NEAR-INFRARED DUAL-COMB SPECTROSCOPY
WITH A CONTINUOUS-WAVE LASER
Guy Millot, Stéphane Pitois
Nathalie Picqué
FRISNO 13 – Aussois, France – March 17 – March 22, 2015
Outline
1 - Motivation
Dual-Comb spectroscopy with a continuous-laser
Generation of two mutually-coherent frequency combs
from a single continuous-wave tunable laser
3 - Results
2 – Experiment
DUAL COMB SPECTROSCOPY
AUSSOIS
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2015
2
1 – Motivation : generalities on frequency combs
Pulse train
E(t)
2∆φ
∆φ
t
1/frep
FT
Spectrum
I(f)
fo= ∆φ frep/2π
fn= nfrep+ fo
frep
DUAL COMB SPECTROSCOPY
f
AUSSOIS
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1 – Motivation : basic concept of dual-comb spectroscopy
Comb 2
∆f rep = f rep 2 − f rep1
Comb 1
1/frep1
Gas sample
1/∆frep
Detector
No moving mechanical part !
Improvement up to six orders of magnitude in acquisition time, sensitivity, resolution and
accuracy compared to Michelson-based Fourier transform spectroscopy
DUAL COMB SPECTROSCOPY
AUSSOIS
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1 – Motivation : basic concept – Frequency domain
λ0 ∼ 1570 nm
λ0
Comb 1 : frep1 = 300 MHz
300 MHz
300,1 MHz
Comb 2 : frep2 = frep1 + ∆frep= 300,1 MHz
TeraHertz Domain
∆frep (=100 kHz) << frep1
…
Frequency (THz)
Down converted ∆f rep
=
= 1 / 3000
frequency factor f
rep1
…
Low frequency detection
RadioFrequency domain
RF
100
kHz
200
kHz
300
kHz
Frequency (kHz)
N x 100
kHz
We have thus achieved a down frequency conversion equivalent to a heterodyne detection
DUAL COMB SPECTROSCOPY
AUSSOIS
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1 – Motivation : basic concept – Temporal domain
Temporal
Coincidence
Temporal
Coincidence
T1=1/frep1 (3.3333 ns)
50 ps
N pulses
T2=1/frep2 (3.3322 ns)
N+1 pulses
(N = 3 000)
∆T ≅
∆f rep
2
f rep
1
(1.11 ps )
Period = 1 / ∆frep
(10 µs)
Interferogram : Ι(t)
DUAL COMB SPECTROSCOPY
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1 – Motivation : state of the art
Principle of dual-comb spectroscopy
firstly proposed by S. Schiller (Düsseldorf)
“Spectrometry with frequency combs,”
Opt. Lett., vol. 27, no. 9, p. 766, 2002.
