69 th International Symposium on Molecular Spectroscopy June

69 th. International Symposium on Molecular Spectroscopy June 17, 2014, The University of Illinois at Urbana-Champaign TJ 13 MID-IR SUB-DOPPLER RESOLUTION SPECTROMETER USING AN ENHANCED-CAVITY ABSORPTION CELL COUPLED WITH A WIDE BEAM M. ABE, K. IWAKUNI, S. OKUBO* and H. SASADA Department of Physics, Faculty of Science and Technology, Keio University, Yokohama, Japan. *Current address: National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST)

Outline • Motivation • Design of an enhanced-cavity absorption cell (ECAC) • Spectroscopy of 12 CH 4 • Spectroscopy of 12 CH 3 D with a new ECAC of the A 1 -A 2 components • Summary and absolute frequency determination

Our spectrometer pump Motivation enhanced-cavity absorption cell (ECAC) Nonlinear crystal signal idler the n 3 band of 12 CH 4 Q(10) Linewidth of Lamb dip ≒ Pressure broadening Transit-time broadening Source width Natural linewidth Power broadening

Our spectrometer pump Motivation enhanced-cavity absorption cell (ECAC) Nonlinear crystal signal idler the n 3 band of 12 CH 4 Q(10) Linewidth of Lamb dip ≒ Pressure broadening Transit-time broadening Source width Natural linewidth Power broadening light molecules: The transit-time broadening is dominant. ⇒introduce a novel ECAC coupled with a wide beam ⇒The linewidth of the Lamb dip reduces by decreasing power broadening.

Transit time in a Gaussian beam Intensity distribution of a Gaussian beam Transverse Beam Size v 2 w 0 I

Transit time in a Gaussian beam Intensity distribution of a Gaussian beam Transverse Beam Size v 2 w 0 I Curved Wavefront

Transit time in a Gaussian beam Intensity distribution of a Gaussian beam Transverse Beam Size v 2 w 0 I Curved Wavefront v Doppler effect k

Transit time in a Gaussian beam Intensity distribution of a Gaussian beam Transverse Beam Size v 2 w 0 I Curved Wavefront The total broadening due to these effects is constant for any longitudinal position. Transit-time broadening: v Doppler effect k

Comparison of ECACs 3. 8 mm 1. 4 mm 2 m 24 cm 38 cm 7 m

Lamb dip Triangle Intensity Experimental setup Frequency 1. 06 mm Sample gas Nd: YAG Laser Linewidth: a few k. Hz PZT FA Waveguide Periodically poled lithium niobate (PPLN) EOM Extended cavity laser diode（ECLD） Linewidth< 500 k. Hz 1. 5 mm Feedback Signal 3. 4 mm enhanced-cavity absorption cell (ECAC) In. Sb detector

Transmission spectrum of ECAC FSR: 400 MHz TEM 00 ＋freqency modulation 520 k. Hz finesse = 770 sideband

Observed spectrum the Q(12) transition of 12 CH 4 in the n 3 band

Mid-IR frequency control Er-OFC n Nd: YAG pump (1. 06 mm) EOM ECLD FA frep = 67 MHz PZT PPLN signal (1. 5 mm) idler ECAC (3. 4 mm) Feedback Signal

Mid-IR frequency control Er-OFC n Nd: YAG pump (1. 06 mm) EOM ECLD FA frep = 67 MHz PZT PPLN signal (1. 5 mm) idler ECAC (3. 4 mm) Feedback Signal

Mid-IR frequency control Synthesizer dsignal Er-OFC n Nd: YAG pump (1. 06 mm) EOM ECLD FA frep = 67 MHz PZT PPLN signal (1. 5 mm) idler ECAC (3. 4 mm) Feedback Signal

the n 1 band A 1 -A 2 splittings of 12 CH 3 D Q(J = 3, K = 3) D = 0. 29 MHz Pressure: 0. 1 – 0. 4 Pa Q(J = 4, K = 3) linewidth ~ 300 k. Hz Q(J = 5, K = 3) D = 0. 92 MHz D = 3. 84 MHz The transition frequency list Trans. Q (3, 3) Q (4, 3) Q (5, 3) 　 　 Comp. n (MHz) Trans. R (3, 3) the n 1 band A 2 89 012 513. 25 (0. 06) A 1 89 012 513. 54 (0. 05) p. P (4, 3) A 1 89 023 546. 89 (0. 10) A 2 89 023 547. 81 (0. 12) p. P (3, 3) A 2 89 037 747. 43 ( - ) A 1 89 037 751. 27 ( - ) 　 　 Comp. A 1 A 2 n (MHz) 89 953 473. 19 (0. 09) 89 953 474. 08 (0. 14) the n 4 band A 2 89 344 491. 73 ( - ) A 1 89 344 505. 05 ( - ) A 1 89 579 142. 15 ( - ) A 2 89 579 144. 59 ( - )

Summary – We have introduced a novel enhanced-cavity absorption cell to reduce the transit-time broadening. It enables us to decrease the input power and the sample pressure and eventually to observe narrow Lamb dips. – We have determined the absolute frequencies of six A 1 -A 2 pairs of 12 CH 3 D using the optical frequency comb with a relative uncertainty of 10– 9.

Summary – We have introduced a novel enhanced-cavity absorption cell to reduce the transit-time broadening. It enables us to decrease the input power and the sample pressure and eventually to observe narrow Lamb dips. – We have determined the absolute frequencies of six A 1 -A 2 pairs of 12 CH 3 D using the optical frequency comb with a relative uncertainty of 10– 9. Thank you for your attention.

Appendix

CH 3 D A 1 -A 2 splitting Vibrational mode CH 4 CH 3 D Td C 3 v n 3 Triply degenerate n 1 (DK=0) non-degenerate n 4 (DK=± 1) Doubly degenerate E K-l=2 K-l=3 K-l=4 J=5 Vibrational excited state J=4 J=3 J=2 Q(J=3, K=3) J=5 Vibrational ground state J=4 J=3 J=2 K=3 K=4 E A 1 A 2

Tunable range DFG (idler) = Nd: YAG laser (pump) − tunable 1. 5 mm ECLD (signal) 85 90 95 R(8) P(12) Phys. Rev. 48, 864 (1935) CH 4 ECLD (signal) Fiber Amp. (signal) mid-IR Freq. / THz C-Band L-Band PPLN 2900 3000 3100 Wavenumber / cm-1

previous enhanced-cavity absorption cell bellows PZT Mirrors are optical windows. mirror separation (FSR) reflectivity (transmittance) finesse (FWHM) 23. 6 cm (636 MHz) 99. 0% (0. 7%) 300 (2. 1 MHz) effective absorption length sensitivity optical field strength at antinodes × 198 × 139 × 17 From Sasada presentation in International Symposium on Molecular Spectroscopy 67 th Meeting

DFG frequency measurement using a frequency comb Er fiber comb rep. freq. : 65 MHz 1. 545 ~ 1. 57 mm A nonlinear fiber broadens comb spectrum. 1. 0 ~ 2. 0 mm From Sasada presentation in International Symposium on Molecular Spectroscopy 67 th Meeting