Effect Of Frequency Chirp Research Articles

Effect of Frequency Chirp and Pulse Length on Laser Wakefield Excitation in Under-Dense Plasma

Effect of Frequency Chirp and Pulse Length on Laser Wakefield Excitation in Under-Dense Plasma

GeV Electron Acceleration by Trapezoidal Laser Pulse Envelope Under Combined Effect of Frequency Chirp and Axial Magnetic Field in Vacuum

In the present article, we have investigated efficient electron acceleration by employing trapezoidal laser pulse envelope under the combined effect of linear chirp and external axial magnetic field in vacuum. In this envelope, the electric signal linearly rises to a certain level, stay there, and then falls back to its initial level. So, there will be an opportunity to increase the interaction time between laser pulse and electron for efficient energy transfer, which has been reflected in this study. A few megaelectronvolt electrons have been injected axially to the front of a trapezoidal short laser pulse. In all calculations, the front end of each laser pulse captures electron at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$t= 0$ </tex-math></inline-formula> . The dynamics arise from analytical and numerical solutions of Newton–Lorentz force equations of motion. Electron energy gain has been calculated for initial laser phase varying from 0 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\pi $ </tex-math></inline-formula> . For the optimum set of laser parameters, we have observed electro’s energy gain of the order of GeV.

Frequency chirp effects on stimulated Raman scattering in inhomogeneous plasmas

Previous studies have shown that the use of laser bandwidth may mitigate the growth of stimulated Raman scattering (SRS) in laser plasma interaction experiments, in particular, when the spectrum of the driving (or pump) laser is composed of uniformly distributed frequency components with a well-chosen bandwidth [for example, Luo et al., Phys. Plasmas 29, 032102 (2022); Wen et al., ibid. 28, 042109 (2021); and Follett et al., ibid. 26, 062111 (2019)]. Here, we investigate the effects of frequency chirp in the pump laser on backward SRS in inhomogeneous plasmas, taking into account kinetic effects associated with the nonlinear detuning of the parametric resonance due to high-amplitude electron plasma waves (EPW). Through theoretical considerations and numerical simulations, using a multi-dimensional particle-in-cell (PIC) code, it is shown that positive frequency chirp rates lead to a displacement of the resonance in the plasma profile. For a sufficiently strong positive chirp rate, such that the resonance displacement is faster than the EPW group velocity, the EPWs prove to remain limited in amplitude such that SRS is suppressed. The required frequency chirp rate corresponds to a laser bandwidth of about 1%–2%.

Laser wakefield and direct laser acceleration of electron by chirped laser pulses

Chirped laser pulses are investigated theoretically for laser wakefield (LW) and direct laser (DL) acceleration of electrons in the plasma-bubble regime. Chirping of laser pulse with a suitable frequency allows a better trapping of electron in bubble regime which enforces a strong betatron resonance inside the plasma bubble. Linear and non-linear effects of frequency chirp determine electron acceleration, de-phasing, beam quality in terms of energy spread and high energy from the direct laser field and its plasma wake in bubble regime. Gaussian chirped laser pulse exhibit high energy to the electrons compared with linear and other non-linear chirped pulses in plasma-bubble regime. The investigation with chirped laser pulse opens new dimension to hybrid LW-DL acceleration mechanism in bubble regime.

Enhancement effect of frequency chirp on vacuum electron-positron pair production in strong field

