Transverse Instability Research Articles

Improved energy spread in the radiation pressure acceleration of protons with a linearly polarized laser.

Degradation in the energy spread of accelerated protons due to the transverse instability induced transparency is one of the critical issues in the laser-driven radiation pressure acceleration (RPA) scheme. This issue is more severe for linearly polarized lasers due to enhanced heating of electrons. Therefore, in spite of being experimentally challenging, most of the numerical studies are performed with circularly polarized lasers. In this work, through particle-in-cell simulations, we demonstrate a significant improvement in the energy spread of the accelerated protons when a multilayered target is irradiated by a linearly polarized laser. This multilayered target consists of a near-critical-density (NCD) layer, sandwiched between a thick metallic foil and a thin RPA target. The role of the NCD target is to suppress the laser transparency to increase the coupling of laser momentum to the RPA protons. On the other hand, the metallic foil utilizes the kinetic energy of the escaping fast electrons to form an electrostatic sheath to filter the low-energy RPA protons. This results in significant improvement in the accelerated proton spectrum, even with a linearly polarized laser.

Customized palatal guide and splint for maxillary expansion

Customization in orthognathic surgery allows better precision and a reduced surgical time. In Le Fort I osteotomy surgery, the maxillary segmentation is considered one of the most unstable procedures due to transverse instability. Various different types of palatal device have been proposed to address this instability. This note describes a customized bone-borne palatal guide and splint that may help surgeons shorten the surgical time and achieve better three-dimensional repositioning, with more postoperative comfort for the patient and occlusal control for the surgeon.

Transverse instability in nonparaxial systems with four-wave mixing.

We present a two-dimensional coupled nonlinear Schrödinger-like system with spatial diffractions, degree of birefringence, and four-wave mixing. This system describes two physical contexts: optical pulse propagation beyond the paraxial approximation in a weakly birefringence waveguide and light propagation near exciton-polariton resonance in semiconductor superlattice materials. We find that such systems naturally support different types of diffraction profiles, including spherical, ellipsoidal, and hyperbolic structures. We then study the transverse instability of the two-dimensional system caused by an infinitesimal perturbation-induced continuous-wave solution. Also, we find out how various physical parameters, such as nonparaxiality, degree of birefringence, power, and four-wave mixing, affect the modulational instability (MI) process, in particular. We explore the existence of bright solitary wave solutions for the proposed system as the influence of MI is closely related to the latter in a nutshell.

Towards a tapered Yb-doped fiber-based narrow linewidth single-mode fiber laser with a high signal to Raman ratio.

We demonstrate an all-fiber high-power narrow-linewidth fiber laser based on a homemade tapered Yb-doped fiber (T-YDF). The laser performance is investigated and systematically compared with different seed powers and pump manners. The experimental results reveal that the injected seed power requires a trade-off designed to take into account the impact of spectral broadening, nonlinear effects, and transverse mode instability (TMI). Compared with the co-pump manner, the counter-pump manner performs well in inhibiting nonlinearities, spectral broadening, and improving the TMI threshold. Under the counter-pump manner, this narrow-linewidth T-YDF amplifier realized a 2.09 kW output power with a 3 dB spectral linewidth of ∼0.34 nm, a beam quality of M2∼1.28 and a high Raman suppression ratio over 53.5 dB, the highest reported power for such a T-YDF-based narrow-linewidth single-mode laser, to the best of our knowledge. This work provides a promising pathway towards implementing monolithic high-power narrow-linewidth single-mode fiber lasers.

Mitigation of transverse mode instability by heat-load modulation.

We present the first experimental realization of a new mitigation strategy for TMI based on controlling the phase shift between the modal intensity pattern and the thermally induced refractive index grating. If specific modulation parameters are applied while pulsing the seed and/or pump radiation, the direction of energy transfer is forced from the higher-order modes into the fundamental mode. In this way, the fiber amplifier can operate at an average output power significantly higher than the TMI threshold with a diffraction-limited beam profile. A stable beam profile is observed at an average output power that is 83% higher than the TMI threshold of the free-running system, with an intra-burst average power that is 4.15 times higher than the TMI threshold.

Investigation of the transverse mode instability in an isolator-free quasi-unidirectional spatiotemporal self-mode locked fiber laser

A spatiotemporal (ST) self-mode locked (ML) Er-doped fiber laser based on the nonlinear multimode interference (NL-MMI) effect without an intracavity isolator has been proposed. Such a STML laser could operate in a quasi-unidirectional regime in both clockwise (CW) and counter-clockwise (CCW) directions. Different spatial dynamics of the STML solitons in the opposite directions are observed. For the STML solitons in the CCW direction, the beam profile is discretely distributed and the transverse mode instability (TMI) is observed. While for the STML solitons in the CW direction, the beam profile is relatively concentrated and can be partially controlled by adjusting the intracavity multimode (MM) fiber at the front-end of the MM coupler. These investigations would help to gain further understanding of the spatial and temporal dynamics and mechanism in the STML lasers.

