1. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 1 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

2. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 2 Outline Cavity enhanced spectroscopy Broad band cavity enhanced methods Adding phase sensitivity The optical vernier Conclusion

3. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 3 Fabry perot resonators light source

4. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 4 … enhance sensitivity Cavity enhanced absorption spectroscopy (CEAS) –Increased interaction length ( ), i.e. sensitivity Cavity ring down (CRD) –Rejects source noise

5. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 5 Broad band CEAS Broadband input source –Low transm. (1 ) –Sens. gain ~ Frequency comb input* –Sens. gain ~ –Ringdown method using streak camera possible** –Narrow probe frequencies (if resolved) BB-Source (S) Spectrometer S R R T 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. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 6 Comb matching In general r and ' will be complicated functions of ! laser frequency comb passive cavity … and the two combs can not be lined up

7. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 7 Adding phase sensitivity to CEAS

8. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 8 Moiré pattern

9. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 9 Scanning the comb

10. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 10 With bad resolution

11. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 11 Extract the information

12. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 12 Some results Yields both loss and dispersion Frequency comb is a “dispersion free” reference Sensitivity ~ Finesse Demonstrated sens.: 10 -6 /cm, 1fs 2 @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. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 13 What about the comb? The optical Vernier

14. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 14 Idea  n+1 nn  n = n  r +  CE cc Requirements: Finesse > m m  r > spec. resolution

15. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 15 Model k=0 1 … 2 l=0 1 2 3 … 3 Close to a spot ( k, l ) the contributions of all other frequencies can be neglected: Y calibration: Identified comb modes:  k+m,l =  k,l+1 ! 2  =(y k+m,l -y k,l+1 )  /c Scanning length: Assuming: n(  k,l+1 )=1 Sample absorbtion: Steady state condition: one line width in more than one lifetime: Scanspeed < (  FSR) 2 /Finesse 2

16. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 16 Implementation Air Resonator Finesse ~ 3000 grating CCD lens

17. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 17 Data Single scan (10ms) Blue box: unique data Red boxes: identified features Gaussian PSF much larger than airy ! Brightness~Int egral of airy

18. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 18 Results* Absorbtion: Noisefloor < 10 -5 /cm (100 Hz) 1/2 = < 10 -6 /cm Hz 1/2 > 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 * To be published in the near future ** Rothman, L. S. et al., J. Quant. Spect. Rad. Trans., 96, 139-204 (2005) No Free parameters (except frequency offset, which was not measured here) O 2 A-Band

19. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 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 ~ Finesse 1/2 only (for shot noise limited detection) Thank you for your attention!

20. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 20 Thanks

21. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 21 Optical Resonators

22. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 22 … enhance nonlinear conversion P c =F/  –Output power grows with finesse 2 or higher! Example: –SHG 560nm->280nm –900mW driving power –20% conversion: 900mW->200mW

23. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 23 Fs-Frequency Comb Spectroscopy

24. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 24 Basics Optical clockwork, connects optical and radio frequency 10 6 phaselocked cw-lasers for high accuracy spectroscopy  0  cosine-pulsesine-pulse - cosine-pulse  /2 ! n = n! r + ! CE ! CE =Á CE /T I()I() 1 cc E(t)=A(t)e i  c t = ++ m=-m=-  A m e -im  r t-i  c t

25. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 25 Spectroscopy with Combs 300 THz I(1)I(1) 1 300 THz band width and 100 MHz mode spacing. 3,000,000 modes with 0.3  W power 1 spectrosopy with a single mode hard but possible: V.Gerginov et al. Optics Letters, 30, 1734 (2005)

26. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 26 Two photon spectroscopy 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) I(1)I(1) 1 all modes contribute.like a cw laser.

27. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 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. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 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. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 29 High Accuracy at high Energy? Planck Scale Frequency measurements –Optical atomic clocks

30. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 30 Hydrogen like He + He + is an ion –Can be trapped and cooled –Long interaction times –Reduced (eliminated) Doppler broadening & shift –Control over other systematics –Reduced (no) recoil HydrogenZ - ScalingHelium Energy levels1S-2S: 10eVZ2Z2 40 eV ~ 60 nm Lamb shift1S: 8GHzZ4Z4 128 GHz Unverified QED correc.Z6Z6 64 times stronger

31. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 31 Optical Resonators for Frequency combs

32. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 32 Fs-Buildup resonator Enhance entire frequency comb Produce XUV frequency comb –Via high order harmonic generation

33. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 33 Real resonator seed laser: P avg =700 mW  =20 fs P peak =300 kW intracavity: P avg = 38 W  = 28 fs P peak =12 MW x55 x40

34. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 34 XUV Output C. Gohle et al., Nature, 436, 234 (2005) R. J. Jones et al., PRL, 94, 193201 (2005)

35. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 35 High Harmonics Hierarchy

36. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 36 Coherence (of the 3rd harm.) C. Gohle et al., Nature, 436, 234 (2005) R. J. Jones et al., PRL, 94, 193201 (2005)

37. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 37 Real resonator seed laser: P avg =700 mW  =20 fs P peak =300 kW intracavity: P avg = 38 W  = 28 fs P peak =12 MW x55 x40

38. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 38 Complete resonator characterization With high sensitivity

39. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 39 f-to-2f interferometer photodiode+counter silica wedges in laser 2x piezo-actuated mirrors Experimental Setup

40. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 40 Data from an “empty ” cavity A. Schliesser et al., Optics Express, 14, 5975-5983 (2006)

41. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 41 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 Result

42. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 42 Measurement of cavity before and after insertion of additional components yields individual contributions. Sapphire plate @ Brewster‘s angle 2 identical high- reflectivity dielectric stack mirrors Verification

43. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 43 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 Empty cavity?

44. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 44 L. S. Rothman et al., The HITRAN 2004 molecular spectroscopic database," J. Quant. Spect. Rad. Trans. 96, 139- 204, (2005) HITRAN data (RT, 1atm, 21%) HITRAN data, convoluted with spectrometer ILS and multiplied with 0.98 Comparison with simulation Phase excursion ~10 -3 rad (on top of a simple quadratic phasedep.)  n ~ 5 £ 10 -11

45. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 45 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 O2O2 H2OH2O Air filled resonator!

46. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 46 Outlook

47. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 47 High power XUV comb seed laser: 10 MHz CPO (120 nJ; 30 fs) enhancement cavity: vacuum setup (3.5 m length) Input: 120nJ, 30fs, 4MW peak 12µJ, 30fs, 400MW peak x 100

48. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 48 Cooling laser system

49. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 49 Helium Spectroscopy

50. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 50 … provide stable references Narrow Markers in Frequency space –If high finesse High stability –~10 -14 @ 1 s –Few Hz linewidth @ 1 PHz

51. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 51 Experimental Setup

52. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 52 Mutual fluctuations of laser/high-F cavity length make a lock at one frequency necessary. Active feedback keeps both on resonance at  lock : Laser Lock

53. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 53 Analysis when locked

54. Snowbird, 2007Broad band cavity enhanced Vernier spectroscopy 54 O2?O2? Air: 21% Oxygen Molecular oxygen „A“ band ~760 nm M. J. Thorpe et al.: Precise measurements of optical cavity dispersion and mirror coating properties via femtosecond combs. Opt. Exp. 13, 882 (2005) J. Zhang et al.: Precision measurement of the refractive index of air with frequency combs. Opt. Lett. 30, 3314 (2005)