Laser Frequency Chirp Research Articles

Controlled transition to different proton acceleration regimes: Near-critical-density plasmas driven by circularly polarized few-cycle pulses

A controlled transition between two different ion acceleration mechanisms would pave the way to achieving different ion energies and spectral features within the same experimental set up, depending on the region of operation. Based on numerical simulations conducted over a wide range of experimentally achievable parameter space, reported here is a comprehensive investigation of the different facets of ion acceleration by relativistically intense circularly polarized laser pulses interacting with thin near-critical-density plasma targets. The results show that the plasma thickness, exponential density gradient, and laser frequency chirp can be controlled to switch the interaction from the transparent operating regime to the opaque one, thereby enabling the choice of a Maxwellian-like ion energy distribution with a cutoff energy in the relativistically transparent regime or a quasi-monoenergetic spectrum in the opaque regime. Next, it is established that a multispecies target configuration can be used effectively for optimal generation of quasi-monoenergetic ion bunches of a desired species. Finally, the feasibility is demonstrated for generating monoenergetic proton beams with energy peak at E≈20–40 MeV and a narrow energy spread of ΔE/E≈18%–28.6% confined within a divergence angle of ∼175 mrad at a reasonable laser peak intensity of I0 ≃ 5.4 × 1020 W/cm2.

Laser Frequency Chirping With Silicon Photonic Circuit Comprising an IQ Modulator and a III-V/Si SOA

We report on the performance of a silicon photonic integrated circuit for laser frequency chirping, developed for FMCW LIDAR applications. An external laser source is fiber coupled to the photonic circuit and frequency chirping is achieved with a silicon IQ modulator and a monolithically integrated III-V/Si semiconductor optical amplifier to compensate for IQ modulator losses. A chirp distortion of 1.8% over 5% to 95% of the chirp ramp period was measured with a delayed self-heterodyne interferometric method. The high chirp linearity of our circuit is confirmed by FMCW ranging measurements using 2-18 m long fiber delay lines. The resulting beat frequency follows a linear dependency with the fiber delay line length (R²=0.999).

Single-Lane 54-Gbit/s PAM-4/8 Signal Transmissions Using 10G-Class Directly Modulated Lasers Enabled by Low-Complexity Nonlinear Digital Equalization

This work presents the use of a directly modulated laser (DML) in time- and wavelength division multiplexed passive optical networks (TWDM-PONs), in which an EDFA-based booster amplification and an SOA-based pre-amplification are utilized to improve the optical power budget. We experimentally demonstrate the C-band optically amplified data transmissions and compare the performance of the non-return-to-zero on-off keying (NRZ-OOK), four and eight-level pulse amplitude modulation (PAM-4/PAM-8) using a 10-G class DML. With optical amplification, the uplink and downlink power budgets of about 28 dB and 35 dB, respectively, are achieved for the PAM-4 54-Gbit/s signals at the hard-decision forward error correction (HD-FEC) limit of 3.8x10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> after transmissions over a 25-km single-mode fiber link. To mitigate waveform distortions caused by the limited bandwidth, nonlinear dynamics, memory effects, and strong laser frequency chirp of the DML, especially during multi-level signal modulations, artificial neural network-based machine learning equalizers and low-complexity Volterra nonlinear equalizers are applied, which operate at the signal baud rate. The bit error ratio performance in conjunction with an enhanced power budget through the use of a low-complexity nonlinear equalizer can justify the validity of using a DML in the next-generation PONs.

Long distance high resolution FMCW laser ranging with phase noise compensation and 2D signal processing.

A long distance high resolution frequency-modulated continuous wave (FMCW) laser rangefinder with phase noise compensation and two-dimensional (2D) data processing skills is developed. Range-finding ladar consists of a continuously chirped laser source, an auxiliary reference interferometer, and a monostatic optical transceiver for target illumination and return photon collection. To extend the range unambiguity and lower the electronic processing bandwidth, a two-step laser frequency chirping scheme is adopted, where a long pulse width, small frequency bandwidth laser chirping signal are used in step 1 for coarse distance estimation, and a short pulse width and large frequency bandwidth laser chirping signal are applied afterwards for step 2 high resolution distance realization. An auxiliary reference interferometer is to record the phase noise originated from the laser source to compensate for phase errors induced in the target return photons. The 2D data processing skill helps to coherently sum up all the phase noise removed echo photons to achieve high resolution range peak extraction with high detection sensitivity. Experimental demonstration shows that the proposed FMCW ladar at 1550 nm wavelength with a laser chirping bandwidth of 10 GHz and electronic processing bandwidth of 200 MHz can measure a corner cube test target in an outdoor atmospheric environment, and the measurement results are 12013.905 m with a 2.4 cm range resolution under strong return photon levels and 12013.920 m with a 2.5 cm range resolution under weak return photon levels.

