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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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