Axel Ruehl

Deutsches Elektronen-Synchrotron, Hamburg, Hamburg, Germany

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Publications (76)117.79 Total impact


  • No preview · Article · Jan 2016 · Optics Letters
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    ABSTRACT: We demonstrate a Ho:YLF regenerative amplifier (RA) overcoming bifurcation instability and consequently achieving high extraction energies of 6.9 mJ at a repetition rate of 1 kHz with pulse-to-pulse fluctuations of 1.1%. Measurements of the output pulse energy, corroborated by numerical simulations, identify an operation point that allows high-energy pulse extraction at a minimum noise level. Complete suppression of the onset of bifurcation was achieved by gain saturation after each pumping cycle in the Ho:YLF crystal via lowering the repetition rate and cooling the crystal. Even for moderate cooling, a significant temperature dependence of the Ho:YLF RA performance was observed.
    Preview · Article · Oct 2015 · Optics Letters
  • Peng Li · Axel Ruehl · Colleen Bransley · Ingmar Hartl
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    ABSTRACT: We present a passively mode-locked, tunable soliton Ho:fiber ring oscillator, optimized for seeding of Ho:YLF amplifiers. The oscillator is independently tunable in central wavelength and spectral width from 2040 nm to 2070 nm and from 5 nm to 10 nm, respectively. At all settings the pulse energy within the soliton is around 800 pJ. The soliton oscillator was optimized to fully meets the spectral requirements for seeding Ho:YLF amplifiers. Its Kelly sidebands are located outside the amplifier gain spectrum, resulting in a train of about 1 ps long pedestal-free pulses with relative intensity noise (RIN) of only 0.13 % RMS when integrated from 1 Hz to Nyquist frequency.
    No preview · Article · Sep 2015

