Effect Of Group Velocity Mismatch Research Articles

Solid-state Mamyshev oscillator

We present the first design and analysis of a solid-state Mamyshev oscillator. We utilize the phase-mismatched cascaded quadratic nonlinear process in a periodically poled lithium niobate waveguide to generate substantial spectral broadening for Mamyshev mode locking. The extensive spectral broadening bridges the two narrowband gain media in the two arms of the same cavity, leading to a broadband mode locking not attainable with either gain medium alone. Two pulses are coupled out of the cavity, and each of the output pulses carries a pulse energy of 25.3 nJ at a repetition rate of 100 MHz. The 10 dB bandwidth of 2.1 THz supports a transform-limited pulse duration of 322 fs, more than 5 times shorter than what can be achieved with either gain medium alone. Finally, effects of group velocity mismatch, group velocity dispersion, and nonlinear saturation on the performance of Mamyshev mode locking are numerically discussed in detail.

Second order ultrasonic guided wave mutual interactions in plate: Arbitrary angles, internal resonance, and finite interaction region

The sensitivity of ultrasonic wave interactions to material and geometric nonlinearities makes them very useful for nondestructive characterization. The ability of guided waves to interrogate inaccessible material domains, be emitted and received from a single surface, and penetrate long distances provides capabilities that bulk waves do not. Furthermore, mutual interactions between waves propagating in collinear or non-collinear directions provide excellent flexibility as to which types of waves are used, as well as their frequencies and interaction angles. While the interaction of bulk waves is well established, the mutual interaction of guided waves traveling in arbitrary directions in a plate is not and requires a general vector-based formulation. Herein, by vector-based calculations, the internal resonance criteria are formulated and evaluated for waves propagating in arbitrary directions in a plate. From the analysis, it is found that non-collinear guided wave interactions transfer power to secondary guided wave modes that is impossible for collinear interactions, which is completely analogous to bulk waves. For the case of tone burst-pulsed wave packets at nonzero interaction angles, the wave interaction zone has a finite size, and its size is dictated by many factors, including, for example, the group velocities of the waves, interaction angle, pulse duration, and dispersion. An analytical model is introduced for finite-sized interaction zones and used to demonstrate the effect of group velocity mismatch on the generation of secondary waves. In addition, finite element simulations are compared to the analytical model and provide additional insight into secondary wave generation and propagation.

Controlling Amplitude and Radiation Pattern of Emitted Terahertz by Group Velocity Mismatch

In this paper, the effect of group velocity mismatch on the characteristic of terahertz (THz) radiation from a plasma irradiated by two-color laser pulses is investigated. The group velocity mismatch is directly related to the slippage-limited interaction length (SLIL). In order to control the characteristic of generated THz radiation, the SLIL effect on the radiation pattern and electric field amplitude is investigated. The SLIL can be controlled by the background plasma density. By rising the plasma density, the SLIL is decreased. Consequently, the THz electric field amplitude is reduced, and the radiation pattern is modified. In addition, by tuning the plasma density, the power pattern directivity and the sidelobe level are controlled.

Planar light bullets under conditions of second-harmonic generation.

We study solutions to second-harmonic-generation equations in two-dimensional media with anomalous dispersion. The analytical solution is obtained in an approximate form of the planar spatiotemporal two-component soliton by means of the averaged Lagrangian method. It is shown that a decrease in the amplitudes of both soliton components and an increase in the value of the transverse coordinate are accompanied by an increase in their temporal duration. Within this variational approach, we have managed to find a stability criterion for the light bullet and a period of oscillations of soliton parameters. Then, we use the obtained form as an initial configuration to carry out the direct numerical simulation of soliton dynamics. We demonstrate stable propagation of spatiotemporal solitons undergoing small oscillations predicted analytically for a long distance. The formation of a two-component light bullet is shown when we launch a pulse only at the fundamental frequency. In addition, we investigate the phase and group-velocity mismatch effects on the propagation of pulses.

