1. Sub-Doppler Resolution Spectroscopy of the fundamental band of HCl with an Optical Frequency Comb ○ K. Iwakuni, M. Abe, and H. Sasada Department of Physics, faculty of Science and Technology, Keio University, Japan 1 68 th. International Symposium on Molecular Spectroscopy June 18, 2013 The Ohio State University TJ01

2. 2 He-Ne laserQuantum cascade laser OPOFT-IRDFG linewidth tunability power Light sources for spectroscopy in the mid-infrared frequency region He-Ne laser Quantum cascade laser Optical parametric oscillator FT-IR

3. Outline 1. DFG spectrometer 2. A new technique to improve the sensitivity of the spectrometer 3. Absolute frequency measurements of the fundamental vibration band of HCl with an OFC 3

4. Outline 4 1. DFG spectrometer 2. A new technique to improve the sensitivity of the spectrometer 3. Absolute frequency measurements of the fundamental vibration band of HCl with an OFC

5. 5 50 mW Basic setup of DFG(Difference-Frequency Generation) spectroscopy InSb detector Nd:YAG laser 1.06 μm ECLD 1.55 μm Wavelength conversion module cavity enhanced absorption cell 3.4 μm Pump wave Signal wave

6. 6 50 mW ECLD 1.55 μm Wavelength conversion module cavity enhanced absorption cell Basic setup of DFG spectroscopy 3.4 μm Nd:YAG laser  1.06 μm  linewidth; 1 kHz InSb detector Pump wave Signal wave

7. 7 50 mW Wavelength conversion module cavity enhanced absorption cell Basic setup of DFG spectroscopy 3.4 μm Nd:YAG laser  1.06 μm  linewidth; 1 kHz extended cavity laser diode (ECLD) + fiber amp.  1.55 μm  linewidth; 500 kHz  frequency tuning range; 400 cm -1 InSb detector Pump wave Signal wave

8. 8 50 mW cavity enhanced absorption cell 8 cm Basic setup of DFG spectroscopy 3.4 μm 250 μW Nd:YAG laser  1.06 μm  linewidth; 1 kHz ECLD + fiber amp.  1.55 μm  linewidth; 500 kHz  frequency tuning range; 400 cm -1 waveguide-type periodically polarized LiNbO 3 (PPLN) with a pigtail  conversion efficiency; 10%/W  frequency tuning range; 2900 - 3050 cm -1 InSb detector DFG light source  3.4 μm  linewidth; ECLD limit Pump wave Signal wave

9. 9 50 mW Nd:YAG laser  1.06 μm  linewidth; 1 kHz ECLD + fiber amp.  1.55 μm  linewidth; 500 kHz  frequency tuning range; 400 cm -1 8 cm PZT mirror cavity enhanced absorption cell (CEAC)  cavity length; 25 cm  mirror reflectance; 99.0 %  effective absorption length; 198 times  electric field; 17 times Basic setup of DFG spectroscopy 3.4 μm 250 μW InSb detector Pump wave Signal wave waveguide-type periodically polarized LiNbO 3 (PPLN) with a pigtail  conversion efficiency; 10%/W  frequency tuning range; 2900 - 3050 cm -1 DFG light source  3.4 μm  linewidth; ECLD limit

10. 10 50 mW Nd:YAG laser  1.06 μm  linewidth; 1 kHz ECLD + fiber amp.  1.55 μm  linewidth; 500 kHz  frequency tuning range; 400 cm –1 8 cm DFG 3.4 μm light source  500 μW  linewidth; 50 kHz PZT mirror cavity enhanced absorption cell (CEAC)  cavity length; 25 cm  mirror reflectance; 99.0 %  effective absorption length; 198 times  electric field; 17 times Basic setup of DFG spectroscopy 3.4 μm 250 μW InSb detector Narrow linewidth Wide tunable range Enough power for saturation spectroscopy Pump wave Signal wave waveguide-type periodically polarized LiNbO 3 (PPLN) with a pigtail  conversion efficiency; 10%/W  frequency tuning range; 2900 - 3050 cm –1

