Mode Instability Threshold Research Articles

Weak Incoherent Optical Signal Amplification Based on Modulation Instability for Imaging Through Fog

Weak optical signal processing based on nonlinear effects offers new approaches for imaging through scattering media. A novel incoherent optical signal amplification method based on spatial modulation instability is proposed for imaging through fog. We experimentally demonstrated the amplification and recovery of degraded weak incoherent optical image signals after passing through dense fog in a photorefractive crystal. Our experimental results indicate that the intensity profiles of the output images can be redistributed from disordered to ordered when the nonlinear strength exceeds the threshold of incoherent modulation instability, which shows that the partially disordered incoherent probe light intensities are orderly transferred to enhance the signal intensity profiles and the residuals become a uniform background. The restored nonlinear output images with high visibility were observed for a proper optical thickness of fog, and weak optical imaging from undetectable to detectable with relatively poor visibility for a larger optical thickness was also realized in the experiment. This incoherent optical signal amplification method based on modulation instability has a potential application for image recovery in atmospheric scattering imagings.

Comparisons between high power fiber systems in the presence of radiation induced photodarkening

Performance of high power fiber lasers and amplifiers with different pump sources are evaluated in the presence of radiation induced photodarkening in post-irradiated active fibers. Evolutions of output power and thermal mode instability threshold under different radiation doses are examined and analyzed. Severe degradation in both output power and thermal mode instability threshold is recorded for high power fiber systems with their active fibers exposed to radiations. Comparisons show that, amplifiers present a relatively better performance than lasers in adverse environments. Besides, 976 nm pump source is more favorable for high power laser sources in radioactive applications, even though 915 nm pump ensures a much lower temperature profile inside the active fiber.

Thermo-optical numerical modal analysis of multicore fibers for high power lasers and amplifiers

Lasers for modern industrial and research applications are required to provide a high average beam power and at the same time be reliable, efficient, and compact. Rare-earth doped single-mode fiber lasers are the most promising solution. Large mode area fibers are effectively used to reduce nonlinear effects and scaling up the beam power which is now bounded by thermal effects. A new cutting-edge approach to the issue involves the use of Multi-Core Fibers (MCFs), coherently combining several lower power beams into a higher power one, and thus pushing the threshold of nonlinearities and transverse mode instabilities to higher power. The amplification process involves heat generation in the doped cores due to quantum defect, which propagates radially and creates a temperature gradient across the fiber cross-section. Even though the cores are optically uncoupled, the refractive index gradient due to thermo-optical effects could cause cross-talk and core mode coupling. In this work, we numerically analyze the performances of 9-core MCFs for high power fiber lasers by taking into account the coupling and bending effects due to the heat load generated by the quantum defect between pump and laser radiation. MCFs show very low sensitivity to heat load and bending, with effectively single-mode behaviour up to 15 μm core diameter (effective area 181 μm2) and down to 35 μm pitch.

Experimental study on the impact of signal bandwidth on the transverse mode instability threshold of fiber amplifiers.

In this work, we conduct a detailed experimental study on the impact of signal bandwidth on the TMI threshold of fiber amplifiers. Both the filtered superfluorescent fiber sources and the phase-modulated single-frequency lasers are employed to construct seed lasers with different 3 dB spectral linewidths ranging from 0.19 nm to 7.97 nm. The TMI threshold of the fiber amplifier employing those seed lasers are estimated through the intensity evolution of the signal laser, and different criteria have been utilized to characterize the spectral linewidth of the seed lasers. Notably, the experimental results reveal that the TMI threshold of fiber amplifiers grows, keeps constant, and further grows as a function of spectral linewidth of seed lasers. Our experimental results could provide a well reference to understand the mechanism of the TMI effect and optimize the TMI effect in high-power fiber amplifiers.

Pump RIN reduction of the TMI threshold powers in Yb-doped kW class fiber amplifiers.