Interaction dynamics of bright solitons in linearly coupled asymmetric systems

We present an extensive numerical investigation on the interaction dynamics of optical bright solitons in asymmetric nonlinear directional couplers taking into account the effects of group velocity mismatch, phase-velocity mismatch and differences in group velocity dispersions and effective mode areas between two cores. We also numerically explore the stability of bright solitons in the presence of harmonic infinitesimal perturbation, which is seeded in the form of uniform white noise. To have a comprehensive picture, we finally emphasize the influence of every individual asymmetric parameters on the soliton interaction in asymmetric nonlinear directional couplers.

Pump phase transfer and its control in hybrid seeded optical parametric amplifiers

Abstract Due to the existence of spatial walk-off and/or group-velocity mismatch effects, pump-to-signal phase transfer becomes inevitable during parametric amplification. We experimentally demonstrate that in hybrid seeded optical parametric amplifiers (OPAs) that include two OPA stages seeded by the signal and idler waves, respectively, the phase of the output signal can be restored to its initial value, although there are spatial and temporal phase fluctuations on the pump source. This method significantly relaxes the requirement for high pump beam quality, which is always very stringent in parametric amplification systems. With the introduction of this scheme into birefringent phase-matching OPAs or chirped-pulse OPAs, it should be promising to achieve intense femtosecond laser pulses that are close to the diffraction limit in space and ultra-high contrast in time, simultaneously.

Numerical study on pulse contrast enhancement in a short-pulse-pumped optical parametric amplifier

We investigate pulse cleaning behaviors in short-pulse-pumped optical parametric amplifiers (OPA). We theoretically study the contrast enhancement of amplified signal pulse and generated idler pulse, and reveal their dependence on the parametric gain in both the regimes of small signal and saturated amplifications. The signal contrast enhancement is nearly equal to the parametric gain, while the idler contrast is approximately equal to the product of the contrasts of the pump and signal pulses in a low gain OPA and increases with the gain. The effects of group-velocity mismatch and group-velocity dispersion on the contrast enhancement are also investigated. The results presented in this paper are of value for pulse cleaning.

Temporal coherence in mirrorless optical parametric oscillators

One of the unique features of mirrorless optical parametric oscillators based on counterpropagating three-wave interactions is the narrow spectral width of the wave generated in the backward direction. In this work, we investigate experimentally and numerically the influence that a strong phase modulation in the pump has on the spectral bandwidths of the parametric waves and on the efficiency of the nonlinear interaction. The effects of group-velocity mismatch and group-velocity dispersion are elucidated. In particular, it is shown that the substantial increase in temporal coherence of the backward-generated wave can be obtained even for pumping with a temporally incoherent pump. A configuration of a mirrorless optical parametric oscillator is proposed where this gain in spectral coherence is maximized without a penalty in conversion efficiency by employing group-velocity matching of the pump and the forward-generated parametric wave.

Fiber four-wave mixing source for coherent anti-Stokes Raman scattering microscopy

We present a fiber-format picosecond light source for coherent anti-Stokes Raman scattering microscopy. Pulses from a Yb-doped fiber amplifier are frequency converted by four-wave mixing (FWM) in normal-dispersion photonic crystal fiber to produce a synchronized two-color picosecond pulse train. We show that seeding the FWM process overcomes the deleterious effects of group-velocity mismatch and allows efficient conversion into narrow frequency bands. The source generates more than 160 mW of nearly transform-limited pulses tunable from 775 to 815 nm. High-quality coherent Raman images of animal tissues and cells acquired with this source are presented.

Pulsed- and continuous-wave difference-frequency generation in AlGaAs Bragg reflection waveguides

Pulsed- and continuous-wave type-I and type-II difference-frequency generations (DFGs) in monolithic AlGaAs Bragg reflection waveguides were comparatively investigated. Phase matching bandwidth of exceeding 40 nm was observed in all the processes. Highest difference-frequency (DF) power of 2.45 nW was obtained in continuous-wave type-II interaction with the average external pump and signal powers of 62.9 and 2.9 mW, respectively. The corresponding nonlinear conversion efficiency is about 1.3×10−3% W−1 for a sample with a length of 1.5 mm. Using split-step Fourier method, the impacts of third-order nonlinearities including two-photon absorption and self-phase modulation on the efficiency of the DFG are numerically investigated. Furthermore, the adverse effects of group velocity mismatch and group velocity dispersion of the interacting frequencies on the efficiency of the pulsed nonlinear process are theoretically studied. Simulations indicate that the dominant factors in limiting the efficiency of the pulsed interaction are group velocity mismatch between pump and DF signal and two-photon absorption of the interacting waves.

