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A frequency comb generator (FCG) based on dual-cavity Brillouin random fiber lasing oscillation in the 1.5 μm telecon spectral window is established and experimentally demonstrated. In the half-open main cavity of the dual cavity, the stimulated Brillouin scattering in highly nonlinear fiber (HNLF) and Rayleigh scattering in single-mode fiber are employed to provide sufficient Brillouin gain and the randomly distributed feedback, respectively, for random mode resonance. The sub-cavity includes an Er-doped fiber amplifier to couple back and boost lower-order Stokes and anti-Stokes light for the cascade of stimulated Brillouin scattering to generate multiple higher-order Stokes and anti-Stokes light. Meanwhile, efficient four-wave mixing is stimulated in the HNLF-based main cavity, further enhancing the number and intensity of the resonant Stokes and anti-Stokes light. By taking advantages of the unique transmission characteristics of nonlinear optical loop mirrors, the power deviation between Stokes and anti-Stokes lines is further optimized with 17 orders of stable Stokes lines and 15 orders of stable anti-Stokes lines achieved within the 10 dB power deviation, with maximum optical signal-to-noise ratio (OSNR) of ~22 dB and ~17 dB and minimum OSNR of ~10 dB and ~7.5 dB for Stokes and anti-Stokes lines, respectively. In addition, the dynamic characteristics of the proposed FCG have been experimentally investigated. Such an FCG with fixed frequency spacing will find promising applications in fields of optical communication, microwave, optical sensing, etc. [ABSTRACT FROM AUTHOR]

A new D-shaped tellurite photonic crystal fiber sensor based on the four-wave mixing (FWM) effect with the surface plasmon resonance (SPR) effect is designed and optimized. The substrate of the D-shaped photonic crystal fiber (D-PCF) is tellurite glass, and the polished surface is plated with the gold film and hydrogen gas-sensitive film. An air hole of the inner cladding, which is plated with the gold film and methane gas-sensitive film, is selected as the second sensing channel to simultaneously measure the concentration of hydrogen and methane. Based on the four-wave mixing, the wavelength shifts of the Stokes and anti-Stokes spectra resulting from the variation of the gas concentration can be used to accurately detect the concentrations of methane and hydrogen. Meanwhile, it is found that the SPR effect can increase the wavelength shifts, which means the sensitivity of methane and hydrogen augment. After parameter optimization, the maximum sensitivities of methane and hydrogen are 4.03 nm/% and −14.19 nm/%, respectively. Both the linearities are up to 99.9%. The resolution of methane is 1.25×10−2% and hydrogen is 7.14×10−3%. Moreover, the fiber length of this sensor is only 20 mm, which is conducive to the construction of a compact or ultra-compact embedded FWM fiber sensor. [ABSTRACT FROM AUTHOR]

Water-based terahertz (THz) coherent detection scheme has been recently proposed, which overcomes the bandwidth limitation or high probe laser power demand in solid-/gas-based schemes. Here, we report a highly efficient THz coherent detection method using ethanol with superior performances under the mechanism of four-wave mixing. We systematically investigate the energy ratios and relative polarizations between the THz-induced second harmonic and control second harmonic (CSH) beams and reveal that ethanol always exhibits significantly higher detection sensitivity than water. The coherent, incoherent, or hybrid detection mode can be flexibly switched by changing the CSH beam polarization. The enhanced sensitivity derives from the much larger third-order nonlinear coefficient and lower ionization potential of ethanol. In addition, for the ethanol–water mixtures with various concentrations, the THz coherent detection signals can always be decomposed into the linear superposition of those from pure ethanol and neat water at the sub-picosecond timescale, indicating the synergistic contribution of ethanol and water molecules in the mixture during the detection process. This work provides a valid method to significantly improve the sensitivity of the liquid-based coherent detection scheme and a research perspective for exploring the solute–solvent molecular interactions. [ABSTRACT FROM AUTHOR]

A picosecond synchronously pumped terahertz optical parametric oscillator based on silicon membrane waveguide (Si‐based WTPO) is proposed and numerically simulated. Through designing the structure parameters of the silicon membrane waveguide, a broadband terahertz frequency range is obtained by satisfying the phase matching condition. The parametric oscillation process and output characteristics of Si‐based WTPO are numerically analyzed by solving the four‐wave mixing coupled‐wave equations, and the result has indicated that the parametric oscillation within the resonant cavity can apparently enhance the output terahertz wave power. Meanwhile, high bandwidth and efficiency output terahertz wave can be obtained, and too high pump power will lead to a reaction of reverse conversion. Finally, the threshold pump power is calculated. The Si‐based WTPO system we proposed is expected to make some contributions to the study of compact and efficient terahertz wave sources. [ABSTRACT FROM AUTHOR]

Recently, the nonlinear optical response of graphene has been widely investigated, as has the integration of this 2D material onto dielectric waveguides so as to enhance the various nonlinear phenomena that underpin all-optical signal processing applications at telecom wavelengths. However, a great disparity continues to exist from these experimental reports, depending on the used conditions or the hybrid devices under test. Most importantly, hybrid graphene-based waveguides were tested under relatively low powers, and/or combined with waveguide materials that already exhibited a nonnegligible nonlinear contribution, thereby limiting the practical use of graphene for nonlinear applications. Here, we experimentally investigate the nonlinear response of Si 3 N 4 waveguides that are locally covered by submillimeter-long graphene patches by means of pulsed degenerate four-wave mixing at telecom wavelength under 7 W peak powers. Our measurements and comparison with simulations allow us to estimate a local change of the nonlinearity sign as well as a moderate increase of the nonlinear waveguide parameter (γ∼−10 m−1W−1) provided by graphene. Our analysis also clarifies the tradeoff associated with the loss penalty and nonlinear benefit afforded by graphene patches integrated onto passive photonic circuits, thereby providing some guidelines for the design of hybrid integrated nonlinear devices, coated with graphene, or, more generally, any other 2D material. [ABSTRACT FROM AUTHOR]