Electron quantum path control in high harmonic generation via chirp variation of strong laser pulses

The quantum phases of the electron paths driven by an ultrafast laser in high harmonic generation in an atomic gas depends linearly on the instantaneous cycle-averaged laser intensity. Using high laser intensities, a complete single ionisation of the atomic gas may occur before the laser pulse peak. Therefore, high harmonic generation could be localised only in a temporal window at the leading edge of laser pulse envelope. Varying the laser frequency chirp of an intense ultrafast laser pulse, the centre, and the width of the temporal window, that the high harmonic generation phenomenon occurs, could be controlled with high accuracy. This way, both the duration and the phase of the electron trajectories, that generate efficiently high harmonics, is fully controlled. A method of spectral control and selection of the high harmonic extreme ultraviolet light from distinct quantum paths is experimentally demonstrated. Furthermore, a phenomenological numerical model enlightens the physical processes that take place. This novel approach of the electron quantum path selection via laser chirp is a simple and versatile way of controlling the time-spectral characteristics of the coherent extreme ultraviolet light with applications in the fields of attosecond pulses and soft x-ray nano-imaging.

High-Field Coherent Terahertz Radiation Generation From Chirped Laser Pulse Interaction With Plasmas

This article presents a theoretical analysis of high-field terahertz (THz) radiation generation from chirped laser pulse interaction with a plasma of slanting density modulation. The mechanism is based on strong nonlinear transverse current produced by the beating of two chirped laser pulses, where the plasma density modulation is inclined at angle to the propagation of the lasers. We claim that oblique spatial density ripple plays a crucial role in field enhancement of THz. At resonance (due to the phase-matching), the field strength of THz becomes maximum for a particular slating angle of the plasma density modulation. We claim that the oblique spatial density ripple and laser frequency-chirp combinedly play a crucial role in field enhancement of THz field. We observe that stable THz is generated with a maximum field strength of 100 MV/cm and a relatively broad spectrum extending out to 2.4 THz, with a strong dependence on the selection of slating angle of plasma density ripple and laser frequency chirp parameter, which corresponds to the conversion efficiency in order of 10−2. The efficiency of the process may be optimized and controlled by the laser and plasma parameters, which may be useful in various applications of these kind of high-field THz.

Angular interferences of sequentially ionized double-continuum wave packets

Following the recent report of energy and angle interferences in below-threshold photoexcitation Auger processes due to postcollision interactions [Wang and Robicheaux, Phys. Rev. A 98, 013421 (2018)], similar interferences are also found in double-continuum wave packets from a two-step time-delayed photoionization of a two-electron atom. Properties of the interferences are studied in detail with respect to laser pulse time widths, laser frequency chirping, and ionization time delay. The effects of these physical quantities on the resulting interferences are discussed.

Generation of an isolated attosecond pulse via current modulation induced by a chirped laser pulse in an x-ray free-electron laser

Aiming for the future upgrade of a hard x-ray beamline at the Pohang accelerator laboratory x-ray free-electron laser (PAL-XFEL), we first analyze the scheme recently proposed for the attosecond-terawatt (TW) x-ray pulse [Phys. Rev. Lett.110, 084801 (2013)PRLTAO0031-900710.1103/PhysRevLett.110.084801]. An x-ray pulse with 32 attosecond full-width half maximum (FWHM) pulse duration and ∼4 TW in power is shown in simulations using the PAL-XFEL parameters. Furthermore, to remove neighboring radiation pulses and to generate an isolated attosecond-TW x-ray pulse, the role of optical laser frequency chirp is examined on the electron beam current modulation as well as on the x-ray pulse generation in the undulator. Our simulations show an isolated x-ray pulse with 42 attosecond FWHM pulse duration and ∼3.5 TW in power for the optimal frequency chirp of a 400 nm optical laser. Recently, in a simpler method [J. Synchrotron Radiat.23, 1273 (2016)JSYRES0909-049510.1107/S1600577516013345] and [Phys. Rev. ST Accel. Beams8, 040701 (2005)PRABFM1098-440210.1103/PhysRevSTAB.8.040701], using a frequency-chirped optical laser and the electron beam delays in between undulator sections, it has been reported that an isolated attosecond-TW x-ray pulse can be obtained at the undulator end. For optimal chirp in this simple setup, we show 50 attosecond FWHM pulse duration and ∼3 TW in power with ∼7.5×1010 photons per pulse at 0.1 nm radiation wavelength. Generation of such inherently synchronized, powerful single attosecond x-ray pulses at PAL-XFEL will be advantageous to the pump-probe experiments in the study of ultrafast dynamics.

