Vernier spectroscopy A broad band cavity enhanced spectroscopy

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 Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 1

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

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

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

Broad band CEAS R BB-Source (S) T • Broadband input source – Low transm. (1 – Sens. gain ~ ) Spectrometer S R • Frequency comb input* – Sens. gain ~ – Ringdown method using streak camera possible** – Narrow probe frequencies (if resolved) T S R T *Gherman, T. & Romanini, D. , Optics Express, 1033 -1042 (2002) **Thorpe, M. J. et al. , Science, 311, 1595 -1599 , 2006 Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 5

Comb matching laser frequency comb passive cavity • In general r and ' will be complicated functions of ! … and the two combs can not be lined up Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 6

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

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

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

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

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

Some results • Yields both loss and dispersion • Frequency comb is a “dispersion free” reference • Sensitivity ~ Finesse • Demonstrated sens. : 10 -6/cm, 1 fs 2@2 THz resolution • Resolution limited by spectrometer • May be useful for survey trace gas detection A. Schliesser et al. , Optics Express, 14, 5975 -5983 (2006) Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 12

What about the comb? The optical Vernier Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 13

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

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/Finesse 2 Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 15

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

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

Results* Absorbtion: • Noisefloor O 2 A-Band t p e c as < 10 /cm Hz x (e h w • > 4 THz bandwidth s ic r 1 GHz sampling (>4000 res. te wh e Datapoints in 10 ms) m et, ) a r fs re a • Quantitative agreement in p of he e y Amplitude and Frequency d e e r c F en ur to HITRAN** database o s N equ ea r m f t Phase: o n *looks good (dispersive features) < 10 -5/cm (100 Hz)1/2= -6 1/2 *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) Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 18

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

Thanks Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 20

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

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

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

Basics f=0 f= /2 cosine-pulse f= +¥ - cosine-pulse E(t)=A(t)ei ct = S Am e-im rt-i ct m=-¥ !n = n!r + !CE=ÁCE/T I( ) c 1 • Optical clockwork, connects optical and radio frequency • 106 phaselocked cw-lasers for high accuracy spectroscopy Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 24

Spectroscopy with Combs 300 THz band width and 100 MHz mode spacing. I(1) 300 THz 1 3, 000 modes with 0. 3 m. W power spectrosopy with a single mode hard but possible: V. Gerginov et al. Optics Letters, 30, 1734 (2005) 1 Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 25

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

… recent results • Cs 6 S-8 S two photon transition Peter Fendel et al. , (… almost submitted) Similar method: A. Marian et al, PRL, 95, 023001 (2005) Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 27

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? Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 28

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

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 Hydrogen Z - Scaling Helium Energy levels 1 S-2 S: 10 e. V Z 2 40 e. V ~ 60 nm Lamb shift 1 S: 8 GHz Z 4 128 GHz Z 6 64 times stronger Unverified QED correc. Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 30

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

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

Real resonator intracavity: seed laser: Pavg = t = Ppeak = Snowbird, 2007 700 m. W 20 fs 300 k. W x 55 x 40 Pavg = t = Ppeak = 38 W 28 fs 12 MW Broad band cavity enhanced Vernier spectroscopy 33

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

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

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

Real resonator intracavity: seed laser: Pavg = t = Ppeak = Snowbird, 2007 700 m. W 20 fs 300 k. W x 55 x 40 Pavg = t = Ppeak = 38 W 28 fs 12 MW Broad band cavity enhanced Vernier spectroscopy 37

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

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

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

Result • to cover entire spectrum, perform multiple measurements with different lock points (here 780. 5 and 801. 0 nm) • wide bandwidth: 150 nm • „wiggles“ at 760 and 825 nm? • empirical reproducibility: 1 fs² in GDD (1. 6 THz BW) and 4*10 -4 in r Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 41

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

Empty cavity? • to cover entire spectrum, perform multiple measurements with different lock points (here 780. 5 and 801. 0 nm) • wide bandwidth: 150 nm • „wiggles“ at 760 and 825 nm? • empirical reproducibility: 1 fs² in GDD (1. 6 THz BW) and 4*10 -4 in r Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 43

Comparison with simulation HITRAN data, convoluted with spectrometer ILS and multiplied with 0. 98 HITRAN data (RT, 1 atm, 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, 139204, (2005) Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 44

Air filled resonator! O 2 H 2 O • to cover entire spectrum, perform multiple measurements with different lock points (here 780. 5 and 801. 0 nm) • wide bandwidth: 150 nm • „wiggles“ at 760 and 825 nm? • empirical reproducibility: 1 fs² in GDD (1. 6 THz BW) and 4*10 -4 in r Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 45

Outlook Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 46

High power XUV comb seed laser: 10 MHz CPO (120 n. J; 30 fs) enhancement cavity: vacuum setup (3. 5 m length) Input: 120 n. J, 30 fs, 4 MW peak x 100 12µJ, 30 fs, 400 MW peak Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 47

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

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

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

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

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

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

O 2? 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) Snowbird, 2007 J. Zhang et al. : Precision measurement of the refractive index of air with frequency combs. Opt. Lett. 30, 3314 (2005) Broad band cavity enhanced Vernier spectroscopy 54