A 3D spatially resolved coupled mode and perturbation analysis for the transverse mode instability threshold powers in Yb-doped fiber amplifiers is extended to include threshold power reduction induced by the pump laser relative intensity noise (RIN). A static intensity grating combines with the fluctuating pump intensity to generate an inversion standing wave. The forward component of the standing wave contributes to the nonlinear coupling between the fundamental mode and the higher-order mode (HOM), thereby reducing the threshold power. It is estimated that a pump RIN level <-150dB/Hz is required to achieve quantum limited performance. The model also quantifies the impact of unwanted laser power in the HOM on the threshold power reduction. The threshold power calculations as a function of RIN and laser power in the HOM are in qualitative agreement with recently published experimental measurements.

Increasing Mode Instability Threshold by Mitigating Driver Noise in High Power Fiber Amplifiers

Reduction of mode instability threshold by driver noise has been observed in a 3kW level high power fiber amplifier system for the first time, which has been confirmed by monitoring the temporal characteristic of the beam profile and the output current. After the onset of mode instability induced by driver noise, the laser beam profiles fluctuated at a characteristic frequency of 0.3kHz with 50Hz-spaced frequency lines in the background. Obvious fluctuation in the signal peak of the optical spectrum has also shown up with the onset of mode instability. By employing an alternative driver with built-in filter, the diver noise has been suppressed by nearly 30dB, and the mode instability threshold has been increased by 25.8%. The evidence of electrical noise lines was absent from the temporal characteristic of the beam profile, only regular frequency component, which demonstrate that the driver noise can be suppressed efficiently by properly designing the driver. The results revealed that the driver noise should be taken into consideration in designing the high power fiber amplifier systems.

Spatial mode control based on photonic lanterns

We demonstrate two critical rules of designing photonic lanterns for applications in adaptive spatial mode control: (1) optimized input fiber arrangements to effectively control modes; (2) appropriate input fiber core-cladding ratio to expand the optional range of the output fiber. The 3×1 and 5×1 photonic lanterns according to above design requirements have been fabricated. Using stochastic parallel gradient descent algorithm, the phases of the inputs are actively modulated to stabilize the output of novel 5×1 photonic lantern with 30/125 µm output fiber. When the control target is the fundamental mode, the M2 factor of output beam is below 1.2 stably, which will provide a possible technical solution to increase the mode instability threshold in large mode area fiber laser systems. Furthermore, we obtain single orbital angular momentum mode (OAM01 or OAM02 mode) and high order linearly polarized mode (LP11 or LP21 mode) with the purity of the corresponding modes over 0.85 by altering evaluation function, which will be of benefit in optical communication and atomic optics.

The effect of population inversion saturation on the transverse mode instability threshold in high power fiber laser oscillators

To achieve a 3.02 kW Yb-doped fiber laser oscillator, the behavior of transverse mode instability (TMI) is experimentally studied in different pumping configurations; co, hybrid, counter, and bidirectional. A comparative analysis showed that population inversion saturation has a substantial impact on TMI threshold enhancement in high power fiber oscillators. Monitoring the dynamic power exchange of fundamental mode and higher-order mode of laser output beam indicates that in a hybrid pumping scheme, simultaneous pumping with two different wavelengths enhances the TMI threshold to a great stand. Moreover, injecting a few watts of pumping light in the counter direction mitigates the TMI caused by pumping in the co-direction. Calculation of population inversion in different pumping configurations using simulation shows that higher population inversion saturation leads to increasing the TMI threshold.

Experimental Demonstration of the Influence of Cooling Temperature on the Thermal Mode Instability in the YB-Doped Fiber Oscillator

