Coherent Pulses Research Articles

Multi-channel integration of radiolocation signals reflected from accelerating high-speed targets

The paper determines the parameters of a coherent pulse-burst integrator with multi-channel capabilities interms of velocity and acceleration. We indicate the singularities of the pulse burst ambiguity function in the velocity-acceleration plane. We present algorithms for estimating velocity and acceleration. We propose a system for multi-channel matched filtering of an unmodulated coherent pulse burst with preliminary non-coherent integration, using which significantly reduces computational expenses. We determine the conditions that lead to the system exhibiting no threshold signal loss.

Broadband gain induced Raman comb formation in a silica microresonator

A high-Q silica whispering-gallery mode microresonator is an attractive platform on which to demonstrate a broad and phase-locked Raman comb in various wavelength regimes. Raman combs can be used for applications such as compact pulse laser sources, sensors, optical clocks, and coherence tomography. However, the formation dynamics of a Raman comb has not been well exploited. Here we study the dynamics of the Raman comb formation in silica rod microresonators, which have cavity free spectral ranges in microwave rates. We generated a Raman comb with a smooth spectral envelope and also observed the transition between two Raman combs located at different center wavelengths. The transition behavior was obtained when we changed the pump detuning and the coupling strength between the microresonator and fiber. We also explain these phenomena by using a simple model based on coupled mode equations.

Design and Processing of a Novel Chaos-Based Stepped Frequency Synthesized Wideband Radar Signal.

The linear stepped frequency and linear frequency shift keying (FSK) signal has been widely used in radar systems. However, such linear modulation signals suffer from the range–Doppler coupling that degrades radar multi-target resolution. Moreover, the fixed frequency-hopping or frequency-coded sequence can be easily predicted by the interception receiver in the electronic countermeasures (ECM) environments, which limits radar anti-jamming performance. In addition, the single FSK modulation reduces the radar low probability of intercept (LPI) performance, for it cannot achieve a large time–bandwidth product. To solve such problems, we propose a novel chaos-based stepped frequency (CSF) synthesized wideband signal in this paper. The signal introduces chaotic frequency hopping between the coherent stepped frequency pulses, and adopts a chaotic frequency shift keying (CFSK) and phase shift keying (PSK) composited coded modulation in a subpulse, called CSF-CFSK/PSK. Correspondingly, the processing method for the signal has been proposed. According to our theoretical analyses and the simulations, the proposed signal and processing method achieve better multi-target resolution and LPI performance. Furthermore, flexible modulation is able to increase the robustness against identification of the interception receiver and improve the anti-jamming performance of the radar.

Analytic random-walk model for the coherence of a frequency comb

We present an analytical study of the frequency comb coherence due to random noise in the pulses phases. We derive a simple expression for the comb lineshape, which depends on a single parameter ${N}_{\text{eff}}$ with the physical meaning of number of coherent comb pulses, inversely proportional to the variance of the phase jumps between subsequent comb pulses. A comparison to the case of a cw-monomode laser with white noise frequency fluctuations is also presented.

Partially coherent noise-like pulse generation in amplified spontaneous Raman emission.

Amplified spontaneous Raman emission under picosecond pulse pumping is studied in detail both experimentally and numerically. It is found that the Raman output pulses are noise-like with partial coherence, which has important implications for various applications. Numerical simulation reproduces the finding well, along with the temporal and spectral dependence of the Raman pulse on the pump pulse energy. The numerical simulations of the temporal, spectral, and energy evolution of the Raman and pump pulse along the fiber gives insight on how to design such sources.

