Temporal Walk-off Research Articles

Exploring the limits to energy scaling and distant-target delivery of high-intensity midinfrared pulses

We numerically investigate the scaling behavior of midinfrared filaments at extremely high input energies. It is shown that, given sufficient power, kilometer-scale, low-loss atmospheric filamentation is attainable by prechirping the pulse. Fully resolved four-dimensional ($xyzt$) simulations show that, while in a spatially imperfect beam the modulation instability can lead to multiple hot-spot formation, the individual filaments are still stabilized by the recently proposed mechanism that relies on the temporal walk-off of short-wavelength radiation.

Dual-soliton Stokes-based background-free coherent anti-Stokes Raman scattering spectroscopy and microscopy.

We propose an all-fiber-generated, dual-soliton, Stokes-based scheme for background-free coherent anti-Stokes Raman scattering (CARS) under the spectral focusing mechanism. Owing to the strong birefringence and high nonlinearity of a polarization-maintaining PCF (PM-PCF), two soliton pulses can be simultaneously emitted along different eigenpolarization axes and both serve as Stokes pulses, while allowing feasible tunability of frequency distance and temporal interval between them. This proposed scheme, based on an all-fiber light source, exploits a unique combination of slight frequency-shift temporal walk-off of these two solitons to achieve efficient suppression of the nonresonant background and beat the inaccessibility and complexity of the excitation source. Capability is experimentally demonstrated by background-free CARS spectroscopy and unambiguous CARS microscopy in the fingerprint region.

Multimode supercontinuum generation in chalcogenide glass fibres.

Mid-infrared supercontinuum generation is considered in chalcogenide fibres when taking into account both polarisations and the necessary higher order modes. In particular we focus on high pulse energy supercontinuum generation with long pump pulses. The modeling indicates that when only a single polarisation in the fundamental mode is considered the obtainable supercontinuum bandwidth is substantially exaggerated compared to when both polarisations are taken into account. Our modeling shows that if the pump pulse is short enough (≤ 10 ps) then higher order modes are not important because of temporal walk-off. In contrast long pump pulses (≥ 40 ps) will efficiently excite higher order modes through Raman scattering, which will deplete the fundamental mode of energy and limit the possibility of obtaining a broadband supercontinuum.

Walk-Off-Induced Modulation Instability, Temporal Pattern Formation, and Frequency Comb Generation in Cavity-Enhanced Second-Harmonic Generation.

We derive a time-domain mean-field equation to model the full temporal and spectral dynamics of light in singly resonant cavity-enhanced second-harmonic generation systems. We show that the temporal walk-off between the fundamental and the second-harmonic fields plays a decisive role under realistic conditions, giving rise to rich, previously unidentified nonlinear behavior. Through linear stability analysis and numerical simulations, we discover a new kind of quadratic modulation instability which leads to the formation of optical frequency combs and associated time-domain dissipative structures. Our numerical simulations show excellent agreement with recent experimental observations of frequency combs in quadratic nonlinear media [Phys. Rev. A 91, 063839 (2015)]. Thus, in addition to unveiling a new, experimentally accessible regime of nonlinear dynamics, our work enables predictive modeling of frequency comb generation in cavity-enhanced second-harmonic generation systems. We expect our findings to have wide impact on the study of temporal and spectral dynamics in a diverse range of dispersive, quadratically nonlinear resonators.

Numerical investigations of non-collinear optical parametric chirped pulse amplification for Laguerre–Gaussian vortex beam

We present for the first time a scheme to amplify a Laguerre–Gaussian vortex beam based on non-collinear optical parametric chirped pulse amplification (OPCPA). In addition, a three-dimensional numerical model of non-collinear optical parametric amplification was deduced in the frequency domain, in which the effects of non-collinear configuration, temporal and spatial walk-off, group-velocity dispersion and diffraction were also taken into account, to trace the dynamics of the Laguerre–Gaussian vortex beam and investigate its critical parameters in the non-collinear OPCPA process. Based on the numerical simulation results, the scheme shows promise for implementation in a relativistic twisted laser pulse system, which will diversify the light–matter interaction field.

Second-harmonic generation of focused ultrashort x-ray pulses

We study the process of x-ray second-harmonic generation (SHG) from focused ultrashort pulses. We numerically simulate and derive approximated analytical expressions for the efficiency and tolerances of the SHG process, considering the effects of temporal and spatial walk-off, and the coupling to the linear Bragg scattering. We show that for the recently observed x-ray SHG in diamond, the simultaneous Bragg diffraction of the generated second harmonic is negligible. However, spatiotemporal walk-off effects are crucial components regarding the wave propagation aspects of this process, due to the highly noncollinear phase-matching geometry.

