International Symposium on Molecular Spectroscopy Ohio State University

International Symposium on Molecular Spectroscopy Ohio State University June, 18, 2012 Sub-Doppler resolution differencefrequency-generation infrared spectrometer with high sensitivity and wide tunability Keio University Sho Okubo, Hirotaka Nakayama, Kana Iwakuni, and Hiroyuki Sasada National Metrology Institute of Japan Hajime Inaba

Contents • 1 st talk by H. S. 1) A 3. 4 - m DFG spectrometer for sub-Doppler resolution spectroscopy 2) Frequency measurements using an optical comb • 2 nd talk by K. I. 1) Comb-referenced spectroscopy 2) Comb-referenced spectroscopy + Frequency modulation spectroscopy

Spectrometer at Keio 3. 4 m region 1) wide tunable range 2) high sensitivity 3) sub-Doppler resolution 4) accurate frequency measurement

DFG source 1. 06 μm 100 m. W Nd: YAG Line width < 2 k. Hz Tunability: 6 GHz 3. 4 μm 100 W Waveguide PPLN Efficiency: 10%/W Tunability: 2. 6 THz F. A. ECLD Line width < 500 k. Hz Tunability: 12 THz 1. 55 μm 100 m. W

Tunable Range Nd: YAG pump + tunable 1. 55 m signal 80 Frequency/ THz 85 Waveguide PPLN 90 95 single element ECLD Fiber amp. 2700 S-band 2800 C-band 2900 L-band 3000 3100 3200 Wavenumber/ cm– 1

Pound-Drever-Hall technique Si Brewster plate Nd: YAG 1. 06 μm ECLD 1. 55 μm PZT CS λ/4 optical cavity waveguide PPLN EOM 10 MHz FA fast In. Sb detector OBPF servo EOM: electro-optical modulator FA: fiber amplifier OBPF: optical band-pass filter CS: current source

Pound-Drever-Hall technique Si Brewster plate Nd: YAG 1. 06 μm ECLD 1. 55 μm PZT CS λ/4 optical cavity waveguide PPLN EOM 10 MHz FA fast In. Sb detector OBPF servo Bandwidth of the servo loop > 200 k. Hz Linewidth of ECLD 500 k. Hz < 25 k. Hz EOM: electro-optical modulator FA: fiber amplifier OBPF: optical band-pass filter CS: current source

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

Sub-Doppler resolution spectrum of methane CH 4 (0. 5 Pa) OBPF In. Sb detector P(7) F 2(2) PZT 1 ECAC PZT 2 sweep sig. PC HWHM: 300 k. Hz S/N: ~100

Sub-Doppler resolution spectrum of methane 3 f signal CH 4 (0. 5 Pa) OBPF In. Sb detector P(7) F 2(2) PZT 1 ECAC 3 k. Hz sweep sig. PZT 2 function gen. 9 k. Hz PC 3 f lock-in amplifier

Sub-Doppler resolution spectrum of methane 3 f signal CH 4 (0. 5 Pa) OBPF In. Sb detector P(7) F 2(2) PZT 1 ECAC 3 k. Hz PZT 2 function gen. servo 9 k. Hz 3 f lock-in amplifier lock

DFG frequency measurement using a frequency comb Er fiber comb rep. freq. : 65 MHz 1. 545 ~ 1. 57 m A nonlinear fiber broadens comb spectrum. 1. 0 ~ 2. 0 m

DFG frequency measurement using a frequency comb mode number difference Dn f. CEO 1. 55 m fbeat ・・・ 1. 06 m fbeat ・・・ ≈≈ ≈ 0 frep comb mode ECLD 1. 55 μm n. DFG = Dn × frep + f 1. 06 m – f 1. 55 m beat f. CEO free n Nd: YAG 1. 06 μm Dn ~1. 32 × 106 frep~ 65 MHz Dfrep~ 0. 1 m. Hz

Frequency Measurement ECAC

Result 56 lines from 88. 2 to 90. 5 THz transitio sym. n sp. measured fr. / k. Hz stnd. dev. / k. Hz previous works / k. Hz discrepancy / k. Hz F 2(2) 88 236 103 826. 8 5. 1 88 236 106. 08 x 103 – 2. 26 x 103 P(7) * A 2 88 238 845 036. 0 2. 5 88 238 846. 25 x 103 – 1. 21 x 103 F 1(2) 88 368 863 359. 9 2. 5 88 368 863 353. 9(94) 6. 0 P(6) ** A 1 88 369 735 528. 0 3. 3 88 369 732. 91 x 103 2. 62 x 103 P(7) E 88 373 149 029. 9 2. 4 88 373 149 028. 55(20) 1. 3 P(7) F 2(2) 88 376 181 600. 3 2. 1 88 376 181 600. 5(20) – 0. 2 … … … Q(1) F 1 90 502 080 707. 9 2. 1 90 502 079. 49 x 103 P(7) * forbidden transition ** 13 CH 4 CIPM recommendation Relative uncertanty: 2 x 10– 11 … P(7) * 1. 21 x 103

Energy Combination difference v 3=1 allowed tr. A 1 JR=66 F 1 ≈ (1) forbidden tr. 2 195 510 271. 6(32) k. Hz *FT-IR JR=77 2 195. 514 GHz F 2(2) 2 195 965 229. 8(60) k. Hz A 2 ground JR=66 A 2 F 2 (1) *FT-IR 2 195 965. 36(140) MHz Optics Express, 19, 23878 (2011) *Champion, et. al. JMS, 133, 244 (1989)

Conclusion 1. A 3. 4 m tunable spectrometer for sub-Doppler resolution spectroscopy has been developed. ２. Absolute frequencies of 56 saturated absorption transitions of CH 4 have been measured using a fiber comb with a relative uncertainty of 10– 11 level.

Acknowledgement S. Okubo H. Nakayama K. Iwakuni Money JST (Japan Science and Technology Agency) Ministry of Education, Culture, Sports, Science and Technology H. Inaba H. Sasada