An on-chip quantum light source based on spontaneous four-wave mixing is an essential element for developing quantum photonic integrated circuit technology, which has the advantage of no connection loss owing to the integration of the source into photonic circuits. The waveguide-based quantum light source inevitably causes propagation loss owing to imperfections in the fabrication process, but the propagation loss effects on photon-pair generation have not been extensively studied. In this study, propagation loss effects were examined using theoretical and experimental methods. In theory, the performance of quantum light sources, such as brightness, heralding efficiency, and coincidence-to-accidental ratio, strongly depend on propagation loss. We fabricate several waveguides with a moderate propagation loss of 2.2 dB/cm to investigate the loss dependence and ascertain that the brightness, heralding efficiency, and coincidence-to-accident ratio strongly correlate with the length of the optical waveguide. The maximum coincidence-count brightness occurred at an optimization length of 1/α, where α is the absorption coefficient. In contrast, the single-count brightness shows slightly different waveguide length dependence owing to loss-induced one-photon states. We expect that the results obtained in this study will greatly assist in determining the proper waveguide length for photon-pair generation according to the source's application fields. The results will be helpful in the development of a quantum light source suitable for practical and quantum optical integrated circuits and will lead to the development of high-fidelity quantum technologies. [ABSTRACT FROM AUTHOR]

Two-port feedback has been theoretically studied and proved to be effective in enhancing the degree of entanglement of the output beams for a four-wave mixing process. Here, to further consider the loss effects and phase delays, we use a model that is closer to practical implementation. By reasonably tuning the feedback coefficients, we find that a higher degree of entanglement and higher power of the output beams can be achieved, and a loose requirement for phase-locking accuracy can be obtained. This scheme may have promising applications in practical quantum computation and quantum communication.

Carrier scattering processes are studied in CH3NH3PbI3 using temperature-dependent four-wave mixing experiments. Our results indicate that scattering by ionized impurities limits the interband dephasing time (T2) below 30 K, with strong electron-phonon scattering dominating at higher temperatures (with a time scale of 125 fs at 100 K). Our theoretical simulations provide quantitative agreement with the measured carrier scattering rate and show that the rate of acoustic phonon scattering is enhanced by strong spin-orbit coupling, which modifies the band-edge density of states. The Rashba coefficient extracted from fitting the experimental results (γc = 2 eV Å) is in agreement with calculations of the surface Rashba effect and recent experiments using the photogalvanic effect on thin films. [ABSTRACT FROM AUTHOR]

A new nonlinear optical scheme relying on sum-frequency mixing in a KBe2BO3F2 crystal has been used to generate intense, broadly tunable, narrow-bandwidth, coherent vacuum-ultraviolet (VUV) radiation beyond 16 eV by resonance-enhanced four-wave mixing in Ar. The VUV radiation was used to record high-resolution pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the N2O+ A+ ← N2O X photoionizing transition in the wave-number range from 132 000 cm−1 to 135 000 cm−1. The rotational structure of almost all vibrational levels of the A+ state with vibrational term values up to 2700 cm−1 could be resolved, and improved values of the first two adiabatic ionization energies of N2O, corresponding to the formation of the X+2Π3/2(000) J+ = 3/2 and A+2Σ+(000) N+ = 0 levels of N2O+ from the X 1Σ+(000) J″ = 0 ground state [103 969.30(12) cm−1 and 132 197.70(12) cm−1, respectively], were derived. The rotational intensity distributions of the bands were found to depend strongly on the value of the vibrational angular momentum of the ionic levels. The vibrational structure is discussed in terms of previously reported effective-Hamiltonian analyses. [ABSTRACT FROM AUTHOR]

Two-dimensional resonance Raman (2DRR) spectroscopies have been used to investigate the structural heterogeneity of ensembles and chemical reaction mechanisms in recent years. Our previous work suggests that the intensities of artifacts may be comparable to the desired 2DRR response for some chemical systems and experimental approaches. In a type of artifact known as a "cascade," the four-wave mixing signal field radiated by one molecule induces a four-wave mixing process in a second molecule. We consider the susceptibility of 2DRR spectroscopy to various types of signal cascades in the present work. Calculations are conducted using empirical parameters obtained for a molecule with an intramolecular charge-transfer transition in acetonitrile. For a fully impulsive pulse sequence, it is shown that "parallel" cascades involving two solute molecules are generally more intense than that of the desired 2DRR response when the solute's mode displacements are 1.0 or less. In addition, we find that the magnitudes of parallel cascades involving both solute and solvent molecules (i.e., a solute-solvent cascade) may exceed that of the 2DRR response when the solute possesses small mode displacements. It is tempting to assume that solute-solvent cascades possess negligible intensities because the off-resonant Raman cross sections of solvents are usually 4–6 orders of magnitude smaller than that of the electronically resonant solute; however, the present calculations show that the difference in solute and solvent concentrations can fully compensate for the difference in Raman cross sections under common experimental conditions. Implications for control experiments and alternate approaches for 2DRR spectroscopy are discussed. [ABSTRACT FROM AUTHOR]

Three-wave mixing spectroscopy of chiral molecules, which exist in left-handed and right-handed conformations, allows for enantioselective population transfer despite random orientation of the molecules. This is based on constructive interference of the three-photon pathways for one enantiomer and the destructive one for the other. We prove here that three mutually orthogonal polarization directions are required to this end. Two different dynamical regimes exist to realize enantioselective population transfer, and we show that they correspond to different phase conditions in the three-wave mixing. We find the excitation scheme used in current rotational three-wave mixing experiments of chiral molecules with C1 symmetry to be close to optimal and discuss the prospects for rovibrational three-wave mixing experiments of axially chiral molecules. Our comprehensive study allows us to clarify earlier misconceptions in the literature. [ABSTRACT FROM AUTHOR]

