FREQUENCY-AGILE DIFFERENTIAL CAVITY RING-DOWN SPECTROSCOPY Z.D Reed, J.T. Hodges National Institute of Standards and Technology

Frequency Stabilized Cavity Ringdown Spectroscopy I = I0 exp-(t/t) + const time frequency 1 𝑐τ =α0 +α(n)

Limitations of CRDS Signal Drift Variations in experimental environment (air pressure, mechanical vibration, temperature, etc) Cavity losses vary with optical alignment Polarization dependent losses Optical feedback Limits averaging time and SNR

Coupled cavity effects Letalon ring-down cavity external optic Phase of etalon shifts with temperature and pressure Leads to variations in cavity base loss Courtois, Bielska and Hodges, Differential CRDS, JOSA B, 30, 1486-1495 (2013).

Differential CRDS DLq,q+Dq = L(nq+Dq) – L(nq) DLq = L(nq) – L(n0) fixed reference mode1 q DLq = L(nq) – L(n0) Reference frequency fixed frequency difference2 DLq,q+Dq = L(nq+Dq) – L(nq) Reduce drift and increase optimal averaging time Reduce etalon amplitudes Removes baseline shifts (from changes in alignment or cavity length) 1Huang & Lehmann, “Long-term stability in continuous wave cavity CRDS experiments”, Appl. Opt. 49, 1378-1387 (2010). 2Courtois, Bielska and Hodges, “Differential cavity ring-down spectroscopy,” JOSA B 30, 1486-1495 (2013).

Fixed Frequency Difference DLq,q+1 = L(nq+1) – L(nq) Δq=1

Fixed Frequency Difference DLq,q+2 = L(nq+2) – L(nq) Δq=2

Fixed Reference Mode DLq = L(nq) – L(n0) q

Differential CRDS fixed reference mode1 fixed frequency difference2 q Reference frequency fixed frequency difference2 Can be fit in same fashion as traditional spectrum Increases optimal averaging time Removes typical baseline and baseline drift Reduces etalon amplitudes

Differential CRDS method schematic Courtois, Bielska and Hodges, Differential CRDS, JOSA B, 30, 1486-1495 (2013).

Differential-CRDS Results SNR = 170,000:1 SNR = 68,000:1 Conventional spectrum with no etalon fitting/subtraction Differential spectrum showing reduction in etalon amplitude Spectrum residuals

Frequency-agile, rapid scanning (FARS) spectroscopy Limitations A solution Thermal or mechanical tuning Slow and often non-linear Discrete frequencies required Laser not narrowed, ringdown events acquired as laser momentarily becomes resonant with cavity modes Limited Δq due to AOM bandwidth Use electro-optic modulator (EOM) to set laser frequency microwave level accuracy Pound Drever Hall (PDH) lock to narrow laser Rapid tuning, acquisition rate limited by τ, wide and variable Δq Frequency-agile, rapid scanning (FARS) spectroscopy MW source side-band spectrum ring-down cavity Detector cw laser gas analyte EOM

Frequency Agile Differential Spectroscopy (FADS-CRDS) 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)

Averaging Statistics: FADS-CRDS vs FARS-CRDS Averaging Statistics: FARS-CRDS vs FADS-CRDS st/t = 0.014% NEA = 9x10-12 cm-1 Hz-1/2 NEA = 4x10-12 cm-1 Hz-1/2 st/t = 0.023% 30 s Optimal averaging time increased by three orders of magnitude!

FADS-CRDS etalons

FADS-CRDS and FARS-CRDS spectra SNR = 12,500:1 SNR = 9,800:1 SDNGP fit, no etalons fit Total measurement time 5.6 s SDNGP fit, one etalon and baseline fit Total measurement time 4.8 s SNR limited by structure in baseline for both cases

Higher point density spectra Lock to local mode Acquire comb of differential points Shift entire comb with AOM Laser tracks comb, and differential removes baseline shift Acquire new comb of differential points

FARS FADS SNR = 12,500:1 SNR = 3,600:1 2500 ppm CO in N2 SDNGP profile

Scanned Cavity Differential CRDS profile: SDNGP

Future Direction Further reduce etalons to remove limiting factor of SNR (>106:1 possible in 5 s spectrum) Utilize scanned cavity tuning with D-CRDS and FADS High SNR with high point density provides stringent tests for line profiles Further investigate effects of polarization drift and cross polarization on losses

Thanks to J.T Hodges, D.A. Long, A.J. Fleisher Guest Researchers K. Bielska, H. Lin, V. Sironneau, G.W. Truong Funding: NIST Greenhouse Gas Measurements and Climate Research Program