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Evaluating the Potential of Multi-Color Heterodyne Absorption Spectroscopy in Turbulent Combustion Thilo Krätschmer, Joachim W. Walewski, and Scott T. Sanders Merits of Fourier-transform spectroscopy (FTS): • High throughput Overcome obstacles by: • Multicolor heterodyne spectroscopy (no moving parts) • Use of lasers (high spectral radiance) 3. Generation of closely spaced frequency pairs a) Two cavities, mode spacing fcavity and fcavity+Δ b)Two combs m fcavity and m(fcavity+Δ) 6. Generation of interlaced frequency combs c) Beating at corresponding frequencies mΔ a) Layout of frequency comb generator (FCG) • Broad wavelength coverage • Sufficient spectral resolution for most gas-phase spectroscopy •Shortcomings of traditional FTS: • Rather slow (> 1 ms per scan) PMC: Polarization maintaining controller Objectives: • Does it work in turbulent flow (combustion)? YES • How to generate the needed frequency combs? MANY FEASIBLE WAYS, PRESENT ONE • Low signal-to-noise due to low spectral radiance of light sources used (incandescent) LOA: Linear optical amplifier VOA: Variable optical attenuator b) Combination of two FCGs to produce interlaced combs 4. Absorption spectroscopy with frequency combs 1. Principle of heterodyne spectroscopy • Mix optical wave of unknown frequency and amplitude with wave of known frequency and amplitude OSA: Optical spectrum analyzer • Emission in the telecom band OSC: Oscilloscope • Etalon finesse = 40 FFT: Fast Fourier transformation 7. First results • Power of beating signal = geometric mean of power of both input waves Absorption frequencies are inferred from beating frequencies • Beating frequency = difference in frequency of both waves • 25 GHz fringe spacing • D = 10 MHz • 190 THz -> 1580 nm 5. Heterodyne spectroscopy in turbulent flows: Does it work? 2. Multicolor heterodyne spectroscopy • Mix optical waves with closely spaced frequencies (THz regime) and known amplitude • Resulting wave beats at known frequencies (rf region) and known amplitudes • Let waves interact with, e.g., molecular absorber • Light at certain frequencies gets absorbed. Change in amplitude is reflected in amplitude change of beating signal • Measurement time: 400 ms • Emulation of beam steering with phase scrambling due to multi-mode dispersion • Spectral resolution ~ 500 GHz (4 nm) • Comparison of FFT of time trace around beating frequency with and without mode dispersion • No noticeable impact of dispersion Measurement in turbulences feasible 8. Work in the near future • Suppress etalon noise • Reduce spectral width of modes -> increase spectral resolution (high-finesse etalons) University of Wisconsin Engine Research Center • Investigate cross-talk of etalon modes

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