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Andreas Tünnermann - One of the best experts on this subject based on the ideXlab platform.

  • pump modulation induced beam stabilization in high power fiber laser systems above the mode Instability Threshold
    Optics Express, 2018
    Co-Authors: Cesar Jauregui, Andreas Tünnermann, Christoph Stihler, Jens Limpert
    Abstract:

    A new way of stabilizing the output beam of a fiber laser system operating above the mode Instability Threshold is described and the first proof-of-principle experimental results are presented. This technique, which relies on a modulation of the pump power, works by washing the thermally-induced refractive index grating out, which weakens the coupling efficiency between transverse modes. One of the main advantages of this simple, yet powerful, approach is that it can be easily incorporated in already existing fiber laser systems since it does not require any additional optical elements. Using this beam stabilization strategy, a significant pointing stability and beam quality improvement has been demonstrated up to an average power of ~600W, which is a factor of 2 above the mode Instability Threshold.

  • optimizing high power yb doped fiber amplifier systems in the presence of transverse mode instabilities
    Optics Express, 2016
    Co-Authors: Cesar Jauregui, Hans-jürgen Otto, Sven Breitkopf, Jens Limpert, Andreas Tünnermann
    Abstract:

    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.

  • Temperature measurements in an ytterbium fiber amplifier up to the mode Instability Threshold
    Fiber Lasers XIII: Technology Systems and Applications, 2016
    Co-Authors: F. Beier, Matthias Heinzig, Bettina Sattler, Till Walbaum, Nicoletta Haarlammert, Thomas Schreiber, Ramona Eberhardt, Andreas Tünnermann
    Abstract:

    We report on the measurement of the longitudinal temperature distribution in a fiber amplifier fiber during high power operation. The measurement signal of an optical frequency domain reflectometer is coupled to an ytterbium doped amplifier fiber via a wavelength division multiplexer. The longitudinal temperature distribution was examined for different pump powers with a sub mm resolution. The results show even small temperature variations induced by slight changes of the environmental conditions along the fiber. The mode Instability Threshold of the fiber under investigation was determined to be 480W and temperatures could be measured overall the measured output power values.

  • Optimizing the mode Instability Threshold of high-power fiber laser systems
    Fiber Lasers XIII: Technology Systems and Applications, 2016
    Co-Authors: Cesar Jauregui, Hans-jürgen Otto, Sven Breitkopf, Jens Limpert, Andreas Tünnermann
    Abstract:

    In this work we present guidelines to increase the transverse mode Instability Threshold of high power fiber amplifiers and also, for the first time to the best of our knowledge, of fiber oscillators. These guidelines do not involve changes in the composition of the active material (except for its doping concentration), but they can still lead to a significant increase of the transverse mode Instability Threshold. The dependence of this parameter on the active ion concentration, the core conformation, the pump configuration and the mirror reflectivities in a fiber oscillator will be studied and discussed.

  • simplified modelling the mode Instability Threshold of high power fiber amplifiers in the presence of photodarkening
    Optics Express, 2015
    Co-Authors: Cesar Jauregui, Hans-jürgen Otto, Jens Limpert, Fabian Stutzki, Andreas Tünnermann
    Abstract:

    In this paper we present a simple model to predict the behavior of the transversal mode Instability Threshold when different parameters of a fiber amplifier system are changed. The simulation model includes an estimation of the photodarkening losses which shows the strong influence that this effect has on the mode Instability Threshold and on its behavior. Comparison of the simulation results with experimental measurements reveal that the mode Instability Threshold in a fiber amplifier system is reached for a constant average heat load value in good approximation. Based on this model, the expected behavior of the mode Instability Threshold when changing the seed wavelength, the seed power and/or the fiber length will be presented and discussed. Additionally, guidelines for increasing the average power of fiber amplifier systems will be provided.

Jens Limpert - One of the best experts on this subject based on the ideXlab platform.