Stability of the modulator in a plasma-modulated plasma accelerator.

We explore the regime of operation of the modulator stage of a recently proposed laser-plasma accelerator scheme [Phys. Rev. Lett. 127, 184801 (2021)0031-900710.1103/PhysRevLett.127.184801], dubbed the plasma-modulated plasma accelerator (P-MoPA). The P-MoPA scheme offers a potential route to high-repetition-rate, GeV-scale plasma accelerators driven by picosecond-duration laser pulses from, for example, kilohertz thin-disk lasers. The first stage of the P-MoPA scheme is a plasma modulator in which a long, high-energy "drive" pulse is spectrally modulated by copropagating in a plasma channel with the low-amplitude plasma wave driven by a short, low-energy "seed" pulse. The spectrally modulated drive pulse is converted to a train of short pulses, by introducing dispersion, which can resonantly drive a large wakefield in a subsequent accelerator stage with the same on-axis plasma density as the modulator. In this paper we derive the 3D analytic theory for the evolution of the drive pulse in the plasma modulator and show that the spectral modulation is independent of transverse coordinate, which is ideal for compression into a pulse train. We then identify a transverse mode instability (TMI), similar to the TMI observed in optical fiber lasers, which sets limits on the energy of the drive pulse for a given set of laser-plasma parameters. We compare this analytic theory with particle-in-cell (PIC) simulations and find that even higher energy drive pulses can be modulated than those demonstrated in the original proposal.

Transverse spectral instabilities in Konopelchenko–Dubrovsky equation

AbstractWe study the transverse spectral stability of the one‐dimensional small‐amplitude periodic traveling wave solutions of the (2+1)‐dimensional Konopelchenko–Dubrovsky (KD) equation. We show that these waves are transversely unstable with respect to two‐dimensional perturbations that are periodic in both directions with long wavelength in the transverse direction. We also show that these waves are transversely stable with respect to perturbations which are either mean‐zero periodic or square‐integrable in the direction of the propagation of the wave and periodic in the transverse direction with finite or short wavelength. We discuss the implications of these results for special cases of the KD equation—namely, KP‐II and mKP‐II equations.

Transverse mode instability in fiber laser oscillators.

What we believe to be a first theoretical study of transverse mode instability (TMI) in oscillators based on a stimulated thermal Rayleigh scattering (STRS) model is conducted. Higher order mode (HOM) lasing is found to happen at high powers. Further fundamental mode (FM) growth is limited once HOM lasing takes place, with further increase of pump power mostly going to HOM growth, a fundamentally different phenomenon from that in fiber amplifiers. TMI thresholds defined as when the HOM lasing condition is met is studied. The results are consistent with the measured TMI thresholds and their dependence on pumping configurations and pump wavelengths.

TMI and polarization static energy transfer in Yb-doped low-NA PM fibers.

In this work, we conduct experimental investigations of transverse mode instabilities (TMI) in a large mode area ultra-low numerical aperture polarization maintaining fiber amplifier. This fiber is few mode in the slow-axis (conventional operation mode), but single mode in the fast-axis. We test the stability of the output beam by changing the input polarization angle and systematically investigate the transverse mode instability threshold in the two principal polarization axes. The lowest TMI threshold at 300 W was found when the input polarization angle was aligned parallel to the slow-axis. Detuning the input polarization angle from the slow-axis led to increased TMI thresholds. For input polarization angle of 90° (parallel to the fast-axis), the output signal was stable up to 475 W and further scaling was limited by the available pump power. However, for fast-axis operation a lower polarization ratio compared to slow-axis operation was observed as well as an unexpected static energy transfer from the fast-axis into the slow-axis above 400 W.

Mitigation of TMI in an 8 kW tandem pumped fiber amplifier enabled by inter-mode gain competition mechanism through bending control.