Extended-bandwidth frequency sweeps of a distributed feedback laser using combined injection current and temperature modulation.

This article details the generation of an extended-bandwidth frequency sweep using a single, communication grade distributed feedback (DFB) laser. The frequency sweep is generated using a two-step technique. In the first step, injection current modulation is employed as a means of varying the output frequency of a DFB laser over a bandwidth of 99.26 GHz. A digital optical phase lock loop is used to lock the frequency sweep speed during current modulation, resulting in a linear frequency chirp. In the second step, the temperature of the DFB laser is modulated, resulting in a shifted starting laser output frequency. A laser frequency chirp is again generated beginning at this shifted starting frequency, resulting in a frequency-shifted spectrum relative to the first recorded data. This process is then repeated across a range of starting temperatures, resulting in a series of partially overlapping, frequency-shifted spectra. These spectra are then aligned using cross-correlation and combined using averaging to form a single, broadband spectrum with a total bandwidth of 510.9 GHz. In order to investigate the utility of this technique, experimental testing was performed in which the approach was used as the swept-frequency source of a coherent optical frequency domain reflectometry system. This system was used to interrogate an optical fiber containing a 20 point, 1-mm pitch length fiber Bragg grating, corresponding to a period of 100 GHz. Using this technique, both the periodicity of the grating in the frequency domain and the individual reflector elements of the structure in the time domain were resolved, demonstrating the technique's potential as a method of extending the sweeping bandwidth of semiconductor lasers for frequency-based sensing applications.

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.

Quasimonoenergic collimated electrons from the ionization of low density gases by a chirped intense Gaussian laser pulse

The spectrum of energy and angle of emittance of the electrons generated during ionization of neon ions Ne8+, krypton ions Kr32+, and argon ions Ar16+ by a laser pulse have been obtained for different values of laser frequency chirp and normalized laser pulse duration. The energy of the electron beam shifts to higher energy with the introduction of frequency chirp. The energy peak shifts towards lower energy with an increase in frequency chirp, and the electron beam becomes more quasi-monoenergetic. The energy peak shifts to higher energy with decreasing laser pulse duration due to increase in asymmetry of the pulse, however, the quasi-monoenergetic property of the electron beam decreases. We can obtain MeV, MeV/GeV, and GeV electron beams using neon, krypton, and argon gases as target. The scattering of the electrons decreases with decreasing laser pulse duration and increasing laser intensity. The energy peak is sharper and at higher energy for the ions located after laser focus than that for the ions located before laser focus for a tightly focused laser pulse.

Characterization of electrons and x-rays produced using chirped laser pulses in a laser wakefield accelerator

The electron injection process into a plasma-based laser wakefield accelerator can be influenced by modifying the parameters of the driver pulse. We present an experimental study on the combined effect of the laser pulse duration, pulse shape, and frequency chirp on the electron injection and acceleration process and the associated radiation emission for two different gas types—a 97.5% He and 2.5% N2 mixture and pure He. In general, the shortest pulse duration with minimal frequency chirp produced the highest energy electrons and the most charge. Pulses on the positive chirp side sustained electron injection and produced higher charge, but lower peak energy electrons, compared with negatively chirped pulses. A similar trend was observed for the radiant energy. The relationship between the radiant energy and the electron charge remained linear over a threefold change in the electron density and was independent of the drive pulse characteristics. X-ray spectra showed that ionization injection of electrons into the wakefield generally produced more photons than self-injection for all pulse durations/frequency chirp and had less of a spread in the number of photons around the peak x-ray energy.

Fourier transform-limited optical frequency-modulated continuous-wave interferometry over several tens of laser coherence lengths.

We report on a versatile optical frequency-modulated continuous-wave interferometry technique that exploits wideband phase locking for generating highly coherent linear laser frequency chirps. This technique is based on an ultra-short delay-unbalanced interferometer, which leads to a large bandwidth, short lock time, and robust operation even in the absence of any isolation from environmental perturbations. In combination with a digital delay-matched phase error compensation, this permits the achievement of a range window about 60 times larger than the intrinsic laser coherence length with a 1.25 mm Fourier transform-limited spatial resolution. The demonstrated configuration can be easily applied to virtually any semiconductor laser.