Mode instability (MI) has become a major factor that restricts the brightness enhancement of fiber laser. This paper studies the MI threshold characteristics of the fiber oscillators with refrigerated ytterbium-doped fiber (YDF). In the experiments, two fiber lasers that can operate at low temperature were built. We rigorously tested the MI threshold of the lasers with the YDF placed in two different environments. At room temperature environment, the MI threshold of the oscillator was measured with the temperature of the YDF water cooling plate ranging from 25 °C to 5 °C. When the cooling temperature of the YDF drops from 20 °C to 5 °C, the MI threshold of the laser rises gradually, but the increase is small. In another experiment, the YDF is assembled in a constant temperature test chamber, in which the cooling of the YDF is more efficient. It is found that as the cooling temperature drops, the MI threshold of the laser has an obvious upward trend. In the process of the cooling temperature of the YDF dropped from 20 °C to −10 °C, the MI threshold of the laser was increased from 867 W to above 931 W, an increase of nearly 10%. It is the first detailed experimental demonstration which shows that decreasing the cooling temperature of the gain fiber can indeed increase the MI threshold of the fiber oscillator. This work can clarify the influence of cooling temperature on the laser thermal effect, which is conducive to perfecting the theoretical model of the MI effect.

High-power tandem-pumped fiber amplifier with beam quality maintenance enabled by the confined-doped fiber.

The high absorption confined-doped ytterbium fiber with 40/250 μm core/inner-cladding diameter is proposed and fabricated, where the relative doping ratio of 0.75 is selected according to the simulation analysis. By employing this fiber in a tandem-pumped fiber amplifier, an output power of 6.2 kW with an optical-to-optical efficiency of ∼82.22% is realized. Benefiting from the large-mode-area confined-doped fiber design, the beam quality of the output laser is well maintained during the power scaling process with the beam quality factor of ∼1.7 of the seed laser to ∼ 1.89 at the output power of 5.07 kW, and the signal-to-noise ratio of the output spectrum reaches ∼40 dB under the maximum output power. In the fiber amplifier based on the 40/250 μm fully-doped ytterbium fiber, the beam quality factor constantly degrades with the increasing output power, reaching 2.56 at 2.45 kW. Moreover, the transverse mode instability threshold of the confined-doped fiber amplifier is ∼4.74 kW, which is improved by ∼170% compared with its fully-doped fiber amplifier counterpart.

Optimizing the pump wavelength to improve the transverse mode instability threshold of fiber laser by 3.45 times

Transverse mode instability (TMI) in fiber lasers refers to the phenomenon of dynamic coupling between the fundamental mode (FM) and high-order modes (HOMs) when the pumping power exceeds a certain threshold, which seriously affects the power scaling of fiber laser. In order to improve the TMI threshold in the fiber laser, we optimized the pump wavelength both consider the quantum defect and the pump absorption. In the experiment, the pump wavelength is optimized and a wavelength-stabilized laser diode (LD) with a central wavelength of 969 nm is proposed as the pump source to suppress the TMI. A forward-pumped Yb-doped fiber laser oscillator with a core/cladding diameter of 30/400 µm was built experimentally. Experimental results show that the TMI threshold of the laser is 719 W when pumped by 969 nm LDs, which is 3.45 times that value when pumped by commonly used 976 nm LDs.

Spacially resolved coupled mode analysis for TMI threshold powers in quantum and Rayleigh scattering limits.

A 3D spatially resolved coupled mode and perturbation analysis for the transverse mode instability (TMI) threshold powers in Yb-doped fiber amplifiers is presented in this paper. Threshold powers are computed in the quantum and thermal Rayleigh scattering limits and are compared with those calculated by other coupled mode analyses. Quantum-limited threshold powers are found to be more than three times greater than those calculated with coupled-mode analyses that use uniform and/or average gain approximations. The analysis presented here includes pump depletion, gain saturation, and transverse hole burning. Simulations are applied to co-, cnt-, and bidirectionally pump amplifier configurations. The appearance of TMI is attributed to the formation of a dynamic thermal grating, which enables the exchange of optical power between the fundamental mode (FM) and higher-order mode (HOM). The sole approximation made is that the power in the HOM is much less than that in the FM. A distributed thermal Rayleigh scattering model is introduced that includes a ray-optic representation of the fiber mode structure that relates the Rayleigh power captured by the HOM to the waveguide structure. The location and strength of the thermal gratings are identified to assist in the application of mitigation techniques.

