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Techniques for High-Bandwidth (> 30 GHz) Chirped-PulseMillimeter/Submillimeter Spectroscopy
Justin L. Neill, Amanda L. Steber, Brent J. Harris, Brooks H. Pate University of Virginia, Department of Chemistry, University of Virginia, McCormick Rd, PO Box 400319, Charlottesville, VA 22904
Kevin O. Douglass, David F. Plusquellic NIST, Optical Technology Division, Gaithersburg, MD 20899
Eyal Gerecht NIST, Electromagnetics Division, Boulder, CO 80305
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Extending Chirped Pulse Spectroscopy into the Millimeter/Submillimeter
More difficult due to lower power levels available
Field group (MIT): 70-100 GHz, 30 mWG.B. Park et al., 64th ISMS, 2009, RH07
NIST: 540-620, 810-870 GHz, ~1-10 mW (trace gas sensing)K. Douglass, 65th ISMS, 2010, WH09E. Gerecht, K.O. Douglass, D.F. Plusquellic, Optics Express, 19, 8973 (2011)
Pulse Generation Detection
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Extending Chirped Pulse Spectroscopy into the Millimeter/Submillimeter
Power increasing in solid-state active multiplier chains
System bandwidth exceeds that of currently available oscilloscopes
Fast digitization rates limit data throughput speed
Goal: Techniques that can measure broadband (>30 GHz) spectra in < 1 ms
Virginia Diodes 840 GHz Active Multiplier Chain
http://vadiodes.com/images/stories/systems/788-860-S155-wr12.pdf
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AWG Can Be the Local Oscillator
Two-channel AWGs are available with up to 6 GHz of bandwidth per channel (more than enough for mmw/sub-mmw spectroscopy)
Single system can measure both absorption and emission spectra: Absorption: chirped-pulse frequency combs Emission: segmented chirped-pulse Fourier transform spectroscopy
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Fie
ld A
mp
litu
de
(V)
Chirped Pulse Frequency Combs (13.5-18.0 GHz)Time Domain Spectrogram
Fourier Transform
trep
Fie
ld A
mp
litu
de
(mV
)
Fie
ld A
mp
litu
de
(mV
)
1/trep
Expanded View
(roll-off due to digitizer)
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Multiplication of Frequency Comb (27-36 GHz)
Bandwidth is extended; frequency comb spacing remains the same
Before Multiplication
After Multiplication
1/trep
1/trep
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Measuring Absorption Spectra with Chirped Pulse Frequency Combs
3,3,3-trifluoropropyne1,000 signal averages(pulsed jet)
1 ms freq. combs, 9 GHz bandwidth 50 kHz comb spacing
J=5-4 J=6-5
single pulsenoise level
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Measuring Absorption Spectra with Chirped Pulse Frequency Combs
Methyl cyanide, 10 mTorr (J=30-29, K=3, 2, 1, 0)
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Segmented Chirped-Pulse Spectroscopy
black: chirped pulses (Ch1)
blue: local oscillator (Ch2)
-detection bandwidth: 100-350 MHz -in weak pulse limit, equal sensitivity per measurement time segmenting
-many fewer data points collected because of low digitization rate
-issue: spectral purity of arb (need good pulse filtering)
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Segmented Chirped-Pulse Spectroscopy
-65 GHz spectrum, 60 s data collection (“1,000,000 GHz/s” scan rate)
FASSST: ~10 GHz/s
(Inset: 1.5 s, 1.728 GHz)
804.063-804.351 GHz
804.351-804.639 GHz
804.639-804.927GHz
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Segmented Chirped-Pulse Spectroscopy
-room temperature-25 ns polarization, 125 ns detection (elevated pressure)-S/N ratio ~200:1 on strongest transition
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Segmented Chirped-Pulse Spectroscopy
From time-domain FID can fit Lorentzian and Gaussian components in time domain (need near-zero dead time!)
Fit of 8 unblended methanol lines from 510-580 GHz system (4 parameters: amplitude, Doppler width, frequency, phase) 9 mTorr pressure; ~250 ns dead time (due to echo); fixed Lorentzian component
G = 2.375(12) MHz (~5% spread) mass = 29.4(29) amu (actual: 32 amu)
Could offer at least “heavy atom” (~10%) mass resolution for each molecule in a complex mixture
Sample single-frame fit(FID filtered in frequency domain)
R. Coerdt and H. Gronig, Appl. Opt. 28 (1989) 3021.S.L. Whittenburg, Spectrochim. Acta. A 52 (1996) 1169.
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Conclusions and Future Applications
-single instrument design for absorption and emission spectroscopy -low digitizer demands -bandwidth limited only by the device bandwidth -“video frame rate” monitoring of molecular concentrations
-coherent detection: signal averaging to improve sensitivity can be performed
-Applications: -characterization of complex mixtures (with mass resolution: library-free detection) -broadband detection of transient species -kinetics monitoring
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Acknowledgements
Funding: NSF CCI (Center for Chemistry of the Universe) CHE-0847919 NSF MRI-R2
CHE-0960074
-Kevin Lehmann-Brian Drouin
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Amplitude Stability of Chirped Pulse Frequency Comb
(frequencies shifted to show amplitude variation) Two single CPFC pulses
Single shot fluctuations are ~2%.
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Upconversion of Chirped-Pulse Frequency Comb:Filtration of Micropulses with Tukey (Tapered Cosine) Filter Suppresses Comb Wings
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Upconversion of Chirped-Pulse Frequency Comb:Filtration of Macropulse with Kaiser-Bessel Window Improves Baseline Resolution
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Compression of Chirped Pulse Frequency Combs
Unfold comb
cis-trans ethyl formate, 606-505
4100 averages27-36 GHz, detected in 923 MHz
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Frequency Shifting of Chirped-Pulse Frequency Comb
The overall frequency of the comb can be shifted by adding in a variable phase shift to the chirped pulses,which can be accomplished because of the complete phase control of the arbitrary waveform generator.
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+
Because there are 5 teeth in the mix comb, the resulting comb is compressed by 10.
Compression of Chirped Pulse Frequency Combs
Coddington, I., Swann, W.C., Newbury, N.R. Phys. Rev. 82, 2010, 043817
/2
Expanded View
Problem: Noise Folding
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Bandwidth Compression
Mixer
Compresses 2.4 GHz of spectral coverage(160 combs) into 52.6 MHz
Coddington, I., Swann, W.C., Newbury, N.R. Phys. Rev. A 82, 2010, 043817.