1. Direct Absorption Spectroscopy with Electro-optic Frequency Combs
Adam J. Fleisher,1 David A. Long,1 David F. Plusquellic,2 Joseph T. Hodges1
1Material Measurement Laboratory
National Institute of Standards & Technology
Gaithersburg, MD 20899, USA
2Physical Measurement Laboratory
Boulder, CO 80305, USA

2. Electro-optic frequency combs
Relatively simple, frequency-agile, and robust optical combs for direct absorption spectroscopy
Interrogate sample at multiple frequencies simultaneously, thus performing MULTIPLEXED spectroscopy
Rapid and highly sensitive measurements

3. Optical frequency comb generators
OFCGs were originally designed to bridge gaps in frequency chains connecting RF references with optical frequencies
Early application – high-speed OCT
Scanning cart Fourier transform replacement.
M. Kourogi, “Optical frequency comb generators and their applications,” in Frequency Control of Semiconductor Lasers, M. Ohtsu, ed. (Wiley, 1996), pp
S.-J. Lee et al., Jpn. J. Appl. Phys. 40, L878 (2001).
I. Coddington et al., Optica 3, 414 (2016)

4. New modulator technology− +
6dB
<10 kHz – 18 GHz
Dual-Drive MZM
<4 Vrms
eospace.com
Enabling technology: fiber-coupled waveguide modulators with low Vπ
1) “Overdrive” the modulator with reasonable RF powers
2) Supply “custom waveforms” with excellent dynamic range
DD-MZM reference. Useful for generating spectrally flat OFCs. OFC generators of references in conference proceeding. Perform MH-CEAS at 1 THz bandwidth. We drive at 1 W (7 V), as compared to Pockel’s cells requiring > 100 V to induce a phase change of pi (V-pi). Cavity design can be painstaking and significantly limit usable bandwidth.
Δ𝜔≈𝜋 𝐴 𝜔
T. Sakamoto et al., Opt. Lett. 32, 1515 (2007)

5. AJF, DAL et al., Opt. Express 24, 10424 (2016).
EO comb generation
RF Overdrive
DAL, AJF et al., Opt. Lett. 39, 2688 (2014).
Custom Waveform
AJF, DAL et al., Opt. Express 24, (2016).
Add picture of modulator. “Brute force.” Low V-pi is the key, new modulator technology available from 400 nm to 2000 nm.

6. Carbon dioxide (12C16O2) 30012 Band of 12C16O2 at 𝜈 0 =6347.851 cm−1
R16e line at 𝜈 0 = cm−1
A.J. Fleisher et al., in preparation.

7. Nitrous Oxide (14N216O)
Analysis of the 4200 band of N2O currently in progress
65 transitions at low pressure
Acquisition time of 1 s per transition
SNR ≥ 100:1
A.J. Fleisher et al., in preparation.

8. AJF, DAL et al., Opt. Express 24, 10424 (2016).
EO comb generation
DAL, AJF et al., Opt. Lett. 39, 2688 (2014).
AJF, DAL et al., Opt. Express 24, (2016).
herotek.com
Add image of SRD.

9. Multi-species spectroscopy
Coupling these EO combs to any enhancement cavity is straightforward (unlike with MLLs)
F ≈ 19,000
Leff ≈ 5 km
P = 13 kPa (100 Torr)
Sufficient bandwidth for multi-species detection. Deep averaging required. Water, CO2, CO, etc.
A.J. Fleisher et al., Opt. Express 24, (2016)

10. Coherent averaging for more than 2 hours!
Without the AOM phase lock, successively triggered interferograms can not be coherently averaged
With the AOM phase lock, the signal-to-noise on a single comb tooth amplitude improves by 𝑁
Coherent averaging for more than 2 hours!
6 TB/h
PI corner of 100 Hz to 1kHz. 6 TB/h, coherent averaging is the only way. Hardware-based solution. Software-based solutions are already under way in our lab.
A.J. Fleisher et al., Opt. Express 24, (2016)

11. Pseudo random bit sequences
Digitally controlled waveforms generated using FPGA devices
Application to rapid pump-probe spectroscopy
Bao et al., Light Sci. Appl. 4, e300 (2015)
Hebert et al., Opt. Express 23, (2015)
Hebert et al., Phys. Rev. Appl. 6, (2016)

12. Chirped waveforms Sub-Doppler Spectroscopy Free Induction Decays
Narrowly spaced comb teeth
Multiplexed sub-Doppler spectroscopy
Precision high-resolution spectroscopy
Pseudo CW electronic signals
Atomic sub-Doppler spectroscopy.
D.A. Long et al., Phys. Rev. A 94, R (2016).
D.A. Long et al., in preparation.