  • the sensitivity of the mode Instability Threshold to different types of intensity noise
    Fiber Lasers XVII: Technology and Systems, 2020
    Co-Authors: Christoph Stihler, Cesar Jauregui, Jens Limpert, Sobhy E Kholaif
    Abstract:

    In this work we experimentally and theoretically investigate the impact of seed intensity-noise on the Threshold of transverse mode Instability (TMI) in Yb-doped, high-power fiber laser systems and compare it to the impact of pump intensity-noise. Former studies have shown that pump intensity-noise significantly decreases the TMI Threshold due to the introduction of a phase shift between the modal interference pattern and the thermallyinduced refractive index grating in the fiber. However, it can be expected that fluctuations of the seed power will also induce such phase shifts due to a change of the extracted energy and the heat load in the fiber. Thus, it is important to investigate which one, i.e. the seed- or the pump intensity-noise, has a severer impact on the TMI Threshold. Our experiments have shown that the TMI Threshold of a fiber amplifier was decreased by increasing the seednoise amplitude. However, contrary to conventional belief, the impact of seed intensity-noise was much weaker than the one of pump intensity-noise. The measurements are in good agreement with our simulations and can be well explained with previous studies about the noise transfer function. The reason for the weaker impact of seed intensity-noise on the TMI Threshold is the attenuation of its frequency components below 20 kHz in saturated fiber amplifiers, which includes the frequencies relevant for TMI. Thus, the main trigger for TMI in saturated fiber amplifiers can be considered to be pump intensity-noise. A suppression of this noise below 20 kHz represents a promising way to increase the TMI Threshold of fiber laser systems.

  • pump modulation induced beam stabilization in high power fiber laser systems above the mode Instability Threshold
    Optics Express, 2018
    Co-Authors: Cesar Jauregui, Andreas Tünnermann, Christoph Stihler, Jens Limpert
    Abstract:

    A new way of stabilizing the output beam of a fiber laser system operating above the mode Instability Threshold is described and the first proof-of-principle experimental results are presented. This technique, which relies on a modulation of the pump power, works by washing the thermally-induced refractive index grating out, which weakens the coupling efficiency between transverse modes. One of the main advantages of this simple, yet powerful, approach is that it can be easily incorporated in already existing fiber laser systems since it does not require any additional optical elements. Using this beam stabilization strategy, a significant pointing stability and beam quality improvement has been demonstrated up to an average power of ~600W, which is a factor of 2 above the mode Instability Threshold.

  • optimizing high power yb doped fiber amplifier systems in the presence of transverse mode instabilities
    Optics Express, 2016
    Co-Authors: Cesar Jauregui, Hans-jürgen Otto, Sven Breitkopf, Jens Limpert, Andreas Tünnermann
    Abstract:

    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.

  • Optimizing the mode Instability Threshold of high-power fiber laser systems
    Fiber Lasers XIII: Technology Systems and Applications, 2016
    Co-Authors: Cesar Jauregui, Hans-jürgen Otto, Sven Breitkopf, Jens Limpert, Andreas Tünnermann
    Abstract:

    In this work we present guidelines to increase the transverse mode Instability Threshold of high power fiber amplifiers and also, for the first time to the best of our knowledge, of fiber oscillators. These guidelines do not involve changes in the composition of the active material (except for its doping concentration), but they can still lead to a significant increase of the transverse mode Instability Threshold. The dependence of this parameter on the active ion concentration, the core conformation, the pump configuration and the mirror reflectivities in a fiber oscillator will be studied and discussed.

  • simplified modelling the mode Instability Threshold of high power fiber amplifiers in the presence of photodarkening
    Optics Express, 2015
    Co-Authors: Cesar Jauregui, Hans-jürgen Otto, Jens Limpert, Fabian Stutzki, Andreas Tünnermann
    Abstract:

    In this paper we present a simple model to predict the behavior of the transversal mode Instability Threshold when different parameters of a fiber amplifier system are changed. The simulation model includes an estimation of the photodarkening losses which shows the strong influence that this effect has on the mode Instability Threshold and on its behavior. Comparison of the simulation results with experimental measurements reveal that the mode Instability Threshold in a fiber amplifier system is reached for a constant average heat load value in good approximation. Based on this model, the expected behavior of the mode Instability Threshold when changing the seed wavelength, the seed power and/or the fiber length will be presented and discussed. Additionally, guidelines for increasing the average power of fiber amplifier systems will be provided.

