1. CHIRPED-PULSE TERAHERTZ SPECTROSCOPY FOR BROADBAND TRACE GAS SENSING
Eyal Gerecht, Kevin O. Douglass, David F. Plusquellic
National Institute of Standards and Technology
Optical Technology Division, Gaithersburg, MD
Recently developed solid state sources and heterodyne detectors for the terahertz frequency range have made it possible to generate and detect precise digitally synthesized waveforms at THz frequencies with ultra-low phase noise. The sample gas is polarized using sub-?s chirped THz pulses and both the absorption and the free inductive decay (FID) signals are detected using a mixer amplifier multiplier chain. This approach allows for a rapid broadband multi-component detection with low parts-per-billion sensitivities and high frequency accuracy. Current acquisition time is 30 seconds for 10.6 GHz of bandwidth. Such a system can be configured into a portable, robust, and easy to use sensing platform. A full description of broadband trace gas sensor operating at 540 GHz to 620 GHz will be presented.

2. Multi-Component Gas Monitor GHGs, VOCs, or breath analysis
Formaldehyde CO
Methanol
Acetone
Ethanol
CO2 (18O)
N2O NO
0.805
0.875
THz
in addition to making measurements to support the hitran database and doing fundamental spectroscopy our Goal is to develop rapid multi chemical gas sensor
Variety of applications the common theme is small molecule with a dipole moment
With high ppb sensitivity
L.S. Rothman et al, “The HITRAN 2004 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 96, (2005).

3. Current Multiplier Chain Coverage
255 – 315 GHz 4 mW
520 – 630 GHz 0.8 mW
850 – 945 GHz 0.1 mW

4. Chirped-Pulse THz Spectrometer
AWG 12 GS/s LO
YIG
9 GHz Source
x48
YIG
Mix AMC
x48
White Cell
E. Gerecht, K.O. Douglass, D.F. Plusquellic, Optics Express, April 22, 2011, Vol. 19, Issue 9, pp (2011)
Field group (MIT): GHz,G.B. Park et al., 64th ISMS, 2009, RH07

5. High Purity MW Chirped Pulse

6. 100 ns - 10 GHz Chirped THz pulse12
Frequency (GHz)
140 MHz in the MW
100
Time (ns)

7. High Speed Data Transfer
Remote Data Transfer
Wired x4 PCI Express
360 MB/s  MS/s
Record and Transfer 
80 MS in 3 s
At 40 GS/s:
4000 records in 3 s
80,000 averages in 60 sec.
Factor of 4 over just using the scope
This method improves the phase stability 1 trigger for every 80 M points – 4000 averages or 8000 avgs

8. White Cell Power Transmission ~20%
Currently aligned for 25 meter path length
M M5 M1 M2 M4 M6
0.5 m
V. B. Podobedov, D. F. Plusquellic, and G. T. Fraser, J. Quant. Spectrosc. Radiat. Transf. 91(3), 287–295 (2005).
V. B. Podobedov, D. F. Plusquellic, K. E. Siegrist, G. T. Fraser, Q. Ma and R. H. Tipping, J. Quant. Spectrosc. Radiat. Transf. 109(3), (2008).

9. Artifact of White Cell Alignment 50 Meter path pulse
Signal
Background
Subtracted

10. Artifact of White Cell Alignment 50 Meter path pulse
Signal
Background
Subtracted

11. Artifact of White Cell Alignment 50 Meter path pulse
Signal
Background
Subtracted

12. 25 ns - 10 GHz Chirped THz pulse12
Frequency (GHz)
100
Time (ns)

13. Direct Absorption of a 5 Component Gas Mix
L.S. Rothman et al, “The HITRAN 2004 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 96, (2005).
H. M. Pickett, R. L. Poynter, E. A. Cohen, M. L. Delitsky, J. C. Pearson, and H. S. P. Muller, "Submillimeter, Millimeter, and Microwave Spectral Line Catalog," J. Quant. Spectrosc. Radiat. Transf. 60, (1998).

14. Absorption - N2O at GHz
Intensity
Pressure (mTorr)

15. Absorption Data Results
The Voigt profile is in good agreement for all of the observed lines. However, the residuals (shown below the line in the insert) indicate the observed line is slightly broader. The best fit line shape gives a Gaussian component of ≈1.49 MHz compared to the HITRAN value of 1.20 MHz. The additional 0.9 MHz Gaussian width contribution is very close to that expected for the transform limit of the 500 ns interval.

16. Correcting Simulated Intensities for FID Response

17. FID Detection of a 5 Component Gas Mix
10.6 GHz in 500 nsec – 80K averages in 60 sec
100,000:1
H2O
2 ppb
x500
x500
OCS
280 ppb
FID Signal (a.u.)
Acetone
8000 ppb
EtOH
1600 ppb
MeOH
100 ppb
N2O
170 ppb
THz
L.S. Rothman et al, “The HITRAN 2004 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 96, (2005).
H. M. Pickett, R. L. Poynter, E. A. Cohen, M. L. Delitsky, J. C. Pearson, and H. S. P. Muller, "Submillimeter, Millimeter, and Microwave Spectral Line Catalog," J. Quant. Spectrosc. Radiat. Transf. 60, (1998).

18. FID Detection of MeOH 10 GHz Bandwidth
Field Magnitude
2.5 mTorr Pure * *
Signal Scaling?
In between limits
Intensity
CPT66_ _AgilentTest – SG32_FID_Final.JBW * *
541060
ν / GHz
552505

19. Extending to Higher Bandwidths
Please See Justin Neil RC06
CPT59_ SGBG00_FID_Final.JBW - Not Squared
90 GHz FID near 850 GHz
MeOH -1.2 mTorr Pure
2ms acquisition time
ν / GHz

20. Measuring lineshapes from the FID
See RB08

21. Conclusions
Demonstrated Chirped pulse THz spectrometer operating in 530 – 630 GHz and 780 – 870 GHz regions
Demonstrated phase stable operation
Demonstrated high sensitivity and rapid throughput for both absorption and FID emission measurements
We continue to make progress on the quantification of FID signals and sensitivities

22. Acknowledgements Virginia L. Perkey – SURF student
Eric M. Vess - SURF student
Tektronix – equipment loan
NIST National Research Council Program
Post Doctoral Research Opportunities

23. 5 Component Gas Mix – 3 GHz Section
Acetone
N2O
MeOH
EtOH