B. Bernhardt, PhD thesis
Th. Hänsch & N. Picqué MPQ Garching
N. R. Newbury et al. NIST Boulder
The need to synchronize femtosecond lasers with an interferometric precision
requires advanced experimental techniques of optical metrology
DUAL COMB SPECTROSCOPY
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1 – Motivation : basic concept
Difficulties and possible solutions
Dual-comb spectroscopy requires a high temporal stability
Possible solutions :
A: Use of an ultra-stable reference cavity and a fast locking system for stabilization of
the two combs
Pb : complex and expensive devices
eg PRL 100, 013902 (2008)
B: Observation and recording of the time fluctuations, a posteriori mathematical
correction of the interferograms Pb : costly time data analysis eg Opt. Express 18,
23358 (2010)
C: Use of an adaptive clock signal for data recording Pb : complex devices
eg Nat. Comm. 5, 3375 (2014)
D: Design of a novel system with intrinsic mutual coherence between the two combs
DUAL COMB SPECTROSCOPY
AUSSOIS
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2 – Experimental breakthrough : generation of two
mutually-coherent combs with a single continuous laser
Electrical Clock
300 MHz
Electrical Pulse Generator ( 50 ps)
Laser Diode
1570 nm / 4 mW
Optical Comb 300 MHz
EDFA
Intensity
Modulators
Optical Comb 300,1 MHz
Electrical Clock
300,1 MHz
Electrical Pulse Generator (50 ps)
As both combs are generated from the same initial laser, they have very good mutual
coherence, so there is no need to synchronize them
DUAL COMB SPECTROSCOPY
AUSSOIS
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2 – Experiment : need for optical spectral filtering
Problem of optical spectral overlapping : double contribution of the optical lines
λ0
Optical spectrum
300 MHz
300,1 MHz
…
…
Frequency (THz)
…
Down converted spectrum
100
kHz
200 kHz 300 kHz
DUAL COMB SPECTROSCOPY
AUSSOIS
Low frequency detection
( RF domain)
N x 100
kHz
– FRISNO13
2015
Frequency (kHz)
2
10
3
2 – Experiment : need for optical spectral filtering
Solving the problem of spectral overlapping with an optical filter
λ0
Optical filter
…
…
Frequency (THz)
…
Down converted spectrum
100
kHz
200
kHz
DUAL COMB SPECTROSCOPY
300
kHz
AUSSOIS
Low frequency detection
( RF domain )
Frequency (kHz)
N x 100
kHz
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2015
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2 – Experiment : spectral detection limit
λ0
Frequency domain of the down converted spectrum
Optical spectrum
…
Frequence (THz)
Limitation of the number of lines : N
x ∆frep < frep2 / 2 = 150 MHz
Useful
0 MHz domain
Order 1
Order 2
600 MHz
300 MHz
Order 0
…
100
kHz
…
…
…
…
Low frequency detection
( RF domain )
N x 100 frep2/2
kHz
DUAL COMB SPECTROSCOPY
Frequency (MHz)
AUSSOIS
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12
3
2 – Experiment : frequency comb characterization
Temporal pulse profile
directly measured
with an ultrafast oscilloscope (33 GHz)
Intensity (arb. units)
Characterization of the optical pulses generated at the output of the intensity modulators
6
5
4
3
∼ 50 ps
2
1
50
100
150
200
250
Time (ps)
∼ 0.2 nm @ -10dB
Characterization with an
optical spectrum analyzer (OSA)
DUAL COMB SPECTROSCOPY
AUSSOIS
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2015
2
13
3
2 – Experiment : spectral broadening of the two frequency
combs by wave-breaking
EDFA
Intensity Modulator
PC
Dispersion Compensated Fiber (DCF)
FC
Numerical simulation of the NLSE
Principle of Wave Breaking
P
•
•
t
δω = ω − ωo
•
chirp
t
•
Normal dispersion leads to flat-top spectrum and maintains high level of spectral coherence
DUAL COMB SPECTROSCOPY
AUSSOIS
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2 – Experiment : spectral broadening of the two frequency
combs by wave-breaking
Spectral broadening versus power at the DCF input
∼ 3 nm (400 GHz) @ -10dB
Dispersion Compensated Fiber
with normal dispersion
@ 1569 nm
L = 1.38 km
γ= 3 W-1 km-1
α= 1 dB/km
D = -94 ps/nm/km
S = -0,12 ps/nm2/km
• Wave-breaking leads to flat-top spectra with low amplitude noise which span up to about 3 nm
(400 GHz) when input power reaches 24 dBm
• The 3-nm spectrum is composed of 1350 individual lines with a power of 0.18 mW per comb line
DUAL COMB SPECTROSCOPY
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2 – Experiment : spectral broadening of the two frequency
combs by wave-breaking
Spectral broadening of the two combs in a single DCF
with counter-propagating beams
EDFA
Intensity Modulator
Comb 1
PC
Circulator
Tunable
Optical
Filter
1.378 km DCF
Intensity Modulator
Comb 2
PC
EDFA
Filter
50/50
Circulator
narrows the comb spectrum to avoid aliasing around the carrier line and
to reject spontaneous emission generated by the different amplifiers
DUAL COMB SPECTROSCOPY
AUSSOIS
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2 – Experiment : comb coherence measurement
Mutual coherence between the two combs - Characterization with a RF spectrum analyzer