The two-dimensional electronic and infrared spectroscopy of oriented single crystals is sensitive to structure and point group symmetry. The third order response of crystals is generally different from measurements of isotropic solutions because each coherence path that contributes to the measured field scales to the ensemble average of the four-point correlation functions of the four field-dipole interactions involved in the respective Feynman paths. An analytical evaluation of 2D optical crystallography which depends on the crystal symmetry, laboratory orientation, and the orientation in the crystallographic frame is presented. Applying a symmetry operator in the basis of the allowed polarised radiation modes provides a method for evaluation of non-zero fourth rank tensor elements alternative to direct inspection methods. Uniaxial and biaxial systems are distinguished and the contributions to the rephasing and non-rephasing directions are evaluated for isolated and coupled oscillators. By exploiting coordinate analysis, the extension of non-linear electronic and infrared crystallography for coupled oscillators demonstrates the structural, directional, and symmetry dependent selection of coherences to the four-wave mixing signal. [ABSTRACT FROM AUTHOR]

Abstract A new D-shaped tellurite photonic crystal fiber sensor based on the four-wave mixing (FWM) effect with the surface plasmon resonance (SPR) effect is designed and optimized. The substrate of the D-shaped photonic crystal fiber (D-PCF) is tellurite glass, and the polished surface is plated with the gold film and hydrogen gas-sensitive film. An air hole of the inner cladding, which is plated with the gold film and methane gas-sensitive film, is selected as the second sensing channel to simultaneously measure the concentration of hydrogen and methane. Based on the four-wave mixing, the wavelength shifts of the Stokes and anti-Stokes spectra resulting from the variation of the gas concentration can be used to accurately detect the concentrations of methane and hydrogen. Meanwhile, it is found that the SPR effect can increase the wavelength shifts, which means the sensitivity of methane and hydrogen augment. After parameter optimization, the maximum sensitivities of methane and hydrogen are 4.03 nm/% and −14.19 nm/%, respectively. Both the linearities are up to 99.9%. The resolution of methane is 1.25×10−2% and hydrogen is 7.14×10−3%. Moreover, the fiber length of this sensor is only 20 mm, which is conducive to the construction of a compact or ultra-compact embedded FWM fiber sensor.

Coherent terahertz (THz) wave emission from indium tin oxide (ITO) in epsilon-near-zero region had been reported recently as a second-order nonlinear process. However, THz radiation from ITO through third-order nonlinear process has not been reported yet. Here, we demonstrate coherent THz radiation from an ITO film via four-wave mixing, a third-order nonlinear process, under the excitation of two-color pulsed laser fields. Experimentally, an in-line phase compensator is utilized to synthesize the asymmetric optical fields to induce photocurrent in an ITO film and consequently emit THz waves. Our experimental results show that THz radiation from ITO films can be coherently controlled by the relative phase between the two-color laser fields. Such a THz wave generation process can be attributed to the fast intra-band nonlinearity in the conduction band. Our demonstration reveals that the fast nonlinearity in ITO can be used to generate THz waves and promotes the investigation of ITO in ultrafast photonics. [ABSTRACT FROM AUTHOR]

Guided waves have been extensively studied in the past few years, and more recently nonlinear guided waves have attracted significant research interest for their potential for early damage detection and material state characterization. Combined harmonic generation due to wave mixing can offer some advantages over second harmonic generation. However, studies focused on Lamb wave mixing are still very limited, and have mainly focused on collinear wave mixing and used plane wave assumption. In this paper, numerical simulations and experiments are conducted to understand the interaction of mixing non-collinear Lamb wave pulses with non-planar wavefronts. The results demonstrate that the generated secondary wave is cumulative under internal resonance conditions and the sum-frequency component of the combined harmonics is useful for characterizing material nonlinearities. [ABSTRACT FROM AUTHOR]

We propose a three-beam stimulated resonant photon collider with focused laser fields in order to directly produce an axion-like particle (ALP) with the two beams and to stimulate its decay by the remaining one. The expected sensitivity around the eV mass range has been evaluated. The result shows that the sensitivity can reach the ALP-photon coupling down to O (10 − 14) GeV−1 with 1 J class short-pulsed lasers. [ABSTRACT FROM AUTHOR]

Nonlinear waveguides with two distinct domains of anomalous dispersion can support the formation of molecule-like two-color pulse compounds. They consist of two tightly bound subpulses with frequency loci separated by a vast frequency gap. Perturbing such a two-color pulse compound triggers periodic amplitude and width variations, reminiscent of molecular vibrations. With increasing strength of perturbation, the dynamics of the pulse compound changes from harmonic to nonlinear oscillations. The periodic amplitude variations enable coupling of the pulse compound to dispersive waves, resulting in the resonant emission of multi-frequency radiation. We demonstrate that the location of the resonances can be precisely predicted by phase-matching conditions. If the pulse compound consists of a pair of identical subpulses, inherent symmetries lead to degeneracies in the resonance spectrum. Weak perturbations lift existing degeneracies and cause a splitting of the resonance lines into multiple lines. Strong perturbations result in more complex emission spectra, characterized by well separated spectral bands caused by resonant Cherenkov radiation and additional four-wave mixing processes. [ABSTRACT FROM AUTHOR]

Four-wave mixing (FWM) was demonstrated using polymer-coated Iron phosphorus trisulfide (FePS3) arc-shaped fiber as a device for nonlinear application. The FWM-based system had a maximum FWM conversion efficiency and wavelength tuning range of −37.5 dB and 6 nm, respectively. The FWM-based laser output was highly stable, with a maximum fluctuation in FWM conversion efficiency of only 0.1 dB. The overlaying of low refractive index (RI) polymer on the FePS3 arc-shaped fiber enhanced the device's performance, with an effective nonlinear coefficient, , of the device was calculated to be 666.7 W− 1 km− 1. The proposed nonlinear device is an exciting device for integration in optical network applications. [ABSTRACT FROM AUTHOR]

The random fiber laser (RFL) has been an excellent platform for exploring novel optical dynamics and developing new functional optoelectronic devices. However, it is challenging for RFLs to regulate their emission into regular narrow pulses due to their intrinsic randomness. Here, through engineering the laser configuration (cavity Q value, gain distribution and nonlinearity), we demonstrate that narrow (~2.5 ns) pulses with record peak power as high as 64.3 kW are achieved from a self-Q-switched random ytterbium fiber laser. Based on high intracavity intensity and efficient interplay of multiple nonlinear processes (stimulated Brillouin scattering, stimulated Raman scattering and four-wave mixing), an over-one-octave visible-near-infrared (NIR) Raman-frequency comb is generated from single-mode silica fibers for the first time. After spectrally filtering the Raman peaks, wavelength-tunable pulses with durations of several hundreds of picoseconds are obtained. Such a high-peak-power random Q-switched fiber laser and wide frequency comb in the visible-NIR region can find applications in diverse areas, such as spectroscopy, biomedical imaging and quantum information. [ABSTRACT FROM AUTHOR]

