Amanda L. Steber, Brent J. Harris, Justin L. Neill, Kevin K. Lehmann, Brooks H. Pate
Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box 400319, Charlottesville, VA 22904.
260-296 GHz Schematic
2-3.5GHz
Frequency Combs Traditionally been used in IR and Optical
spectroscopy 1,2,3
Good for high pressure systems: pressure broadened lines (as opposed to emission techniques)
Potential for broadband absorption data with fast collection rates
Split the power over many frequenciesFASSST: µW4 CPFC: mW
1 F Adler, M.J. Thorpe, K.C. Cossel, J. Ye, Annu. Rev. Anal. Chem. 3 (2010) 175-205.2 P. Mddaloni, P. Cancio, P. De Natale, Meas. Sci. Technol. 20 (2009) 052001.3 I. Coddington, W.C. Swann, N.R. Newbury, Phys. Rev. 82, 2010, 043817.4 I.R. Medvedev, C.F. Neese, G.M. Plummer, F.C. De Lucia, Opt. Lett. 35 (2010) 1533 – 1535.
Laser Frequency Combs Low duty cycle Three independent paramenters: T, trep, Δ
Repeat n times
T
trep
Δt
1/trep
Bandwidth 1/Δtνsp
1/trep = νsp
1/ T = Δ
1 F Adler, M.J. Thorpe, K.C. Cossel, J. Ye, Annu. Rev. Anal. Chem. 3 (2010) 175-205.
Chirped-Pulse Frequency Combs
100% duty cycle
E(t)
Repeat n times
t
T
tchirps
f1f2
http://www.nanotron.com/EN/CO_techn-css.php
ϕ1Φ1 + Δϕ11/tchirp = νsp
1/ T = Δ
Δf = f2 – f1
Bandwidth Δf x 24Δν/ νsp = 1/n
Demonstration of Frequency Combs
Fie
ld A
mp
litu
de
(V)
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)
From Neill, J.L. et al. International Symposium on Molecular Spectroscopy, 66 th meeting, Talk RC06
Demonstration of Frequency Combs
Bandwidth is extended; frequency comb spacing remains the same
Before Multiplication
After Multiplication
1/trep
1/trep
From Neill, J.L. et al. International Symposium on Molecular Spectroscopy, 66 th meeting, Talk RC06
Frequency Comb Shape
Micropulse: 20 sMacropulse: 1 msTooth Spacing: 50 kHzTooth Width: 1 kHz
Acrylonitrile 5 mTorr 400 MHz
bandwidth for the frequency combs
Absorption vs Emission
S/N: 7:1Single Acquisition: 1 msPressure: 5 mTorr
S/N: 45:1Single Acquisition: 2 µsPressure: 2 mTorr
High Pressure Spectrum
OCS5 Torr
spectrum5 GHz
bandwidth
Transients
~10ns for this spectrum
Challenges
Phase Stability Tooth to tooth fluctuations Transients (reduce efficiency) Time resolution is set by the tooth
resolution Large Data sets
Potential Solutions
Shape the pulse in the Arb Can use filters and windows in post
processing to remove transients Use smaller bandwidths
Three Possible Techniques Fullband Spectra
36GHz of spectrum in a single chirpLarge variations in the overall profile
Comb Compression3
Segmented Frequency Combs
3 I. Coddington, W.C. Swann, N.R. Newbury, Phys. Rev. 82, 2010, 043817
Advantages of Segmented Frequency Combs Noise won’t fold over LO purity not much of an issue Make smaller data files that can be manipulated
much faster Sweeping across the bandwidth would be easy with
an Arb Due to device performances there seems to be: Reduced spurious signals (FC08) Better power response for smaller bandwidths More consistent response of device for smaller
bandwidths
Summary
Pressure Broadened lines that would not be measurable by emission can be detected.
Allows for fast collection of broadband absorption spectra
Can potentially be collected by 3 methods, using one instrumental setup
Acknowledgements
Pate Lab NSF CCI (Center for Chemistry of the
Universe)
CHE-0847919
+
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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