Transcript CPFT2.pptx

Chirped-Pulse Fourier Transform
mm-Wave Spectroscopy from 260295GHz
Brent J. Harris, Amanda L. Steber, Justin L. Neill*, Brooks H. Pate
University of Virginia,
Department of Chemistry, University of Virginia,
McCormick Rd, PO Box 400319, Charlottesville, VA 22904
*University of Michigan
Department of Astronomy, University of Michigan
500 Church St., Ann Arbor, MI 48109
mm-Chirped Pulse Spectrometer
Low IF (MHz – GHz)
10 MHz Rb
Standard
2.5 GS/s
Digitizer
x12 Multiplier
Chain
Sub-harmonic mixer
12 GS/s
AWG
8.80 GHz
PDRO
220 – 325 GHz
WR 3.4
2-3.5 GHz
10.8-12.3 GHz
x24 Multiplier
Chain
Chamber
258 – 295 GHz
Static Gas Experiments
DC –
33 GHz
Using the Power
Translating power into speed
Molecular transitions saturate upon
absorption on the order of μWatts.
(mTorr)
For single transform limited pulses,
the rest of the power is unused.
Fast frequency sweeps “spread” the
power across a broad frequency
range. (no saturation)
For CPFT, weak pulse limit:
more power = more signal
Chirped-Pulse at 1mm
Time domain signal after 45dB amplification
Doppler dephasing dominates
below 1mTorr (~1.5 us)
Scan rate: 36 GHz / 2 μs
18,000,000 GHz/s
Short recovery from excitation
compared to measurement time
Sensitivity is achieved by time
domain averaging
Maximum Bandwidth Spectroscopy
Tektronix: DPO/DSA/MSO70000 Series
100GS, 33GHz Bandwidth
At 100GS, each trace 200k points
Essentially 100% duty cycle for
up to 250 Million points (1250 FIDs).
Compatible for coupling
measurement to transient
events like 10Hz LASER
Acetaldehyde, 100shot
Adetaldehyde,
100shot (200
(200mms),
s), FullBand sweep
Adetaldehyde,
sweep
40:1 atat 281083.4
269852.3MHz
MHz (15
(14 06 15
9 - -13
40:1
146 08)14)
0.5
Experiment
JPL Simulation
0.3
0.4
Acetaldehyde,
Adetaldehyde, 100shot (200ms), FullBand sweep
40:1 at 281083.4 MHz (15 0 15 - 14 0 14)
0.5
0.2
0.4
0.2
0.1
0.3
Intensity (mV)
Intensity
Intensity (mV)
(mV)
0.3
0.0
-0.1
0.1
Experiment
JPL Simulation
0.2
0.1
0.0
-0.1
-0.2
-0.2
-0.3
-0.3
0.0
-0.4
-0.4
260000 270000
270000
280000
271000
Frequency(MHz)
(MHz)
Frequency
290000
260000
270000
280000
Frequency (MHz)
290000
Applications
Compatible for coupling to transient events
Pulse-jet synthesis
Discharge, short-lived species
Double-resonance spectroscopy
LASER, dynamical studies
(Can probe the time domain through 1250 FIDs)
Suppose you want to interrogate one line
Narrow Band Sweeps
Acetaldehyde,
Signal scales as (BW)1/2 in weak pulse limit
Acetaldehyde,
Acetaldehyde: 130:1
Methanol:
90:1
Methyl Formate 60:1
So, how to get the advantage out smaller
bandwidth chirps???
Segmenting vs Fullband
Segmenting: better signal strength, but longer experiment (50 FIDs)
Fullband: can signal average in equivalent time
The result is : Same sensitivity
Segmented CP & Real Time Averaging
Approach 100% duty cycle: Trace detections of analytes
Number of data points per FID:
8,000 (seg) vs 200,000 (full)
Essentially 100% duty cycle up to
16 Million back-to back acquisitions
Signal averaging very stable!
Agilent: U1084 Acquiris 8-bit High Speed PCIe Digitizer
with on-board Signal Processing (4GS/s)
Segmented CPFT vs Absorption
FASST Absorption Spectroscopy*
6 m path length
~70GHz in 40 s
CPFT Spectroscopy
4 m path length
~30GHz in 10 ms
(1000X faster for equivalent
sensitivity)
* S.M. Fortman, I.R. Medvedev, C. F. Neese, F.C. De Lucia, Ap J, 2010, 725, 1682
Segmented CPFT vs Absorption
Tradeoff: Resolution
3X line width compared to FASSST
Improvement by phase unwrapping of the
magnitude spectrum to recover the absorption
and dispersion line shapes. Could see 2X
better line resolution for CPFT.
Tradeoff: Spectral Purity
AWG LO purity creates spurs and images
Improvement by advances in AWGs. Also, fast
Hallmarks of Segmented CPFT:
- Measured against zero background switching MW synthesizers.
- Simple frequency calibration
- Minimal data manipulation/processing
FFT (parallelizable for segmented)
gain correction
- Sensitivity
Conclusions and Future Directions
Chirped-Pulse Fourier Transform spectroscopy translates the high
power available in THz devices into speed. Sensitivity is achieved
1000X faster than the fastest absorption techniques.
Full band swept experiment rep rate makes the technique compatible
for coupling with transient laser events.
Segmented sweeping of the spectrum results in equal sensitivity in the
weak pulse limit and is accompanied with cost reduction in signal
processing (both time and $).
Essentially 100% duty cycle in time domain averaging can be achieved
with real time digitizers. The speed of broadband detection of weak
emitting analytes makes mm-Wave spectrum a good space for
analytical chemistry.
Acknowledgements
Pate Lab
NSF CCI (Center for Chemistry of the Universe)
CHE-0847919
This material is based upon work supported by the National
Science Foundation Graduate Research Fellowship under Grant
No. DGE-0809128