69th. International Symposium on Molecular Spectroscopy June 17, 2014, The University of Illinois at Urbana-Champaign TJ13 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 12CH4 with a new ECAC Spectroscopy of 12CH3D and absolute frequency determination of the A1-A2 components Summary

Transit-time broadening Motivation Our spectrometer enhanced-cavity absorption cell (ECAC) pump Nonlinear crystal signal idler the n3 band of 12CH4 Linewidth of Lamb dip Q(10) Pressure broadening Transit-time broadening Source width Natural linewidth ≒ Power broadening

Transit-time broadening Motivation Our spectrometer enhanced-cavity absorption cell (ECAC) pump Nonlinear crystal signal idler the n3 band of 12CH4 Linewidth of Lamb dip Q(10) 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 2w0 I

Transit time in a Gaussian beam Intensity distribution of a Gaussian beam Transverse Beam Size v 2w0 I Curved Wavefront

Transit time in a Gaussian beam Intensity distribution of a Gaussian beam Transverse Beam Size v 2w0 I Curved Wavefront v k Doppler effect

Transit time in a Gaussian beam Intensity distribution of a Gaussian beam Transverse Beam Size v 2w0 I Curved Wavefront v k The total broadening due to these effects is constant for any longitudinal position. Doppler effect Transit-time broadening:

Comparison of ECACs 3.8 mm 1.4 mm 24 cm 38 cm 2 m 7 m ミラー損失を0.3%と同じにしているため

Experimental setup Feedback Signal Intensity Triangle Frequency Lamb dip Intensity Triangle Frequency 1.06 mm Sample gas InSb detector Nd:YAG Laser Linewidth: a few kHz PZT 3.4 mm Waveguide Periodically poled lithium niobate (PPLN) enhanced-cavity absorption cell (ECAC) FA 12 T=LandC band EOM Extended cavity laser diode（ECLD） Linewidth< 500 kHz 1.5 mm Feedback Signal 10 10

Transmission spectrum of ECAC FSR: 400 MHz TEM00 ＋freqency modulation 520 kHz sideband sideband finesse = 770

Observed spectrum the Q(12) transition of 12CH4 in the n3 band

Mid-IR frequency control Er-OFC frep = 67 MHz n PZT pump (1.06 mm) Nd:YAG idler (3.4 mm) PPLN ECAC FA signal (1.5 mm) EOM Feedback Signal ECLD

Mid-IR frequency control Er-OFC frep = 67 MHz n PZT pump (1.06 mm) Nd:YAG idler (3.4 mm) PPLN ECAC FA signal (1.5 mm) EOM Feedback Signal ECLD

Mid-IR frequency control Synthesizer dsignal Er-OFC frep = 67 MHz n PZT pump (1.06 mm) Nd:YAG idler (3.4 mm) PPLN ECAC FA signal (1.5 mm) EOM Feedback Signal ECLD

A1-A2 splittings of 12CH3D The transition frequency list the n1 band Pressure: 0.1 – 0.4 Pa linewidth ~ 300 kHz Q(J = 3, K = 3) Q(J = 4, K = 3) Q(J = 5, K = 3) D = 0.29 MHz D = 0.92 MHz D = 3.84 MHz The transition frequency list Trans. Comp. n (MHz) the n1 band R (3, 3) A1 89 953 473.19 (0.09) Q (3, 3) A2 89 012 513.25 (0.06) 89 953 474.08 (0.14) 89 012 513.54 (0.05) the n4 band Q (4, 3) 89 023 546.89 (0.10) pP (4, 3) 89 344 491.73 ( - ) 89 023 547.81 (0.12) 89 344 505.05 ( - ) Q (5, 3) 89 037 747.43 ( - ) pP (3, 3) 89 579 142.15 ( - ) 89 037 751.27 ( - ) 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 A1-A2 pairs of 12CH3D using the optical frequency comb with a relative uncertainty of 10–9.

Thank you for your attention. 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 A1-A2 pairs of 12CH3D using the optical frequency comb with a relative uncertainty of 10–9. Thank you for your attention.

Appendix

CH3D A1-A2 splitting E K-l=2 K-l=3 K-l=4 Vibrational mode CH4 CH3D J=5 Vibrational excited state Td C3v J=4 n1 (DK=0) non-degenerate J=3 J=2 n3 Q(J=3,K=3) n4 (DK=±1) Doubly degenerate Triply degenerate J=5 Vibrational ground state J=4 J=3 E J=2 A1 K=2 K=3 K=4 A2

Tunable range R(8) P(12) CH4 ECLD (signal) Fiber Amp. (signal) PPLN DFG (idler) = Nd:YAG laser (pump) − tunable 1.5 mm ECLD (signal) mid-IR Freq. / THz 85 90 95 R(8) P(12) Phys. Rev. 48, 864 (1935) CH4 ECLD (signal) ECLD tunable range~30THz Fiber Amp. (signal) C-Band L-Band PPLN Wavenumber / cm-1 2900 3000 3100

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) The optical cavity is also used as an absorption cell. This picture shows an enhanced cavity absorption cell. A pair of mirrors are also optical windows to seal the sample gas. The mirror separation is 23.6 cm corresponding to the free spectral range of 636 MHz. The mirrors have the reflectivity of 99.0% and the transmittance of 0,7 %. The absorption length effectively increases 200 times. Because the field strength is enhanced 17 times at the anti nodes, the sub-Doppler spectral lines are observed easily. effective absorption length sensitivity optical field strength at antinodes ×198 ×139 ×17 From Sasada presentation in International Symposium on Molecular Spectroscopy 67th 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 We measure the DFG frequency using a frequency comb. This picture shows an Er fiber comb made by Kana Iwakuni for this measurement. The repetition rate is 65 MHz, and the comb spectrum is spread between 1.545 and 1.57 micrometer. It is broadened further using a nonlinear fiber. Then the comb spectrum covers 1 to 2 micrometer. From Sasada presentation in International Symposium on Molecular Spectroscopy 67th Meeting