13. IM, HNLF, Filter Cavities …
G. Millot et al., Nature Photon (2016).
K. Beha et al., Optica 4, 406 (2017).
Carving pulses out of your CW laser. Not all that efficient, but still frequency agile.
Pulse carving and non-linear broadening

14. EO comb literature
Background: Lee et al., Jpn. J. Appl. Phys. 40, L878 (2001).
Background: Ferdous et al., Opt. Lett. 34, (2009).
Review: Torres-Company et al., Laser Photonics Rev. 8, 368 (2014).
Long et al., Opt. Lett. 39, 2688 (2014)
Martin-Mateos et al., IEEE Photon. Technol. Lett. 27, (2015)
Martin-Mateos et al., Opt. Express 23, (2015)
Bao et al., Light Sci. Appl. 4, e300 (2015)
Hebert et al., Opt. Express 23, (2015)
Duran et al., Opt. Express 23, (2015)
Millot et al., Nat. Photonics 10, 27 (2016)
Duran et al., Opt. Lett. 41, 4190 (2016)
Posado-Roman et al., Sensors 16, 2007 (2016)
Fleisher et al., Opt. Express 24, (2016)
Hebert et al., Phys. Rev. Appl. 6, (2016)
Long et al., Phys. Rev. A 94, R (2016)
Jerez et al., Opt. Express 24, (2016)
Jerez et al., Opt. Lett. 41, 4293 (2016).
Bonilla-Manrique et al., IEEE Quant. Elec. 23, (2017)
Beha et al., Optica 4, 406 (2017)
Long et al., in preparation
Fleisher et al., in preparation
Carlos III, Chalmers, etc. Review article.

15. Time-resolved spectroscopy Accurate reference data (line-lists)
NIST applications
Time-resolved spectroscopy
Remote sensing
Fundamental physics
Accurate reference data (line-lists)
Physical sensing
Fleisher et al., J. Phys. Chem. Lett. 5, 2241 (2014)
Plusquellic et al., CLEO 2017, AM1A.5.

16. Acknowledgements MML PML David Long David Plusquellic Zachary Reed
Kevin Douglass
Joseph Hodges
Stephen Maxwell
NIST Greenhouse Gas Measurements and Climate Research Program
NRC Postdocs opportunities:
Especially NRC postdocs.

17. Blank
A.J. Fleisher et al., Opt. Express 24, (2016)

18. Local Oscillator (LO) Reference (Ref) Probe Heterodyne RF Signal
RF Detuning
Amplitude
Optical Detuning
Local Oscillator (LO)
fn,LO = nfmod + f0
Reference (Ref)
fn,Ref = n(fmod + δfmod)
+ f0 + fAOM,Ref
Probe
fn,Probe = n(fmod + δfmod)
+ f0 + fAOM,Probe
Real-time molecular spectroscopy.
Heterodyne RF Signal
fn,RF = nδfmod + fAOM,Ref +
nδfmod + fAOM,Probe
I. Coddington et al., Optica 3, 414 (2016)

19. Relative phase stabilization
10 MHz Clock
RF Reference
Phase Frequency Detector
Loop Filter
Detector
BPF
Amp
AOM
VCO
Simply due to differential phase noise in the probe/ref/LO fiber and free space paths. The reference path has been removed, and a fiber optic switch (1 ms) toggles between the reference and the probe.
Local Oscillator
Probe or Ref.
Laser
RF Cable
Optical Fiber
A.J. Fleisher et al., Opt. Express 24, (2016)

20. Reduction in RF linewidth
t = 10 s
RBW = 10 Hz
inset:
RBW = 200 mHz
A.J. Fleisher et al., Opt. Express 24, (2016)

21. Coherent averaging tacq = 200 μs Δfmod = 300 kHz N = 1000
noise reduction by 𝑁

22. Fast acquisition tacq = 10 μs Δfmod = 300 kHz N = 1000
noise reduction by 𝑁

23. Model
A.J. Fleisher et al., in preparation.