  • No preview · Conference Paper · Jun 2015

  • No preview · Conference Paper · May 2015
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    ABSTRACT: We demonstrate a compact 290 fs, 0.5 mJ laser source at 2-μm wavelength generated from mJ-level 3.4-ps pulses from a fiber laser seeded Ho:YLF regenerative amplifier system via pulse compression in a gas-filled Kagome type HC-PCF.
    No preview · Article · May 2015
  • P. Li · A. Ruehl · C. Bransley · I. Hartl
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    ABSTRACT: We present soliton operation of holmium-doped fiber oscillators with independently tunable central wavelengths from 2040nm to 2070nm and spectral widths from 5nm to 10nm, which can be matched to the requirements of Ho:YLF amplifiers.
    No preview · Article · May 2015
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    ABSTRACT: We present a novel full 3+1D split-step numerical model for white-light generation including all major physical effects. We present a detailed spatial mode analysis and derive imaging systems to re-shape the spatial distribution.
    No preview · Article · May 2015
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    ABSTRACT: We demonstrate a simple and compact Holmium-doped fiber femtosecond oscillator, in-band pumped by a commercial Tm-doped fiber laser. The oscillator operates in the dispersion managed soliton regime at net zero intracavity dispersion and delivers >1 nJ pulse energy at 35 MHz repetition rate. The pulse duration directly at the oscillator output is 160 fs FWHM, close to the Fourier-limit of 145 fs FWHM. Using an additional nonlinear compressor stage, sub-100 fs FWHM pulse durations could be achieved. The nonlinear fiber compressor is implemented by a solid core highly nonlinear fiber for spectral broadening and a single mode fiber for pulse compression.
    No preview · Article · Dec 2014 · Optics Letters
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    ABSTRACT: Ultrashort multi-mJ laser pulses at 2 µm are of great importance to e.g. advance HHG in the water window energy range and THz generation [1]. A Ho:YAG regenerative amplifiers were recently demonstrated with mJ-level pulses energies [2], but required complex optical parametric amplifiers for seeding. Here we report on a compact Ho:YLF chirped pulse amplifier system seeded with a home-built Ho:fiber oscillator. To compensate for gain narrowing, intracavity gain shaping was employed as a simple and flexible solution. In a proof of principle experiment, we inserted an un-optimized etalon in the cavity of the regenerative amplifier and were able to significantly decrease the compressed pulse duration. The basis of the setup was a prototype Ho:YLF regenerative amplifier followed by a single pass amplifier (both from Q-peak Inc.) [3]. The amplifier was seeded with a home-built Ho:fiber oscillator; (whose details can be found in [4]) stretched and compressed with chirped volume Bragg gratings (CVBG) exhibiting a stretching factor of 19 ps/nm. The seed spectrum was centred at 2.05 µm with a bandwidth of 19 nm at FWHM. A total amplification of ~10 7 was achieved leading to an output energy of 1.3 mJ at 1 kHz. The pulses were compressed using a second identical CVBG with an efficiency of 88% leading to a pulse energy of 1.1 mJ. The beam profile observed with a camera showing an output beam diameter of 1.3 mm (inset Fig.1.a). To measure the pulse duration, we generated the second harmonic in a BBO crystal and then used a commercial auto-correlator at 1 µm. We derived a pulse duration of 3.5 ps for the 2 µm pulses assuming a sech 2 pulse shape (See Fig. 1.a). For gain shaping, a 120-µm thick uncoated (reflection of 4%) fused quartz-etalon was placed inside the cavity. The wavelength dependant transmission of the etalon reshapes the effective gain [5]. The etalon was tuned to provide maximum loss at the gain peak of 2050 nm and minimum loss at 2065 nm. To compensate for the reduced overall gain and the additional losses the output coupling was reduced from 11% to 2%. The result of this gain shaping technique is not only observable by the reduced pulse duration of 2.5 ps (see Fig. 1.a) but also by broadened optical spectrum (See Fig. 1.b). An optimized configuration with a single face reflection of 28% and a thickness of 150 µm can almost double the gain-bandwidth to ~22nm as shown by simulations in Fig.1.c. Thereby, approaching the Fourier-limit, such method of gain shaping can generate sup-ps pulses Fig. 1 (a) Autocorrelation trace of the corresponding frequency doubled pulses; in the inset, a far-field beam profile of the regen output after compression is shown. (b) Spectrum without and with the etalon (c) Simulation results of single pass gain spectrum shaping with (red) and without (black) the etalon of 28% reflectivity. In summary, we demonstrated a compact Ho:fiber/Ho:YLF-amplifier system with an amplification factor of 10 7 delivering 1.3 mJ pulses at 3.5 ps. Intracavity gain shaping was applied for the first time to a Ho-based amplifier system leading to a reduction of the compressed pulse duration to 2.5 ps. Further reduction with an optimized intracavity filtering is planned for the near future. The authors would like to acknowledge the contributions of Anne-Laure Calendron, Peter Krötz and Alex Dergachev. References
    Full-text · Conference Paper · Oct 2014
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    ABSTRACT: Ultrashort multi-mJ laser pulses at 2 µm are of great importance to e.g. advance HHG in the water window energy range and THz generation [1]. A Ho:YAG regenerative amplifiers were recently demonstrated with mJ-level pulses energies [2], but required complex optical parametric amplifiers for seeding. Here we report on a compact Ho:YLF chirped pulse amplifier system seeded with a home-built Ho:fiber oscillator. To compensate for gain narrowing, intracavity gain shaping was employed as a simple and flexible solution. In a proof of principle experiment, we inserted an un-optimized etalon in the cavity of the regenerative amplifier and were able to significantly decrease the compressed pulse duration. The basis of the setup was a prototype Ho:YLF regenerative amplifier followed by a single pass amplifier (both from Q-peak Inc.) [3]. The amplifier was seeded with a home-built Ho:fiber oscillator; (whose details can be found in [4]) stretched and compressed with chirped volume Bragg gratings (CVBG) exhibiting a stretching factor of 19 ps/nm. The seed spectrum was centred at 2.05 µm with a bandwidth of 19 nm at FWHM. A total amplification of ~10 7 was achieved leading to an output energy of 1.3 mJ at 1 kHz. The pulses were compressed using a second identical CVBG with an efficiency of 88% leading to a pulse energy of 1.1 mJ. The beam profile observed with a camera showing an output beam diameter of 1.3 mm (inset Fig.1.a). To measure the pulse duration, we generated the second harmonic in a BBO crystal and then used a commercial auto-correlator at 1 µm. We derived a pulse duration of 3.5 ps for the 2 µm pulses assuming a sech 2 pulse shape (See Fig. 1.a). For gain shaping, a 120-µm thick uncoated (reflection of 4%) fused quartz-etalon was placed inside the cavity. The wavelength dependant transmission of