Mid-infrared Frequency Comb Spanning an Octave Based on an Er Fiber Laser and Difference-Frequency Generation
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Mid-infrared Frequency Comb Spanning an Octave Based on an Er Fiber Laser and Difference-Frequency Generation Fritz Keilmann & Sergiu Amarie
Received: 26 February 2012 / Accepted: 3 April 2012 / Published online: 17 April 2012 # Springer Science+Business Media, LLC 2012
Abstract We describe a coherent mid-infrared continuum source with 700 cm-1 usable bandwidth, readily tuned within 600–2500 cm-1 (4–17 μm) and thus covering much of the infrared "fingerprint" molecular vibration region. It is based on nonlinear frequency conversion in GaSe using a compact commercial 100-fs-pulsed Er fiber laser system providing two amplified near-infrared beams, one of them broadened by a nonlinear optical fiber. The resulting collimated mid-infrared continuum beam of 1 mW quasi-cw power represents a coherent infrared frequency comb with zero carrier-envelope phase, containing about 500,000 modes that are exact multiples of the pulse repetition rate of 40 MHz. The beam's diffraction-limited performance enables long-distance spectroscopic probing as well as maximal focusability for classical and ultraresolving near-field microscopies. Applications are foreseen also in studies of transient chemical phenomena even at ultrafast pump-probe scale, and in high-resolution gas spectroscopy for e.g. breath analysis. Keywords Infrared laser . Infrared continuum source . Mid-infrared supercontinuum . Frequency-comb beam . Mid-infrared frequency comb The mid-infrared region spanning the decade of wavelengths from ca. 2.5 to 25 μm covers nearly all fundamental vibration frequencies of molecules and solids. This is why midinfrared spectroscopy is widely used to analyze and identify chemical compounds. The workhorse instrument for this task is the Fourier-transform infrared (FTIR) spectrometer. It relies on an incoherent thermal light source that covers not only the mid-infrared but also the near-infrared and far-infrared regions [1]. Much stronger brightness over also the full infrared spectrum is available in the incoherent beam lines of many electron synchrotrons [2]. Mid-infrared lasers have been serving important applications in materials processing [3], sensing [4], metrology [5], and communication [6] even though they allow only restricted F. Keilmann (*) LASNIX, Sonnenweg 32, 82152 Berg, Germany e-mail: [email protected] S. Amarie Max Planck Institute of Quantum Optics, Am Coulombwall 1, 85748 Garching, Germany
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J Infrared Milli Terahz Waves (2012) 33:479–484
ranges of step tuning (for example 9–11 μm in the case of the CO2 laser, or 5–7 μm for the CO laser) or of continuous tuning (for example 10 % in the case of quantum cascade lasers). But these lasers cannot support modern extensions of mid-infrared spectroscopy requiring coherent beams with a continuum of frequencies over one or several octaves. Spectroscopic ellipsometry [7, 8], for example, can benefit from coherent beams providing a well-defined, shallow incidence angle with small samples, while spectroscopic near-field microscopy of scattering type (s-SNOM) [9, 10] needs d
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