Enabling Complex Modulation of Directly Modulated Signals Using Laser Frequency Chirp

Directly modulated laser (DML) with direct detection (DD) dominates the commercial optical short reach applications. When upgrading the DML-DD to higher data rate, the frequency chirp is commonly regarded as a performance barrier. On the contrary, in this letter, we take advantage of the chirp to generate phase modulation along with the intensity, namely, realize complex modulation at transmitter using a single DML. Coherent detection (CD) is applied to recover this 2 dimensional (2-D) signal. A modified Viterbi algorithm (VA) conducts maximum likelihood decoding utilizing the signal intensity and differential phase in receiver digital signal processing (DSP). By making decision using 2-D information, VA provides more than 9-dB optical signal to noise ratio (OSNR) sensitivity advantage over the conventional DML-CD with intensity only decision, in a 10-GBaud pulse-amplitude modulation (PAM)-4 system. We demonstrate the first PAM-8 experiment over 160-km standard single mode fiber using the phase diversity DML.

Electron heating enhancement by frequency-chirped laser pulses

Propagation of a chirped laser pulse with a circular polarization through an uprising plasma density profile is studied by using 1D-3V particle-in-cell simulation. The laser penetration depth is increased in an overdense plasma compared to an unchirped pulse. The induced transparency due to the laser frequency chirp results in an enhanced heating of hot electrons as well as increased maximum longitudinal electrostatic field at the back side of the solid target, which is very essential in target normal sheath acceleration regime of proton acceleration. For an applied chirp parameter between 0.008 and 0.01, the maximum amount of the electrostatic field is improved by a factor of 2. Furthermore, it is noticed that for a chirped laser pulse with a0 = 5, because of increasing the plasma transparency length, the laser pulse can penetrate up to about ne ≈ 6nc, where nc is plasma critical density. It shows 63% increase in the effective critical density compared to the relativistic induced transparency regime for an unchirped condition.

Nanocontrol of single dense energetic electron sheet in a chirped pulse with critical relativistic intensity

We investigate the effect of laser frequency chirp on the generation of a dense energetic electron sheet. The direct acceleration regime of laser driven dense electron sheet requires high laser intensity to function efficiently. We demonstrate this requirement can be relaxed by applying chirped lasers. The acceleration becomes more efficient while driven by a pulse with a proper linear chirp, leading to a dense energetic electron sheet in nanoscale, and therefore requires lower laser intensity.

Relativistic effects in atom and neutron interferometry and the differences between them

In recent years there has been an enormous progress in matter wave interferometry. The Colella- Overhauser-Werner (COW) type of neutron interferometer and the Kasevich-Chu (K-C) atom interferometer, are the prototype of such devices and the issue of whether they are sensitive to relativistic effects has recently aroused much controversy. We examine the question of to what extent the gravitational red shift, and the related twin paradox effect can be seen in both of these atom and neutron interferometers. We point out an asymmetry between the two types of devices. Because of this, the non-vanishing, non-relativistic residue of both effects can be seen in the neutron interferometer, while in the K-C interferometer the effects cancel out, leaving no residue, although they could be present in other types of atom interferometers. Also, the necessary shifting of the laser frequency (chirping) in the atom interferometer effectively changes the laboratory into a free-fall system, which could be exploited for other experiments.

Effect of the frequency chirp on laser wakefield acceleration

The role of laser frequency chirps in the laser wakefield accelerator is examined. We show that in the linear regime, the evolution of the laser pulse length is affected by the frequency chirp, and that positive (negative) chirp compresses (stretches) the laser pulse, thereby increasing (decreasing) the peak vector potential and wakefield amplitude. In the blowout regime, the frequency chirp can be used to fine-tune the localized etching rates at the front of the laser. In our simulations, chirped laser pulses can lead to 15% higher self-trapped electrons and 10% higher peak energies as compared to the transform-limited pulse. Chirps may be used to control the phase velocity of the wake and to relax the self-guiding conditions at the front of the laser. Our predictions are confirmed by multi-dimensional particle-in-cell simulations with OSIRIS.

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.

Link Design Rules for Cost-Effective Short-Range Radio Over Multimode Fiber Systems

Referring to short-range radio over multimode fiber links, we find out important guidelines for the realization of cost-effective intensity modulated directly detected systems. Since the quality of today's connectors is considerably higher than in the past, we demonstrate that two important parameters of the system are the finite detecting area of the photodiode and the laser frequency chirp. Furthemore, we show that the use of the central launch technique inherently determines a lower impact of modal noise fluctuations with respect to the offset launch one. This makes CL more convenient for short-range applications, where the transmittable bandwidth does not constitute the main limitation.