11. Outline 11 1. DFG spectrometer 2. A new technique to improve the sensitivity of the spectrometer 3. Absolute frequency measurements of the fundamental vibration band of HCl with an OFC

12. ν f rep ECLD Nd:YAG laser 1.06 μm 1.55 μm PPLN 3.4 μm CEAC InSb detector synthesizer absolute frequency Comb-referenced DFG spectrometer Lamb dip OFC TAI The linewidth of the idler wave (HWHM); 25 kHz sweep condition  0.01 Hz/step

13. The method of absolute frequency measurement of the DFG light 13 ν idler = Δn f rep + ( f beat1 – f beat2 ) + f ceo  f ceo Δn = n 1 – n 2 ≈ 1, 310, 000 ν pump = f ceo + n 1 f rep + f beat1 ν signal = f ceo + n 2 f rep + f beat2 ν signal pump f beat2 f beat1 ν n1 = f ceo + n 1 f rep νn2νn2 f rep ・・・ 0 Hz f ceo f rep = 67 MHz ν idler

14. ν f rep ECLD Nd:YAG laser 1.06 μm 1.55 μm PPLN 3.4 μm CEAC InSb detector synthesizer absolute frequency sweep condition  0.01 Hz/step Comb-referenced DFG spectrometer Lamb dip OFC TAI The linewidth of the idler wave (HWHM); 25 kHz

15. ν f rep ECLD Nd:YAG laser 1.06 μm 1.55 μm PPLN 3.4 μm CEAC InSb detector synthesizer absolute frequency sweep condition  0.01 Hz/step Comb-referenced DFG spectrometer Lamb dip OFC TAI The linewidth of the idler wave (HWHM); 25 kHz The S/N should be increased by accumulating spectrum data for a long time. However, the power fluctuation reduces the sensitivity. A wavelength-modulation technique is applied.

16. 1.55 μm Wavelength-modulation spectrometer ν f rep ECLD Nd:YAG laser 1.06 μm PPLN 3.4 μm CEAC InSb detector synthesizer absolute frequency signal lock in amp. modulation 3 kHz demodulation 3 kHz 21.4 MHz TAI

17. Outline 17 1. DFG spectrometer 2. A new technique to improve the sensitivity of the spectrometer 3. Absolute frequency measurements of the fundamental vibration band of HCl with an OFC

18. Sub-Doppler resolution spectrum of H 35 Cl the fundamental vibration band v = 1, J’ = 1 v = 0, J ”= 0 F’ = 1/2 F’ = 5/2 F’ = 3/2 F” = 3/2 R(0) measurement conditions ●sweep step; 0.01 Hz/step (13.1 kHz/step in the mid-infrared frequency) ●averaged over 20 frequency sweeps ●sweep time; 20 ms/step ●pressure; a few mTorr ●measurement time; 20min. ●linewidth (HWHM); 230 kHz measurement conditions ●sweep step; 0.01 Hz/step (13.1 kHz/step in the mid-infrared frequency) ●averaged over 20 frequency sweeps ●sweep time; 20 ms/step ●pressure; a few mTorr ●measurement time; 20min. ●linewidth (HWHM); 230 kHz F’ = 3/2 F’ = 5/2 F’ = 1/2 * * * *cross-over resonance the energy levels associated with the R (0) transition

19. Absolute frequency measurement of the R(0) transition *cross-over resonance F’F’Measured frequency / kHzUncertainty / kHz 3/287 127 059 4365 5/287 127 076 9056 1/287 127 090 5236 F’ = 3/2 F’ = 1/2 * * * F’ = 5/2 Data Fitting HITRAN data87 127 083. 36 MHz HITRAN data weighted mean of the present data

20. R(2) R(1) * *cross-over resonance * ** * * * * * Sub-Doppler resolution spectrum of H 35 Cl ☆ ΔF ＝ ＋ 1 ☆ ΔF ＝ 0 ☆ ☆ ☆ ☆ ☆ ☆ ☆ ☆ ☆ ☆ ☆☆ ☆ * *

