High-power sub-two-cycle mid-infrared pulses at 100 MHz repetition rate

Journal name:
Nature Photonics
Year published:
Published online

Powerful coherent light with a spectrum spanning the mid-infrared (MIR) spectral range is crucial for a number of applications in natural as well as life sciences, but so far has only been available from large-scale synchrotron sources1. Here we present a compact apparatus that generates pulses with a sub-two-cycle duration and with an average power of 0.1 W and a spectral coverage of 6.8–16.4 μm (at −30 dB). The demonstrated source combines, for the first time in this spectral region, a high power, a high repetition rate and phase coherence. The MIR pulses emerge via difference-frequency generation (DFG) driven by the nonlinearly compressed pulses of a Kerr-lens mode-locked ytterbium-doped yttrium–aluminium–garnet (Yb:YAG) thin-disc oscillator. The resultant 100 MHz MIR pulse train is hundreds to thousands of times more powerful than state-of-the-art frequency combs that emit in this range2, 3, 4, and offers a high dynamic range for spectroscopy in the molecular fingerprint region4, 5, 6, 7 and an ideal prerequisite for hyperspectral imaging8 as well as for the time-domain coherent control of vibrational dynamics9, 10, 11.

At a glance


  1. Driving laser.
    Figure 1: Driving laser.

    a, Kerr-lens mode-locked Yb:YAG thin-disc oscillator. The 0.1 mm thin disc is wedged and curved with a radius of curvature of about 20m. The disc is pumped at 940nm in a multipass cavity configuration (green beam), which results in a pump spot diameter of 2.5mm. The oscillator beam diameter on the disc is 2.2mm (1/e2 intensity). The 250 fs pulses (centre wavelength of 1,030nm) produced by the oscillator are broadened to about 125nm FWHM in an 80-mm-long large-mode-area PCF with a mode-field diameter of 35 µm. Temporal compression is subsequently achieved by 20 bounces on two types of chirped mirrors in a double-angle configuration, which accumulates a total group-delay dispersion of about −2,200 fs2. The footprint of the oscillator and compression stage is 50 × 150 cm2. HD, highly dispersive mirrors; OC, output coupler; KM, Kerr medium. b, Temporal intensity and phase of the compressed pulse measured by FROG (see Methods), which indicates a FWHM pulse duration of 19 fs.

  2. MIR generation and detection set-up.
    Figure 2: MIR generation and detection set-up.

    The driving pulses are focused with a 250 mm focal-length lens onto the LGS crystal. a, The 1/e2 intensity diameter of the NIR focus is 130µm, which leads to a peak intensity of 3.5 × 1011 W cm–2. The NIR and MIR beams are collimated after the crystal with a 100mm focal-length off-axis parabolic mirror. b, A 5mm thick Ge filter reflects the NIR beam and transmits the MIR beam towards the diagnostics, thermal power meter and beam profiler (Fig. 3). c, EOS set-up. A low-power copy of the NIR pulse is obtained from a reflection off a thin fused-silica wedge in the main NIR beam path (and with negligible effect on the transmitted beam) and serves as the probe pulse for the EOS. The probe-pulse beam is combined collinearly with the MIR beam at the Ge filter and both beams are focused onto a 30-µm-thin GaSe crystal with a 100mm focal-length off-axis parabolic mirror. The electric field of the MIR pulse modulates the birefringence of the crystal10, 25. The NIR pulse probes the birefringence as a function of the delay between the two pulses, by a polarization-state detection scheme that consists of a quarter-wave plate (QWP), a Wollaston prism and a balanced detector using two Si photodiodes. The DFG-driving NIR beam is chopped at 2 kHz with a mechanical chopper, which enables lock-in detection. The measurements of the integrated power and of the beam profile are performed without the chopper.

  3. MIR beam profile.
    Figure 3: MIR beam profile.

    The collimation of the MIR beam was achieved by adjusting the distance of the DFG focus to the collimating parabolic mirror and its orientation (Fig. 2). The profile of the MIR beam was measured with a microbolometer-array-based beam profiler. After achieving collimation, the MIR beam profile did not change notably along a few metres of propagation. The 1/e2 intensity diameter of the collimated beam is 1.897 mm in the horizontal (x) and 1.653mm in the vertical (y) direction.

  4. DFG results.
    Figure 4: DFG results.

    a, Raw EOS time-domain data (black line) of the MIR pulse consists of 3,000 measurement points with a lock-in integration time of 100ms per point, which results in five minutes of measurement time. The SNR, determined in the time domain as the mean magnitude of the signal amplitude divided by the standard deviation of the peak signal, evaluated from several measurements, was 101. Retrieved MIR electric field data (red line) after deconvolution with the probe pulse (Methods). The FWHM of the intensity envelope is 66 fs. b, Normalized PSD of the Fourier transforms of the EOS time-domain trace, of the retrieved field and of the NIR probe pulse. The dynamic range of the measurement, determined as the peak of the signal PSD divided by the average detector noise floor (blue, continuous line), is 2.7 × 104. The absolute power per comb line of the generated MIR radiation, obtained by calibrating the normalized PSD of the retrieved MIR power spectrum by the independently measured total power and considering the pulse repetition frequency, is shown on the right axis. c, Spectral phase of the Fourier transforms of the EOS time-domain trace, of the retrieved field and of the NIR probe pulse. The spectral resolution of frequency-domain data is 0.64 THz.


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Author information

  1. Present addresses: TOPTICA Photonics AG, Lochhamer Schlag 19, Gräfelfing 82166, Germany

    • T. Paasch-Colberg


  1. Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, Garching 85748, Germany

    • I. Pupeza,
    • J. Zhang,
    • N. Lilienfein,
    • M. Seidel,
    • N. Karpowicz,
    • I. Znakovskaya,
    • E. Fill,
    • O. Pronin,
    • F. Krausz &
    • A. Apolonski
  2. ICFO – Institut de Ciencies Fotoniques, Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain

    • I. Pupeza,
    • D. Sánchez &
    • J. Biegert
  3. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

    • J. Zhang &
    • Z. Wei
  4. Fakultät für Physik, Ludwig-Maximilians-Universität München, Am Coulombwall 1, Garching 85748, Germany

    • N. Lilienfein,
    • M. Seidel,
    • T. Paasch-Colberg,
    • I. Znakovskaya,
    • M. Pescher,
    • W. Schweinberger,
    • V. Pervak,
    • E. Fill,
    • O. Pronin,
    • F. Krausz &
    • A. Apolonski
  5. Department of Physics & Astronomy, King Saud University, PO Box 2455, Riyadh 11451, Saudi Arabia

    • T. Paasch-Colberg &
    • W. Schweinberger
  6. ICREA – Instituciò Catalana de Recerca i Estudis Avancats, Barcelona 08010, Spain

    • J. Biegert


I.P., D.S., J.Z., N.L., M.S., N.K., T.P., W.S., V.P., E.F., O.P., Z.W., F.K., A.A. and J.B. conceived and designed the experiments. I.P., D.S., J.Z., N.L., M.S., T.P., I.Z., M.P., W.S. and V.P. performed the experiments. I.P., D.S., N.L., M.S., N.K., T.P., I.Z. and W.S. analysed the data. I.P., D.S., J.Z., N.L., M.S., N.K., T.P., I.Z., V.P., E.F., O.P., Z.W., F.K., A.A. and J.B. contributed materials and/or analysis tools. I.P., D.S., J.Z., N.L., M.S., N.K., T.P., M.P., W.S., E.F., F.K., A.A. and J.B. wrote the paper.

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