Experimental research on measuring the fine structure of long pulse in time domain by synchronized ultrashort pulse

We propose a method of measuring the fine structure of a long pulse in time domain by a synchronized ultrashort pulse, in which the basic principle is the cross-correlation interaction between the long pulse and the ultrashort pulse in a nonlinear crystal. After a theoretical analysis on the principle and resolution, as an example, the fine structure of a picosecond Nd:YLF laser pulse is characterized by a synchronized femtosecond pulse experimentally. Further, the effect of group velocity mismatch on measurement results is discussed.

Femtosecond second-harmonic generation in AlGaAs Bragg reflection waveguides: theory and experiment

The impact of third-order nonlinearities including self-phase modulation and two-photon absorption on the efficiency of the second-harmonic generation is numerically investigated using the split-step Fourier method in phase-matched Bragg reflection waveguides. Also using the same technique, the adverse effects of group velocity mismatch and group velocity dispersion of the interacting frequencies on the efficiency of the nonlinear process are examined and contrasted for optimal sample design. Using an optimized structure, we report efficient femtosecond second-harmonic generation in monolithic AlGaAs Bragg reflection waveguides for a type II nonlinear interaction. For a 190 fs pulsed pump around 1555 nm with an average power of 3.3 mW, a peak second-harmonic power of 25.5 μW is measured in a sample with a length of 1.1 mm. The normalized conversion efficiency of the process is estimated to be 2.0×104% W−1 cm−2. Pump depletion is clearly observed when operating at the phase-matching wavelength.

Modeling and Optimization of Quasi-Phase Matching Via Domain-Disordering

Second-harmonic generation in a quasi-phase matched waveguide produced using a domain-disordered GaAs-AlAs superlattice is modeled including the effects of group velocity mismatch, nonlinear refraction, two-photon absorption, and linear loss. The model predicts our experimentally observed second-harmonic powers within an order of magnitude. Self-phase modulation and two-photon absorption led to reduced conversion efficiencies of up to 33% at input peak powers >50 W. Group velocity mismatch results in a reduction of 23% in conversion efficiency using estimated group velocities calculated from the measured effective refractive index. The modeling also shows that the conversion efficiency peaks at propagation lengths longer than the pulse walk off length and that duty cycle variations induced shifts in the tuning curves. Group velocity mismatch also increased the conversion bandwidth by ~ 30%. Incomplete modulation of chi <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2)</sup> in disordered regions reduced the output conversion efficiency by up to two orders of magnitude. Grating-assisted phase matching led to a 7% efficiency drop for a Deltan of 0.045 at the second-harmonic and 0.01 at the fundamental. This model serves as a valuable tool to provide insight into the optimization of these devices.

Limits to compression with cascaded quadratic soliton compressors

We study cascaded quadratic soliton compressors and address the physical mechanisms that limit the compression. A nonlocal model is derived, and the nonlocal response is shown to have an additional oscillatory component in the nonstationary regime when the group-velocity mismatch (GVM) is strong. This inhibits efficient compression. Raman-like perturbations from the cascaded nonlinearity, competing cubic nonlinearities, higher-order dispersion, and soliton energy may also limit compression, and through realistic numerical simulations we point out when each factor becomes important. We find that it is theoretically possible to reach the single-cycle regime by compressing high-energy fs pulses for wavelengths lambda = 1.0-1.3 microm in a beta -barium-borate crystal, and it requires that the system is in the stationary regime, where the phase mismatch is large enough to overcome the detrimental GVM effects. however, the simulations show that reaching single-cycle duration is ultimately inhibited by competing cubic nonlinearities as well as dispersive waves, that only show up when taking higher-order dispersion into account.