In this review article, we show an important aspect of electron-positron pair production from vacuum under strong background field where the frequency chirping plays a key role in enhancing the pair production. A series of researches on the enhancement effect of frequency chirp on electron-positron pair production in strong field is summarized. Three approaches are introduced, i.e. the Dirac-Heisenberg-Wigner formalism used to treat the spatial inhomogeneous field or/and multidimensional homogeneous time-dependent field, quantum Vlasov equation to cope with the one-dimensional homogeneous time-dependent field, and the computation quantum field theory employed to study the problem with external potential. Some interesting results about the momentum spectrum structure of created particle and the yielding of pair numbers are demonstrated for various different field parameters such as field strength and central frequency, in particular their significant influence on results when the frequency chirping form or/and strength are changed. In general, the number density can be improved by 2-3 orders of magnitude with the strengthening of frequency chirping in comparison with that without chirping for low frequency field, which is attributed to the effect that the dynamically assisted mechanism plays a significant role since the chirping expands the frequency spectrum of field. For high frequency field, however, this effect is suppressed so that the number density is enhanced by about a few times. For spatially inhomogeneous field, field changing on a small scale does not make the number density so high and the frequency chirping can enhance the yield in the order of magnitude, while the field changing on a large scale makes the number density to approach to that of homogeneous field and the chirping increases the yield by a few times. These numerical results can be understood by the Wentzel-Kramer-Brillouin (WKB) approximation and the structure of turning points. Finally the possible applicable prospects of the electron-positron pair production by the frequency chirping are presented briefly.

Terahertz radiation generation by beating of two chirped laser pulses in a warm collisional magnetized plasma

Analytical equations of terahertz (THz) radiation generation based on beating of two laser beams in a warm collisional magnetized plasma with a ripple density profile are developed. In this regard, the effects of frequency chirp on the field amplitude of the terahertz radiation as well as the temperature and collision parameters are investigated. The ponderomotive force is generated in the frequency chirp of beams. Resonant excitation depends on tuning of the plasma beat frequency, magnetic field frequency, thermal velocity, collisional frequency, and effect of the frequency chirp with the plasma density. For optimum parameters of frequency and temperature the maximum THz amplitude is obtained.

50 Gbps PAM-4 Over Up to 80-km Transmission With C-Band DML Enabled by Post-Equalizer

In this letter, transmission of directly-modulated PAM-4 in C-band is experimentally demonstrated. Instead of introducing any optical devices for dispersion management, we employ receiver-side equalization consisting of Volterra-based FFE and Volterra-based DFE, to mitigate the signal distortions induced by the frequency chirp effect and chromatic dispersion. Results indicate that, PAM-4 transmissions of 56 Gbps over 60-km and 60 Gbps over 80-km are achieved respectively under the HD-FEC limit ( $3.8\times 10^{-3}$ ) and SD-FEC limit ( $2.2\times 10^{-2}$ ).

Improving the Link Availability of an Underwater Wireless Optical Communication System Using Chirped Pulse Compression Technique

The effect of frequency chirping on the link availability of an underwater wireless optical communication (UWOC) system is studied theoretically using a simple and realistic model. It is observed, both theoretically and numerically, that a light pulse can be compressed in temporal axis when propagating in the water medium and can attain the minimum value at a prespecified location by appropriately choosing the initial pulse chirp parameter. The pulse compression takes place due to the interplay between the initial frequency chirp and the group velocity dispersion (GVD) effect. An increase in peak intensity, due to pulse compression, at the prespecified receiver location results in an improvement in the link availability compromised due to both attenuation and turbulence. The link availability is quantified by estimating the fading probability for various cases of initially chirped (+ve, 0, and -ve) Gaussian pulses under the influence of attenuation and turbulence and the results are compared for different scenarios. To have a complete channel description, the effect of particulate scattering on the GVD-induced pulse compression and hence on the fading probability is studied and quantified using the Monte Carlo numerical simulation technique. It is shown that, for the same system and channel parameters, the fading probability of a UWOC system is a few orders of magnitude lower (at a prespecified location) for the case with the negatively chirped pulse than for the cases when the initial pulse chirp is either zero or positive. In addition, the analytical expressions for estimating the focus distance and minimum pulse length, as functions of the channel properties, incident pulse length, and initial chirp parameters, are presented and discussed.

