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

Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 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


Outline• Cavity enhanced spectroscopy• Broad band cavity enhanced

methods• Adding phase sensitivity• The optical vernier• Conclusion


Fabry perot resonatorslight source


… enhance sensitivity• Cavity enhanced

absorption spectroscopy (CEAS)– Increased

interaction length ( ), i.e. sensitivity

• Cavity ring down (CRD)– Rejects source

noise

/ ¡ r


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

SR

RT

T

SRT

pF

F

*Gherman, T. & Romanini, D., Optics Express, 10, 1033-1042 (2002) **Thorpe, M.J. et al., Science, 311, 1595-1599 , 2006


Comb matching• In general r and ' will be complicated functions of !

lase

r fre

quen

cy

com

b

pass

ive

cavi

ty

… and the two combs can not be lined up


Adding phase sensitivity to CEAS


' Moiré pattern


Scanning the comb


Á !

/ ¡ r !

With bad resolution


Áes E !

Extract the information

r ! Á !


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)


What about the comb?The optical Vernier


Idean+1n

n = n r +CE

c

Requirements:• Finesse > m• m r > spec. resolution

! rmn


Model

k=01

…

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=(yk+m,l-yk,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/Finesse2


Implementation

Air

Resonator Finesse ~ 3000

grating

CCD

lens


Data• Single scan

(10ms)

• Blue box: unique data

• Red boxes: identified features

• Gaussian PSF much larger than airy ! Brightness~Integral of airy


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

* To be published in the near future** Rothman, L. S. et al., J. Quant. Spect. Rad. Trans., 96, 139-204 (2005)

No Free

parameter

s (exce

pt

frequency

offset, w

hich was

not meas

ured here

)

O2 A-Band


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!


Thanks


Optical Resonators


… enhance nonlinear conversion

• Pc=F/– Output power grows

with finesse2 or higher!

• Example:– SHG 560nm->280nm– 900mW driving

power

– 20% conversion: 900mW->200mW


Fs-Frequency CombSpectroscopy


Basics

• Optical clockwork, connects optical and radio frequency• 106 phaselocked cw-lasers for high accuracy

spectroscopy

0

cosine-pulse sine-pulse - cosine-pulse

/2

!n = n!r + !CE!CE=ÁCE/T

I()

1c

E(t)=A(t)eict = +

m=- Am e-imrt-ict


Spectroscopy with Combs

300 THz

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)


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)

1

all modes contribute.

like a cw laser.


… recent results• Cs 6S-8S two

photon transition

456 nm

822 nm

6s1/2

F= 36s

1/2F= 3

8s1/2

F= 38s

1/2F= 4

7p

4.2 µm

0 20 40 60

50

65

80

fr$qu$ncy shift at 820 nm (MHz)

phot

on c

ount

rat$

(kH

z)

fr$p

/ 2 (a)

7000

9000

11000

phot

on c

ount

rat$

(Hz)

(b)

-4 -2 0 2 4

-505

fr$qu$ncy shift at 820 nm (MHz)da

ta-fi

t (%

)

ºs¡ sF

ºs¡ sF

Peter Fendel et al., (… almost submitted)

Similar method: A. Marian et al, PRL, 95, 023001 (2005)


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?


High Accuracy at high Energy?• Planck Scale

• Frequency measurements– Optical atomic clocks


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 - Scaling Helium

Energy levels 1S-2S: 10eV Z2 40 eV ~ 60 nmLamb shift 1S: 8GHz Z4 128 GHzUnverified QED correc. Z6 64 times stronger


Optical Resonators for Frequency combs


Fs-Buildup resonator

• Enhance entire frequency comb

• Produce XUV frequency comb– Via high order harmonic generation


Real resonator

seed laser:

Pavg = 700 mW = 20 fsPpeak = 300 kW

intracavity:

Pavg = 38 W = 28 fsPpeak = 12 MW

x55

x40

F =¼


XUV Output

C. Gohle et al., Nature, 436, 234 (2005)R. J. Jones et al., PRL, 94, 193201 (2005)


High Harmonics Hierarchy


Coherence (of the 3rd harm.)

C. Gohle et al., Nature, 436, 234 (2005)R. J. Jones et al., PRL, 94, 193201 (2005)


Real resonator

seed laser:

Pavg = 700 mW = 20 fsPpeak = 300 kW

intracavity:

Pavg = 38 W = 28 fsPpeak = 12 MW

x55

x40

F =¼


Complete resonator characterizationWith high sensitivity


f-to-2f interferometer photodiode+countersilica wedges in laser

2x piezo-actuated mirrors

Experimental Setup


Data from an “empty” cavity

A. Schliesser et al., Optics Express, 14, 5975-5983 (2006)


•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


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



Empty cavity?


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



O2 H2O

Air filled resonator!


Outlook


High power XUV combseed laser: 10 MHz CPO (120 nJ; 30 fs)

enhancement cavity: vacuum setup (3.5 m length)

-10 0 10 20 30 40 50

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

Ref

lect

ed P

ower

(AC

-Cou

pled

)

Time [µs]

11s decay -> Finesse 300

750 775 800 825

2

4

6

8

10

12

14

16

Laser Resonator

Pow

ersp

ectru

m [a

u]

Wavelength [nm]Input: 120nJ, 30fs, 4MW peak

12µJ, 30fs, 400MW peakx 100


Cooling laser system


Helium Spectroscopy


… provide stable references• Narrow Markers in

Frequency space– If high finesse

• High stability– ~10-14 @ 1 s– Few Hz linewidth @

1 PHz

=F


Experimental Setup


Mutual fluctuations of laser/high-F cavity length make a lock at one frequency necessary. Active feedback keeps both on resonance at

lock: Á 7!

Ã ¡ !

!

!Á

Laser Lock


r ! Á !

¡ @

@!

Ã ¡ !

!

!Á

! !

Analysis when locked


O2?Air: 21% OxygenMolecular oxygen „A“ band ~760 nm

M. J. Thorpe et al.: Precise measurements of optical cavitydispersion and mirror coating properties viafemtosecond 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)

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

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