Cesar Jauregui - One of the best experts on this subject based on the ideXlab platform.

  • the sensitivity of the mode Instability Threshold to different types of intensity noise
    Fiber Lasers XVII: Technology and Systems, 2020
    Co-Authors: Christoph Stihler, Cesar Jauregui, Jens Limpert, Sobhy E Kholaif
    Abstract:

    In this work we experimentally and theoretically investigate the impact of seed intensity-noise on the Threshold of transverse mode Instability (TMI) in Yb-doped, high-power fiber laser systems and compare it to the impact of pump intensity-noise. Former studies have shown that pump intensity-noise significantly decreases the TMI Threshold due to the introduction of a phase shift between the modal interference pattern and the thermallyinduced refractive index grating in the fiber. However, it can be expected that fluctuations of the seed power will also induce such phase shifts due to a change of the extracted energy and the heat load in the fiber. Thus, it is important to investigate which one, i.e. the seed- or the pump intensity-noise, has a severer impact on the TMI Threshold. Our experiments have shown that the TMI Threshold of a fiber amplifier was decreased by increasing the seednoise amplitude. However, contrary to conventional belief, the impact of seed intensity-noise was much weaker than the one of pump intensity-noise. The measurements are in good agreement with our simulations and can be well explained with previous studies about the noise transfer function. The reason for the weaker impact of seed intensity-noise on the TMI Threshold is the attenuation of its frequency components below 20 kHz in saturated fiber amplifiers, which includes the frequencies relevant for TMI. Thus, the main trigger for TMI in saturated fiber amplifiers can be considered to be pump intensity-noise. A suppression of this noise below 20 kHz represents a promising way to increase the TMI Threshold of fiber laser systems.

  • pump modulation induced beam stabilization in high power fiber laser systems above the mode Instability Threshold
    Optics Express, 2018
    Co-Authors: Cesar Jauregui, Andreas Tünnermann, Christoph Stihler, Jens Limpert
    Abstract:

    A new way of stabilizing the output beam of a fiber laser system operating above the mode Instability Threshold is described and the first proof-of-principle experimental results are presented. This technique, which relies on a modulation of the pump power, works by washing the thermally-induced refractive index grating out, which weakens the coupling efficiency between transverse modes. One of the main advantages of this simple, yet powerful, approach is that it can be easily incorporated in already existing fiber laser systems since it does not require any additional optical elements. Using this beam stabilization strategy, a significant pointing stability and beam quality improvement has been demonstrated up to an average power of ~600W, which is a factor of 2 above the mode Instability Threshold.

  • optimizing high power yb doped fiber amplifier systems in the presence of transverse mode instabilities
    Optics Express, 2016
    Co-Authors: Cesar Jauregui, Hans-jürgen Otto, Sven Breitkopf, Jens Limpert, Andreas Tünnermann
    Abstract:

    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.

  • Optimizing the mode Instability Threshold of high-power fiber laser systems
    Fiber Lasers XIII: Technology Systems and Applications, 2016
    Co-Authors: Cesar Jauregui, Hans-jürgen Otto, Sven Breitkopf, Jens Limpert, Andreas Tünnermann
    Abstract:

    In this work we present guidelines to increase the transverse mode Instability Threshold of high power fiber amplifiers and also, for the first time to the best of our knowledge, of fiber oscillators. These guidelines do not involve changes in the composition of the active material (except for its doping concentration), but they can still lead to a significant increase of the transverse mode Instability Threshold. The dependence of this parameter on the active ion concentration, the core conformation, the pump configuration and the mirror reflectivities in a fiber oscillator will be studied and discussed.

