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

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

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. 95-135. S.-J. Lee et al., Jpn. J. Appl. Phys. 40, L878 (2001). I. Coddington et al., Optica 3, 414 (2016)

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)

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, 10424 (2016). Add picture of modulator. “Brute force.” Low V-pi is the key, new modulator technology available from 400 nm to 2000 nm.

Carbon dioxide (12C16O2) 30012 Band of 12C16O2 at 𝜈 0 =6347.851 cm−1 R16e line at 𝜈 0 =6359.967 cm−1 A.J. Fleisher et al., in preparation.

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.

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, 10424 (2016). herotek.com Add image of SRD.

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, 10424 (2016)

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, 10424 (2016)

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, 27806 (2015) Hebert et al., Phys. Rev. Appl. 6, 044012 (2016)

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, 061801R (2016). D.A. Long et al., in preparation.

IM, HNLF, Filter Cavities … G. Millot et al., Nature Photon. 10 27 (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

EO comb literature Background: Lee et al., Jpn. J. Appl. Phys. 40, L878 (2001). Background: Ferdous et al., Opt. Lett. 34, 3875 (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, 1309 (2015) Martin-Mateos et al., Opt. Express 23, 21149 (2015) Bao et al., Light Sci. Appl. 4, e300 (2015) Hebert et al., Opt. Express 23, 27806 (2015) Duran et al., Opt. Express 23, 30557 (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, 10424 (2016) Hebert et al., Phys. Rev. Appl. 6, 044012 (2016) Long et al., Phys. Rev. A 94, 061801R (2016) Jerez et al., Opt. Express 24, 14986 (2016) Jerez et al., Opt. Lett. 41, 4293 (2016). Bonilla-Manrique et al., IEEE Quant. Elec. 23, 5300107 (2017) Beha et al., Optica 4, 406 (2017) Long et al., in preparation Fleisher et al., in preparation Carlos III, Chalmers, etc. Review article.

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.

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: adam.fleisher@nist.gov Especially NRC postdocs.

Blank A.J. Fleisher et al., Opt. Express 24, 10424 (2016)

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)

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, 10424 (2016)

Reduction in RF linewidth t = 10 s RBW = 10 Hz inset: RBW = 200 mHz A.J. Fleisher et al., Opt. Express 24, 10424 (2016)

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

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

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