Axel Ruehl

Deutsches Elektronen-Synchrotron, Hamburg, Hamburg, Germany

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Publications (67)111.98 Total impact

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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.
    Optics Letters 12/2014; 39(24):6859-62. DOI:10.1364/OL.39.006859 · 3.18 Impact Factor
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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
    International Committee on Ultrahigh Intensity Lasers ICUIL-2014, Goa, India; 10/2014
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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
    International Committee on Ultrahigh Intensity Lasers; 10/2014
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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
    Europhoton 2014; 08/2014
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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.
    Proceedings of IPAC2014, Dresden, Germany; 06/2014
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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.
    The European Physical Journal Conferences 01/2013; 41(11006). DOI:10.1051/epjconf/20134111006
  • SPIENewsroom 07/2012; DOI:10.1117/2.1201206.004297
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    ABSTRACT: We present full phase stabilization of an amplified Yb:fiber femtosecond frequency comb using an intracavity electro-optic modulator and an acousto-optic modulator. These transducers provide high servo bandwidths of 580 kHz and 250 kHz for f(rep) and f(ceo), 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 ultrastable 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.
    Optics Letters 06/2012; 37(12):2196-8. DOI:10.1364/OL.37.002196 · 3.18 Impact Factor
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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.
    Optics Letters 06/2012; 37(12):2232-4. DOI:10.1364/OL.37.002232 · 3.18 Impact Factor
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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.
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    ABSTRACT: Development of the optical frequency comb has revolutionised metrology and precision spectroscopy due to its ability to provide a precise and direct link between microwave and optical frequencies. A novel application of frequency comb technology that leverages both the ultrashort duration of each laser pulse and the exquisite phase coherence of a train of pulses is the generation of frequency combs in the extreme ultraviolet (XUV) via high harmonic generation (HHG) in a femtosecond enhancement cavity. Until now, this method has lacked sufficient average power for applications, which has also hampered efforts to observe phase coherence of the high-repetition rate pulse train produced in the extremely nonlinear HHG process. Hence, the existence of a frequency comb in the XUV has not been confirmed. We have overcome both challenges. Here, we present generation of >200 {\mu}W per harmonic reaching 50 nm, and the observation of single-photon spectroscopy signals for both an argon transition at 82 nm and a neon transition at 63 nm. The absolute frequency of the argon transition has been determined via direct frequency comb spectroscopy. The resolved 10-MHz linewidth of the transition, limited by the transverse temperature of the argon atoms, is unprecedented in this spectral region and places a stringent upper limit on the linewidth of individual comb teeth. Due to the lack of cw lasers, these frequency combs are currently the only promising avenue towards extending ultrahigh precision spectroscopy to below the 100-nm spectral region with a wide range of applications that include spectroscopy of electronic transitions in molecules, experimental tests of bound state and many body quantum electrodynamics in He+ and He, development of next-generation "nuclear" clocks, and searches for spatial and temporal variation of fundamental constants using the enhanced sensitivity of highly charged ions.
    Nature 02/2012; 482(7383):68-71. DOI:10.1038/nature10711 · 42.35 Impact Factor
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    ABSTRACT: We address technical impediments to the generation of high-photon flux XUV frequency combs through cavity-enhanced high harmonic generation. These difficulties arise from mirror damage, cavity nonlinearity, the intracavity plasma generated during the HHG process, and imperfect phase-matching. By eliminating or minimizing each of these effects we have developed a system capable of generating > 200 μW and delivering ~20 μW of average power for each spectrally separated harmonic (wavelengths ranging from 50 nm - 120 nm), to actual comb-based spectroscopy experiments.
    Optics Express 11/2011; 19(23):23483-93. DOI:10.1364/OE.19.023483 · 3.53 Impact Factor
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    ABSTRACT: New developments on fiber-laser-frequency combs, demonstrating improved coherence properties, power scaling to the 80W level and extended spectral coverage from the XUV to the mid-IR will be discussed.
    Frontiers in Optics; 10/2011
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    ABSTRACT: New developments in femtosecond fiber-lasers include ultrabroad coherent supercontinuum generation, power levels up to 80W, at 120fs and 150MHz of an Yb-fiber laser and sub-100fs Tm-fiber-systems. Those sources are CEO-phase stabilized for advanced frequency comb applications.
    Conference on Lasers and Electro-Optics/Pacific Rim; 08/2011
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    ABSTRACT: We present detailed studies of the coherence properties of an ultrabroadband supercontinuum, enabled by a comprehensive approach involving continuous-wave laser sources to independently probe both the amplitude and phase noise quadratures across the entire spectrum. The continuum coherently spans more than 1.5 octaves, supporting Hz-level comparison of ultrastable lasers at 698 nm and 1.54 $$\mu${}$m. We present a complete numerical simulation of the accumulated comb coherence in the limit of many pulses, in contrast to the single-pulse level, with systematic experimental verification. The experiment and numerical simulations reveal the presence of quantum-seeded broadband amplitude noise without phase coherence degradation, including the discovery of a dependence of the supercontinuum coherence on the fiber fractional Raman gain.
    Physical Review A 07/2011; 84(1):011806-. DOI:10.1103/PHYSREVA.84.011806 · 2.99 Impact Factor
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    ABSTRACT: We generate high order harmonics of a femtosecond frequency comb at the focus of a high finesse optical cavity with 150 MHz repetition rate. The resulting table top high average brightness extreme ultraviolet (XUV) light source has promising applications in XUV frequency metrology, strong field and molecular physics studies, and more traditional XUV applications currently served by synchrotron light sources. We will discuss our recent technical achievements and detailed understandings of the intracavity extreme nonlinear processes that have led to XUV output power beyond the10 μW per harmonic level and reduced high frequency optical phase noise. We will also present the latest measurement on the coherence properties of VUV/XUV frequency combs.
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    ABSTRACT: We report on the coherent transfer of optical phase from 698nm to 1542nm below the level of 1.5Hz linewidth. Detailed numerical simulations of the supercontinuum generation process support the observed preservation of phase coherence.
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    ABSTRACT: We demonstrate wavelength tunable coherent Raman soliton generation in a Tm fiber amplifier seeded with a passively mode locked Tm fiber oscillator and subsequent octave spanning continuum generation in highly-nonlinear fibers
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    ABSTRACT: We present a highly coherent Yb-fiber laser based frequency comb spanning nearly 1.5 spectral octaves. The coherence properties are investigated by optical beat experiments and numerical simulations.

Publication Stats

513 Citations
111.98 Total Impact Points

Institutions

  • 2014
    • Deutsches Elektronen-Synchrotron
      Hamburg, Hamburg, Germany
    • University of Hamburg
      Hamburg, Hamburg, Germany
  • 2012
    • University of Colorado at Boulder
      • Department of Physics
      Boulder, Colorado, United States
  • 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