Attosecond pulse walk-off in high-order harmonic generation

We study the influence of the generation conditions on the group delay of attosecond pulses in high-order harmonic generation in gases. The group delay relative to the fundamental field is found to decrease with increasing gas pressure in the generation cell, reflecting a temporal walk-off due to the dispersive properties of the nonlinear medium. This effect is well reproduced using an on-axis phase-matching model of high-order harmonic generation in an absorbing gas.

The factors of spontaneous parametric down conversion process in BiB3O6 crystal with a broadband pump

We investigate the spontaneous parametric down-conversion (SPDC) process in biaxial nonlinear crystal BiB3O6 (BIBO) with a broadband pump. Under the type I and type II phase-matching conditions, we numerically calculate the influencing factors on SPDC process in BIBO crystal, such as temporal and spatial walk-off, the acceptance angles, and spectral acceptance bandwidth. Comparing the two types of phase-matching, we could conclude that the type II phase-matching in BIBO crystal is better for the SPDC because of zero dispersion and bigger acceptance parameters. These results can be used to construct a system to generate ultra-fast entangled photon pairs.

Generating polarization-entangled photon pairs using cross-spliced birefringent fibers

We demonstrate a novel polarization-entangled photon-pair source based on standard birefringent polarization-maintaining optical fiber. The source consists of two stretches of fiber spliced together with perpendicular polarization axes, and has the potential to be fully fiber-based, with all bulk optics replaced with in-fiber equivalents. By modelling the temporal walk-off in the fibers, we implement compensation necessary for the photon creation processes in the two stretches of fiber to be indistinguishable. Our source subsequently produces a high quality entangled state having (92.2 ± 0.2) % fidelity with a maximally entangled Bell state.

State engineering of photon pairs produced through dual-pump spontaneous four-wave mixing

We study theoretically the joint spectral properties of photon-pairs produced through spontaneous four-wave mixing (SFWM) with two spectrally distinct pump pulses in optical fibers. We show that, due to the group velocity difference between the pulses, the signature of the interaction can be significantly different from spontaneous parametric down-conversion or SFWM with a single pump pulse. Specifically, we study the case where temporal walk-off between the pumps enables a gradual turn-on and turn-off of the interaction. By utilizing this property, we develop a new approach towards tailoring the spectral correlations within the generated photon pairs, demonstrating the ability to produce factorable photon-pair states, and hence heralded single photons in a pure wave-packet. We show that the use of two pumps is advantageous over single-pump SFWM approaches towards this goal: the usage of the dual-pump configuration enables, in principle, the creation of completely factorable states without any spectral filtering, even in media for which single-pump SFWM tailoring techniques are unsatisfactory, such as standard polarization-maintaining fiber.

Black-light continuum generation in a silica-core photonic crystal fiber

We report the observation of a broadband continuum spanning from 350 to 470 nm in the black-light region of the electromagnetic spectrum as a result of picosecond pumping a solid-core silica photonic crystal fiber at 355 nm. This was achieved despite strong absorption and a large normal dispersion of silica glass in the UV. Further investigations reveal that the continuum generation results from the interplay of intermodally phase-matched four-wave mixing and cascaded Raman scattering. We also discuss the main limitations in terms of bandwidth and power due to temporal walk-off, fiber absorption, and the photo darkening effect, and we suggest simple solutions.

Temperature-dependent visible to near-infrared optical properties of 8 mol% Mg-doped lithium tantalate

We report the experimental determination of the ordinary and extraordinary refractive index of 8 mol% Mg-doped congruent lithium tantalate (MCLT). Refractive index measurements cover a spectral range from 450nm to 1550nm and temperatures varying from 22°C to 200°C. Experimental data are fitted to a temperature-dependent dispersion relation that has not been previously used with this material family. Based on this relation, various optical properties of MCLT are calculated, including thermo-optic coefficient, group velocity dispersion, phase matching curve and temporal walk-off. In an additional quasi-phase-matching second-harmonic-generation experiment it is shown that the proposed dispersion relation may be used to predict grating period with remarkable nanometer-scale accuracy.

Narrowband pulse-enhanced upconversion of chirped broadband pulses

We propose and demonstrate an efficient sum-frequency mixing scheme based onnarrowband and chirped broadband pulses. It combines the advantages of wider spectralacceptance bandwidth and of alleviating the temporal walk-off, which are both beneficial tohigher conversion efficiency. Chirped sum-frequency pulses at 455 nm with energy up to360 µJ, corresponding to aconversion efficiency of ∼ 40%, are obtained and the pulses can be compressed to ∼ 110 fs. The sum-frequency mixing scheme may provide a promising route to the efficientgeneration of deep-ultraviolet femtosecond pulses.