In this work, the impact of fiber bending and mode content on transverse mode instability (TMI) is investigated. Based on a modified stimulated thermal Rayleigh scattering (STRS) model considering the gain competition between transverse modes, we theoretically detailed the TMI threshold under various mode content and bending conditions in few-mode fibers. Our theoretical calculations demonstrate that larger bending diameters increase the high order mode (HOM) components in the amplifier, which in turn reduces the frequency-shifted Stokes LP11o mode due to the inter-mode gain competition mechanism, thus improving the TMI threshold of few-mode amplifiers. The experimental results agree with the simulation. Finally, by optimizing the bending, an 8.38 kW output tandem pumped fiber amplifier is obtained with a beam quality M2 of 1.8. Both TMI and stimulated Raman scattering (SRS) are well suppressed at the maximum power. This work provides a comprehensive analysis of the TMI in few-mode amplifiers and offers a practical method to realize high-power high-brightness fiber lasers.

Base suction, entrainment flux, and wake modes in the vortex formation region at the rear of a three-dimensional blunt bluff body.

A slitted base cavity of constant depth with a varying filling ratio 0≤R_{f}≤100% is experimentally investigated to reduce the form drag of a three-dimensional blunt body (the so-called squareback Ahmed body) at a Reynolds number Re=2.89×10^{5}. The drag reduction is achieved by a decrease of base suction (or, equivalently, the increase of pressure at the base). The plain cavity (R_{f}=100%) reduces the base suction by 22% compared to the case with no cavity (R_{f}=0). All intermediate filling ratio are obtained by the enlargement of the slits, initially having a zero width for the plain cavity case. It is shown that the gradual base suction change can be related to the level of the entrainment flux of the free shear layers developing from the rear separation and to the suppression of the transverse steady asymmetric instability of the wake. The model of the vortex formation region length of Gerrard [J. Fluid Mech. 25, 401 (1966)0022-112010.1017/S0022112066001721] is shown to provide an insightful interpretation of the drag reduction mechanism using ventilated base cavities.

Tunable mode conversion in a mechanical metamaterial via second harmonic generation

Mode conversion of elastic waves is ubiquitous in solids and has various applications in medical imaging, signal processing, vibration suppression, etc. Nonlinear harmonic generation in mechanical metamaterials is a way to achieve efficient, self-adaptive mode conversion, but is hindered by the required while difficult-to-meet phase matching condition. Moreover, enhanced mode conversion usually works in a narrow frequency band, and lacks of tunability. In this paper, we realize wide band, tunable enhanced mode conversion via second harmonic generation (SHG), with a pre-strained, translation-rotation coupled mechanical metamaterial. We demonstrate the realization experimentally and numerically, along with an analytical discrete model developed to reveal the origin of the tunability. It is shown that the pre-strain gives rise to hardening in stretch and softening in compression, broadening the effective frequency range of the enhanced mode conversion. Moreover, the pre-strain affects not only the geometrical configuration but also the on-site stiffness of the metamaterial, both of which are responsible for the tunability of the enhanced mode conversion. A transverse instability induced by compression is also revealed, showing unusual effect on the enhanced mode conversion during the initial period of time. Our work can inspire new device designs for vibration control, signal processing and non-destructive testing.

Mitigating transverse mode instability in fiber laser oscillator by employing direct pump modulation

The transverse mode instability (TMI) is a thermal-optical effect, inducing a sudden deterioration of beam quality and average power. This non-linear effect will greatly limit the power scaling of fiber lasers. We demonstrate a direct pump modulation to mitigate TMI in a 30 μm-core-diameter all-fiber laser oscillator. It only relies on function generator and pump drivers without the need of feedback control units. The groups of pump LDs and pump drivers and the phase difference between the channels of function generator have a great impact on the beam stability. Compared with the TMI threshold 180 W in continuous wave (CW) mode, the maximum average output power of a stabilized beam is increased up to 310 W via a multi-parameter-based direct pump modulation.

Output characteristics’ static fluctuations versus the pump power in 1018 nm fiber oscillators

This paper investigates the static fluctuating behavior of output parameters in 1018 nm fiber lasers using 20/400 µm and 25/400 µm ytterbium-doped fibers (YDFs). It is seen that by increasing the pump power, some static fluctuations is induced in the output characteristics of the lasers, such as output power, back-reflected power, and beam quality factor (M2). The growth of these parameters fluctuates versus the pump power, without any modulation frequency in the temporal behavior of the output beam profile. This effect, which to the best of our knowledge is reported for the first time, occurs at powers much lower than the threshold for dynamic transverse mode instability (TMI). It was found that the static mode-coupling occurs between two lowest-order modes and causes these fluctuations in the lasers’ output parameters. Conducting the experiment for 1080 nm fiber lasers with different lengths of YDF, in addition to confirm the descriptions about how the static fluctuations occurs, shows that this effect occurs in other wavelengths as well.

3kW forward-pumped fiber laser via pump recycler.

