Ultrasensitive, high accuracy measurements of trace gas species D. A. Long, A. J. Fleisher, J. T. Hodges, and D. F. Plusquellic ISMS 24 June 2015 Figure from HDR Architecture, Inc.

Outline A series of new spectroscopic techniques Each utilizes electro-optic modulators to enable high speed scanning They use optical cavities to achieve high absorption sensitivity They offer a range of complexity, speed, and sensitivity

Single-mode excitation with locked cavity Cavity ring-down spectroscopy (CRDS) incident laser beam recirculating field detector low-loss dielectric mirror ring-down signal Advantages: High effective pathlength and sensitivity Insensitive to laser intensity noise Small sample volume Empty-cavity Absorbing medium t

Measurements with frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) Hyperfine structure Measured CO 2, CO, H 2 O, O 2, CH 4 Line positions linked to OFC O 2 A-band line list for HITRAN Very powerful. But very slow. SNR=1,800,000:1

The problem frequency To record a spectrum you need to tune the laser frequency This generally requires thermal or mechanical tuning This is usually non-linear and very slow Things are even more difficult with cavity-enhanced spectroscopy (discrete frequencies)

One solution Stabilize the cavity’s length (actively or passively) This ensures we know exactly where the cavity mode are located Use an electro-optic modulator (EOM) to program the laser frequency We can do this with microwave-level accuracy These waveguide based devices are amazing: Very high bandwidth (up to 300 GHz!) Ultra-low insertion loss Very low V π (i.e. required microwave power) Very high efficiencies Fiber coupled

Frequency-agile, rapid scanning (FARS) spectroscopy Advantages: Overcomes slow mechanical and thermal scanning Links optical detuning axis link to radio-frequency (RF) standards Wide frequency tuning range (> 130 GHz = 4.3 cm -1 ) Method: Use waveguide electro-optic phase-modulator (EOM) to generate tunable sidebands Drive PM with a rapidly-switchable microwave (MW) source Fix carrier and use ring-down cavity to filter out all but one selected side band G.-W. Truong et al., Nature Photonics, (2013) MW source EOM cw laser ring-down cavity side-band spectrum Detector gas analyte

 C +  +  f C++fC++f C+C+ cavity resonances frequency scanning ff FARS operating principle G.-W. Truong et al., Nature Photonics, (2013)

Very high acquisition rates: scanning No dead time due to tuning Rates as high as 8 kHz limited by cavity response time D. A. Long et al., Appl. Phys. B, (2013)

Lower finesse = faster rates D. A. Long et al., Appl. Phys. B, (2013) t = ns  1.2 ns Finesse = 60 RD acq. rate = 5 MHz   /  = 4 % NEA = 1.9×10 -8 cm -1 Hz -1/2 Finesse = 20,000 RD acq. rate = 8 kHz   /  = % NEA = 1.7× cm -1 Hz -1/2 K. O. Douglass, JOSA B, (2013)

Due to the quality of our frequency axis we can record the individual cavity resonances ~130 Hz relative laser linewidth Uncertainty of the fitted resonance frequency ~1 Hz The width of the resonances provides a complimentary measure of the absorption Uncertainty of the fitted width of the resonances ~0.04% Extremely precise frequency axis D. A. Long et al., Appl. Phys. B, (2013)

High accuracy of frequency axis-linked to OFC cavity dispersion g DD  40 fs 2 Absolute accuracy of 4 kHz Two measurements agree to within 4 Hz

2 THz wide spectra recorded in 45 minutes 425 ppm of CO 2 in air ECDL grating moved every 12 GHz Each point is the average of 100 RDs Spectra of entire absorption bands D. A. Long, et al., JQSRT, (2015)

  /  = 0.014% NEA = 9x cm -1 Hz -1/2 NEA = 4x cm -1 Hz -1/2   /  = 0.023% 30 s Differential approach increases optimal averaging time by more than three orders of magnitude Differential FARS-CRDS  q = 1 See Zach Reed’s talk for more information (WF03)

What if I want higher sensitivity? To reduce 1/f noise, we want to make the measurement away from DC Also need to rapidly compare ring-down time constants at different wavelengths

Improving the sensitivity: Heterodyne measurements J. Ye and J. Hall., Phys. Rev. A, (2000) To do this we adapted the approach of Ye and Hall

Heterodyne measurements: Our approach D. A. Long, et al., Appl. Phys. B, (2014) Replace their two AOMs with a single EOM This enables rapid scanning Our approach also allows for a continuous lock (greatly reduces complexity)

Heterodyne measurements: Reaching the quantum-noise limit Utilized both a traditional InGaAs detector and an APD Able to reach the quantum-noise limit with the APD The traditional InGaAs allows for an NEA of 6E-14 cm -1 Hz -1/2 APDPIN D. A. Long, et al., Appl. Phys. B, (2014)

What if I don’t want to scan? Then multiplex!

Mode-locked femtosecond OFCs Advantages: Wide bandwidth (octave-spanning) Can be self-referenced (absolute freq. axis) Disadvantages: Essentially fixed repetition rate Low power per tooth (nW to µW) Large and expensive Image from Menlo Systems

An alternate approach: electro-optic modulators Ideal for targeted measurements of selected species T. Sakamoto, et al., Electron. Lett., (2007) Dual-drive MZM allows for power-leveling of the comb

Variable pitch Pitch can be changed in <100 µs D. A. Long, et al., Opt. Lett., (2014)

Multiheterodyne spectroscopy Common mode (no need for phase locking) D. A. Long, et al., Opt. Lett., (2014)

Referencing D. A. Long, et al., Opt. Lett., (2014)

Multiheterodyne spectra Acquired in 30 s (average of 10,000 spectra) D. A. Long, et al., Opt. Lett., (2014)

FS MZM LO MZM probe EOM probe EOM LO Frequency (a.u.) Replicate our original DD-MZM OFC generator using a second modulator at high frequency (≈ GHz). Increase optical bandwidth to >150 GHz. Still acquire the down-converted RF spectrum in 2 ms. Dense OFC generation

>280 optical modes Down-converted RF comb

Where are we headed? The mid-infrared.

Mid-infrared CRDS MDA < 6×10 −12 cm −1 at 4 s

Quantum-noise-limited measurements Quantum-Noise-Limited Fit ResidualsDetector-Noise-Limited Fit Residuals low noise PDhigh noise PD More on this in A. J. Fleisher’s talk (WF04)

Mid-infrared sensitivities NIST value

Conclusions Presented several new techniques for rapid, ultrasensitive detection – Frequency-agile, rapid scanning (FARS) spectroscopy Use an EOM to step scan the laser frequency Scanning rates limited only by the cavity response time – Heterodyne-detected cavity ring-down spectroscopy (HD-CRDS) Make the measurement well above DC Leads to quantum-noise-limited sensitivity Does not compromise scanning rate – Multiheterodyne spectroscopy with EOM-generated optical frequency combs Allows for multiplexed detection of several trace gases Far lower costs and complexity than with femtosecond lasers Inherently common-mode

Acknowledgements Joseph Hodges, David Plusquellic, Adam Fleisher, Gar-Wing Truong, Szymon Wojtewicz, Katarzyna Bielska, Hong Lin, Qingnan Liu, Zachary Reed, Kevin Douglass, Stephen Maxwell, Roger van Zee – NIST A NIST Innovations in Measurement Science (IMS) award The NIST Greenhouse Gas Measurements and Climate Research Program Always looking for good post-docs