Interference line between the two combs at 3 MHz
before spectral broadening
after spectral broadening
2.5 Hz
2.5 Hz
2.5 Hz corresponds to a relative coherence time of 400 ms.
Such coherence time appears unaffected by the kilometric length of the
nonlinear fiber, partly due to the use of a single fiber for the broadening of the two combs
The counter-propagation of the two combs in the same nonlinear fiber
is highly suitable for generating a coherent dual-comb
DUAL COMB SPECTROSCOPY
AUSSOIS
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2 – Experiment : use of a Hollow-Core Photonic Crystal Fiber
Hollow Core
photonic bandgap fiber
NKT Photonics HC-1550-02
L = 48 m
Physical properties
Core diameter
Cladding pitch
PCF domain diameter
Cladding diameter
10 ± 1 μm
3.8 ± 0.1 μm
70 ± 5 μ
120 ± 2 μm
Cross section
Intensity profile
Optical properties @ 1550 nm
Design wavelength
Attenuation < 30 dB/km
Typical GVD
Spectral range of use
Mode diameter
1550 nm
90 ps/nm/km
1490-1680 nm
9 ± 1 μm
DUAL COMB SPECTROSCOPY
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2 – Experiment : overview of the experimental setup
Electrical Pulse Generator
300 MHz
Polarization Controler
EDFA
Laser diode (1569 nm)
EDFA
Polarization Controler
Intensity Modulators
1,38 km DCF
(D =- 94 ps/nm/km)
EDFA
Circulator
Electrical Pulse Generator
300,1 MHz
Circulator
Coupler 50/50
Optical filter
Coupler 99/1
Gas cell
Photodiode
Micro Lens
Collimator
MO x20
MO x20
Picoscope 5444B
16 bits with 60 MHz bandwidth
62.5 MS/s
NEP = 10 −15W / Hz
Ampli / Electrical Filter
(32 MHz)
48 m - HC Fiber
DUAL COMB SPECTROSCOPY
16 bits Oscilloscope
Reference
AUSSOIS
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3 – Results : carbon dioxide absorption at
telecommunication wavelengths
Absorption in the
mid-infrared
Absorption in the
near-infrared (telecom)
~ 100 000 times weaker
DUAL COMB SPECTROSCOPY
AUSSOIS
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3 – Results : carbon dioxide absorption at
telecommunication wavelengths
Absorption in the L-band
Band 30012–00001
P
R
12CO
2
1569 nm
P
DUAL COMB SPECTROSCOPY
13CO
2
X 100
R
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3 – Results : carbon dioxide absorption
λo = 1569.00 nm
Interferogram
Spectrum with a mixture C12/C13 90% - 10 %
Total pressure 200 mBar – Carrier wavelength : 1569 nm
9.7 µs
frep = 300 MHz ; ∆frep = 103 kHz
A total optical span exceeding 400 GHz without aliasing
is possible.
Time window = 524 µs
Average over 100 spectra
Recording time 52.4 ms
SNR > 500
35 GHz (115 lines)
FT
The noise-equivalent-absorption (NEA) coefficient
at 1s-time-averaging, defined as (Labs SNR)-1 (T/M)1/2,
is 8.5 x 10-9 cm-1 Hz-1/2.