Effect of frequency-chirped laser pulses on terahertz radiation generation in magnetized plasma

The effect of frequency chirp on the generated terahertz (THz) wave by the interaction between intense laser pulses and under dense plasma in the presence of transverse magnetic field is investigated theoretically in the present model. An expression for the electron Lorentz factor coupling cyclotron motion nonlinearity is derived for static magnetic field perpendicular to the laser propagation axis. The beating lasers produce a nonlinear ponderomotive force due to their oscillatory motion. This force drives a nonlinear current, at beat frequency, which produces the THz radiation. The influence of the external magnetic field on the optimization process of THz field amplitude is investigated numerically. A linear frequency chirp increases the duration of nonlinear interaction of laser pulse with plasma electrons and hence, enforces the interaction for longer duration. The presence of magnetic field further provides the additional momentum to the THz photon to obtain a significant gain in output yield. Our numerical simulations reveal that there is a significant enhancement in the THz field strength for optimized value of the chirp parameter and magnetic field.

Effects of frequency chirping and finite extinction ratio of optical modulators in microwave photonic IFM receivers

Frequency-to-power mapping approach based on chromatic dispersion is frequently used in microwave photonic (MWP) instantaneous frequency measurement (IFM) receivers. In this technique, carrier frequency of an unknown microwave signal is estimated according to a specific relationship between the unknown microwave frequency and the ratio of two optical or microwave power functions which is known as amplitude comparison function (ACF). The ACF can be constructed using chromatic dispersion-induced power fading functions of intensity- or phase-modulated optical signals transmitting through dispersive mediums. Practical non-ideal intensity modulators have finite extinction ratio and suffer from frequency chirping. Despite previously published numerous works, the contribution of these parameter in MWP IFMs have not been already studied. In this paper, effects of chirp parameter and finite extinction ratio of intensity modulators on the performance of chromatic dispersion-based MWP IFM receivers are theoretically investigated. It is shown that in general the ACFs of these MWP IFM receivers depend on the chirp parameter of their intensity modulators which is a function of unknown amplitude of the microwave signal whose frequency is intended to be measured and varies with time. This problem cannot be resolved by calibrations and degrades the measurement accuracy. Besides, it is shown that ACFs are different for different finite extinction ratios which degrades the measurement accuracy. Finally, we propose using Mach–Zehnder interferometer (MZI)- and Sagnac interferometer (SI)-based intensity modulators with zero chirp in these MWF IFMs. Analytical results are verified by computer simulations using OptiSystem software.

Direct digital predistortion technique for the compensation of laser chirp and fiber dispersion in long haul radio over fiber links

Analog Radio-over-Fiber (A-RoF) communication technology constitutes a promising technique for next generation radio access networks thanks to its relatively low bandwidth requirements. Within this context, an efficient predistortion technique is proposed, which can be applied to reduce the impairments of A-RoF systems due to the combined effects of frequency chirp of the laser source and chromatic dispersion of the optical channel. The radio frequency signal is firstly put in digital form through an analog to digital converter, then the predistortion operation is realized, and finally the resulting signal is put again into analog form. A comprehensive analysis on the theoretical basis of the proposed approach is presented, together with the approximations introduced, which makes it practically realizable. The improvements on the quality of the received signal due to the proposed solution are illustrated with reference to scenarios of applicative interest. The performance of proposed technique is evaluated in terms of adjacent channel leakage ratio and error vector magnitude when long term evolution signal is applied at the input.

Chirp evaluation of semiconductor DFB lasers through a simple Interferometry-Based (IB) technique.

Direct modulation of a laser source is often utilized in realizing optical fiber connections where the cost of the entire system must be kept at a low level. An undesired consequence of this choice is the onset of the laser frequency chirp effect, which is detrimental in the case of either digital or analog links, and must be evaluated with precision in order to perform an accurate design of the whole system. Various methods of evaluation of the chirp parameters have been proposed, and the choice among them is typically made on the basis of the laboratory equipment available at the moment. This paper adds a further element to the set of possible choices, since it presents a method for the evaluation of the adiabatic chirp factor in distributed feedback (DFB) laser sources, which exploits a simple interferometric scheme, guarantees low cost, and shows, at the same time, good accuracy of the results.