SUSTAINABILITY OF ADAPTIVE ALGORITHMS FOR BAND REJECTION OF PASSIVE INTERFERENCES

The article is dedicated to the investigation of adaptive algorithms of band rejection filters, which ensure a top efficiency of suppression of correlated (passive) noise in conditions of a priori parametric uncertainty during wobbling of the repetition period. The purpose of the article is the analysis of the stability of algorithms for calculation of weight coefficients of rejection filters and development of recommendations for minimizing the sensitivity of these algorithms to calculation errors. A specific form of adaptive algorithms is given for which the analysis of stability of calculation of the weight coefficients of rejection filters in the wobbling of the repetition period of probing coherent pulses is carried out. Analysis of the stability of adaptive algorithms based on the energy criterion (the coefficient of improving the signalto-noise ratio), depending on the choice of their basic parameters (rows and decomposition element), makes it possible to optimize these parameters by criterion of minimum sensitivity of algorithms to mathematical calculations errors. Recommendations are given on selection of parameters of the algorithms for calculating the weight coefficients and a procedure is proposed for calculating the modified algorithms for adaptive rejection of passive noise during the wobbling of the repetition period. As a result of optimization of algorithms by the criterion of constancy of derivatives it is established that in order to ensure a minimum value of the matrix of the derivative weight coefficients under the selected number of the expansion line, the preference should be given to selection of the element (column) of the expansion under the same number.

Electronic control of different generation regimes in mode-locked all-fibre F8 laser

We demonstrate for the first time an electronically controlled realisation of markedly different generation regimes in a mode-locked all-fibre figure-eight (F8) Yb-doped laser. Electronic adjustment of the ratio of pumping powers of two amplification stages in a nonlinear amplifying loop mirror enables the establishment of stable pulse generation regimes with different degrees of coherence and control over their parameters within relatively broad limits, with the pulse duration range exceeding a factor of two in the picosecond domain for coherent and incoherent pulses, the energy range exceeding an order of magnitude for incoherent pulses (2.2–24.8 nJ) and over a factor of 8 for coherent pulses (1.9–16.2 nJ). Adjustment of the pumping powers allows one to maintain the duration of the coherent pulses and to set their peak power in the range of 32.5–292.5 W. The proposed configuration of electronic control over the radiation parameters of a mode-locked all-fibre F8 laser enables reproducible generation of pulses of different types with specified parameters within a broad range of values.

Target recognition and discrimination based on multiple-frequencies LFM signal with subcarrier hopping

An anti-deception jamming technique is proposed for moving target indication in a pulse-Doppler (PD) radar. The deceive targets are produced by digital radio frequency memory, which tries to pull off the range and velocity gates of real targets. Similar to orthogonal frequency division multiplexing, we use different sets of orthogonal sub-carriers in consecutive coherent pulse intervals (CPIs). By changing sub-carriers in different CPIs, we show that the deceive targets appear as interference in receiving signals. The generalized likelihood ratio test is used for detection and discrimination of real targets. The performance of the proposed method is achieved analytically and by simulations. Furthermore, we implement a hardware block using a TMS6416-DSK DSP for a PD radar prototype exploiting the proposed algorithm to deception discrimination. The presented results demonstrate the good accordance with theoretical predictions.

Generation of coherent two-color pulses at two adjacent harmonics in a seeded free-electron laser

The growing requirements of pump-probe techniques and nonlinear optics experiments greatly promote the studies of two-color free-electron lasers (FELs). We propose a new method to generate coherent two-color pulses in a high-gain harmonic generation (HGHG) FEL. In this scheme, an initial tilted electron beam is sent though the modulator and dispersive section of an HGHG FEL to generate the bunching at harmonics of the seed laser. Then a transverse gradient undulator (TGU) is adopted as the radiator and in such radiator, only two separated fractions of the tilted beam will resonate at two adjacent harmonics of the seed laser and are enabled to emit the coherent two-color pulses simultaneously. The time separation between the two pulses are on the order of hundreds of femtoseconds, and can be precisely controlled by varying the tilted amplitude of the electron beam and/or the transverse gradient of the TGU radiator. Numerical simulations confirm the validity and feasibility of this scheme in the EUV waveband.