In this paper, we have discussed the enhanced three-wave mixing (TWM) nonlinear optical processing in a three-level symmetry broken three-level quantum system. The quantum system interacts with three drivel laser fields out of which two are weak probes and generated signal beams and one of them is strong coupling light. We have shown that due to symmetry broken of quantum system the nonlinear sum frequency generation leads to TWM. By adjusting the different system parameters, the efficiency of TWM mechanism can be improved. Moreover, we have found when the strong coupling light becomes optical vortex light, the efficiency of TWM depends on the orbital angular momentum (OAM) light. Our proposed model may be having useful applications in nonlinear spectroscopy based on OAM. [ABSTRACT FROM AUTHOR]

The article presents a study which explores the ultra-broadband all-optical sampling of optical waveforms. It demonstrate an alternative, all-optical approach for measuring electric fields of broadband laser pulses, which offers an advantage in terms of sensitivity and signal-to-noise ratio and extends the detection bandwidth of optical methods to the petahertzdomain.

Laser feedback-based self-mixing interferometry (SMI) is a promising technique for displacement sensing. However, commercial deployment of such sensors is being held back due to reduced performance in case of variable optical feedback which invariably happens due to optical speckle encountered when sensing the motion of non-cooperative remote target surfaces. In this work, deep neural networks have been trained under variable optical feedback conditions so that interferometric fringe detection and corresponding displacement measurement can be achieved. We have also proposed a method for automatic labelling of SMI fringes under variable optical feedback to facilitate the generation of a large training dataset. Specifically, we have trained two deep neural network models, namely Yolov5 and EfficientDet, and analysed the performance of these networks on various experimental SMI signals acquired by using different laser-diode-based sensors operating under different noise and speckle conditions. The performance has been quantified in terms of fringe detection accuracy, signal to noise ratio, depth of modulation, and execution time parameters. The impact of network architecture on real-time sensing is also discussed. [ABSTRACT FROM AUTHOR]

Multipartite quantum steering has important applications in secure quantum communication networks. In this work, three different energy-level cascaded four-wave mixing (FWM) processes are proposed to generate quadripartite quantum steering. The covariance matrix among different modes combination is constructed. The properties of quantum steering of the three cascaded models were studied. The results show that all three model structures can achieve various types of quantum steering, and the steerability varies with the amplitude gain of the FWM processes. The results provide a theoretical basis for the experimental generation of multipartite quantum steering and a significant reference for secure quantum communication. [ABSTRACT FROM AUTHOR]

Bound states in the continuum (BICs) have received significant attention for their ability to enhance light-matter interactions across a wide range of systems, including lasers, sensors, and frequency mixers. However, many applications require degenerate or nearly degenerate high-quality factor (Q) modes, such as spontaneous parametric down conversion, non-linear four-wave mixing, and intra-cavity difference frequency mixing for terahertz generation. Previously, degenerate pairs of bound states in the continuum (BICs) have been created by fine-tuning the structure to engineer the degeneracy, yielding BICs that respond unpredictably to structure imperfections and material variations. Instead, using a group theoretic approach, we present a design paradigm based on six-fold rotational symmetry (C6) for creating degenerate pairs of symmetry-protected BICs, whose frequency splitting and Q-factors can be independently and predictably controlled, yielding a complete design phase space. Using a combination of resonator and lattice deformations in silicon metasurfaces, we experimentally demonstrate the ability to tune mode spacing from 2 nm to 110 nm while simultaneously controlling Q-factor. Bound states in the continuum can enhance light-matter interactions across a wide range of optical systems. Here, the authors present a design paradigm for creating pairs of bound states whose optical responses can be predictably controlled. [ABSTRACT FROM AUTHOR]

We report photon–phonon atomic coherence (cascade- and nested-dressing) interaction from the various phase transitions of Eu3+: BiPO4 crystal. Such atomic coherence spectral interaction evolves from out-of-phase fluorescence to in-phase spontaneous four-wave mixing (SFWM) by changing the time gate. The dressing dip switch and three dressing dips of SFWM result from the strong photon–phonon destructive cross- and self-interaction for the hexagonal phase, respectively. More phonon dressing results in the destructive interaction, while less phonon dressing results in the constructive interaction of the atomic coherences. The experimental measurements of the photon–phonon interaction agree with the theoretical simulations. Based on our results, we proposed a model for an optical transistor (as an amplifier and switch). [ABSTRACT FROM AUTHOR]

Four-Wave Mixing (FWM) is a performance constraint in WDM Radio over Fiber(RoF) systems. This article uses several equations to analyze & investigate the performance of FWM crosstalk power and Signal to Crosstalk Ratio (SXR) corresponding to channel spacing, transmission distance & input optical power for different kinds of fiber like Alcatel Teralight, Corning Leaf, Furukawa SM-332 and Lucent Truewave under the influence of higher-order dispersion terms up to 10th order. An improved optical network including a polarization rotator at the transmitter and modern state of art optical fibers is proposed to combat FWM-induced crosstalk. It is reported that the use of a polarization rotator reduces FWM crosstalk by 7.5 dBm with an input power of 20 dBm. Further, it is also revealed that Corning LEAF performs better compared to the rest of fibers while minimizing FWM-induced crosstalk. [ABSTRACT FROM AUTHOR]

We derive propagation equations modeling third-order susceptibility-induced nonlinear interaction and linear mode coupling in waveguides. We model material susceptibility with Raman and electronic response which include approximations suited for optical communications. We validate our model by comparing numerical integration of the propagation equations to continuous wave measurements of a silicon on insulator waveguide. [ABSTRACT FROM AUTHOR]