A unified theory of zonal flow shears and density corrugations in drift wave turbulence

A unified theory of zonal flow shears and density corrugations in drift wave turbulence is presented. Polarization and density advection beat excitation are studied in combination with modulational response. Noise is driven by two-time flux correlation. While the effective zonal flow eddy viscosity can go negative, the zonal diffusivity is positive definite. There is no inverse cascade of density corrugation. The connection between avalanches and corrugations is discussed. The zonal cross-correlation is identified and calculated. Conditions for alignment of zonal shears and corrugation gradients are determined, and the implications for staircase structure are discussed. We show that the synergy of beat noise and modulational effects is stronger than either alone. Strong zonal flows can be excited well below the modulational instability threshold. In the context of L–H transition, zonal noise quenches turbulence overshoot by eliminating the threshold for zonal flow excitation. The power threshold for L–H transition is lowered.

Static and dynamic mode evolution in high-power distributed side-coupled cladding-pumped fiber amplifiers

AbstractWe present a theoretical study of mode evolution in high-power distributed side-coupled cladding-pumped (DSCCP) fiber amplifiers. A semi-analytical model taking the side-pumping schemes, transverse mode competition, and stimulated thermal Rayleigh scattering into consideration has been built, which can model the static and dynamic mode evolution in high-power DSCCP fiber amplifiers. The mode evolution behavior has been investigated with variation of the fiber amplifier parameters, such as the pump power distribution, the length of the DSCCP fiber, the averaged coupling coefficient, the number of the pump cores and the arrangement of the pump cores. Interestingly, it revealed that static mode evolution induced by transverse mode competition is different from the dynamic evolution induced by stimulated thermal Rayleigh scattering. This shows that the high-order mode experiences a slightly higher gain in DSCCP fiber amplifiers, but the mode instability thresholds for DSCCP fiber amplifiers are higher than those for their end-coupled counterparts. By increasing the pump core number and reducing the averaged coupling coefficient, the mode instability threshold can be increased, which indicates that DSCCP fibers can provide additional mitigation strategies of dynamic mode instability.

Influence of pedestal diameter on mode instabilities in Yb/Ce/Al-doped fibers.

In this paper we present numerical and experimental results revealing that the mode instability threshold of highly Yb-doped, Ce/Al co-doped pedestal fibers is affected by the size of the index-increased pedestal structure surrounding the core. An alternative preparation technology for the realization of large mode area fibers with very large Al-doped silica pedestals is introduced. Three different pedestal fiber design iterations characterized by low photodarkening were manufactured and tested in counter-pumped amplifier setups. Up to 1.9 kW continuous-wave output power of near-diffraction-limited beam quality (M2 = 1.26) was achieved with an 18/200/420 µm fiber of very low NA = 0.042, limited only by the occurrence of mode instabilities.

Wave-polarization Analysis of the Alfvénic Slow Solar Wind at Kinetic Scales

Abstract This paper reports the first polarization measurement in the Alfvénic slow solar wind. The normalized magnetic helicity is used as a diagnostic parameter for studying the polarization status of the high-frequency magnetic fluctuations, along with an attempt to identify various wave modes in the solar wind turbulence. Clear evidence for the existence of ion cyclotron waves (ICWs) and kinetic Alfvén waves (KAWs) is also found in the Alfvénic low-speed plasma, robustly supporting the idea that the Alfvénic content of the solar wind fluctuations at fluid scales is the key parameter driving wave generation at kinetic scales. By separating the contributions to helicity from the two modes, it is possible to address the thermodynamical properties of ICWs and KAWs and provide the first direct estimate of their magnetic compressibility. In particular, while ICWs are mainly associated with higher levels of anisotropy and appear to be bounded by the threshold of proton–cyclotron kinetic instability, KAWs (which end up being more compressive than ICWs) are found at lower anisotropies and seem to be limited by the mirror mode instability threshold, extending as well to near the parallel fire hose unstable region. These result are relevant to theories of turbulence and dissipation in the solar wind.

