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NEAR-INFRARED DUAL-COMB SPECTROSCOPY WITH A CONTINUOUS-WAVE LASER
FRISNO 13 – Aussois, France – March 17 – March 22, 2015
Guy Millot, Stéphane Pitois Nathalie Picqué
1 - Motivation
2 – Experiment 3 - Results
Dual-Comb spectroscopy with a continuous-laser Generation of two mutually-coherent frequency combs
from a single continuous-wave tunable laser
2 2 DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
Outline
3
1 – Motivation : generalities on frequency combs
2 3
E(t) Pulse train
Spectrum FT
t
∆φ 2∆φ
1/frep
f
I(f) fo= ∆φ frep/2π
frep
fn= nfrep+ fo
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
1 – Motivation : basic concept of dual-comb spectroscopy
2 4
12 repreprep fff −=∆
Comb 1
Comb 2
Gas sample
1/frep1
1/∆frep
Detector
No moving mechanical part !
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
Improvement up to six orders of magnitude in acquisition time, sensitivity, resolution and accuracy compared to Michelson-based Fourier transform spectroscopy
3
1 – Motivation : basic concept – Frequency domain
2 5
300 MHz 300,1 MHz λ0
Frequency (THz)
Comb 1 : frep1 = 300 MHz
Comb 2 : frep2 = frep1 + ∆frep= 300,1 MHz
Frequency (kHz) 100 kHz
200 kHz
N x 100 kHz
300 kHz
…
…
TeraHertz Domain
Down converted frequency factor 3000/1
1
=∆
=rep
rep
ff
Low frequency detection RadioFrequency domain
RF
∆frep (=100 kHz) << frep1 λ0 ∼ 1570 nm
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
We have thus achieved a down frequency conversion equivalent to a heterodyne detection
3
1 – Motivation : basic concept – Temporal domain
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Period = 1 / ∆frep (10 µs)
Temporal Coincidence
Interferogram : Ι(t)
N pulses
N+1 pulses
T1=1/frep1 (3.3333 ns)
(N = 3 000)
T2=1/frep2 (3.3322 ns)
)11.1(21
psff
Trep
rep∆≅∆
ps50
Temporal Coincidence
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
1 – Motivation : state of the art
2 7 DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
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
3
1 – Motivation : basic concept Difficulties and possible solutions
2 8
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 – FRISNO13 2015
3
2 – Experimental breakthrough : generation of two mutually-coherent combs with a single continuous laser
2 9
Laser Diode 1570 nm / 4 mW
Electrical Clock 300 MHz
Electrical Clock 300,1 MHz
Electrical Pulse Generator ( 50 ps)
Electrical Pulse Generator (50 ps)
Intensity Modulators
Optical Comb 300 MHz
Optical Comb 300,1 MHz
EDFA
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
As both combs are generated from the same initial laser, they have very good mutual coherence, so there is no need to synchronize them
3
2 – Experiment : need for optical spectral filtering
2 10
300 MHz 300,1 MHz
λ0
Frequency (THz)
… …
Optical spectrum
Problem of optical spectral overlapping : double contribution of the optical lines
Frequency (kHz) 100 kHz
200 kHz N x 100 kHz
Low frequency detection ( RF domain)
300 kHz
… Down converted spectrum
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
2 – Experiment : need for optical spectral filtering
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λ0
Frequency (THz)
… …
Frequency (kHz) 100 kHz
200 kHz
N x 100 kHz
Low frequency detection ( RF domain )
300 kHz
…
Solving the problem of spectral overlapping with an optical filter
Optical filter
Down converted spectrum
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
2 – Experiment : spectral detection limit
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λ0
Frequence (THz)
…
Frequency (MHz) 100 kHz
N x 100 kHz
Low frequency detection ( RF domain ) … … … … …
300 MHz 600 MHz 0 MHz Useful
domain
Limitation of the number of lines : N x ∆frep < frep2 / 2 = 150 MHz
Optical spectrum
Frequency domain of the down converted spectrum
frep2/2
Order 0
Order 1 Order 2
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
2 – Experiment : frequency comb characterization
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Temporal pulse profile
directly measured
with an ultrafast oscilloscope (33 GHz)
Characterization with an
optical spectrum analyzer (OSA)
Characterization of the optical pulses generated at the output of the intensity modulators
∼ 50 ps
50 100 150 200 250
Time (ps)
Inte
nsity
(arb
. uni
ts)
1
2
3
4
5
6
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
∼ 0.2 nm @ -10dB
3
2 – Experiment : spectral broadening of the two frequency combs by wave-breaking
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Principle of Wave Breaking
Normal dispersion leads to flat-top spectrum and maintains high level of spectral coherence
