1. Recent Progress in Chirped-Pulse Fourier Transform THz Spectroscopy
Eyal Gerecht1, Ron Goldfarb1, David Plusquellic2 and Kevin Douglass2
National Institute of Standards and Technology
1. Electromagnetics Division, Boulder, CO
2. Optical Technology Division, Gaithersburg, MD

2. Introduction : Quantifying Greenhouse Gases
There is an initiative at NIST to develop measurement technologies to quantify greenhouse gas emissions over a range of length scales for studying Climate change or to develop carbon mitigation strategies
As well as provide a supportive role such as providing spectroscopic parameters for HITRAN
The technique that I will be talking about could be used as a point source monitor
May be possible to develop an open path version

3. Multi-Component Gas Monitor GHGs, VOCs, or breath analysis
N2O NO
Formaldehyde
Acetone
Methanol
CO2 (18O)
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
Ethanol
0.805
0.875
THz

4. Room Temperature D2O 4000 avgs 10 mTorr 2µs pulse 60 MHz bandwidth
Low Q cavity
4000 avgs
10 mTorr
120:1 RMS
40:1 pp
BW 60 MHz power 1 watt
2 us pulse
Maybe Q 100

5. Room Temperature D2O Simulation using JPL database

6. Predicting Sensitivity in the THz
S/N = 120:1 for D2O at 10 mTorr total pressure at 10 GHz
Signal loss due to limited power -
660 times stronger line intensity for N2O at GHz
Operate at Doppler Limited Pressure ~ 250 mTorr
~ 20 – 200 Net Gain
Does not take into account Q or coupling efficiency (which was likely small in the semi-confocal cavity) not more than 16
Really just tells us we think this should work, this is just for emission measurement
For absorption if we can get the light through the cell and pick a strongly absorbing line we are guaranteed to see signal
4000
Expect : 1 S/N
Low ppm sensitivity if scaling from 20:1 signal 1%D2O in N2

7. Chirped THz Pulse Experimental Setup: Gaithersburg Lab
10 MHz Ru
Standard
ARB
480 MHz bandwidth Chirp at 540 GHz
Synthesizer
10-18 GHz
x48
Digital Oscilloscope - LeCroy 8zi
the next step is actually making chirped pulses – you don’t need much bandwidth in MW so a lower cost ARB is fine.
You also need to make a really clean chirp at MW freqs (with either filtering or special mixers
Explain diagram
One of the great things about the chirp is that the bandwidth gets multiplied up which so you can tell immediately if you have generated a chirp at the final output frequency
IF amp
KVARZ Synthesizer
GHz
THz Harmonic Mixer
G.G.Brown, B.C.Dian, K.O.Douglass, S.M.Geyer, S.Shipman and B.H.Pate, Rev.Sci.Instrum. 79 (2008)

8. Chirped Pulse before Multiplication

9. Chirped THz Pulse Experimental Setup: Gaithersburg Lab - Planned
10 MHz Ru
Standard DG
ARB
Synthesizer
10-18 GHz
x48
Digital Oscilloscope
- LeCroy 8zi
40 GS/s
Receiver is on order as well as an upgrade to existing multiplier chain
Capability will be from 100 GHz to 900 GHz various gaps
Long path
absorption cell
IF amp
HP Synthesizer
GHz

10. White Cell Proof of Principle
D2O Room Temperature 10 mTorr
Room Temperature measurements of D2O
At Microwave Frequencies
220:1 RMS or 80:1 pp
Likely a factor of 10 due to poor coupling

11. FT-THz Setup: Boulder Lab
DPO70604
6 GHz Bandwidth
10 MHz Ru
Standard DG
Synthesizer
10-18 GHz
nx72 = GHz
x72
1 Meter Cell
Room temperature detector!!
x72
Synthesizer 2
10-18 GHz
IF amp
nx72 = GHz
mixer
Room Temp.
nbeat = 1GHz

13. THz Chirp Pulse
dBm

14. N2O: Absorption and Emission
For the Measurement we chose N2O in the range of our multiplier 853 GHz in a 1 meter path length cell which is predicted to absorb nearly 90% of the light
Emission and absorption are square of mag FT
~ 3 MHz FWHM

15. Absorption 40,000 avgs HITRAN Measured 490:1 RMS 175:1 pp
Widths about 2.6 MHz and 3 MHz 10%
Absorbance 1.92 and ~20%
Step size ~380 kHz measured slightly asymetric

16. Free Induction Decay Emission Room temp. N2O 250 mTorr
RMS 100:1
JPL center Freq.
Δ 40 kHz
160,000 avgs – 20 minutes

17. N2O and Methanol: 300 MHz Bandwidth
250 mTorr total avgs

18. FT-THz Setup – Enhanced Detection
DPO70604
6 GHz Bandwidth
10 MHz Ru
Standard DG
ARB
Synthesizer
10-18 GHz
nx72 = GHz
x72
Long path
absorption cell
4K system
nbeat = 1GHz
x72
Synthesizer 2
10-18 GHz
nx72 = GHz
IF amp
8 GHz Bandwidth
HOT Electron Bolometer
A passive heterodyne hot electron bolometer imager operating at 850 gigahertz”, Gerecht, E., Gu, D., You, L., and Yngvesson, K. S. IEEE Trans.Microw.Theory Tech. 2008, 56(5),

19. Conclusions Demonstrated x72 and x48 multiplication of a chirped pulse
Phase locked Chirped Pulse measurements at 850 GHz of both absorption and emission signals of N2O and Methanol with a bandwidth of 300 MHz
Next Generation system expect a factor ~400 improvement in sensitivity

20. Acknowledgements
Virginia L. Perkey – SURF student
Gerald Fraser