We report a single-end forward-pumped fiber laser with a record high output power of 3kW. The laser is assembled exclusively from commercially widespread components such as the Yb-doped fiber with core/cladding diameter of 20/400µm, pump laser diodes at an emission wavelength of 915nm, and a signal and pump fiber combiner that serves as the pump recycler. The record high power arises from the combination of the 915nm pumping and pump recycler with an effective reflectivity of 78%, increasing simultaneously the thresholds for stimulated Raman scattering and transverse mode instability (TMI). The length of the oscillator was also varied experimentally from 20m to 5m, showing a contrast of up to 19% in the TMI threshold. This shows the importance of accurately partitioning the Yb-doped fiber length in between the oscillator and amplifier sections to minimize the impact of TMI.

Transverse mode instability considering bend loss and heat load.

Previously, we developed a highly efficient transverse mode instability model by integrating stimulated thermal Rayleigh scattering and quasi-3D fiber amplifier models, enabling the consideration of the 3D gain saturation effect, with its accuracy verified by reasonable fit to experimental data. Bend loss was however ignored. Higher-order-mode bend loss can be very high especially for fibers with core diameters below 25µm and is sensitive to the local heat load. By using a FEM mode solver to account for bend loss and local heat-load-induced bend loss reduction, the transverse mode instability threshold is studied in detail, resulting in some interesting new insights.

Origin of SRS-induced beam quality distortion under TMI threshold.

In high power fiber lasers, the degradation of beam quality caused by Raman effect has attracted more and more attention in recent years, but its physical mechanism is still unclear. We're going to differentiate between heat effect and nonlinear effect by duty cycle operation. The evolution of beam quality at different pump duty cycles has been studied based on a quasi-continuous wave (QCW) fiber laser. It is found that even if the Stokes intensity is only -6 dB (energy proportion: 26%) lower than that of the signal light intensity, the beam quality has no obvious change with the duty cycle of 5%; on the contrary, when the duty cycle gradually approaches 100% (CW-pumped scheme), the beam quality distortion changes faster and faster with the increase of Stokes intensity. The experimental results are contrary to core-pumped Raman effect theory [IEEE Photon. Technol. Lett.34, 215 (2022)10.1109/LPT.2022.3148999], and further analysis confirms that the heat accumulation in the process of Stokes frequency shift should be responsible for this phenomenon. That is the first time, to the best of our knowledge, for intuitive reveal of the origin of stimulated Raman scattering (SRS)-induced beam quality distortion under transverse mode instability (TMI) threshold in an experiment.

Transverse Mode Instability in Raman Fiber Amplifiers

Hybrid ytterbium-Raman fiber amplifiers have reached multi-kW levels in recent years, showing great promise for kW powers at new wavelengths. Many applications need diffraction-limited output which is limited by transverse mode instability (TMI). In this work, we extended our 3D amplifier model for studying TMI in ytterbium fiber amplifiers to include Raman gain and used this new model to study TMI in two Raman fiber amplifiers: a hybrid ytterbium-Raman fiber amplifier and a passive Raman fiber amplifier. Our previous study shows that gain saturation effect in ytterbium fiber amplifiers plays a significant role in suppressing TMI in kW ytterbium fiber amplifiers. This new study shows that in a Raman fiber amplifier in the absence of similar connection between local Raman gain and local Stokes wave, TMI is not much affected by seed power, fiber length or core diameter in the absence of mode-dependent loss. Mode-dependent loss is a key factor for increasing the TMI threshold in this case.

Suppressing transverse mode instability through multimode excitation in a fiber amplifier

High-power fiber laser amplifiers have enabled an increasing range of applications in industry, science, and defense. The power scaling for fiber amplifiers is currently limited by transverse mode instability. Most techniques for suppressing the instability are based on single- or few-mode fibers in order to output a clean collimated beam. Here, we study theoretically using a highly multimode fiber amplifier with many-mode excitation for efficient suppression of thermo-optical nonlinearity and instability. We find that the mismatch of characteristic length scales between temperature and optical intensity variations across the fiber generically leads to weaker thermo-optical coupling between fiber modes. Consequently, the transverse mode instability (TMI) threshold power increases linearly with the number of equally excited modes. When the frequency bandwidth of a coherent seed laser is narrower than the spectral correlation width of the multimode fiber, the amplified light maintains high spatial coherence and can be transformed to any target pattern or focused to a diffraction-limited spot by a spatial mask at either the input or output end of the amplifier. Our method simultaneously achieves high average power, narrow spectral width, and good beam quality, which are required for fiber amplifiers in various applications.