Spatiotemporal dynamics of Raman coherence in hollow-core fibers for a pump-probe setup

We present an experimental and theoretical study of the stimulated Raman emission in hollow-core kagome waveguides in a pump-probe arrangement. We perform an experimental investigation of the power of the Stokes signal from the probe, which is below the stimulated Raman scattering threshold, as a function the pump-probe delay time. The results show the Stokes power to increase with pump-probe delay, reaching a maximum at 10 ns, and to decrease afterward. In view of a coherence decay time of only 0.25 ns, we demonstrate a surprisingly slow reduction of Stokes signal with the characteristic time much longer than the coherence decay time by a factor of up to 40. The numerical investigations explain the observed phenomenon as a result of the spatiotemporal dynamics of the probe pulse and Raman coherence. We show that the increase of the characteristic time can be related to the spatial position of the intense sideband generation event and its dependence on the pump-probe delay.

Intense broadband mid-infrared pulses of 280 MV/cm for supercontinuum generation in gaseous medium.

We produce extremely bright mid-infrared (mid-IR) pulses with a tunable wavelength of 7μm to 15μm through difference frequency generation. Optimization of beam quality and beam focusing results in an intense mid-IR field spatiotemporally confined in the lambda-cubic volume. A near planar wavefront is achieved through manipulating the wavefront curvature of the pumping pulse in the frequency downconversion process. Coherent mid-IR pulses are produced with the peak field of 280MV/cm at 10μm, and its intensity exceeds 100 TW/cm2, estimated from measured pulse energy, and spatial and temporal pulse profiles. Interaction of such an intense mid-IR field with Xe and Kr gas forms plasma and generates a supercontinuum in the visible range.

Diffraction based Hanbury Brown and Twiss interferometry at a hard x-ray free-electron laser

X-ray free-electron lasers (XFELs) provide extremely bright and highly spatially coherent x-ray radiation with femtosecond pulse duration. Currently, they are widely used in biology and material science. Knowledge of the XFEL statistical properties during an experiment may be vitally important for the accurate interpretation of the results. Here, for the first time, we demonstrate Hanbury Brown and Twiss (HBT) interferometry performed in diffraction mode at an XFEL source. It allowed us to determine the XFEL statistical properties directly from the Bragg peaks originating from colloidal crystals. This approach is different from the traditional one when HBT interferometry is performed in the direct beam without a sample. Our analysis has demonstrated nearly full (80%) global spatial coherence of the XFEL pulses and an average pulse duration on the order of ten femtoseconds for the monochromatized beam, which is significantly shorter than expected from the electron bunch measurements.

Rabi-oscillation-enhanced frequency conversion in quantum-dot semiconductor optical amplifiers

We investigate the nonlinear light propagation in InAs/InGaAs quantum-dot-in-a-well semiconductor optical amplifiers in the limit of strong optical excitation where Rabi oscillations are excited in the active medium. The amplifier is analyzed in a degenerate four-wave-mixing setup and characterized by its frequency conversion and creation performance. Our simulations show that the interplay between the nonlinear four-wave-mixing process and the coherent Rabi oscillations greatly influences the frequency conversion process. Rabi oscillations can be resonantly excited by the correct choice of the frequency detuning between pump and probe signals, which greatly enhances the nonlinear frequency conversion efficiency at frequencies up to several THz. We furthermore show that the coherent pulse shaping of ultrashort optical pulses in the quantum-dot medium can greatly enhance their spectral bandwidth, potentially allowing for ultra-broad-band frequency comb generation.

Concept of quantum timing jitter and non-Markovian limits in single-photon detection

Single atoms form a model system for understanding the limits of single photon detection. Here, we develop a non-Markov theory of single-photon absorption by a two-level atom to place limits on the absorption (transduction) time. We show the existence of a finite rise time in the probability of excitation of the atom during the absorption event which is infinitely fast in previous Markov theories. This rise time is governed by the bandwidth of the atom-field interaction spectrum and leads to a fundamental jitter in time-stamping the absorption event. Our theoretical framework captures both the weak and strong atom-field coupling regimes and sheds light on the spectral matching between the interaction bandwidth and single photon Fock state pulse spectrum. Our work opens questions whether such jitter in the absorption event can be observed in a multi-mode realistic single photon detector. Finally, we also shed light on the fundamental differences between linear and nonlinear detector outputs for single photon Fock state vs. coherent state pulses.

