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

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

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

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

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)

Multiplication of Frequency Comb (27-36 GHz)

Bandwidth is extended; frequency comb spacing remains the same

Before Multiplication

After Multiplication

1/trep

1/trep

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

Measuring Absorption Spectra with Chirped Pulse Frequency Combs

Methyl cyanide, 10 mTorr (J=30-29, K=3, 2, 1, 0)

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)

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

Segmented Chirped-Pulse Spectroscopy

-room temperature-25 ns polarization, 125 ns detection (elevated pressure)-S/N ratio ~200:1 on strongest transition

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.

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

Acknowledgements

Funding: NSF CCI (Center for Chemistry of the Universe) CHE-0847919 NSF MRI-R2

CHE-0960074

-Kevin Lehmann-Brian Drouin

Amplitude Stability of Chirped Pulse Frequency Comb

(frequencies shifted to show amplitude variation) Two single CPFC pulses

Single shot fluctuations are ~2%.

Upconversion of Chirped-Pulse Frequency Comb:Filtration of Micropulses with Tukey (Tapered Cosine) Filter Suppresses Comb Wings

Upconversion of Chirped-Pulse Frequency Comb:Filtration of Macropulse with Kaiser-Bessel Window Improves Baseline Resolution

Compression of Chirped Pulse Frequency Combs

Unfold comb

cis-trans ethyl formate, 606-505

4100 averages27-36 GHz, detected in 923 MHz

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.

+

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

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Expanded View

Problem: Noise Folding

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.