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Vernier spectroscopy A broad band cavity enhanced spectroscopy method with cw laser resolution

Vernier spectroscopy A broad band cavity enhanced spectroscopy method with cw laser resolution. Christoph Gohle , Albert Schliesser, Björn Stein, Akira Ozawa, Jens Rauschenberger, Thomas Udem, Theodor W. Hänsch. Outline. Cavity enhanced spectroscopy Broad band cavity enhanced methods

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Vernier spectroscopy A broad band cavity enhanced spectroscopy method with cw laser resolution

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  1. Vernier spectroscopyA broad band cavity enhanced spectroscopy method with cw laser resolution Christoph Gohle, Albert Schliesser, Björn Stein, Akira Ozawa, Jens Rauschenberger, Thomas Udem, Theodor W. Hänsch

  2. Outline • Cavity enhanced spectroscopy • Broad band cavity enhanced methods • Adding phase sensitivity • The optical vernier • Conclusion

  3. Fabry perot resonators light source

  4. … enhance sensitivity • Cavity enhanced absorption spectroscopy (CEAS) • Increased interaction length ( ), i.e. sensitivity • Cavity ring down (CRD) • Rejects source noise

  5. Broad band CEAS R • Broadband input source • Low transm. (1 ) • Sens. gain ~ • Frequency comb input* • Sens. gain ~ • Ringdown method using streak camera possible** • Narrow probe frequencies (if resolved) Spectrometer BB-Source (S) T S R T S R T *Gherman, T. & Romanini, D., Optics Express, 10, 1033-1042 (2002) **Thorpe, M.J. et al., Science, 311, 1595-1599 , 2006

  6. laser frequency comb Comb matching • In general r and 'will be complicated functions of ! passive cavity … and the two combs can not be lined up

  7. Adding phase sensitivity to CEAS

  8. Moiré pattern

  9. Scanning the comb

  10. With bad resolution

  11. Extract the information

  12. Some results • Yields both loss and dispersion • Frequency comb is a “dispersion free” reference • Sensitivity ~ Finesse • Demonstrated sens.: 10-6/cm, 1fs2@2THz resolution • Resolution limited by spectrometer • May be useful for survey trace gas detection A. Schliesser et al., Optics Express, 14, 5975-5983 (2006)

  13. What about the comb? The optical Vernier

  14. Idea n= nr +CE n+1 n c • Requirements: • Finesse > m • m r > spec. resolution

  15. Model Close to a spot (k,l) the contributions of all other frequencies can be neglected: … 3 2 1 k=0 l=0 1 2 3 … Scanning length: Sample absorbtion: Y calibration: Identified comb modes: k+m,l=k,l+1!2=(yk+m,l-yk,l+1)/c Assuming: n(k,l+1)=1 Steady state condition: one line width in more than one lifetime: Scanspeed < ( FSR)2/Finesse2

  16. Implementation CCD grating lens Air Resonator Finesse ~ 3000

  17. Data • Single scan (10ms) • Blue box: unique data • Red boxes: identified features • Gaussian PSF much larger than airy ! Brightness~Integral of airy

  18. Results* • Absorbtion: • Noisefloor • < 10-5/cm (100 Hz)1/2= • < 10-6/cm Hz1/2 • > 4 THz bandwidth • 1 GHz sampling (>4000 res. • Datapoints in 10 ms) • Quantitative agreement in • Amplitude and Frequency • to HITRAN** database • Phase: • *looks good (dispersive features) • *not optimized for good phase sensitivity O2 A-Band No Free parameters (except frequency offset, which was not measured here) * To be published in the near future ** Rothman, L. S. et al., J. Quant. Spect. Rad. Trans., 96, 139-204 (2005)

  19. Conclusions • Pro’s • Comb resolution (i.e. Hz level if desired) • Fast (partly parallel acquisition) • Simple • Large bandwidth • Amplitude AND Phase sensitivity • Self calibrating • Reproducibility limited by primary frequency standard only • Subdoppler methods easily conceivable • Con • Transmitted power ~ 1/Finesse • Sensitivity Gain ~ Finesse1/2 only (for shot noise limited detection) Thank you for your attention!