The long optical path within the hollow core fiber
leads to high sensitivity without multi-pass cell
DUAL COMB SPECTROSCOPY
AUSSOIS
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3 – Results : carbon dioxide absorption
λo = 1569.00 nm
Computed spectrum with the line parameters
of the HITRAN 2012 database
assuming Lorentzian profiles
Reference spectrum
1
2 3
4
Normalization
1 : 13C16O2
2 : 12C16O2
3 : 12C16O2
4 : 12C16O2
30011-00001 band
31112-01101 band
30012-01101 band
31112-01101 band
R(10) line
R(21) line
R(36) line
R(20) line
λ=1569.419 nm
λ=1569.486 nm
λ=1569,494 nm
λ=1569,544 nm
S=4.4 10-25 cm.molecule-1
S=5.8 10-25 cm.molecule-1
S=5.0 10-24 cm.molecule-1
S=6.0 10-25 cm.molecule-1
v1v2l2v3n
DUAL COMB SPECTROSCOPY
AUSSOIS
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3 – Results : carbon dioxide absorption
Linewidth of spectral modes
Zoom on a line
Temporal window = 134.2 ms
No averaging
12 Hz !
RF spectrum (down converted frequencies)
36 kHz in the
optical domain
For comparison, the width of an individual comb line from free-running
mode-locked erbium-doped fiber lasers was found to be 260 kHz over an integration time of 1.3 s
Ideguchi T., Poisson A., Guelachvili G., Picqué N, Hänsch T.W., Nature Communications 5, 3375 (2014)
DUAL COMB SPECTROSCOPY
AUSSOIS
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3 – Results : carbon dioxide absorption
Wavelength tunability
The frequency agility of the laser diode allows us to readily probe other spectral regions or other molecules
1
12C16O ,
2
31112-01101 band R(16) line
2
12C16O ,
2
30012-00001 band R(30) line
3
12C16O ,
2
31112-01101 band R(15) line
4
12C16O ,
2
31112-01101 band R(14) line
5
12C16O ,
2
30012-00001 band R(28) line
Frequency agility is mainly limited by the bandwidth of the amplifiers
Use of optical amplifiers at other telecom bands
DUAL COMB SPECTROSCOPY
access to a wide spectral range
AUSSOIS
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4- Conclusions and perspectives

Original method using a single continuous diode laser with standard linewidth.

No need to synchronize the two combs and low phase noise.

The set-up only harnesses standard optoelectronic devices at telecom wavelengths and
adapted fibers. This dramatically simplifies the implementation of a dual-comb spectrometer.

No resonant cavity : variable repetition rate ( from 100 to 500 MHz ) and subsequent resolution.

Spectral broadening by wave breaking : flatness of the spectrum and low time and
amplitude jitters.

First use of a Hollow Core fiber in dual-comb spectroscopy: high sensitivity, measurement of
weak absorption lines (almost a billion times less intense than absorption lines in the midinfrared).

Easy self-calibration spectra.

Small spectral window, but significant power per comb line and wavelength agility by using
a tunable continuous laser.
Work in progress : increase the signal to noise ratio, access to other wavelengths
DUAL COMB SPECTROSCOPY
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THANKS FOR YOUR ATTENTION
and THANKS :
to Julien Fatome, Bertrand Kibler, Christophe Finot, Gil Fanjoux,
Vincent Tissot, Philippe Morin
and for financial supports :
•
IXCORE Research Foundation
•
PARI PHOTCOM Regional Council of Burgundy
•
Labex ACTION
DUAL COMB SPECTROSCOPY
AUSSOIS
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2 – Experimental breakthrough : generation of two
mutually-coherent combs with a single continuous laser
