1. National Institute of Standards and Technology, Boulder, CO
A portable dual frequency comb spectrometer for atmospheric applications
Kevin Cossel, Eleanor Waxman, Gar Wing Truong, Fabrizio Giorgetta, William Swann, Sean Coburn, Robert Wright, Gregory Rieker, Ian Coddington, Nathan Newbury
ISMS 2016
National Institute of Standards and Technology, Boulder, CO
where is this

2. Greenhouse gases drive radiative forcing
Radiative forcing = Eabsorbed by Earth – Eradiated back to space
IPCC 5th Assessment Report, 2013

3. System comparison Point sensor Dual Comb Satellite
Second time resolution
Second-minute time resolution
Day time resolution
1-100 m spatial resolution
1-12 km spatial resolution
km spatial resolution

4. Possible applications
City-scale measurements
Leak/source identification
Source
Sensor Hub
Beam
Next talk by Sean Coburn
Continuous emission measurements
Mobile measurement platform

5. The frequency comb Time domain Single pulse Train of pulses 1/frep
fCEO
frep
Look at the Nobel talks. Precision laser spectroscopy and the optical frequency comb.
Frequency domain
Hall and Hänsch, 2005 Nobel Prize
S. T. Cundiff, J. Ye, and J. L. Hall, Scientific American, April 2008

6. Dual comb spectroscopy
1/fr,1 = ns
Comb 1
1/fr,2 = ns
Comb 2
frep+dfrep
frep
Similar to FTIR, but:
No moving parts
~ cm-1 point spacing
<0.001 cm-1 instrument line shape
Rapid scanning (~2 ms per interferogram)
Interfere on photodetector

7. Dual comb spectroscopy – frequency domain
Frequency Comb 1
gas
Detector
Comb 1 &2
tooth spacing differs by Dfrep
absorption
profile
Frequency Comb 2
frep1
frep1 ~ frep2 ~ 200MHz
Comb 2
frep2
frep1 – frep2=Dfrep
Comb 1
Accurate spectra requires:
1. mutual coherence,
2. stabilized comb tooth frequency
Optical frequency (THz)
Magnitude
RF frequency (MHz)
frep/2
Dfrep = 1 kHz

8. Strengths of dual comb spectroscopy
Dual-comb spectroscopy can provide:
High-resolution, broadband near-infrared spectra with negligible instrument lineshape
1-10 km length scales (between point & satellite sensors)
Second to minute time resolution
Eye safe, accurate, continuous, automated measurements
High resolution, high frequency accuracy
Negligible instrument lineshape
Rapid measurement of a single spectrum
Broadband
Multi-species detection and temperature
High brightness single spatial mode
Fiber compatible
Long propagation distances
Compatible with multi-pass cells and cavities

9. Portable, Lower Cost Spectrometers
Proof-of-Concept Spectrometer
Fieldable Dual-Comb Spectrometer
Coddington et al., PRA 82, (2010)
Zolot et al., JQSRT 118, 26 (2013)
Truong et al., in prep
Robust Er:fiber frequency comb
FPGA control electronics
Lock to narrow-linewidth cw laser and quartz crystal oscillator
10x cost reduction from proof-of-concept
Sinclair et al., Rev. Sci. Inst. 86, (2015)

10. Demonstration over 2-km path
6” Ritchey-Chretien Telescope
Detector
Off-axis parabolic collimator
4 mW
Comb 1
Comb 2
2-km path
5” retroreflector
Signal ( uW)
Size of ams

11. Measured 2-km Atmospheric Transmission over Two Bands
1.6 micron band
2 micron band
Spectra
Transmission
138 minute coherent average

12. 1.6 µm band fit

13. 24 hour time series

14. Instrument precision: CO2 and CH4

15. Dual-path comparison 430 425 420 XCO2 (ppm) 415 410 405 400 1.90
1.86
XCH4 (ppm)
1.82
1.78
9:30 PM : : :00 PM
Local Time

16. Preliminary data with 12-km path length

17. Aside: laboratory line-strength studies
Precise measurement of relative HITRAN linestrengths
CO2 Band
Mean (ppm)
Ratio to 2.06 band
1.6 µm
4520(4)
1.018(1)
2.01 µm
4531(1)
1.020(1)
2.06 µm
4442(1) 1
Preliminary data!