Scaling laws for soliton pulse compression by cascaded quadratic nonlinearities

We present a detailed study of soliton compression of ultrashort pulses based on phase-mismatched second-harmonic generation (SHG) (i.e., the cascaded quadratic nonlinearity) in bulk quadratic nonlinear media. The single-cycle propagation equations in the temporal domain including higher-order nonlinear terms are presented. The balance between the quadratic (SHG) and the cubic (Kerr) nonlinearity plays a crucial role; we define an effective soliton number—related to the difference between the SHG and the Kerr soliton numbers—and show that it has to be larger than unity for successful pulse compression to take place. This requires that the phase mismatch be below a critical level, which is high in a material where the quadratic nonlinearity dominates over the cubic Kerr nonlinearity. Through extensive numerical simulations we find dimensionless scaling laws, expressed through the effective soliton number, that control the behavior of the compressed pulses. These laws hold in the stationary regime, in which group-velocity mismatch effects are small, and they are similar to the ones observed for fiber soliton compressors. The numerical simulations indicate that clean compressed pulses below two optical cycles can be achieved in a β-barium borate crystal at appropriate wavelengths, even for picosecond input pulses.

Controllable Self-Steepening of Ultrashort Pulses in Quadratic Nonlinear Media

By analyzing ultrashort optical pulse propagation in quadratic nonlinear media beyond the slowly varying envelope approximation, we find that the sign and magnitude of self-steepening can be controlled through the wave vector mismatch. As an example of this phenomenon's impact on ultrashort pulse propagation, we show that it may be used to cancel the propagation effects of group-velocity mismatch. We obtain quantitative agreement between theory, simulations, and experiments.

Noncollinear phase matching in fluorescence upconversion

We investigate noncollinear sum-frequency generation in a time-resolved fluorescence upconversion experiment to eliminate group-velocity mismatch (GVM), a major mechanism that deteriorates time resolution. The noncollinear geometry inherently causes phase-front mismatch (PFM) that also spoils time resolution. The effects of GVM and PFM on the time resolution are studied by numerical calculations and experiments. Based on the investigation, a fluorescence upconversion apparatus with time resolution better than 45 fs (FWHM) is demonstrated.

Quasi-group-velocity matching using integrated-optic structures

We propose a device to compensate for group-velocity mismatch (GVM) effects that limit the efficiency-bandwidth product in nonlinear frequency-mixing devices. Integrated wavelength-dependent delay lines are introduced periodically in a waveguide containing a series of quasi-phase-matching (QPM) gratings. Appropriate choice of the time delays can compensate for GVM. We have demonstrated a two-stage device in a periodically poled lithium niobate waveguide. Two approximately 150-fs-long pulses generated 6 ps apart by second-harmonic generation in two QPM gratings were resynchronized by a fixed delay line, and their relative phase was fine controlled by temperature tuning. This technique, which can be iterated to more than two segments, permits optical frequency mixers with a higher efficiency-bandwidth product than would be possible in a single grating short enough to avoid GVM effects.

Mode locking by cascading of second-order nonlinearities

We present a comprehensive theoretical and experimental study on a new passive mode-locking technique, called cascaded second-order nonlinearity mode locking (CSM), which exploits cascaded second-order nonlinearities to obtain large third-order susceptibilities from an intracavity second harmonic crystal. The nonlinear phase shift that originates in the nonlinear crystal is converted into a nonlinear amplitude modulation by a suitable intracavity aperture. A numerical model, based on a perturbative approach, allows the nonlinear loss modulation of resonators used for CSM to be calculated as a function of the resonator parameters and of the phase mismatch. The predictions of the model are confirmed by experiments performed on a CW Nd:YAG laser. The effects of group velocity mismatch and the limitations which it poses on the minimum achievable pulsewidth are analyzed both experimentally and theoretically.

Effect of group-velocity mismatch on amplitude and phase modulation of picosecond pulses in quadratically nonlinear media

Ultrafast all-optical effects in quadratically nonlinear media are studied, with particular emphasis being given to the effect of group-velocity mismatch of the optical pulses at different carrier frequencies. The basic consequences of the resulting pulse walk-off for fundamental all-optical concepts based on quadratic nonlinearities, such as phase-insensitive amplification without and with cascading, intensity- and phase-controlled polarization rotation, and switching relying on phase modulation, are studied in detail. We derive optimal parameters for the application of these concepts in all-optical devices.