Backward three-wave optical solitons

Backward symbiotic solitary waves in quadratic media with absorption losses are generated through the nonlinear non-degenerate three-wave interaction. We study these solitary waves in the particular case of a doubly backward quasi-phase matching configuration. The same mechanism responsible for nanosecond solitary wave morphogenesis in the c.w. pumped Brillouin-fiber-ring laser may act for picosecond pulse generation in a quadratic c.w. pumped optical parametric oscillator (OPO). The resonant condition is automatically satisfied in stimulated Brillouin backscattering when the fiber-ring laser contains a large number of longitudinal modes beneath the gain curve. However, in order to achieve quasi-phase matching between the three optical waves in the Χ(2) medium, a nonlinear susceptibility inversion grating of sub-µm period is required. Such a quadratic medium supports solitary waves that result from energy exchanges between dispersionless waves of different velocities. We show, by a stability analysis of the non-degenerate backward OPO in the QPM decay interaction between a c.w. pump and backward signal and idler waves that the inhomogeneous stationary solution exhibits a Hopf bifurcation with a single control parameter. Above OPO threshold, the nonlinear dynamics yields self-structuration of a backward symbiotic solitary wave, which is stable for a finite temporal walk-off (i.e. different group velocities) between signal and idler waves. We also study the dynamics of singly backward mirrorless OPO’s (BMOPO’s) pumped by an incoherent field, in line with the recent experimental demonstration of this OPO configuration. We show that this system is characterized, as a general rule, by the generation of a highly coherent backward field, despite the high degree of incoherence of the pump field. This remarkable property finds its origin in two distinct phase-locking mechanisms that originate respectively in the convection and the dispersion properties of the fields. In both cases we show that the incoherence of the pump is transferred to the co-moving field, which thus allows the backward field to evolve towards a highly coherent state. We propose realistic experimental conditions that may be implemented with currently available technology and in which backward coherent wave generation from incoherent excitation may be observed and studied.

Terahertz wave generation from gas plasma using a phase compensator with attosecond phase-control accuracy

We report the use of a precise phase compensator for the generation of intense terahertz waves from laser-induced gas plasma excited by a femtosecond pulse (ω) and its second harmonic (2ω) at both close contact and standoff distances. The attosecond accuracy phase-control capability of the device enables further optimization of the terahertz emission from gas plasma and elimination of the temporal walkoff between the ω and 2ω pulses traveling in dispersive media, resulting in intense terahertz generation at a distance of over 100 m by sending the optical beams far away and focusing them locally.

Proposal of high quality walk-off compensated sum frequency generation of ultra-short pulses

A multi-crystal and a frequency-selective group-velocity compensating schemes are proposed for the sum frequency-third harmonic generation of low power ultra-short pulses from a Ti:sapphire femtosecond laser. The mechanisms of compensation are analyzed accordingly. It is shown that the temporal walk-off can be effectively compensated without any dispersive element. By use of BBO, calcite crystals together with specially designed composite phase delay plates and dichroic beamsplitters, high quality third harmonic ultra-violet pulses within 300 nm can be generated by the two schemes.

Passively mode-locked slow pump optical parametric oscillators

We predict that employing a Bragg grating to simultaneously phase match and induce large temporal walk-off between the signal and slow propagating pump in a nonlinear crystal can be conducive to development of passively mode locked optical parametric oscillators in which cw near-infrared pump is efficiently converted into short middle-infrared pulses.

Widely tunable dual-pump parametric amplifiers with photonic crystal fibres

This paper theoretically studies the double-pumped fibre-optical parametric amplifiers (FOPAs) in photonic crystal fibres. Two distinct working regimes of FOPAs are researched, which depend on the dispersion at the central wavelength of the two pumps. Extremely broad tuning range can be obtained when the central pump wavelength is in the normal dispersion regime and is insensitive to the wavelength separation between the two pumps, while the tuning range is narrow in the anomalous dispersion regime and can be significantly enhanced by increasing the wavelength separation. Impacts of higher-order dispersions and temporal walk-off on the gain spectra are also discussed.

Experimental investigation of parametric transfer in synchronously pumped optical parametric oscillators

We experimentally investigate the parameters affecting parametric transfer in a synchronously pumped optical parametric oscillator for indirect shaping of mid-infrared ultrashort pulses and make comparisons with previously reported numerical modeling. The individual effects of the parameters are discussed in detail. We conclude that signal bandwidth narrowing, minimal signal amplification, large pump depletion, cavity length tuning, and minimal pump and idler temporal walk-off are required for high fidelity transfer, which we are able to demonstrate for a strongly chirped pump pulse.

Coherent signal from incoherently cw-pumped singly resonant Ti:LiNbO_3 integrated optical parametric oscillators

Incoherent broad-bandwidth cw-pumping singly resonant Ti:LiNbO3 integrated optical parametric oscillators, in the quasi-phase-matched nondegenerate type I {eee} configuration, may efficiently generate coherent signal output by the convection-induced phase-locking mechanism. The incoherence of the pump is absorbed by the idler wave, propagating at the same group velocity, while the finite temporal walk-off of the signal with respect to the pump and idler waves determines a signal coherence gain of more than 4 orders of magnitude.