Transverse Mode Instability Research Articles

Static thermo-optic instability in double-pass fiber amplifiers.

A coupled-mode formalism, earlier used to describe transverse mode instabilities in single-pass optical fiber amplifiers is extended to the case of double-pass amplifiers. Contrary to the single-pass case, it is shown that the thermo-optic nonlinearity can couple light at the same frequency between the LP01 and LP11 modes, leading to a static deformation of the output beam profile. This novel phenomenon is caused by the interaction of light propagating in either direction with thermo-optic index perturbations caused by light propagating in the opposite direction. The threshold power for the static deformation is found to be several times lower than what is typically found for the dynamic modal instabilities observed in single-pass amplifiers.

Large-mode-area fibers operating near single-mode regime.

Lower NA in large-mode-area fibers enables better single-mode operation and larger core diameters. Fiber NA has traditionally been limited to 0.06, mostly due to the control tolerance in the fabrication process. It has been recognized recently that transverse mode instability is a major limit to average power scaling in fiber lasers. One effective method to mitigate this limit is to operate nearer to the single-mode regime. Lower fiber NA is critical in this since it allows relatively larger core diameters which is the key to mitigate the limits imposed by nonlinear effects. We have developed a fabrication process of ytterbium-doped silica glass which is capable of highly accurate refractive index control and sufficient uniformity for LMA fibers. This process is also capable of large-volume production. It is based on a significant amount of post-processing once the fiber preforms are made. We have demonstrated 30/400 and 40/400 LMA fibers with a NA of ~0.028 operating very close to the single-mode regime. The second-order mode cuts off at ~1.2μm and ~1.55µm respectively. We have also studied issues related to bend losses due to the low NA and further optimization of LMA fibers.

Optimizing high-power Yb-doped fiber amplifier systems in the presence of transverse mode instabilities.

The average output power of Yb-doped fiber amplifier systems is currently limited by the onset of transverse mode instabilities. Besides, it has been recently shown that the transverse mode instability threshold can be significantly reduced by the presence of photodarkening in the fiber. Therefore, reducing the photodarkening level of the core material composition is the most straightforward way to increase the output average power of fiber amplifier systems but, unfortunately, this is not always easy or possible. In this paper we present guidelines to optimize the output average power of fiber amplifiers affected by transverse mode instabilities and photodarkening. The guidelines derived from the simulations do not involve changes in the composition of the active material (except for its doping concentration), but can still lead to a significant increase of the transverse mode instability threshold. The dependence of this parameter on the active ion concentration and the core conformation, among others, will be studied and discussed.

Tapered cladding diameter profile design for high-power tandem-pumped fiber lasers

The thermal effect has become the biggest limiting factor regarding the further power scaling of single mode fiber lasers, and it can lead to coating failure and transverse mode instability. A tapered cladding diameter profile design is proposed for the tandem-pumped fiber laser in this work, as it can smooth the temperature profile and reduce the maximum temperature rise within the fiber tremendously. The improvement in performance of the fiber design is verified by analytical and numerical results.

Optimizing Yb concentration of fiber amplifiers in the presence of transverse modal instabilities and photodarkening.

The Yb concentration of double-clad optical fiber amplifiers is numerically optimized with respect to maximizing the transverse modal instability threshold in the presence of absorption arising from photodarkening. The pump cladding area is scaled with the Yb concentration to approximately maintain the pump absorption in operation. It is found that approximate analytical expressions can predict the optimized concentration levels found in numerical simulations with sufficient accuracy to be useful in fiber design.

Numerical Investigation on Primary Atomization Mechanism of Hollow Cone Swirling Sprays

The atomization process of swirling sprays in gas turbine engines has been investigated using a LES-VOF model. With fine grid resolution, the ligament and droplet formation processes are captured in detail. The spray structure of fully developed sprays and the flow field are observed firstly. A central recirculation zone is generated inside the hollow cone section due to the entrainment of air by the liquid sheet and strong turbulent structures promote the breakup of ligaments. At the exit of injector nozzle, surface instability occurs due to disturbance factors. Axial and transverse mode instabilities produce a net-like structure ligament zone. Finally, the generation mechanism of the droplet is analyzed. It is found that the breakup mechanism of ligaments is located at the Raleigh capillary region. Axial symmetry oscillation occurs due to the surface tension force and the capillary waves pinch off from the neck of the ligaments. Secondary breakup and coalescence occur at the “droplet zone,” resulting in a wider distribution curve at the downstream area.

Core-pumped single-frequency fiber amplifier with an output power of 158 W.