  • simplified modelling the mode Instability Threshold of high power fiber amplifiers in the presence of photodarkening
    Optics Express, 2015
    Co-Authors: Cesar Jauregui, Hans-jürgen Otto, Jens Limpert, Fabian Stutzki, Andreas Tünnermann
    Abstract:

    In this paper we present a simple model to predict the behavior of the transversal mode Instability Threshold when different parameters of a fiber amplifier system are changed. The simulation model includes an estimation of the photodarkening losses which shows the strong influence that this effect has on the mode Instability Threshold and on its behavior. Comparison of the simulation results with experimental measurements reveal that the mode Instability Threshold in a fiber amplifier system is reached for a constant average heat load value in good approximation. Based on this model, the expected behavior of the mode Instability Threshold when changing the seed wavelength, the seed power and/or the fiber length will be presented and discussed. Additionally, guidelines for increasing the average power of fiber amplifier systems will be provided.

Shui Wang - One of the best experts on this subject based on the ideXlab platform.

  • electromagnetic ion cyclotron waves Instability Threshold condition of suprathermal protons by kappa distribution
    Journal of Geophysical Research, 2007
    Co-Authors: Fuliang Xiao, Huiyong He, Huinan Zheng, Qinghua Zhou, Shui Wang
    Abstract:

    [1] The well-known generalized Lorentzian (kappa) distribution generally provides a good representation for the high-energy tail population of natural cosmic suprathermal plasmas. In this study we examine the electromagnetic ion cyclotron waves (EMIC) Instability driven by the temperature anisotropy condition (T⊥/T∥ > 1) of suprathermal protons modeled with a typical kappa distribution in a cold multispecies plasma (electron, H+, He+, and O+). Since the EMIC wave Instability is found to be significant typically above the O+ band, we apply a linear theory to study the Instability Threshold condition for the He+ and H+ bands particularly around the geostationary orbit. The Instability Threshold condition, as in the case for a regular bi-Maxwellian, is found to follow a typical form T⊥/T∥ − 1 = S/β∥α, with higher values in the He+ band than those in the H+ band in the case of the strong wave Instability owing to a lower maximum wave growth in the He+ band. As the spectral index κ increases, the Instability Threshold condition generally decreases and tends to the lowest limiting values of the bi-Maxwellian, since the evaluation by the bi-Maxwellian generally overestimates the maximum wave growth. The densities of the cold components (particularly protons) have impacts on the Threshold condition primarily in the H+ band, with a higher density of cold protons leading to a lower value of the Threshold condition. The results above may further reveal the nature of this Instability Threshold condition for the EMIC waves in any other space plasmas where an anisotropic suprathermal ion component and cold multicomponents are present together.

  • whistler Instability Threshold condition of energetic electrons by kappa distribution in space plasmas
    Journal of Geophysical Research, 2006
    Co-Authors: Fuliang Xiao, Huinan Zheng, Qinghua Zhou, Shui Wang
    Abstract:

    [1] Observational studies clearly reveal that natural space plasmas generally possess a pronounced non-Maxwellian high-energy tail distribution that can be well modeled by a generalized Lorentzian (kappa) distribution. In this study we consider the whistler mode wave Instability driven by the anisotropy condition (T⊥/T∥ > 1) of energetic electrons modeled with a typical kappa distribution in the presence of a cold plasma population. We use a linear theory to study the Instability Threshold condition for two typical plasma regions of interest: the higher-density (or a weakly magnetized) region and the lower-density (or a strongly magnetized) region. We find that (1) as in the case for a regular bi-Maxwellian, the energetic electron anisotropy T⊥/T∥ is subject to the Threshold condition of this whistler Instability, and the Instability Threshold condition obeys a general form T⊥/T∥ − 1 = S/β∥α, with a narrow range of the fitting parameter 0.25 ≤ α ≤ 0.52 over 0.01 ≤ β∥ ≤ 2.0; (2) the Instability Threshold condition in the higher-density (or a weakly magnetized) region is generally lower than that in the lower-density (or a strongly magnetized) region, specifically, with the fitting parameter range 0.3 ≤ S ≤ 5.0 in the higher-density (or a weakly magnetized) region, while 0.32 ≤ S ≤ 6.94 in the lower-density (or a strongly magnetized) region; and (3) the Instability Threshold condition for the kappa distribution generally decreases as the spectral index κ increases and tends to the lowest limiting values of the bi-Maxwellian as κ → ∞. The results above may present a further insight into the nature of this Instability Threshold condition for the whistler mode waves in the outer radiation belts of the Earth, the inner Jovian magnetosphere, or other space plasmas where an anisotropic hot electron component and a cold plasma component are both present.