  20. Thanks

  21. Optical Resonators

  22. … enhance nonlinear conversion • Pc=F/ • Output power grows with finesse2 or higher! • Example: • SHG 560nm->280nm • 900mW driving power • 20% conversion: 900mW->200mW

  23. Fs-Frequency CombSpectroscopy

  24. f=p/2 f=p f=0 - cosine-pulse cosine-pulse sine-pulse +¥ S Am e-imwrt-iwct = m=-¥ I(w) 1 wc Basics E(t)=A(t)eiwct !n = n!r +!CE !CE=ÁCE/T • Optical clockwork, connects optical and radio frequency • 106 phaselocked cw-lasers for high accuracy spectroscopy

  25. 3,000,000 modes with 0.3 mW power spectrosopy with a single mode hard but possible: V.Gerginov et al. Optics Letters, 30, 1734 (2005) 1 Spectroscopy with Combs I(1) 300 THz band width and 100 MHz mode spacing. 1 300 THz

  26. all modes contribute. like a cw laser. Two photon spectroscopy I(1) 1 Pionieered by: Ye.V.Baklanov, V.P.Chebotayev, Appl. Phys 12, 97 (1977) and M.J.Snadden, A.S.Bell, E.Riis, A.I.Ferguson, Opt. Comm. 125, 70 (1996)

  27. … recent results • Cs 6S-8S two photon transition Peter Fendel et al., (… almost submitted) Similar method: A. Marian et al, PRL, 95, 023001 (2005)

  28. Comb Spectroscopy? • Fs-frequency combs combine • High peak power of a fs-laser • High spectral quality of cw-laser • Good for applications where there are no continous lasers available • First impressive steps: S. Witte et al., Science, 307, 400 (2005) • Highly nonlinear spectroscopy?

  29. High Accuracy at high Energy? • Planck Scale • Frequency measurements • Optical atomic clocks

  30. Hydrogen likeHe+ • He+ is an ion • Can be trapped and cooled • Long interaction times • Reduced (eliminated) Doppler broadening & shift • Control over other systematics • Reduced (no) recoil

  31. Optical Resonators for Frequency combs

  32. Fs-Buildup resonator • Enhance entire frequency comb • Produce XUV frequency comb • Via high order harmonic generation

  33. intracavity: • Pavg = 38 W • = 28 fs • Ppeak = 12 MW seed laser: Pavg = 700 mW t = 20 fs Ppeak = 300 kW x55 x40 Real resonator

  34. XUV Output Output: 10nW C. Gohle et al., Nature, 436, 234 (2005) R. J. Jones et al., PRL, 94, 193201 (2005)

  35. ? High Harmonics Hierarchy

  36. Coherence (of the 3rd harm.) C. Gohle et al., Nature, 436, 234 (2005) R. J. Jones et al., PRL, 94, 193201 (2005)

  37. intracavity: • Pavg = 38 W • = 28 fs • Ppeak = 12 MW seed laser: Pavg = 700 mW t = 20 fs Ppeak = 300 kW x55 x40 Real resonator Lost a factor of 2!

  38. Complete resonator characterization With high sensitivity

  39. Experimental Setup f-to-2f interferometer 2x piezo-actuated mirrors photodiode+counter silica wedges in laser

  40. Data from an “empty” cavity A. Schliesser et al., Optics Express, 14, 5975-5983 (2006)

  41. Result • to cover entire spectrum, perform multiple measurements with different lock points (here 780.5 and 801.0 nm) • wide bandwidth: 150nm • „wiggles“ at 760 and 825 nm? • empirical reproducibility: 1fs² in GDD (1.6 THz BW) and 4*10-4 in r

  42. Verification Sapphire plate @ Brewster‘s angle 2 identical high-reflectivity dielectric stack mirrors Measurement of cavity before and after insertion of additional components yields individual contributions.

  43. Empty cavity? • to cover entire spectrum, perform multiple measurements with different lock points (here 780.5 and 801.0 nm) • wide bandwidth: 150nm • „wiggles“ at 760 and 825 nm? • empirical reproducibility: 1fs² in GDD (1.6 THz BW) and 4*10-4 in r

  44. Comparison with simulation HITRAN data, convoluted with spectrometer ILS and multiplied with 0.98 HITRAN data (RT, 1atm, 21%) Phase excursion ~10-3 rad (on top of a simple quadratic phasedep.) n ~ 5 £ 10-11 L. S. Rothman et al., The HITRAN 2004 molecular spectroscopic database," J. Quant. Spect. Rad. Trans. 96, 139-204, (2005)

  45. Air filled resonator! O2 H2O • to cover entire spectrum, perform multiple measurements with different lock points (here 780.5 and 801.0 nm) • wide bandwidth: 150nm • „wiggles“ at 760 and 825 nm? • empirical reproducibility: 1fs² in GDD (1.6 THz BW) and 4*10-4 in r

  46. Outlook

  47. Input: 120nJ, 30fs, 4MW peak x 100 12µJ, 30fs, 400MW peak High power XUV comb seed laser: 10 MHz CPO (120 nJ; 30 fs) enhancement cavity: vacuum setup (3.5 m length)

  48. Cooling laser system

  49. Helium Spectroscopy

  50. … provide stable references • Narrow Markers in Frequency space • If high finesse • High stability • ~10-14 @ 1 s • Few Hz linewidth @ 1 PHz

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