snowbird, 2007broad band cavity enhanced vernier spectroscopy 1 vernier spectroscopy a broad band...
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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
Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 2
Outline• Cavity enhanced spectroscopy• Broad band cavity enhanced
methods• Adding phase sensitivity• The optical vernier• Conclusion
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Fabry perot resonators
light source
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… enhance sensitivity• Cavity enhanced
absorption spectroscopy (CEAS)– Increased
interaction length ( ), i.e. sensitivity
• Cavity ring down (CRD)– Rejects source
noise
/ ¡ r
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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
pF
F
*Gherman, T. & Romanini, D., Optics Express, 10, 1033-1042 (2002) **Thorpe, M.J. et al., Science, 311, 1595-1599 , 2006
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Comb matching• In general r and ' will be complicated functions of !
lase
r fr
equ
ency
co
mb
pass
ive c
avit
y
… and the two combs can not be lined up
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Adding phase sensitivity to CEAS
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' Moiré pattern
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Scanning the comb
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Á !
/ ¡ r !
With bad resolution
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Áes E !
Extract the information
r ! Á !
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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)
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What about the comb?The optical Vernier
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Idean+1n
n = n r +CE
c
Requirements:
• Finesse > m
• m r > spec. resolution
! rmn
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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
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Implementation
Air
Resonator Finesse ~ 3000
grating
CCD
lens
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Data• Single scan
(10ms)
• Blue box: unique data
• Red boxes: identified features
• Gaussian PSF much larger than airy ! Brightness~Integral of airy
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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 Fr
ee p
aram
eter
s (ex
cept
frequen
cy o
ffset,
which w
as
not m
easu
red h
ere)
O2 A-Band
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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!
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Thanks
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Optical Resonators
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… enhance nonlinear conversion
• Pc=F/– Output power grows
with finesse2 or higher!
• Example:– SHG 560nm->280nm– 900mW driving
power
– 20% conversion: 900mW->200mW
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Fs-Frequency CombSpectroscopy
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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/TI()
1c
E(t)=A(t)eict = +
m=- Am e-imrt-ict
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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)
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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.
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… recent results
• Cs 6S-8S two photon transition
456 nm
822 nm
6s1/2
F= 36s
1/2F= 3
8s1/2
F= 3
8s1/2
F= 4
7p
4.2 µm
0 20 40 60
50
65
80
fr$qu$ncy shift at 820 nm (MHz)
ph
oto
n c
oun
t ra
t$ (
kHz)
fr$p
/ 2 (a)
7000
9000
11000
ph
oto
n c
oun
t ra
t$ (
Hz)
(b)
-4 -2 0 2 4
-5
0
5
fr$qu$ncy shift at 820 nm (MHz)
da
ta-f
it (%
)
ºs¡ sF
ºs¡ sF
Peter Fendel et al., (… almost submitted)
Similar method: A. Marian et al, PRL, 95, 023001 (2005)
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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?
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High Accuracy at high Energy?• Planck Scale
• Frequency measurements– Optical atomic clocks
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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 nm
Lamb shift 1S: 8GHz Z4 128 GHz
Unverified QED correc. Z6 64 times stronger
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Optical Resonators for Frequency combs
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Fs-Buildup resonator
• Enhance entire frequency comb
• Produce XUV frequency comb– Via high order harmonic generation
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Real resonator
seed laser:
Pavg = 700 mW = 20 fsPpeak = 300 kW
intracavity:
Pavg = 38 W = 28 fsPpeak = 12 MW
x55
x40
F =¼
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XUV Output
C. Gohle et al., Nature, 436, 234 (2005)R. J. Jones et al., PRL, 94, 193201 (2005)
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High Harmonics Hierarchy
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Coherence (of the 3rd harm.)
C. Gohle et al., Nature, 436, 234 (2005)R. J. Jones et al., PRL, 94, 193201 (2005)
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Real resonator
seed laser:
Pavg = 700 mW = 20 fsPpeak = 300 kW
intracavity:
Pavg = 38 W = 28 fsPpeak = 12 MW
x55
x40
F =¼
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Complete resonator characterization
With high sensitivity
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f-to-2f interferometer photodiode+countersilica wedges in laser
2x piezo-actuated mirrors
Experimental Setup
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Data from an “empty” cavity
A. Schliesser et al., Optics Express, 14, 5975-5983 (2006)
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•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
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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
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•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?
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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
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•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
O2 H2O
Air filled resonator!
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Outlook
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High power XUV comb
seed 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
(A
C-C
oupl
ed)
Time [µs]
11s decay -> Finesse 300
750 775 800 825
2
4
6
8
10
12
14
16
Laser Resonator
Pow
ers
pect
rum
[au
]
Wavelength [nm]
Input: 120nJ, 30fs, 4MW peak
12µJ, 30fs, 400MW peak
x 100
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Cooling laser system
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Helium Spectroscopy
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… provide stable references• Narrow Markers in
Frequency space– If high finesse
• High stability– ~10-14 @ 1 s– Few Hz linewidth @
1 PHz
=F
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Experimental Setup
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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
Snowbird, 2007 Broad band cavity enhanced Vernier spectroscopy 53
r ! Á !
¡@
@!
Ã
¡!
!
!
Á ! !
Analysis when locked
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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)