the etalon reshapes the effective gain [5]. The etalon was tuned to provide maximum loss at the gain peak of 2050 nm and minimum loss at 2065 nm. To compensate for the reduced overall gain and the additional losses the output coupling was reduced from 11% to 2%. The result of this gain shaping technique is not only observable by the reduced pulse duration of 2.5 ps (see Fig. 1.a) but also by broadened optical spectrum (See Fig. 1.b). An optimized configuration with a single face reflection of 28% and a thickness of 150 µm can almost double the gain-bandwidth to ~22nm as shown by simulations in Fig.1.c. Thereby, approaching the Fourier-limit, such method of gain shaping can generate sup-ps pulses Fig. 1 (a) Autocorrelation trace of the corresponding frequency doubled pulses; in the inset, a far-field beam profile of the regen output after compression is shown. (b) Spectrum without and with the etalon (c) Simulation results of single pass gain spectrum shaping with (red) and without (black) the etalon of 28% reflectivity. In summary, we demonstrated a compact Ho:fiber/Ho:YLF-amplifier system with an amplification factor of 10 7 delivering 1.3 mJ pulses at 3.5 ps. Intracavity gain shaping was applied for the first time to a Ho-based amplifier system leading to a reduction of the compressed pulse duration to 2.5 ps. Further reduction with an optimized intracavity filtering is planned for the near future. The authors would like to acknowledge the contributions of Anne-Laure Calendron, Peter Krötz and Alex Dergachev. References
    Full-text · Conference Paper · Oct 2014
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    ABSTRACT: High-energy laser sources at 2 µm are of great importance to e.g. advance HHG in the water window energy range and THz generation [1]. Recently, a sub-ps, mJ-level Ho:YAG amplifier has been demonstrated [2] but it utilized as a seed source an optical parametric amplifier, which was not only complex but also had 3 orders of more seed energy hence the net amplification was 3 orders less than the system described here. Here we report on a compact Ho:YLF chirped pulse amplifier system seeded with a home-built Ho-fibre oscillator, stretched and compressed with chirped volume Bragg gratings (CVBG). To our knowledge, for the first time, intracavity gain shaping inside a Ho-based amplifier system was employed, which enabled to decrease the compressed pulse duration from 3.5 to 2.5 ps. The basis of the setup was a prototype Ho:YLF regenerative amplifier followed by a single pass amplifier (both from Q-peak Inc.) [3]. As the seed source, we used a home-built Ho-fiber seed oscillator whose details can be found in [4]. The oscillator pulses were stretched to ~300 ps using a CVBG exhibiting a stretching factor of 19 ps/nm. The optical spectrum was centered at 2.05 μm with a bandwidth of 19 nm at FWHM. The stretched output of the seed laser was mode-matched to the cavity mode using a telescope resulting in a beam diameter of 2.7 mm at 1/e 2 . After stretching, the seed energy was ~60 pJ due to the 21 nm wide acceptance bandwidth of the CVBG. The maximum amplification of the pulse train in the regenerative amplifier was achieved after 12 round trips. The output of the regenerative amplifier was further amplified in a single pass amplifier to 1.3 mJ corresponding to a net gain of ~10 7 . The pulses were compressed in a second identical CVBG with an efficiency of 88% resulting in a final pulse energy of 1.1 mJ. The beam profile observed with a camera was bell-shaped with an output beam diameter of 1.3 mm. To measure the pulse duration, we first generated the second harmonic of the output in a 2 mm long BBO crystal and then used a commercial auto-correlator at 1 µm. By assuming a sech 2 pulse shape, we derived a pulse duration of 3.5 ps of the 2 µm pulses. The spectral bandwidth of the pulses was primarily limited by gain narrowing in the regenerative amplifier. To reduce this effect, we inserted a 120-µm thick uncoated fused quartz-etalon inside the cavity [5]. The etalon created a maximum loss at 2050 nm and minimum loss at 2065 nm to re-shape the effective gain spectrum (Fig1.a). To compensate for the loss of the etalon, the 11% output coupling was reduced to 2%. With this technique, we could shorten the pulse duration from 3.5ps to 2.5 ps (shown in Fig.1.b) at the expense of a reduced pulse energy of 0.8 mJ. The compensation of gain narrowing is also illustrated by the spectral broadening as shown in Fig.1.c. Fig. 1 (a) Single pass gain spectrum shaping without and with the etalon. (b) Autocorrelation trace of the corresponding frequency doubled pulses. (c) Spectrum without and with the etalon. In summary, we demonstrated a compact Ho:fibre/Ho:YLF-amplifier system with an amplification factor of 10 7 delivering mJ-level picosecond pulses. To our best knowledge, intracavity gain shaping was applied for the first time to a Ho-based amplifier system. Further reduction in pulse width through intracavity filtering with an etalon is expected in the near future. The authors would like to acknowledge the contributions of
    Full-text · Conference Paper · Aug 2014
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    ABSTRACT: A dedicated accelerator research and development facility SINBAD (Short INnovative Bunches and Accelerators at DESY) is proposed. This multi-purpose research facility is initially aimed at promoting three major goals: (1) Short electron bunches for ultra-fast science. (2) Construction of a plasma accelerator module with useable beam quality (3) Setup of an attosecond radiation source with advanced technology. Research and development on these topics is presently ongoing at various places at DESY, as add-on experiments at operational facilities. The two research goals are intimately connected: short bunches and precise femtosecond timing are requirements for developing a plasma accelerator module with external injection or staging. The scientific case of a dedicated facility for accelerator research at DESY is discussed. Further options are mentioned, like the use of a 1 GeV beam from Linac II for FEL studies. The presently planned conversion of the DORIS accelerator and its central halls into the SINBAD facility is described. The available space will allow setting up several independent experiments with a cost-effective use of the same infrastructure (for example a central high power laser, a central timing and synchronization lab, etc.). National and international contributions and proposals can be envisaged. A preliminary, possible layout and the design work plan are discussed.
    Full-text · Conference Paper · Jun 2014
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    Full-text · Dataset · Jul 2013
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    T K Allison · A Cingöz · C Benko · D C Yost · A Ruehl · M Fermann · I Hartl · J Ye
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    ABSTRACT: We report the generation of high power extreme ultraviolet frequency combs at 154 MHz repetition rate. The XUV combs are characterized by conducting high resolu-tion spectroscopy and observing the heterodyne beats between two independent systems.
    Full-text · Article · Jan 2013 · The European Physical Journal Conferences