The method of adiabatic frequency conversion, in analogy with a two‐level atomic system, has been put forward recently and verified experimentally to achieve robust frequency mixing processes such as sum and difference frequency generation. Here a comparative study of efficient frequency mixing using various techniques of shortcuts to adiabaticity such as counter‐diabatic driving and invariant‐based inverse engineering is presented. It is shown here that it is possible to perform sum frequency generation by properly designing the poling structure of a periodically poled crystal and the coupling between the input lights and the crystal. The required crystal length for frequency conversion significantly decreases beyond the adiabatic limit. This approach significantly improves the robustness of the process against the variation in temperature as well as the signal frequency. By introducing a single parameter control technique with constant coupling and combining with the inverse engineering, perturbation theory, and optimal control, it is shown that the phase mismatch can be further optimized with respect to the fluctuations of input wavelength and crystal temperature that results into a novel experimentally realizable mixing scheme. [ABSTRACT FROM AUTHOR]

The spectral broadening of a chirped, temporally stretched (up to 200 ps) Ti:sapphire laser pulse propagating together with a broadband low-power seed pulse through BaWO4 crystals was experimentally and numerically studied. Our analysis demonstrated that self-phase modulation could not have affected the pulse spectrum under the experimental conditions. The spectral broadening of the Ti:sapphire laser pulse was found to be due to four-wave mixing, which resulted in transferring the power of the central wavelength to the spectral wings. To the best of our knowledge, this is the first demonstration of such origin of laser pulse spectral broadening in crystals. [ABSTRACT FROM AUTHOR]

We propose a scheme to exchange optical vortices beyond electromagnetically induced transparency (EIT) but based on four-wave mixing (FWM) in a five-level atomic system consisting of two Λ subsystems linked via a weak driving field. When the laser fields are strong enough, the quantum interference responsible for the EIT in each Λ subsystem is washed out, giving rise to the Autler-Townes splitting (ATS). When only one of the control fields carries an optical vortex, it is shown that the generated FWM field obtains the vorticity of the vortex control. We distinguish between three different regimes, i.e., a pure EIT, a joint EIT-ATS, and a dual-ATS, where the optical angular momentum (OAM) translation can take place. Elaborating on the distinction between three regimes through numerical analysis, we find that the maximum energy conversion efficiency is obtained in the joint EIT-ATS and dual-ATS regimes. The latter is more favorable as the absorption losses vanish as the beam propagates into the atomic cloud. The results may find applications in the implementation of high-efficient frequency and OAM conversion devices for quantum information processing. [ABSTRACT FROM AUTHOR]

We report the generation of biphotons, with a temporal full width at the half maximum (FWHM) of 13.4 ± 0.3 µs and a spectral FWHM of 50 ± 1 kHz, via the process of spontaneous four-wave mixing with laser-cooled atoms. The temporal width is the longest, and the spectral linewidth is the narrowest to date. This is also the first biphoton result that obtains a linewidth below 100 kHz, reaching a new milestone. The very long biphoton wave packet has a signal-to-background ratio of 3.4, which violates the Cauchy–Schwarz inequality for classical light by 4.8 folds. Furthermore, we demonstrated a highly tunable-linewidth biphoton source and showed that while the biphoton source's temporal and spectral width were controllably varied by about 24 folds, its generation rate only changed by less than 15%. A spectral brightness or generation rate per pump power per linewidth of 1.2× 106 pairs/(s mW MHz) was achieved at the temporal width of 13.4 µs. The above results were made possible by the low decoherence rate and high optical depth of the experimental system, as well as a novel scheme of classical fields' and biphotons' propagation directions in the experiment. This work has demonstrated a high-efficiency ultranarrow-linewidth biphoton source and has made substantial advancements in quantum technology utilizing heralded single photons. [ABSTRACT FROM AUTHOR]

A system of coupled waves equations for calculation the vector amplitudes of linearly polarized light waves during their degenerate contra-directional four-wave mixing in cubic photorefractive crystals of 23 and symmetry classes in general spatial orientation was obtained. Linear electrooptic, photoelastic, and inverse piezoelectric effects, as well as natural optical activity, absorption and circular dichroism are taken into consideration at the theoretical modelling. The dependencies of intensity of p- and s-components of the phase-conjugated wave on the crystal orientation angle and the thickness of one at different polarization azimuths of the signal wave for GaAs and Bi12SiO20 crystals of (110)-cut are analyzed. It is shown that these dependencies for Bi12SiO20 crystal exhibiting optical activity differ from the ones derived for optically non-active GaAs crystal. It takes place since under the combined action of the induced anisotropy and optical activity, the conditions of diffraction of the light waves during their four-wave mixing by holographic gratings formed in the Bi12SiO20 crystal will be different with respect to ones in GaAs. It is established that the effect of optical activity on intensity of the phase-conjugated wave considerably depends on the values of the crystal orientation angle, thickness, and polarization azimuths of the signal wave. Optical activity can lead to both decreasing and increasing of intensity of the phase-conjugated wave in dependence on the conditions of holographic experiment. [ABSTRACT FROM AUTHOR]

The effect of an in‐mold static mixer on the optical properties of polystyrene (PS) parts was explored within the injection molding process. Several mixers were assessed via simulation and molding trials to identify the mixing ability and effect on optical properties including retardation and birefringence. It was found that the static mixers within the runner could successfully mix the polymer and disrupt property distributions including temperature but that there was only a slight improvement in retardation with some of the mixer cases. The experiments and simulations showed relatively good correlation in results although there were slight differences in the trends that could be due to the experimental measurement resolution or unaccounted‐for variables between the experiments and simulations. The retardation was experimentally measured using a custom‐made polariscope using photography and image processing. These experiments indicated that the use of a static mixer within the runner system of a mold could be used for homogenizing the polymer melt after the plasticizing unit. However, its effect on improving the optical performance of injection molded parts could be offset by the melt flow downstream of the static mixer and the potential increase of residual stresses due to flow restriction, suggesting the importance of mixer location and geometry. [ABSTRACT FROM AUTHOR]

