1. Multiplexed Detection of CO2 using a Novel Dual-Comb Spectrometer
A. J. Fleisher,1 D. A. Long,1 J. T. Hodges,1 D. F. Plusquellic2
1National Institute of Standards and Technology,
Gaithersburg, MD, USA
2National Institute of Standards and Technology,
Boulder, CO, USA

2. Multiplexed approach to spectroscopy
Multiplexing
acquiring data on all available optical channels simultaneously
increase acquisition speed
Fast Scanning
digital control of laser frequency
electro-optic modulators
Traditional Scanning
thermomechanical tuning
laser current tuning
mechanical laser cavity length tuning
Dave Long
MJ02
The ultimate in scanning speed.

3. Experimental overview
Gas Sample PD
RSA
Multiplexed spectra. Sample interrogation. Comb. Readout with a second comb. Real-time spectral analyzer.
Amplitude
RF Detuning

4. Generating an EOM comb Fiber-coupled waveguide EOMs− +
6dB
<10 kHz – 18 GHz
Dual-Drive MZM
<6 Vrms
Fiber-coupled waveguide EOMs
Large tenability (20 GHz)
Low Vπ
Generate optically flat combs
Up to 700 comb modes
Fiber-coupled. Large tenability (20 GHz). Low V-pi. Optically flat output (reference).

5. OFC generation
Changing the mode spacing is limited by the tuning speed of our microwave sources (≤ 100 µs)
Add modulation parameters, OSA specs. Out to 700 comb teeth.
D.A. Long et al., Opt. Lett. 39, 2688 (2014)

6. Optical frequency combs
OFC from EOM of a CW laser
Menlo Systems FOFC
Toptica, Thorlabs, …. disclosure.
Sacrifice on bandwidth
Gain in power per comb mode
Targeted molecular sensing
Toptica, Thorlabs, IMRA …
D.A. Long et al., Opt. Lett. 39, 2688 (2014)

7. Multimode cavity couplingwf
cavity
resonances
Excite multiple modes simultaneously. Tunable, user defined, frequency agile comb mode spacing. ADD LOCKING.
Comb spacing
scanning
wC+d
wC+d+wf
D.A. Long et al., Appl. Phys. B 114, 489 (2014)

8. Multiheterodyne spectroscopy
Local Oscillator (LO)
fn,LO = nfmod + f0
Amplitude
Probe
fn,Probe = n(fmod + δfmod)
+ f0 + fAOM
Optical Detuning
Wavemeter gives us f0.
Heterodyne RF Signal
fn,RF = nδfmod + fAOM
f0 = 193 THz
fmod = 203 MHz
δfmod = 24 kHz
fAOM = 99 MHz
Amplitude
RF Detuning
D.A. Long et al., Opt. Lett. 39, 2688 (2014)

9. Technical drawing FL FL FC FS FS FC Lock (5%) FS Sample
MZMLO FL
AOMref FL FC PD FS FS FC
MZMprobe
Lock (5%) FS
Sample
AOMsample FA
Local Oscillator + Probe
and
Local Oscillator + Reference
Detector reveals two rf combs EC
ECDL
𝑇= 𝐼 𝐼 0 =exp −𝛼𝐿
D.A. Long et al., Opt. Lett. 39, 2688 (2014)

10. Down-converted rf spectrum
𝑇= 𝐼 probe 𝐼 ref =exp −𝛼𝐿
Acquisition speed (bandwidth)? Resolution?
D.A. Long et al., Opt. Lett. 39, 2688 (2014)

11. MH-CEAS of CO2 PCO2 = 14 Pa Finesse = 20,000 FSR = 203 MHz
Acquisition speed? Averaging time? BANDWIDTH? Simultaneously acquire 23 modes at a rate of XX. Observable is electric field vs. rf frequency … therefore we acquire both amplitude and phase data. 75 db SNR.
10,000 spectra in 30 seconds
single element
NEA = 3×10-10 cm-1 Hz-1/2
D.A. Long et al., Opt. Lett. 39, 2688 (2014)

12. Ruggedized spectrometer
Fiber-coupled multipass cell
MH-DAS
PCO2 = 13.3 kPa
L = 80 cm total
Acquisition speed? Averaging time? BANDWIDTH? Not enough to observe CO2 in the atmosphere in the near-IR (order of magnitude). NEEDS: stable power supply (DC), active rf/DC control.
single element
NEA = 4×10-6 cm-1 Hz-1/2
2𝐹 𝜋 𝐿 𝑐𝑎𝑣 𝐿 𝑚𝑝𝑐
D.A. Long et al., Opt. Lett. 39, 2688 (2014)

13. Ruggedized spectrometer
Pump laser
MZMs PD
Dave + cart = ruggedized! Breadboard size?
Multipass cell
AOMs

14. Frequency comb applications
Step-scan
4 orders of magnitude.
EOM
EOM
Step-scan
Electro-optic
mod. combs
T.J. Kippenberg et al. Science, 332, (2011).

15. Mini-comb applications
Nonlinear spec.
Targeted sensing
Molecular dispersion
Cavity GDD
Electro-optic modulator combs
Fractional bandwidth (Δλ/λ)
Mode spacing (Hz)
9 orders of magnitude!