Hans-jürgen Otto - One of the best experts on this subject based on the ideXlab platform.

  • optimizing high power yb doped fiber amplifier systems in the presence of transverse mode instabilities
    Optics Express, 2016
    Co-Authors: Cesar Jauregui, Hans-jürgen Otto, Sven Breitkopf, Jens Limpert, Andreas Tünnermann
    Abstract:

    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.

  • Optimizing the mode Instability Threshold of high-power fiber laser systems
    Fiber Lasers XIII: Technology Systems and Applications, 2016
    Co-Authors: Cesar Jauregui, Hans-jürgen Otto, Sven Breitkopf, Jens Limpert, Andreas Tünnermann
    Abstract:

    In this work we present guidelines to increase the transverse mode Instability Threshold of high power fiber amplifiers and also, for the first time to the best of our knowledge, of fiber oscillators. These guidelines do not involve changes in the composition of the active material (except for its doping concentration), but they can still lead to a significant increase of the transverse mode Instability Threshold. The dependence of this parameter on the active ion concentration, the core conformation, the pump configuration and the mirror reflectivities in a fiber oscillator will be studied and discussed.

  • simplified modelling the mode Instability Threshold of high power fiber amplifiers in the presence of photodarkening
    Optics Express, 2015
    Co-Authors: Cesar Jauregui, Hans-jürgen Otto, Jens Limpert, Fabian Stutzki, Andreas Tünnermann
    Abstract:

    In this paper we present a simple model to predict the behavior of the transversal mode Instability Threshold when different parameters of a fiber amplifier system are changed. The simulation model includes an estimation of the photodarkening losses which shows the strong influence that this effect has on the mode Instability Threshold and on its behavior. Comparison of the simulation results with experimental measurements reveal that the mode Instability Threshold in a fiber amplifier system is reached for a constant average heat load value in good approximation. Based on this model, the expected behavior of the mode Instability Threshold when changing the seed wavelength, the seed power and/or the fiber length will be presented and discussed. Additionally, guidelines for increasing the average power of fiber amplifier systems will be provided.

  • impact of photodarkening on the mode Instability Threshold
    Optics Express, 2015
    Co-Authors: Hans-jürgen Otto, Cesar Jauregui, Jens Limpert, Norbert Modsching, Andreas Tünnermann
    Abstract:

    The Threshold-like onset of mode instabilities is currently the main limitation for the scaling of the average output power of fiber laser systems with diffraction limited beam quality. In this contribution, the impact of a wavelength shift of the seed signal on the mode Instability Threshold has been investigated. Against expectations, it is experimentally shown that the highest mode instabilities Threshold is reached around 1030 nm and not for the smallest wavelength separation between pump and signal. This finding implies that the quantum defect is not the only source of thermal heating in the fiber. Systematic experiments and simulations have helped in identifying photodarkening as the most likely second heat source in the fiber. It is shown that even a negligible photodarkening-induced power loss can lead to a decrease of the mode instabilities Threshold by a factor of two. Consequently, reduction of photodarkening is a promising way to mitigate mode instabilities.

  • scaling the mode Instability Threshold with multicore fibers
    Optics Letters, 2014
    Co-Authors: Hans-jürgen Otto, Cesar Jauregui, Jens Limpert, Fabian Stutzki, Arno Klenke, Andreas Tünnermann
    Abstract:

    Mode instabilities (MIs) have quickly become the most limiting effect for the average power scaling of nearly diffraction-limited beams from state-of-the-art fiber laser systems. In this work it is shown that, by using an advanced multicore photonic crystal fiber design, the Threshold power of MIs can be increased linearly with the number of cores. An average output power of 536 W, corresponding to 4 times the Threshold power of a single core, is demonstrated.