A system of coupled wave equations that can be used to find the components of the vector amplitudes of linearly polarized light waves on amplitude–phase holographic gratings formed in a cubic photorefractive crystal due to four-wave mixing has been presented. The linear electro-optical, photoelastic, and inverse piezoelectric effects, as well as natural optical activity, circular dichroism, and absorption in the crystal, have been taken into account when deriving the coupling equations. The characteristics of energy exchange in a (111) InP:Fe crystal in the presence of four-wave mixing have been analyzed. It has been shown that the best agreement of the theoretically calculated dependences of the intensities of the p and s polarized components of the conjugate light wave on the orientation angle with the known experimental results is achieved under the assumption that several amplitude–phase holographic gratings are formed in the crystal. The dependence of the intensity of the conjugate wave on the orientation angle and the azimuth of the linear polarization of the pump wave has been theoretically studied. It has been shown that this intensity can be significantly increased by choosing the optimal values of these parameters. [ABSTRACT FROM AUTHOR]

As an emerging field, nonlinear topological photonics attracts great attention nowadays, where pioneering works show interesting phenomena including topological edge solitons and nonlinearity‐induced coupling of light into topological edge states. In this work, the nonlinear effects in a photonic topological insulator in the synthetic space including the frequency axis of light, in a ring resonant array under dynamic modulations are studied, where the four‐wave mixing nonlinear process is included. Such nonlinear process not only induces local interactions, but also creates long‐range interactions in the frequency dimension. These results show destruction on topologically protected edge states when the nonlinear effects are increasing. Moreover, it also breaks the symmetry in the synthetic space as well as the band structure, which reflects different propagation behaviors for the different input sources with positive and negative frequency shifts and exhibits short‐time robustness in weak nonlinear cases. This work hence explores the role of nonlinear effects in affecting the edge modes in the synthetic space, which may be useful in quantum many‐body simulations and find possible applications in information processing. [ABSTRACT FROM AUTHOR]

Multiple speckle diffusing wave spectroscopy (MSDWS) can be applied to measure spatially heterogeneous mechanical behavior in soft solids, with high sensitivity to deformation and both spatial and temporal resolution. In this paper, we discuss the mathematical approach behind the quantification of the deformation rate from MSDWS data and provide guidelines for optimizing the selection of experimental parameters in measurements. After validating the method in extensional tests on an elastomer, we provide an example of the potentiality of MSDWS by measuring the spatial distribution of the deformation rate during shear debonding of adhesive tapes. We quantitatively characterize the deformation rate distribution related to shearing and peeling under loading. A highly heterogeneous deformation rate distribution is observed, and time-dependent measurements reveal an increase in deformation localization hundreds of seconds before full debonding. This behavior, previously predicted by theory and simulation, is demonstrated here experimentally for the first time. [ABSTRACT FROM AUTHOR]

Quantum networks, the backbone of the future quantum internet, require flexible and precise control to faithfully share entanglement with different users. These networks will allow multiple applications including distributed quantum computation and information theoretic secure communication. In this scenario, orbital angular momentum (OAM) can be employed to enhance the capacity of quantum information processing and to distribute quantum entanglement in an efficient and scalable way with multiple users. Here, a method for realizing a multiplexed continuous variable (CV) quantum entanglement distribution based on a four‐wave mixing process combined with an active routing technique is experimentally demonstrated. By exploiting Sagnac‐based OAM sorters, the OAM entanglement can be distributed and measured with the different users in a quasi‐complete star network configuration. The generations of multiple sets of OAM multiplexed bipartite CV entanglement with four users and six users are proposed and demonstrated here. In addition, the propagation of one mode through a 1 km of single‐mode fiber shows the applicability of this approach. These results propose and demonstrate a new method for distributing CV entanglement in a quantum network enlarging the range of applicability of CV quantum information protocols. [ABSTRACT FROM AUTHOR]

The analytical modeling for linear and non-linear impairments (LNI) for single mode fiber (SMF) and simulative analysis of extended power budget class-1 i.e. E1-class of next generation passive optic network-2 (NG-PON2) is presented in the paper. The proposed power-budget class time and wavelength division multiplexing (TWDM) based NG-PON2 network configuration is delivering symmetric 2.5 point-to-multi point (PtM) representing time and TWDM XGS-PON along with 10 Gbps point-to-point (PtP) wavelength division multiplexing (WDM) symmetric and coexistent channels (Ch) in worst-case scenario. Channel modelling of LNI is carried out under the incremental and aggregate strain of estimated chromatic dispersion CD, self-phase modulation (SPM), cross-phase modulation (XPM) of single mode fiber (SMF) channel deployed in optical distributing network (ODN) between 20 and 50 km link lengths. Downlink (D/L) and uplink (U/L) ODN is optimized to PTODN = 7.1 dBm and 4.09 dBm, respectively and corresponding channel imparities including Kerr parameter γ, corresponding SPM, XPM, CD, four-wave mixing (FWM) analysis and corresponding component power and efficiency, interchange crosstalk (Cc) attracting power penalties (Pc) are estimated and incorporated in the simulation for the real response of lossy SMF channel. Further, splitter power budget (SP) and slitter configuration for respective PtM and PtP D/L Ch are estimated under the aggregate impact of CD, SPM, XPM and FWM. Analytical modeling of LNI and simulative analysis has confirmed the configuration delivered rise in receiver sensitivity (Rxs) of − 39.75/− 33.45 dBm and − 29/− 35.34 dBm for D/L and U/L Ch respectively. The estimated power of FWM element is between 1.34 × 10−52 to 2.15 × 10−53 and 3.87 × 10−52 to 2.41 × 10−51 contributing crosstalk between Ch (Cc) of − 31.97/− 30.12 dB for down/up spectrum under the impact of SPM and XPM respectively accommodating splitter configuration of 768 at 40 km. [ABSTRACT FROM AUTHOR]