16. Mini-comb applications
Nonlinear spec.
Cavity GDD
Targeted sensing
Molecular disp.
A. Bartels et al. Science 326, 681 (2009)
Laser size?
Fractional bandwidth (Δλ/λ)
Span of femtosecond optical frequency comb laser cavity sizes required to match this range of mode spacing. Length comparisons; 18 kHz frep requires a cavity from here to Rockville and back ( 18 GHz requires a cavity about the size of a US dime (
Electro-optic modulator combs
Mode spacing (Hz)

17. Summary of method Low cost
An order of magnitude cheaper than competing technologies
Does not require a highly trained user
Digital control of comb spacing and power leveling
High power per comb tooth
Built-in reference channel
Common mode signals (no need for complicated locking)

18. Acknowledgements MML PML David Long Kevin Douglas Katarzyna Bielska
Stephen Maxwell
Joseph Hodges
David Plusquellic
NIST Innovations in Measurement Science (IMS) award
NIST Greenhouse Gas Measurements and Climate Research Program

19. Insert title here

20. Multiplexed detection schemes
Streak Camera
Dispersive
M.J. Thorpe et al., Science 311, 1595 (2006)
S.A. Diddams et al., Nature 445, 627 (2007)
Fourier transform
Multiheterodyne
First direct frequency comb spectroscopy was done on Rb atoms held in a magneto-optical trap (MOT).
J. Mandon et al., Nature Photon. 3, 99 (2009)
I. Coddington et al., Phys. Rev. Lett. 100, (2008)

21. Mach-Zehnder modulator− +
6dB
<10 kHz – 18 GHz
Dual-Drive MZM
<6 Vrms
Control of both amplitude and phase
Flatten output over many sideband orders
Up to 700 comb modes from ONE cw laser!
Detuning (GHz)
Transmission
PCH4 = 16 kPa
L = 20 cm
fmod = 20 MHz
tacq = 2 ms
1.0
0.0
-6.5
6.5
Major advantage … optical flat over a reasonable bandwidth. Show some other references.
MZM references here.

22. FARS experimental setup
PD2
PBS
PBS
ring-down cavity
PD1
s-pol
p-pol
PDH servo
signal
acquisition
EOM 1
ECDL
lock leg
EOM 2
probe leg
2f lock
TTL trigger
switch
PDH lock beam
RF source
(0 – 70 GHz)
2f PDH error signal
Cavity modes
D. A. Long et al., Appl. Phys. B, (2013)

23. Experimental Overview
Laser
AOM
MZM PD FS FC
Gas Sample
Local Oscillator
Reference
Probe
(b) − +
6dB
<10 kHz – 18 GHz
Dual-Drive MZM
<6 Vrms
(c)
Local Oscillator (LO)
fn,LO = nfmod + f0
Probe
fn,Probe = n(fmod + δfmod)
+ f0 + fAOM
Heterodyne RF Signal
fn,RF = nδfmod + fAOM
Optical Detuning
RF Detuning
Amplitude

24. Technical Illustrations 1 and 2
EOM PD FC
Laser FS FS
AOM
Gas Sample FC
EOM
AOM
MZM FC PD
ECDL FS
MZM FS
AOM
Gas Sample FC
AOM

25. Argument for multiplexing
Traditional Scanning
Fast Scanning
Multiplexing
New outline: design a novel flat optical frequency comb light source, couple to an optical cavity for high sensitivity, measure relative transmission over all cavity modes simultaneously.
Down-convert the optical frequency comb spectrum to radiofrequencies using another optical frequency comb.
Multiheterodyne Cavity-Enhanced Absorption Spectrosocpy
MH-CEAS

26. Femtosecond optical frequency comb
Hall and Hänsch, 2005 Nobel Prize
Optical frequency comb
Time domain
Single pulse
Train of pulses
Distribution of comb teeth intensities defined partly by gain material.
Frequency domain
S. T. Cundiff, J. Ye, and J. L. Hall, Scientific American, April 2008

27. Frequency comb application
Book Chapters
Femtosecond Laser Spectrosocpy,
ed. P. Hannaford, Springer (2005).
Femtosecond Optical Frequency Comb Technology, eds. J. Ye and S.T. Cundiff, Springer (2005).
Select Reviews
A. Schliesser et. al. Nature Photonics, 6, 440 (2012).
F. Adler et. al. Annu. Rev. Anal. Chem., 3, 175 (2010).
Distribution of comb teeth intensities defined partly by gain material.
us!
N.R. Newbury, Nature Photonics 5, 186 (2011).

28. Technical Scheme 3 FC FS FS FC Sample EC EOM1 AOM1 PD EOM2 AOM2 ECDLFA
Sample
AOM2 EC
ECDL

29. Frequency Comb Applications
Targeted sensing
Non-linear spec.
Molec. disp.
Mirror disp.
Telecom
Fractional bandwidth (Δλ/λ)
Add application bubbles like in previous plot. Targeted molecular sensing, non-linear molecular spectroscopy, mirror dispersion, telecom applications. Span of femtosecond optical frequency comb laser cavity sizes required to match this range of mode spacings.
Electro-optic modulator combs
Mode spacing (GHz)

30. Potential application
Environmental Monitoring: monitor carbon dioxide and other greenhouse gas emissions to facilitate accurate assessment of impacts on global climate change and carbon mitigation.
Safety and Homeland Security: detection of Toxic Industrial Chemicals (TICS) and Toxic Industrial Materials (TIMs), explosives and other hazardous materials.
Manufacturing: detection and profiling of ion and radical concentrations during electrospray ionization, chemical vapor deposition and etching processes.
Biomedical Analysis: breath analysis.

31. Dual-Mini-Comb Spectroscopy
Inexpensive alternative to optical frequency combs created using phase-stable mode-locked lasers
Acquisition time and bandwidth can be adjusted on the fly
Sensitive to individual optical cavity mode dispersion
Different from DCS with MLL OFCs.