550 W single frequency fiber amplifiers emitting at 1030 nm based on a tapered Yb-doped fiber.

In this paper, we report a high power single frequency 1030 nm fiber laser with near-diffraction-limited beam quality based on a polarization-maintaining tapered Yb-doped fiber (T-YDF). The T-YDF has advantages of effectively suppressing stimulated Brillouin scattering (SBS) while maintaining good beam quality. As a result, a record output power of 379 W single frequency, linearly polarized, nearly single-mode fiber amplifier operating at 1030 nm is demonstrated. The polarization extinction ratio is as high as 16.3 dB, and the M2 is measured to be 1.12. Further, the dependence of the thermal-induced mode instability (TMI) threshold on the polarization state of an input signal laser is investigated for the first time. By changing the polarization state of the injected seed laser, the output power can increase to 550 W while the beam quality can be maintained well (M2=1.47). The slope efficiency of the whole amplifier is about 80%. No sign of SBS appears even at the highest output power and the further brightness scaling of both situations is limited by the TMI effect. To the best of our knowledge, this result is the highest output power of all-fiberized single frequency fiber amplifiers.

Thermal modeling of high-power Yb-doped fiber lasers with irradiated active fibers.

With both radiation effects and thermal effects taken into consideration, a multiphysics thermal model concerning high-power Yb-doped fiber lasers operated with post-irradiated active fibers is established. Radiation-related parameters, including propagation losses, refractive indexes and lifetime, are considered. And, with the temperature profile of the active fiber, temperature-dependent parameters, including absorption and emission cross-sections, refractive indexes and lifetime, are updated every loop to simulate the output parameters. Simulation results show that radiation induces great changes to the thermal profiles of the active fiber. And severe performance degradation of high-power Yb-doped fiber lasers are recorded, featuring a remarkable drop in output power and an even steeper decline in the transverse mode instability threshold, which is a predominant limitation at high radiation doses. With a deposited radiation of 100 Gy, an output decline of about 50% and a mode instability threshold drop over 85% are observed. And it's shown that, with the exploited active fiber, it's hardly possible for the investigated fiber laser to generate stable single-mode output at kilowatt levels with accumulated radiation doses beyond 50 Gy. At low radiation doses within 20 Gy, to maintain safe and stable single-mode operation of the laser system, longer active fibers with lower absorption coefficients are preferred despite a small rollover of the output power.

Effective suppression of the photodarkening effect in high-power Yb-doped fiber amplifiers by H2 loading

The radical suppression of the photodarkening effect and laser performance deterioration via H 2 loading were demonstrated in high-power Yb-doped fiber (YDF) amplifiers. The photodarkening loss at equilibrium was 114.4 dB/m at 702 nm in the pristine fiber, while it vanished in the H 2 -loaded fiber. To obtain a deeper understanding of the impact of photodarkening on laser properties, the evolution of the mode instability threshold and output power in fiber amplifiers was investigated. After pumping for 300 min, the mode instability threshold of the pristine fiber dropped from 770 to 612 W, and the periodic fluctuation of the output power became intense, finally reaching 100 W. To address the detrimental effects originating from photodarkening, H 2 loading was applied in contrast experiments. The output power remained stable, and no sign of mode instability was observed in the H 2 -loaded fiber. Moreover, the transmittance at 638 nm confirmed the absence of the photodarkening effect. The results pave the way for the further development of high-power fiber lasers.

Mode instabilities in Yb:YAG crystalline fiber amplifiers.

Mode instabilities (MI) threshold in the Yb:YAG crystalline fiber amplifier is simulated by a full numerical model. The propagation of signal fields is simulated by the finite-difference beam-propagation method combined with the rate equations, and the time-dependent heat equation is solved by the alternating-direction-implicit method. Considering the strong temperature-dependent laser performance of Yb:YAG, an iterative method is applied to reach the steady state of Yb:YAG, the crystalline fiber amplifier, before the simulation of MI behavior. The simulated MI thresholds in Yb:YAG crystalline fiber amplifiers are found to be at least 28 times of those in Yb-doped silica-glass fiber amplifiers, up to tens of kilowatts. Simulation results show that, in addition to the expected higher thermal conductivity and lower thermo-optic coefficient, strong gain saturation also plays an important role in the high MI threshold of the Yb:YAG crystalline fiber.