18. Conclusions Performance: 20 second time resolution
1-12 km open-air path
2 ppm CO2 precision
20 ppb CH4 precision
Additionally, can detect H2O, HDO, 13CO2
Outlook:
Better collection efficiency for long path lengths
Additional species (CO, NH3, O2)
Portable system with high spectral resolution, broad bandwidth and no moving parts

19. Acknowledgements NRC Postdoctoral fellowship program
Newbury group at NIST
Additional thanks to:
Greg Rieker (CU)
Caroline Alden (CU)
Sean Coburn (CU)
Robbie Wright (CU)
Scott Diddams (NIST)
Colm Sweeney (NOAA)
Anna Karion (NIST)
James Whetstone (NIST)
NRC Postdoctoral fellowship program

21. 6” Ritchey-Chretien Telescope
Transceiver Optics
6” Ritchey-Chretien Telescope
Combined comb light
Detector
12.5cm hollow corner cube retroreflector
5.8 km (11.6km total)
Off-axis parabolic collimator
Show view of penthouse?
Better photo of telescope (remove cam)
Launch: 3.8cm beam diameter, 3.1mW, to 1.69μm (5920 to 6370 cm-1)
Receive: 2.1μW average

22. Erbium fiber frequency comb
10 db
Fiber
amp
2 cm
16 cm
11 cm
0.7 liter package (2 x 0.35L)
Simple reproducible design
Reasonable cost
fs laser & amplifier
supercontinuum generation & offset frequency detection
1.5 mm
1.6 mm
1.3 mm
Mode locked Er:fiber laser
200 MHz rep rate
~100 fs pulses
~10 mW output
All fiber
Non-linear fiber
1.3 mm
1.5 mm
1.7mm
Currently a good option for practical sensing is Er:fiber based combs. Emit in the near infrared where many molecules absorb, but more importantly, at the same wavelengths that are used for telecom optical fiber transmission. So a phenomenal array of robust, low-cost fiber components exist at these wavelengths.
The output of the Erbium fiber comb is actually only ~100 nm wide centered around 1560 nm. But, as I mentioned on the last slide, we have very high peak power during the pulse, which is good for the non-linear processes that can be used to broaden the output of the combs. So we can add non-linear fiber or highly non-linear fiber to the output to generate continuum that still retains the perfectly spaced comb structure.
Highly non-linear fiber
1 mm
2.0 mm
“octave-spanning” comb
Sinclair et al. (2015) Rev. Sci. Inst., 86, ,
A Compact Optically-Coherent Fiber Frequency Comb

23. Comb Stabilization: From Lab to Field
Mutual coherence through common high bandwidth lock to diode laser
0.02rad at 1565nm
Absolute frequency tied to fieldable quartz oscillator
~2MHz absolute optical frequency fo
frep
fdiode
Wouldn’t be able to do a frep lock, because then we would lose the optical coherence ~
Loop Filter
Diode Driver
Low bandwidth
Quartz Osc.

24. Near-IR dual comb spectroscopy
1.6 mm interferogram
2 mm interferogram
Fourier Transform

25. Defining the frequency comb
Measuring fo via f-2f ν1 ν2
m=2n
ν2 - ν1 = 2(nfr + f0) - (mfr +f0) = f0
Two RF frequencies exactly determine all optical frequencies:
repetition frequency fr: cavity length
carrier envelope offset frequency fo: pump power and cavity phase shifts

26. Fits with Other Models Hitran 2012:
CO2 - Similar residuals to Hitran 2008
CH4 – Larger residuals compared with Hitran 2008
where is this

27. Concentration Retrievals
Model-independent systematic uncertainty is based on sensitivity of retrieved concentration to:
Maximum pressure and temperature inhomogeneities along path
Uncertainty in pathlength and pressure
Baseline correction (10× larger contribution than other factors)
where is this

28. Comparison with Other Techniques
TCCON x x x
GOSAT
OCO-2
Solar FTIR ground stations
Satellite Systems x
Current Measurement
Dual Comb
Spectroscopy
Open-path mobile FTIR
Atmospheric
CO2 Linewidth
TCCON=Total Carbon Column Observing Network (fixed, up-looking FTIRs)
Picarro
CRDS
cm-1
Resolution
“fully” resolved
partially resolved
unresolved CO2 lines