Four-wave mixing processes are ubiquitous in ultrafast optics and the determination of the coefficients of the χ(3) tensor is thus essential. We introduce a novel time-resolved ultrafast spectroscopic method to characterize the third-order nonlinearity on the femtosecond time-scale. This approach, coined as "nonlinear chirped interferometry," makes use of the variation of the optical group delay of a transmitted probe under the effect of an intense pump pulse in the nonlinear medium of interest. The observable is the spectral interference between the probe and a reference pulse sampled upstream and the metric is the transient swing of the probe group delay. We show that the detected signal is enhanced when the pulses are weakly chirped, and that, although interferometric, the method is intrinsically less sensitive to environmental phase fluctuations and drifts. By chirping adequately the reference pulse, the transient frequency shift of the probe pulses is also detected in the time domain and the detected nonlinear signal is enhanced. Nonlinear phase shifts as low as 10 mrad, corresponding to a frequency shift of 30 GHz, i.e., 0.01% of the carrier frequency, are detected without heterodyne detection or active phase-stabilization. The diagonal and/or non-diagonal terms of reference glasses (SiO2) and crystals (Al2O3, BaF2, CaF2) are characterized. The method is finally applied to measure the soft vibration mode of KTiOAsO4 (KTA). [ABSTRACT FROM AUTHOR]

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The dependence of the correlations between two symmetric four-wave mixing (FWM) signals on detuning is discussed through a simple theoretical model. The model consists of an ensemble of two-level atoms interacting with two electric fields in the semi-classical approach. Using the density matrix formalism, stochastic equations for the component of coherence responsible for the FWM processes are obtained. Here, the stochastic nature of the problem arises from the phase of the fields. Through Ito's calculus, the stochastic differential equations are solved and the dependence of the cross-correlation function G (2) on the detuning of one field is analyzed. The emergence of Rabi oscillations in the G (2) function, consistent with previous experimental findings, is observed. What is more, for weak detuning, there is a crossover from the correlation to the anticorrelation regime, the latter persisting for a relatively large range of detuning. This paper could be of interest for the optimization of the experimental parameters for the correlations between FWM signals. • Simple model for correlations between two Four Wave Mixing Signals. • Rabi oscillations in the correlation function are a consequence of oscillations in the non-DC population. • There is a switch from the correlation to the anti-correlation regime when the detuning is slightly changed. • It is suggested that the anticorrelation region is experimentally a better regime for potential research. [ABSTRACT FROM AUTHOR]

• Comprehensive analysis of generation and separation of both frequency degenerate and non-degenerate FWM twin beams in an OPLM which consists of an optical coupler with adjustable splitting ratio, a nonlinear fiber, a dispersion element, and two tunable attenuators placed at different positions in the loop is demonstrated. • With the splitting ratio of the coupler being 1:1 and no loss in the loop, both the frequency degenerate and non-degenerate twin beams can propagate through different ports completely by introducing proper dispersion, and switching between phase-sensitive and phase-insensitive amplification can be realized by changing the dispersion within the loop. • When the beam splitting ratio of the coupler deviates from 1:1, adjusting the loss in the loop can partially compensate for the asymmetry of the beam splitting ratio and promote the separation of signal and idler beams. • Our research is of great significance for the preparation of twin beams and the creation of switchable phase-sensitive amplifiers by Sagnac fiber loop. We theoretically demonstrate the propagating performance of twin beams generated in a Sagnac fiber loop by developing a practical model of optical parametric loop mirror consisting of an optical coupler with adjustable splitting ratio, a nonlinear fiber, a dispersion element, and two tunable attenuators placed at different positions in the loop. Two situations of frequency degenerate and frequency non-degenerate four-waving mixing are considered and various of factors including dispersion conditions, internal losses in the loop, and splitting ratios of the coupler are investigated in detail. When the splitting ratio of the coupler is 1:1 and no loss is in the loop, both the frequency degenerate and non-degenerate twin beams can propagate from different ports completely by introducing proper dispersion, and switching between phase-sensitive and phase-insensitive amplification can be realized by changing the dispersion within the loop. In general, the propagation of signal and idler beams at the two output ports of the fiber loop are influenced by both the splitting ratio of the coupler and the internal loss within the loop. Adjusting the internal loss can partially compensate for the asymmetry of the splitting ratio of the coupler, facilitating spatial separation of the signal and idler beams. Our research is useful for the generation of twin beams using Sagnac fiber loop and the realization of switchable phase-sensitive amplifiers. [ABSTRACT FROM AUTHOR]

• Describe the development status of optical network and common security protection technologies. • Describe the development status of all-optical mode matching technology in photonic firewall. • The working principle of squarer is introduced, and two all-optical pattern matching systems based on squarer are proposed for QPSK and 8PSK respectively. • Conduct simulation verification and anti-noise performance analysis of the two systems mentioned. Photonic firewall which can conduct the data identification and intrusion detection directly at the optical layer is a high-speed and low-energy network security equipment. For the photonic firewall, the all-optical matching system is the core part. This paper focuses on designing all-optical matching systems for high-order phase modulated optical signals. Based on the squarer, two all-optical matching systems are designed for Quadrature Phase Shift Keying (QPSK) and 8 Phase Shift Keying (8PSK) optical signals respectively. The systems mainly consist of squarers, multiplexers, AND gates and regenerators. The squarer is implemented based on the four-wave mixing (FWM) effect of highly nonlinear fiber (HNLF). The theoretical analysis and simulation verification are conducted for the proposed system. The performances of feasibility and noise impact are evaluated for the proposed system with the data rate of 100 Gbaud. The results show that the system can correctly identify and localize the target sequence with the length of 4 and 8 symbols in the data sequence with the length of 16 symbols. The impact of noise is evaluated by the extinction ratio (ER) and contrast ratio (CR). The ER of two all-optical matching systems reaches 0 dB when the optical signal-to-noise ratio (OSNR) of input optical signal is about 5.59 dB and 9.24 dB respectively. The CR of two all-optical matching systems reaches 0 dB when the OSNR of input optical signal is about 4 dB and 8.62 dB respectively. [ABSTRACT FROM AUTHOR]

