10 THz we measured the two-dimensional (2D) distributions of fluence and polarization of terahertz (THz) emission from a single-color femtosecond filament. At the majority of frequencies studied, the THz beam has a donut-like shape with azimuthal modulations and radial polarization. At the maximal modulation, THz beam takes the form of the two lobes and polarization of the THz field degenerates into orthogonal to the laser pulse polarization direction. Violation of the radially polarized donut beam shape is due to destructive interference of THz waves driven by light pressure directed along the laser beam propagation axis and ponderomotive force parallel to the laser polarization.

We observed huge, almost two orders of magnitude, increase in the energy of the forward terahertz (THz) emission by the long femtosecond filament if transverse electrostatic field is applied along its full length. This proves summation of emission both from different parts of a single filament and from multiple filaments. The observed THz radiation has a narrow angular shape with a maximum in the direction of laser radiation and a spectrum in the range of 0.05–0.3 THz with a maximum at about 0.1 THz. Numerical simulations based on the UPPE well explain the experimental findings.

The terahertz (THz) radiation emitted by an air-based femtosecond filament biased by a static electric field is known to have on-axis shape and relatively low frequency spectrum in contrast to the unbiased single-color and two-color schemes. Here, we measure the THz emission of a 15-kV/cm-biased filament in air produced by a 740-nm, 1.8-mJ, 90-fs pulse and demonstrate that a flat-top on-axis THz angular distribution of the emission at 0.5-1 THz transforms into a contrast ring-shaped one at 10 THz.

Квантовомеханические расчеты нелинейного отклика одномерной квантовой системы, воспроизводящей энергетическую структуру ксенона, на ультрафиолетовый фемтосекундный импульс с интенсивностью 1-100 ТВт/см2 показали дисперсию коэффициента кубической нелинейности в диапазоне 266- 400 нм и его зависимость от интенсивности, исключающую описание отклика связанных электронов в виде χ(3)E3. Вычисление поляризации на базе одномерной квантовой модели может быть использовано при моделировании распространения ультрафиолетового фемтосекундного излучения в газе.

Focused femtosecond beam filamentation after amplitude masks has been studied experimentally and numerically. We deduced conditions (energy per hole, diameter and geometrical composition of holes, focal length) providing for the formation of the regularized bundle of filaments or single on-axis filament at the given pulse duration and beam diameter. We showed that a light channel with small diameter (∼200 μm) and overcritical peak power may be formed well before both the focal distance and the Marburger length, and this channel collapses due to self-focusing and forms the filament. The start position of such a filament can be predicted based on the linear propagation equation, while a more sophisticated non-linear approach that takes into account the Kerr nonlinearity, plasma effects, etc., helps to describe the temporal structure of a filament, its frequency, and its angular spectrum.

Quantum-mechanical simulations of the nonlinear response of a one-dimensional quantum system with the energy structure close to that of the xenon atom to an ultraviolet femtosecond pulse with an intensity of 1–100 TW/cm2 reveal the dispersion of the cubic nonlinearity coefficient in the range of 266–400 nm and its intensity dependence. This excludes the description of the response of bound electrons as χ^(3)E^3 . The calculation of the polarization with this one-dimensional quantum model can be used to simulate the propagation of ultraviolet femtosecond radiation in a gas.

Recent measurements of time delays during tunnelling of cold atoms through an optically created potential barrier have fuelled an ongoing debate about possible time delays during light-induced tunnelling of an electron from an atom. Yet, such a delay—whether it is present or not—is only one quantity characterizing the tunnelling wavepacket, whilst the underlying dynamics are richer. Here we show how to complement photo-electron detection in laser-induced tunnelling by measuring the light emitted by the tunnelling electron—the so-called Brunel radiation. Using a combination of single- and two-colour driving fields, we identify the all-optical signatures of the reshaping of the tunnelling wavepacket as it emerges from the tunnelling barrier and moves away from the core. This reshaping includes not only an effective time delay but also the time-reversal asymmetry of the ionization process, which we describe theoretically and observe experimentally. We show how both delay and reshaping are mapped onto the polarization properties of the Brunel radiation, with different harmonics behaving as different hands of a clock moving at different speeds. The all-optical detection may also allow time-resolved measurements of optical tunnelling in condensed matter systems on the attosecond time scale. Whether or not an electron wavepacket accumulates a time delay when tunnelling out of an atom is still under debate. Improved all-optical characterization of the tunnelling dynamics by combining one- and two-colour driving fields may shed light on this question.