Shot-by-shot characterization of focused X-ray free electron laser pulses

X-ray free electron lasers (XFEL) provide intense and almost coherent X-ray pulses. They are used for various experiments investigating physical and chemical properties in materials and biological science because of their complete coherence, high intensity, and very short pulse width. In XFEL experiments, specimens are irradiated by XFEL pulses focused by mirror optics. The focused pulse is too intense to measure its coherence by placing an X-ray detector on the focal spot. Previously, a method was proposed for evaluating the coherence of focused pulses from the visibility of the diffraction intensity of colloidal particles by the speckle visibility spectroscopy (SVS). However, the visibility cannot be determined exactly because the diffraction intensity is integrated into each finite size detector pixel. Here, we propose a method to evaluate the coherence of each XFEL pulse by using SVS in combination with a theory for exact sampling of the diffraction pattern and a technique of multiplying the diffraction data by a Gaussian masks, which reduces the influence of data missing in small-angle regions due to the presence of a direct beamstop. We also introduce a method for characterizing the shot-by-shot size of each XFEL pulse by analysing the X-ray irradiated area.

Passive state preparation in the Gaussian-modulated coherent-states quantum key distribution

In the Gaussian-modulated coherent states (GMCS) quantum key distribution (QKD) protocol, Alice prepares quantum states \emph{actively}: for each transmission, Alice generates a pair of Gaussian-distributed random numbers, encodes them on a weak coherent pulse using optical amplitude and phase modulators, and then transmits the Gaussian-modulated weak coherent pulse to Bob. Here we propose a \emph{passive} state preparation scheme using a thermal source. In our scheme, Alice splits the output of a thermal source into two spatial modes using a beam splitter. She measures one mode locally using conjugate optical homodyne detectors, and transmits the other mode to Bob after applying appropriate optical attenuation. Under normal conditions, Alice's measurement results are correlated to Bob's, and they can work out a secure key, as in the active state preparation scheme. Given the initial thermal state generated by the source is strong enough, this scheme can tolerate high detector noise at Alice's side. Furthermore, the output of the source does not need to be single mode, since an optical homodyne detector can selectively measure a single mode determined by the local oscillator. Preliminary experimental results suggest that the proposed scheme could be implemented using an off-the-shelf amplified spontaneous emission source.

Optical nutation in acetylene-filled hollow-core photonic crystal fiber

We report detailed experimental results on the optical nutation effect in acetylene-filled hollow-core photonic crystal fibers (HC-PCFs). This coherent quantum optical effect is manifested by the appearance of attenuated oscillations in the profiles of transmitted step-like coherent optical pulses. The experiments used 15 ns optical pulses with peak powers up to ∼4 W and centered at 1530.37 nm, which corresponds to the acetylene (12C2H2) vibration-rotation resonance absorption line P9. The all-fiber cell made from a HC-PCF with internal diameter ∼10.3 μm contained a gas pressure of about 0.12 Torr. The Maxwell-Bloch equations are used to analyze the influence of the homogeneous relaxation, the inhomogeneous Doppler broadening of the absorption line, and the random orientation of the acetylene molecules. By comparing the experimental results with numerical simulations, we obtain the longitudinal and the transverse relaxation times to be close to 10 ns and find that the transition dipole moment of the acetylene P9 absorption line is ∼1.36×10−32 Cm.

Soft X-Ray Second Harmonic Generation as an Interfacial Probe.

Nonlinear optical processes at soft x-ray wavelengths have remained largely unexplored due to the lack of available light sources with the requisite intensity and coherence. Here we report the observation of soft x-ray second harmonic generation near the carbon K edge (∼284 eV) in graphite thin films generated by high intensity, coherent soft x-ray pulses at the FERMI free electron laser. Our experimental results and accompanying first-principles theoretical analysis highlight the effect of resonant enhancement above the carbon K edge and show the technique to be interfacially sensitive in a centrosymmetric sample with second harmonic intensity arising primarily from the first atomic layer at the open surface. This technique and the associated theoretical framework demonstrate the ability to selectively probe interfaces, including those that are buried, with elemental specificity, providing a new tool for a range of scientific problems.