21. R(2) R(1) * *cross-over resonance * ** * * * * * Sub-Doppler resolution spectrum of H 35 Cl ☆ ΔF ＝ ＋ 1 ☆ ΔF ＝ 0 ☆ ☆ ☆ ☆ ☆ ☆ ☆ ☆ ☆ ☆ ☆☆ ☆ * *

22. Absolute frequency measurement of the R(1) and R(2) transitions 22 F” → F’Measured frequency / kHzFitting uncertainty / kHz ☆ 1/2 → 3/287 716 141 8564 ☆ 5/2 → 5/287 716 142 9292 ☆ 1/2 → 1/2 87 716 159 101.11.0 ☆ 3/2 → 5/2 87 716 159 871.21.6 ☆ 5/2 → 7/287 716 160 4683 ☆ 3/2 → 3/287 716 172 2003 R(1) R(2) F” → F’Measured frequency / kHzFitting uncertainty / kHz ☆ 7/2 → 7/288 286 255 3116 ☆ 3/2 → 5/288 286 268 3513 ☆ 1/2 → 3/288 286 268 7104 ☆ 5/2 → 7/288 286 272 4103 ☆ 7/2 → 9/2 88 286 272 913.01.5 ☆ 5/2 → 5/2 88 286 280 289.61.4 ☆ 3/2 → 3/288 286 285 5354

23. Summary We developed highly sensitive comb-referenced DFG spectrometer. We recorded sub-Doppler resolution spectrum of the fundamental vibration band of H 35 Cl and H 37 Cl, and determined the transition frequencies with a few kHz uncertainty. 23 Acknowledgments This research is financially supported by Grand-in-Aid for Scientific Research (A) and the Photon Frontier Network Program of the Ministry of Education, Culture, Sports, Science and Technology, Japan. At TJ03, Masashi Abe will talk about the frequency measurements of CH 4 using this technique.

24. Setup of the OFC f rep = 67 MHz spectral range; 1 μm– 2 μm spectrum analyzer InGaAs-detector EDF: Er-doped fiber TEC: temperature controller HNLF: highly non-linear fiber f rep f ceo f rep - f ceo 10 dB/div

25. pressure #1 pressure #2 pressure #3 pressure #4 F” = 3/2 F” = 5/2 F” = 1/2 *1 *2 *3 Pressure dependent intensity of the cross-over resonances *1 *2 *3 *1 *2 *3 *1 *2 *3 R (0) low normalized intensity #1 #2 #3 #4 high pressure *1 *2 *3 F”= 1/2 3/2 5/2

26. Stabilization the Nd:YAG laser to the OFC Stabilization the Nd:YAG laser to the OFC Span: 500 kHz RBW: 1 kHz VBW: 1 kHz 50 kHz 30 dB The beat signal between the Nd:YAG laser and the OFC InGaAs detector Nd:YAG synthesizer OFC servo PZT,current 10 dB before the improvement after the improvement

27. Tunable Range 85909580 ECLD Frequency/ THz C-band L-band Fiber amp. 2900300031002800 27003200 Wavenumber/ cm –1 Waveguide PPLN single element S-band Nd:YAG pump + tunable 1.55  m signal

28. Enhanced-cavity absorption cell effective absorption length sensitivityoptical field strength at antinodes ×198×139×17 mirror separation (FSR) reflectivity (transmittance) finnesse (FWHM) 23.6 cm (636 MHz)99.0% (0.7%)300 (2.1 MHz) Mirrors are optical windows. bellows PZT

29. DFG frequency measurement using a frequency comb Er fiber comb rep. rate: 65 MHz 1.545 ~ 1.57  m Er fiber comb rep. rate: 65 MHz 1.545 ~ 1.57  m A nonlinear fiber broadens comb spectrum. 1.0 ~ 2.0  m A nonlinear fiber broadens comb spectrum. 1.0 ~ 2.0  m