Two-dimensional distribution patterns of terahertz radiation generated in a laser single-color filament plasma are measured at several frequencies. In the low-frequency region (0.1–0.5 THz), the radiation propagates in a cone with a minimum on the axis. At higher frequencies, the terahertz radiation pattern depends significantly on the laser pulse polarization. In the case of linear polarization, the axial symmetry is broken: terahertz radiation propagates into two maxima located along the axis perpendicular to the laser polarization. In the case of circular polarization, the axial symmetry of the terahertz radiation distribution is restored.

We study the propagation of ultrashort laser pulse in filamentation and postfilamentation regimes at the distances up to 95 m. In order to control the start of the filament and spectrum broadening we insert meshes inside the beam. For all beam configurations we found distances range where laser pulse triggers high-voltage discharge.

We performed full characterization of postfilament formed by the radiation of the Ti: Sa laser system on an extended atmospheric path. Single-shot angle-wavelength spectra, beam diameter and self-correlation function measurements have been employed for this purpose. Using angle-wavelength spectra, the evolution of on-axis red-shifted humps has been traced, showing that their divergence does not exceed 0.5 mrad. Two zones in the postfilamentation process have been revealed: the Stockes zone with soliton-like propagation, where the number of the Stockes humps and their shift increases while the pulse duration remains almost constant, and the zone where the temporal and spectral postfilament characteristics changes like in linear propagation mode, while the beam divergence is negligible due to the Kerr nonlinearity.

We experimentally investigate the low-frequency (below 1 THz) spectral content of broadband terahertz (THz) emission from two-color femtosecond filament formed by the 2.7-mJ, 40-fs, 800+400-nm pulse focused into air. For incoherent detection, we screened the Golay cell by the bandpass filters and measured the THz angular distributions at the selected frequencies ν=0.5, 1, 2 and 3 THz. The measured distributions of THz fluence were integrated over the forward hemisphere taking into account the transmittance of the filters, thus providing the estimation of spectral power at the frequencies studied. The spectral power decreases monotonically with the frequency increasing from 0.5 to 3 THz, thus showing that the maximum of THz spectrum is attained at ν≤0.5 THz. The THz waveform measured by electro-optical sampling (EOS) based on ZnTe crystal and transformed into the spectral domain shows that there exists the local maximum of the THz spectral power at ν≈1 THz. This disagrees with monotonic decrease of THz spectral power obtained from the filter-based measurements. We have introduced the correction to the spectral power reconstructed from EOS measurements. This correction takes into account different focal spot size for different THz frequencies contained in the broadband electromagnetic pulse. The corrected EOS spectral power is in semi-quantitative agreement with the one measured by a set of filters.

In 3D + time numerical simulations, we study the wavelength scaling law for the energy of terahertz (THz) radiation emitted from a two-color femtosecond filament, which forms during cofocusing into air the fundamental and second harmonics of the laser pulse. In our simulations, the central wavelength of the fundamental harmonic varied from 0.8 to 8 μm and the numerical aperture varied from 0.006 to 0.03. While the harmonics and supercontinuum development are not extreme, so the harmonics spectra are clearly separated, the energy of the generated THz radiation is proportional to the oscillation energy of the electrons, which grows as the squared pump wavelength, and the total number of free electrons in the filament, which decreases quasi-exponentially with the pump wavelength. As a result, the scaling law for the THz energy on the pump wavelength is nonmonotonic with the maximum at 1.6–4 μm depending on the focusing conditions.