Single-frequency laser sources at a wavelength of 1μm are typically scaled in power with Ytterbium-doped double-clad fiber amplifiers. The main limitations are stimulated Brillouin scattering, transversal mode instabilities and, from a technical point of view, the degree of fiber integration for a rugged setup. Addressing these limitations, we propose an alternative high-power single-frequency amplifier concept based on core pumping. A nonplanar ring oscillator with 2W of output power at 1kHz spectral linewidth was scaled by a fiber amplifier up to a power of 158W without any indication of stimulated Brillouin scattering-using a standard Ytterbium-doped single-mode fiber with a mode field area of only ∼100 μm2. A short active fiber length and a strong temperature gradient along the gain fiber yield to efficient suppression of stimulated Brillouin scattering. For deeper understanding of the Brillouin scattering mitigation mechanism, we studied the Brillouin gain spectra with a Fabry-Perot interferometer at different output power levels of the fiber amplifier.

Maximizing power output from continuous-wave single-frequency fiber amplifiers.

This Letter reports on a method of maximizing the power output from highly saturated cladding-pumped continuous-wave single-frequency fiber amplifiers simultaneously, taking into account the stimulated Brillouin scattering and transverse modal instability thresholds. This results in a design figure of merit depending on the fundamental mode overlap with the doping profile, the peak Brillouin gain coefficient, and the peak mode coupling gain coefficient. This figure of merit is then numerically analyzed for three candidate fiber designs including standard, segmented acoustically tailored, and micro-segmented acoustically tailored photonic-crystal fibers. It is found that each of the latter two fibers should enable a 50% higher output power than standard photonic crystal fiber.

Transverse mode instability induced by stimulated Brillouin scattering in a pulsed single-frequency large-core fiber amplifier.

We report the observation of transverse mode instability (TMI) in a pulsed single-frequency ytterbium-doped large-core fiber amplifier in which stimulated Brillouin scattering (SBS) is generated easily owing to the high peak power and narrow linewidth of the laser pulses. It was shown experimentally that the threshold of TMI is almost the same as that of SBS and that the suppression of SBS also increases the threshold of TMI, which indicates that the TMI originates from SBS in the fiber.

Modal instability in high power solid-state lasers with an unstable cavity

A phenomenon of modal instability is investigated theoretically in some high power solid-state lasers with an unstable cavity. This modal instability is caused by a thermo-optic-thermo feedback coupled with an asymmetrical laser oscillation inside some unstable cavities. Numerical investigations indicate that this instability can appear in all lasers with Nd3+-doped disk scheme, Yb3+-doped disk scheme and direct-liquid-cooled slab scheme, but the physical mechanisms of optic-thermo feedback in the three schemes are completely different. It is shown that the modal instability can appear within a special power range only in the Nd- or Yb-doped disk lasers due to certain saturation phenomenon, but exhibits a power threshold in the direct-liquid-cooled slab lasers. Therefore different strategies to mitigate the modal instability in different lasers are suggested. Finally, the presence of transverse modal instability in the Nd:YAG thin-disk laser is confirmed qualitatively by an experiment.

High Power Fiber Lasers: A Review

In this paper, we summarize the fundamental properties and review the latest developments in high power fiber lasers. The review is focused primarily on the most common fiber laser configurations and the associated cladding pumping issues. Special attention is placed on pump combination techniques and the parameters that affect the brightness enhancement observed in single-mode and multimode high power fiber lasers. The review includes the major limitations imposed by fiber nonlinearities and other parasitic effects, such as optical damage, transverse modal instabilities and photodarkening. Finally, the paper summarizes the power evolution in continuous-wave and pulsed ytterbium-doped fiber lasers and their impact on industrial applications.

Short-Term and Long-Term Stability in Ytterbium-Doped High-Power Fiber Lasers and Amplifiers

In this paper we discuss recent progress in exploring short-term and long-term stability of high power Yb-doped fiber lasers and amplifiers. Long-term stability is associated with photodarkening effects that can significantly reduce operational lifetime of a high-power laser system. Short-term stability is associated with so-called transverse modal instabilities that degrade output beam quality at average powers above a certain threshold. In this paper we review ongoing studies that provide experimental characterization, and explore physical causes and mitigation strategies of the different physical phenomena involved. Those studies are critical for achieving stable operation of high power fiber lasers.

Impact of gain saturation on the mode instability threshold in high-power fiber amplifiers.

We present a coupled-mode model of transverse mode instability in high-power fiber amplifiers, which takes the effect of gain saturation into account. The model provides simple semi-analytical formulas for the mode instability threshold, which are valid also for highly saturated amplifiers. The model is compared to recently published detailed numerical simulations of mode instability, and we find reasonably good agreement with our simplified coupled-mode model.