AGILENT MGX N5181A-501
Sinusoidal electrical wave : tunable frequency from 100 kHz to 1 GHz by step of 1 Hz
Temporal jitter < 1ps at 100 MHz and < 500 fs at 500 MHz
Output power > 13 dBm
OSICS TLS-50 YENISTA
1568,77 nm - 1607,47 nm
4mW - 10 mW
Linewidth ∼1 MHz typical
Electrical Pulse Generato ( 50 ps)
Electrical Clock - 300 MHz
PPG50 PHOTLINE
Gaussian to super-Gaussian
Pulse Width: 50 ps
Repetition rate : 100 MHz to 500 MHz
Rise time: 15 ps
RMS jitter < 2 ps
Electrical Comb 300 MHz
Laser Diode
1570 nm / 4 mW
MANLIGHT
L Band - 20 dBm
Optical Comb 300 MHz
EDFA
Intensity
Modulator
Optical Comb 300,1 MHz
Electrical Clock - 300,1 MHz
Electrical Comb 300,1 MHz
MODBOX PHOTLINE
Signal wavelength tunable from 1520 to 1600 nm
Maximum input power : 100 mW (20 dBm)
Optical modulator : bandwidth 18 GHz
Extension rate : 30 dB
Optical pulse duration : 50 ps
Electrical Pulse Generator (50 ps)
As both combs are generated from the same initial laser, we will see later that they have
very good mutual coherence, so there is no need to synchronize them
DUAL COMB SPECTROSCOPY
AUSSOIS
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9
2 – Experiment : need for optical spectral filtering
Solving the problem of spectral overlapping with an optical filter
Filter YENISTA :
Spectral range : 1480 nm to 1620 nm
Spectral bandwidth : 32 pm to 650 pm
Filtering slope : 800 dB/nm
λ0
Optical filter
…
∼ 4 to 80 GHz @ 1570 nm
that to say
∼ 1 to 26 MHz for the
down converted spectrum
…
Frequency (THz)
…
Down converted spectrum
100
kHz
200
kHz
DUAL COMB SPECTROSCOPY
300
kHz
AUSSOIS
Low frequency detection
( RF domain )
Frequency (kHz)
N x 100
kHz
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2015
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11
3
2 – Experiment : frequency comb characterization
Characterization of the electrical combs with the RF spectrum analyzer
RF comb with frep= 300 MHz
Zoom on the peak at 300 MHz
∼ 2 Hz
Characterization of the optical pulses generated at the output of the intensity modulators
Intensity (arb. units)
Direct measurement of the temporal profile of an
optical pulse with the ultrafast oscilloscope (33 GHz)
Characterization with the optical spectrum analyzer (OSA)
6
∼ 0.2 nm @ -10dB
5
4
3
∼ 50 ps
2
1
50
100
150
200
250
Time (ps)
DUAL COMB SPECTROSCOPY
AUSSOIS
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2 – Experiment : comb characterization
Electrical
Spectrum Analyzer
AGILENT N9010A
Range : 9kHz – 26,5 GHz
Resolution : 2 Hz
Optical
Spectrum Analyzer
OSA
YOKOGAWA AQ6370
Resolution : 20 pm -> 2 nm
Dynamical range : 45 dB ->
57 dB
Ultrafast
Oscilloscope
DSO
DUAL COMB SPECTROSCOPY
AGILENT Infiniium DSO-X 93304Q
Bandwidth : 33 GHz
80 GSa/s
AUSSOIS
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2
13
3
2 – Experiment : spectral broadening of the two frequency
combs by self-phase modulation
EDFA
PC
High Nonlinear Fiber
Intensity Modulator
Frequency Comb
Spectral broadening versus power at the HNLF input
∼ 15 nm @ -10dB
High NonLinear Fiber with
anomalous dispersion
@ 1569 nm
L = 1000 m
γ= 10 W-1 km-1
α= 0,4 dB/km
D = 0,2 ps/nm/km
S = 0,045 ps/nm2/km
DUAL COMB SPECTROSCOPY
AUSSOIS
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2
15
3
2 – Experiment : spectral broadening of the two frequency
combs by wave-breaking
Spectro-temporal representation of a pulse at different propagation distances
Fig.2 from : C. Finot et al., JOSAB 25, p. 1938 (2008).