• An all-optical, real-time, nonlinear temporal compression method is presented. • An extreme time-interleaving technique, enabling the multiplexing of a 10-Mbit/s data packet within a 10-Gbit/s data sequence is demonstrated. • This is achieved through an extreme × 4350-temporal reduction of the initial µs-signal Mbit/s data waveform. • A good agreement between numerical simulations and experiments is observed. This study expands upon an innovative, all-optical, real-time, nonlinear temporal compression technique based on a counter-propagating degenerate four-wave mixing interaction in a birefringent optical fibre. Specifically, we provide a proof-of-principle demonstration of an advanced time-interleaving procedure, enabling the multiplexing of a 10-Mbit/s data packet within a 10-Gbit/s data sequence, achieving a record compression factor of 4350 alongside the demonstration of time-reversal capabilities. The proposed approach is scalable, holding promise for dynamic optical networks, buffering applications, and the generation of ultrafast waveforms. [ABSTRACT FROM AUTHOR]

Dispersion-Shifted Fiber (DSF) is essential for reducing chromatic dispersion in high-speed optical communication systems. This study investigates the influence of Four Wave Mixing (FWM) on the quality of signals in orthogonal channels. We examine the advantages of DSF technology and analyze the impact of modulation formats such as On-Off Keying with Return-to-Zero (OOK-RZ) and Duo Binary Modulation class-1 (DBM-1) on transmission performance at different distances. This research assesses the efficacy of orthogonal channels in mitigating four-wave mixing (FWM) effects and improving the overall performance of eight-channel systems at distances of 100 km and 200 km through computer simulations. The results of our study show notable enhancements, namely in optimizing the Q-factor (a metric for signal quality) and reducing bit error rates when employing orthogonal channels compared to previous work. By integrating orthogonal channels with OOK-RZ modulation, we achieved higher performance and reduced nonlinear impairments in a simulated eight-channel system with 50 GHz spacing and 80 Gb/s data rates. This effect was particularly pronounced at high input power levels. At an input power of 20 dBm and a distance of 200 km, this particular combination yielded a maximum Q-factor of 27.25 and a minimum FWM power of −54 dBm. In comparison, under the same conditions, the use of OOK-RZ alone resulted in an FWM power of −24 dBm and a Q-factor of only 1.63. This research provides vital insights into enhancing the efficiency and dependability of optical communication systems, hence facilitating breakthroughs in high-speed data transfer and network scalability. • FWM crosstalk is greatly influenced by modulation quality, channel orthogonality, and dispersion conditions. • A new approach involving the combined utilization of orthogonal dense channels with OOK-RZ modulation has demonstrated superior system performance and minimized nonlinear defects, • New technique showcases improved efficiency and reliability in data transmission, offering potential advancements in various applications where signal integrity and robustness are paramount. • A technique for suppressing Four-Wave Mixing (FWM) crosstalk has been successfully implemented at dispersion values close to zero, with high pumping power, and dense channel spacing. • Interference is reduced under extreme conditions such as dense channel spacing, high pump power and low dispersion condition. [ABSTRACT FROM AUTHOR]

We suggest a scheme for the excitation and amplification of surface plasmon polaritons (SPPs) along the interface between metal and semiconductor quantum well (SQW), employing a four-wave mixing (FWM) process. The SQW consists of four-level asymmetric double quantum wells that exhibit quantum interference effects, which leads to the coupler-free excitation of SPPs. In our proposed system, the inherent losses of SPPs are compensated by introducing gain through the FWM process, resulting in a significant enhancement in the propagation length and penetration depth of SPPs. We further analyze the effect of gain on the long-range and short-range SPPs and observe that the propagation distance and lifetime of both types of SPPs are increased. • A method for exciting and amplifying surface plasmon polaritons (SPPs) at the interface of a metal-semiconductor quantum well (SQW) is proposed. • The suggested model compensates for SPP losses via a four-wave mixing (FWM) method, increasing SPP propagation length and penetration depth. • Due to FWM, the proposed technique generates long-range and short-range SPPs with increased propagation length and lifespan. [ABSTRACT FROM AUTHOR]

• Stress measurement by laser grating technique is proposed. • Laser grating source demonstrates enhanced capabilities in maintaining superior signal quality and stability compared with line source. • A cross-correlation method is applied for the time-of-flight measurement. • The impact of temperature for stress measurement is discussed and corrected. • The uncertainty budgets for the measured results are analyzed. This paper introduces an innovative laser ultrasonic method designed for the assessment of surface stress. Specifically, a two-wave mixing interferometer is employed to detect Rayleigh waves generated by an Nd:YAG laser, offering a non-contact detection technique that mitigates errors introduced by coupling conditions. In contrast to pulse-line source excitation, a laser grating is proposed for ultrasound generation. This configuration avoids system biases in sound velocity measurement caused by attenuation and dispersion, leading to higher signal-to-noise ratio and increased stability. After data acquisition, a cross-correlation method is applied for the time-of-flight measurement. Upon determination of the acoustoelastic coefficient and ultrasonic velocity in a stress-free state, other aluminium blocks from the same batch are used for validation experiments. Subsequently, the temperature correction is implemented for measurement results and the corrected results exhibit a reduced uncertainty of less than 5 MPa, thus providing a reliable non-destructive solution for stress detection. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the general localized waves on the nonvanishing background for a coupled generalized nonlinear Schrödinger system with four wave mixing (FWM) effect. Using a uniform Darboux transformation, we construct a unified analytical solution formula which can be used not only to generate some reported localized waves, but also to yield some new results. Based on whether the relative wave vector of the two seed continuous-waves is zero, we respectively obtained various localized waves on the plane wave and periodic backgrounds, including bright/dark solitons, breathers, and rogue waves. Especially, we assort these localized waves and provide detailed parameter conditions for generating them, and find that the FWM term b plays a decisive role in generating the periodic background for those localized waves. In addition, interaction dynamics of the hybrid localized waves on both the plane wave and periodic backgrounds composed of the bright/dark soliton, breather and rogue wave are analyzed. We expect that these results will shed light on the understanding of localized waves on the periodic background owing to the interference of multiple continuous-wave fields. • We construct a unified analytical solution formula for a CGNLSE with FWM effect. • Assort the localized waves and provide parameter conditions for generating them. • Interaction dynamics of the hybrid localized waves are analyzed. [ABSTRACT FROM AUTHOR]