Multi-GeV electron acceleration by a periodic frequency chirped radially polarized laser pulse in vacuum

Linear and periodic effects of frequency chirp on electron acceleration by radially polarized (RP) laser pulse in vacuum have been investigated. A frequency chirp influences the electron dynamics, betatron resonance, and energy gain by electron during interaction with the RP laser pulse and ensures effective electron acceleration with high energy gain (~GeV). The electron energy gain with a periodic frequency chirped laser pulse is about twice as high as with a linear chirp. Our observations reveal electron energy gain of about 10.5 GeV with a periodic chirped RP petawatt laser pulse in vacuum.

Effect of frequency chirp on supercontinuum generation in silicon waveguides with two zero-dispersion wavelengths

The influence of initial chirp on supercontinuum generation in SOI rib waveguide with two zero-dispersion wavelengths was studied numerically, based on the generalized nonlinear Schrödinger equation (GNSE). The full-width at half-maximum (FWHM) and the peak power of the pre-chirped hyperbolic secant in the simulation are 50fs and 50W, respectively. The simulation results indicate that a positive initial chirp makes the energy transfer to the normal dispersion zone by affecting self-phase modulation (SPM) and four-wave mixing (FWM) processes, and therefore enhances the supercontinuum bandwidth as well as improves the spectral flatness. In particular, at the optimal initial chirp parameter of C=3, the bandwidth at −10dB level increases to about 1620nm (from 1140 to 2760nm), exceeding an octave-spanning.

Enhanced electron—positron pair creation by the frequency chirped laser pulse

Based on the quantum Vlasov equation, the effect of frequency chirp on electron—positron pair production is investigated. The cycle parameter, which characterizes the laser field cycle degree within the pulse, is also considered. In both supercycle and subcycle laser pulses the frequency chirp can greatly enhance the momentum distribution function of created pairs and the pair number density. The pair number density created by a supercycle laser pulse is larger than that by a subcycle pulse under the same laser frequency and chirping. There exists an optimal cycle parameter corresponding to the maximum value of the created pair number density for different chirp rates. It is found that the pair number density is sensitive/insensitive to chirping rate when the cycle parameter lies below/above the optimal one.

Frequency chirp and mode partition induced mutual constraint on the side-band phase noise of a mode-locking WRC-FPLD fiber ring self-started with a lengthened feedback loop

The specific mutual constraint effect of frequency chirp and mode partition on the side-band phase noise and timing jitter is theoretically and experimentally analyzed in a harmonically mode-locked weak-resonant-cavity Fabry–Perot laser diode (WRC-FPLD) fiber ring self-started by fiber-coupled optoelectronic feedback at 10 GHz with a lengthened loop. By inserting a 100 m long single-mode fiber into the feedback loop to self-start the mode-locking, minimal single-side-band phase noise of −70 dBc Hz−1 and −125 dBc Hz−1 are obtained at 100 Hz and 1 MHz offset, respectively. The optimized pulse-train exhibits a timing jitter of 0.67 ps and a pulsewidth of 18.5 ps. A significant degradation is observed for feedback loop lengths longer than 200 m due to the enhanced mode-partition noise contributed by the fiber-ring cavity incorporated in the WRC-FPLD. The theoretical model shows that the mutually coupled fiber-ring cavity and the optoelectronic feedback loop could provide a mode-locking pulse with a minimum phase noise of −135 dBc Hz−1 under the compromise of phase noise responses between the fiber-ring cavity and the feedback loop. These two factors hinder the improvement expected from the longer feedback loop and higher Q-value because the mode-partition noise is greatly enlarged when the multi-mode pulse experiences serious dispersion during distant transmission in the fiber. Under such a mutual constraint, the optimized return-to-zero (RZ) pulsed carrier is on–off keying (OOK) non-RZ data by 10 Gbit s−1, showing a high signal to noise ratio of 11 dB to achieve a receiving sensitivity of −19.2 dBm at a bit error rate (BER) of 10−9. When using the RZ pulsed carrier for data transmission at 10 Gbit s−1 in a passive optical network with a dense wavelength division multiplexing channelization of 200 GHz, the RZ-OOK data-stream still exhibits an error-free performance at a receiving power sensitivity as low as −17 dBm, even though a power penalty of 2.5 dB is added into the BER response.