Noisy propagation of coherent states in a lossy Kerr medium

We identify and discuss nonlinear phase noise arising in Kerr self-phase modulation of a coherent light pulse propagating through an attenuating medium with third-order nonlinearity in a dispersion-free setting. This phenomenon, accompanying the standard unitary Kerr transformation of the optical field, is described with high accuracy as Gaussian phase diffusion with parameters given by closed expressions in terms of the system properties. We show that the irreversibility of the nonlinear phase noise ultimately limits the ability to transmit classical information in the phase variable over a lossy single-mode bosonic channel with Kerr-type nonlinearity. Our model can be also used to estimate the amount of squeezing attainable through self- phase modulation in a Kerr medium with distributed attenuation.

Гидроакустические закономерности пищевого поведения дельфинов

Гидроакустические закономерности пищевого поведения дельфинов определяются характеристиками и функциональностью их акустических сигналов. Все акустические сигналы дельфинов классифицированы в соответствии с их физическими характеристиками в свете теории сигналов и эхолокации следующим образом: последовательности ультракоротких сверхширокополосных когерентных импульсов — «щелчки», частотно-модулированные (ЧМ) симултоны с равномерно распределёнными тонами — «свисты», пачки взаимно когерентных импульсов (КИ), пачки взаимно некогерентных импульсов (НИ) и пачки универсальных импульсов (УИ). Они играют роль зондирующих сигналов шести сонаров дельфина, оптимизированных для решения разных эхолокационных задач. Возможности использования дельфинами различных акустических сигналов для эхолокационного поиска и классификации объектов питания изучены в настоящей работе на основе собственных и литературных данных. Последовательности «щелчков» дельфины могут применять для эхолокации объектов питания (отдельной рыбы ― на расстоянии до 70–110 м, косяка рыбы ― на расстоянии до 450–650 м) и сородичей (на расстоянии до 450–650 м), а также для их классификации. «Свисты» дельфины могут использовать для обнаружения объектов питания (отдельной рыбы ― на расстоянии до 2 км, косяка рыбы ― на расстоянии до 9–13 км) и сородичей (на расстоянии до 9–13 км), для определения их дальности, а также для измерения их относительной радиальной скорости (приближение или удаление). Дальность и точность эхолокационного обнаружения сородичей и объектов пищи сонаром дельфина с применением ЧМ-сигналов лучше, чем при использовании «щелчков», более чем на порядок. Кроме того, ЧМ-сонар обеспечивает измерение радиальной скорости приближения или удаления подводного объекта относительно дельфина. Между тем преимуществом сонара, использующего «щелчки», является тонкий анализ амплитудно-временных особенностей спектра эха рыб с целью их классификации. Пачки КИ дельфины могут применять для отслеживания динамики изменения положения объекта питания на дистанциях менее 2,5 м для точного захвата жертвы. Высокая разрешающая способность слуха дельфина по времени (около 0,02 мс) позволяет обрабатывать тонкую временную динамику эхосигналов, приходящих с малых расстояний. Пачки НИ ― сигналы разговорного языка ― дельфины могут использовать для организации различных видов сотрудничества и сложной кооперации между собой во время поиска и ловли рыбы. Пачки УИ дельфины могут применять для расширения пространства эхолокационного обзора вокруг особи, с целью улучшения качества мониторинга за объектами питания, независимо от положения головы дельфина в пространстве, для определения дальности и относительной радиальной скорости, а также для классификации объектов питания на малых расстояниях. Развитие и совершенствование разных типов акустических сигналов, сонаров и различных методов обработки эхосигналов у дельфинов вызвано, прежде всего, необходимостью оптимизации гидроакустических закономерностей их пищевого поведения и ориентации в трёхмерном пространстве. Можно предполагать наличие аналогичных гидроакустических закономерностей пищевого поведения у Odontoceti исходя из подобия их акустических сигналов и морфологии. Акустические закономерности пищевого поведения дельфинов и летучих мышей схожи несмотря на то, что у них различная среда обитания (водная и наземно-воздушная) и что эти млекопитающие относятся к разным отрядам царства животных (китообразные и рукокрылые).