Theoretical and numerical investigation of filament onset distance in atmospheric turbulence

We analyze and simulate the propagation of laser beams with powers larger than the nonlinear self-focusing power through atmospheric turbulence. Turbulence-induced filamentation is theoretically described in terms of whole-beam self-focusing and transverse modulational instability. We describe a numerical simulation method to model nonlinear focusing in turbulence. We find good agreement between our simulation and a previously published laboratory-scale experiment. Simulations are then performed to calculate probability distributions for filament onset and wander over kilometer distances, and are compared with theory. The theoretical model can be made to agree with the simulations for physically meaningful choices of a few fitting parameters. We also examine the use of focusing optics to control filamentation range of modulationally unstable beams.

Intense femtosecond shaped laser beams for writing extended structures inside transparent dielectrics

We review recent results on the propagation and self-focusing of intense femtosecond laser pulses with shaped beam profiles in transparent dielectric media. At sufficiently high optical power, beam shaping seeds into the transverse modulation instability and results in the deterministic placement of intense laser filaments within the beam profile. Resulting spatial filament distributions may be utilized for writing complex extended structures inside transparent dielectrics. Specific examples of beam shapes we will discuss are Bessel beams, optical vortices, Bessel beams of higher order, and Airy beams.

Frequency resolved transverse mode instability in rod fiber amplifiers

Frequency dynamics of transverse mode instabilities (TMIs) are investigated by testing three 285/100 rod fibers in a single-pass amplifier setup reaching up to ~200W of extracted output power without beam instabilities. The pump power is increased well above the TMI threshold to uncover output dynamics, and allowing a simple method for determining TMI threshold based on standard deviation. The TMI frequency component is seen to appear on top of system noise that may trigger the onset. A decay of TMI threshold with test number is identified, but the threshold is fully recovered between testing to the level of the pristine fiber by thermal annealing the fiber output end to 300°C for 2 h.

Kinetic simulations of the self-focusing and dissipation of finite-width electron plasma waves.

Two-dimensional simulations, both Vlasov and particle-in-cell, are presented that show the evolution of the field and electron distribution of finite-width, nonlinear electron plasma waves. The intrinsically intertwined effects of self-focusing and dissipation of field energy caused by electron trapping are studied in simulated systems that are hundreds of wavelengths long in the transverse direction but only one wavelength long and periodic in the propagation direction. From various initial wave states, both the width at focus Δm relative to the initial width Δ0 and the maximum field amplitude at focus are shown to be a function of the growth rate of the transverse modulational instability γTPMI divided by the loss rate of field energy νE to electrons escaping the trapping region. With dissipation included, an amplitude threshold for self-focusing γTPMI/νE∼1 is found that supports the analysis of Rose [Phys. Plasmas 12, 012318 (2005)].

Estimating modal instability threshold for photonic crystal rod fiber amplifiers

We present a semi-analytic numerical model to estimate the transverse modal instability (TMI) threshold for photonic crystal rod amplifiers. The model includes thermally induced waveguide perturbations in the fiber cross section modeled with finite element simulations, and the relative intensity noise (RIN) of the seed laser, which seeds mode coupling between the fundamental and higher order mode. The TMI threshold is predicted to ~370 W - 440 W depending on RIN for the distributed modal filtering rod fiber.

Observation of surface dark photovoltaic solitons

Surface dark solitons in photovoltaic nonlinear media are reported. Taking advantage of diffusion and photovoltaic nonlinearities we demonstrated the surface dark solitons and their behaviors near surface theoretically and experimentally in LiNbO₃ crystal. It is very interesting that surface dark soliton is just half of dark soliton in bulk. Another interesting thing is that transverse modulation instability can be perfectly suppressed by surface dark soliton in virtue of surface. In addition, surface waveguides were written successfully utilizing surface dark soliton.

Theoretical analysis of mode instability in high-power fiber amplifiers

We present a simple theoretical model of transverse mode instability in high-power rare-earth doped fiber amplifiers. The model shows that efficient power transfer between the fundamental and higher-order modes of the fiber can be induced by a nonlinear interaction mediated through the thermo-optic effect, leading to transverse mode instability. The temporal and spectral characteristics of the instability dynamics are investigated, and it is shown that the instability can be seeded by both quantum noise and signal intensity noise, while pure phase noise of the signal does not induce instability. It is also shown that the presence of a small harmonic amplitude modulation of the signal can lead to generation of higher harmonics in the output intensity when operating near the instability threshold.