  • No preview · Article · Jul 2012 · SPIENewsroom
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    ABSTRACT: We present full phase stabilization of an amplified Yb:fiber femtosecond frequency comb using an intra-cavity electro-optic modulator and an acousto-optic modulator. These transducers provide high servo bandwidths of 580 kHz and 250 kHz for frep and fceo, producing a robust and low phase noise fiber frequency comb. The comb was self-referenced with an f - 2f interferometer and phase locked to an ultra-stable optical reference used for the JILA Sr optical clock at 698 nm, exhibiting 0.21 rad and 0.47 rad of integrated phase errors (over 1 mHz - 1 MHz) respectively. Alternatively, the comb was locked to two optical references at 698 nm and 1064 nm, obtaining 0.43 rad and 0.14 rad of integrated phase errors respectively.
    Full-text · Article · Jun 2012 · Optics Letters
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    ABSTRACT: We report on a mid-IR frequency comb source of unprecedented tunability covering the entire 3-10 μm molecular fingerprint region. The system is based on difference frequency generation in a GaSe crystal pumped by a 151 MHz Yb:fiber frequency comb. The process was seeded with Raman-shifted solitons generated in a highly nonlinear suspended-core fiber with the same source. Average powers up to 1.5 mW were achieved at the 4.7 μm wavelength.
    Full-text · Article · Jun 2012 · Optics Letters
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    ABSTRACT: We demonstrate a mid-infrared frequency comb of unprecedented tunability covering the entire 3-10 μm fingerprint region. The comb is based on a Raman shifted Yb:fiber laser and difference frequency generation.
    No preview · Article · May 2012
  • Axel Ruehl

    No preview · Article · May 2012 · Optics and Photonics News

Publication Stats

670 Citations
117.79 Total Impact Points

Institutions

  • 2014
    • Deutsches Elektronen-Synchrotron
      Hamburg, Hamburg, Germany
    • University of Hamburg
      Hamburg, Hamburg, Germany
  • 2010-2012
    • VU University Amsterdam
      • LaserLaB Amsterdam-Institute for Lasers, Life and Biophotonics Amsterdam
      Amsterdamo, North Holland, Netherlands
  • 2009-2012
    • IMRA America, Inc.
      Fremont, California, United States
  • 2005-2009
    • Laser Zentrum Hannover e.V.
      Hanover, Lower Saxony, Germany