Effect of junction temperature on the large-signal properties of a 94 GHz silicon based double-drift region impact avalanche transit time device

The authors have developed a large-signal simulation technique extending an in-house small-signal simulation code for analyzing a 94 GHz double-drift region impact avalanche transit time device based on silicon with a non-sinusoidal voltage excitation and studied the effect of junction temperature between 300 and 550 K on the large-signal characteristics of the device for both continuous wave (CW) and pulsed modes of operation. Results show that the large-signal RF power output of the device in both CW and pulsed modes increases with the increase of voltage modulation factor up to 60%, but decreases sharply with further increase of voltage modulation factor for a particular junction temperature; while the same parameter increases with the increase of junction temperature for a particular voltage modulation factor. Heat sinks made of copper and type-IIA diamond are designed to carry out the steady-state and transient thermal analysis of the device operating in CW and pulsed modes respectively. Authors have adopted Olson's method to carry out the transient analysis of the device, which clearly establishes the superiority of type-IIA diamond over copper as the heat sink material of the device from the standpoint of the undesirable effect of frequency chirping due to thermal transients in the pulsed mode.

Compensation of directly modulated distributed feedback laser frequency chirps in optical orthogonal frequency division multiplexing intensity-modulation and direct-detection passive optical network systems

A simple and effective technique capable of compensating frequency chirps associated with directly modulated distributed feedback (DFB) lasers (DMLs) is proposed, which utilises an electrical analogue circuit (EAC) and an optical phase modulator. The EAC produces a phase signal mimicking the original phase of the DML-modulated optical signal, and the optical phase modulator driven by the phase signal compensates for the DML frequency chirp in the optical domain. In optical orthogonal frequency division multiplexing (OOFDM) passive optical network (PON) systems utilising DMLs and intensity-modulation and direct-detection (IMDD), the proposed technique can almost completely alleviate the DML frequency chirp effect. In particular, compared to uncompensated conventional DML-based OOFDM IMDD PON systems, the proposed technique can not only improve the transmission capacity by approximately 25% over a wide transmission distance range but also considerably enhance the system performance robustness to variations in DML operating conditions. In addition, optimum EAC design parameters are also identified, based on which the present technique offers system operating condition-independent transmission performance.

Adaptive-Modulation-Enabled WDM Impairment Reduction in Multichannel Optical OFDM Transmission Systems for Next-Generation PONs

The transmission performance of multichannel adaptively modulated optical orthogonal frequency-division multiplexing (AMOOFDM) signals is investigated numerically, for the first time, in optical-amplification-free and chromatic-dispersion-compensation-free intensity-modulation and direct-detection systems directly incorporating modulated distributed feedback (DFB) lasers (DMLs). It is shown that AMOOFDM not only significantly reduces the nonlinear wavelength-division multiplexing (WDM) impairments induced by the effects of cross-phase modulation and four-wave mixing but also effectively compensates for the DML-induced frequency chirp effect. In comparison with conventional modulated optical orthogonal frequency-division multiplexing (OFDM), which uses an identical signal modulation format across all the subcarriers, AMOOFDM improves the maximum achievable signal transmission capacity of a central WDM channel by a factor of 1.3 and 3.6 for 40- and 80-km standard single-mode fibers, respectively, with the corresponding dynamic input optical power ranges being extended by approximately 5 dB. In addition, AMOOFDM also causes the occurrence of cross-channel complementary modulation format mapping among various WDM channels, leading to considerably improved transmission capacities for all individual WDM channels.

Effects of laser frequency chirp on modal noise in short-range radio over multimode fiber links

An important effect of the frequency chirp of the optical transmitter in radio over multimode fiber links is put into evidence experimentally and modeled theoretically for the first time, to our knowledge. This effect can have an important impact in short-range connections, where, although intermodal dispersion does not generally cause unacceptable limitations to the transmittable bandwidth, the presence of modal noise must be accurately kept under control, since it determines undesired real-time fluctuations of the link.