ξ=z/LD
τ=t/To
Flat-top spectrum
High degree of
coherence
DUAL COMB SPECTROSCOPY
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2 – Experiment : spectral characterization examples
Optical spectra directly measured with the OSA
After the DCF
After the optical filter
After absoption in CO2
DUAL COMB SPECTROSCOPY
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2 – Experiment : jitters of the two combs
Direct observation of the timing and amplitude jitters
with the 33 GHz oscilloscope
10 000 optical pulses – Trigger by the electrical pulse (data b)
50 ps
Before
100 ps
spectral
broadening
After
100 ps
162 ps
DUAL COMB SPECTROSCOPY
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2 – Experiment : jitters of the two combs
Histogram obtained with the 33 GHz oscilloscope
8417 optical pulses – Trigger by the electrical pulse
After
spectral
broadening
100 ps
DUAL COMB SPECTROSCOPY
AUSSOIS
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2 – Experiment : comb coherence measurement
Comb coherence after spectral broadening - Characterization with the RF spectrum analyzer
Comb at the DCF input ( peak at 300 MHz )
Comb at the DCF output ( peak at 300 MHz )
1 Hz
1 Hz
Coherence
of each
Comb
The nonlinear fiber does not induce any additional jitter on each comb
Interference line between the two combs at 3 MHz at the DCF
input
2.5 Hz
2.5 Hz
output
Mutual
coherence
between the
two Combs
The counter-propagation of the two combs in the same nonlinear fiber
is highly suitable for generating a coherent dual-comb
DUAL COMB SPECTROSCOPY
AUSSOIS
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3 – Results : carbon dioxide absorption
Different pressure and different C12/C13 ratios
C12 : R38
1569,250 nm
C12 : R36
1569,494 nm
C12 : R20
1569,544 nm
λo = 1569,00 nm
Spectrum 100 mBar / natural CO2 (1 % C13)
Carrier wavelength : 1569 nm
frep = 300 MHz ; ∆frep = 103 kHz
Reference
Temporal window = 525 µs
Average over 100 spectra
Recording time ∼ 50 ms
DUAL COMB SPECTROSCOPY
AUSSOIS
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3 – Results : carbon dioxide absorption
Wavelength tunability
C12 : R14
1572,66 nm
λo = 1572,45 nm
Reference
Spectrum with a mixture C12/C13 90% - 10 %
Total pressure 200 mBar – Carrier wavelength : 1572,45 nm
frep = 300 MHz ; ∆frep = 103 kHz
Temporal window = 525 µs
Average over 100 spectra
Recording time ∼ 50 ms
DUAL COMB SPECTROSCOPY
AUSSOIS
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3 – Results : carbon dioxide absorption
Wavelength tunability
C12 : R6
1574,034 nm
λo = 1573,90 nm
Reference
Spectrum with a mixture C12/C13 90% - 10 %
Total pressure 200 mBar – Carrier wavelength : 1573,90 nm
frep = 300 MHz ; ∆frep = 103 kHz
Temporal window = 525 µs
Average over 100 spectra
Recording time ∼ 50 ms
DUAL COMB SPECTROSCOPY
AUSSOIS
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3 – Results : carbon dioxide absorption
Wavelength tunability
C13 :
1569,075 nm
C12 :R40
C12 :
C12 :
1569,012 nm
1569,19 nm 1569,25 nm
C12 :R38
1569,228 nm
λo = 1568,80 nm
Reference
Spectrum with a mixture C12/C13 90% - 10 %
Total pressure 200 mBar – Carrier wavelength : 1573,90 nm
frep = 300 MHz ; ∆frep = 103 kHz
Temporal window = 525 µs
Average over 100 spectra
Recording time ∼ 50 ms
DUAL COMB SPECTROSCOPY
AUSSOIS
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3 – Results : carbon dioxide absorption
Wavelength tunability
frep1
Electric field
Absorption
line
frep2
Optical frequency (THz)
∆frep
Radio frequency (MHz)
DUAL COMB SPECTROSCOPY
AUSSOIS
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3 – Biomedical application : diagnosis of air breathing
DUAL COMB SPECTROSCOPY
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