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
t
P
oω−ω=δω
•
•
t
chirp •
•
Numerical simulation of the NLSE
Intensity Modulator
EDFA PC Dispersion Compensated Fiber (DCF)
FC
3
2 – Experiment : spectral broadening of the two frequency combs by wave-breaking
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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
Spectral broadening versus power at the DCF input
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
∼ 3 nm (400 GHz) @ -10dB
• 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
3
2 – Experiment : spectral broadening of the two frequency combs by wave-breaking
2 16 DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
Spectral broadening of the two combs in a single DCF with counter-propagating beams
narrows the comb spectrum to avoid aliasing around the carrier line and to reject spontaneous emission generated by the different amplifiers
Intensity Modulator
EDFA
1.378 km DCF
Intensity Modulator
EDFA
50/50
PC
PC
Circulator
Circulator
Comb 1
Comb 2
Tunable Optical Filter
Filter
3
2 – Experiment : comb coherence measurement
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Mutual coherence between the two combs - Characterization with a RF spectrum analyzer
Interference line between the two combs at 3 MHz
2.5 Hz 2.5 Hz
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
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
after spectral broadening before spectral broadening
3
2 – Experiment : use of a Hollow-Core Photonic Crystal Fiber
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Physical properties Core diameter 10 ± 1 μm Cladding pitch 3.8 ± 0.1 μm PCF domain diameter 70 ± 5 μ Cladding diameter 120 ± 2 μm
Optical properties @ 1550 nm Design wavelength 1550 nm Attenuation < 30 dB/km Typical GVD 90 ps/nm/km Spectral range of use 1490-1680 nm Mode diameter 9 ± 1 μm
Cross section Intensity profile
Hollow Core photonic bandgap fiber
NKT Photonics HC-1550-02 L = 48 m
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
2 – Experiment : overview of the experimental setup
HzWNEP /10 15−=
Laser diode (1569 nm)
EDFA
Electrical Pulse Generator 300 MHz
Intensity Modulators
Electrical Pulse Generator 300,1 MHz
EDFA
EDFA
Polarization Controler
Polarization Controler 1,38 km DCF
(D =- 94 ps/nm/km)
Circulator Circulator
Optical filter
Reference Coupler 99/1
Coupler 50/50
Collimator MO x20 MO x20
Micro Lens
Photodiode
Ampli / Electrical Filter (32 MHz)
16 bits Oscilloscope Picoscope 5444B
16 bits with 60 MHz bandwidth 62.5 MS/s
48 m - HC Fiber
Gas cell
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3
3 – Results : carbon dioxide absorption at telecommunication wavelengths
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Absorption in the mid-infrared
Absorption in the
near-infrared (telecom)
~ 100 000 times weaker
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
3 – Results : carbon dioxide absorption at telecommunication wavelengths
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Absorption in the L-band
12CO2
13CO2 X 100
1569 nm
P R
P R
Band 30012–00001
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
3 – Results : carbon dioxide absorption
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Spectrum with a mixture C12/C13 90% - 10 %
Total pressure 200 mBar – Carrier wavelength : 1569 nm
frep = 300 MHz ; ∆frep = 103 kHz
A total optical span exceeding 400 GHz without aliasing
is possible.
Recording time 52.4 ms SNR > 500
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
FT
Interferogram
λo = 1569.00 nm
9.7 µs
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
Time window = 524 µs Average over 100 spectra
35 GHz (115 lines)
3
3 – Results : carbon dioxide absorption
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1 : 13C16O2 30011-00001 band R(10) line λ=1569.419 nm S=4.4 10-25 cm.molecule-1
2 : 12C16O2 31112-01101 band R(21) line λ=1569.486 nm S=5.8 10-25 cm.molecule-1 3 : 12C16O2 30012-01101 band R(36) line λ=1569,494 nm S=5.0 10-24 cm.molecule-1 4 : 12C16O2 31112-01101 band R(20) line λ=1569,544 nm S=6.0 10-25 cm.molecule-1
λo = 1569.00 nm
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
Reference spectrum
1 2 3 4
v1v2l2v3n
Normalization
Computed spectrum with the line parameters of the HITRAN 2012 database assuming Lorentzian profiles
3
3 – Results : carbon dioxide absorption Linewidth of spectral modes
2 24
Zoom on a line
12 Hz !
Temporal window = 134.2 ms
No averaging
RF spectrum (down converted frequencies)
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
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)
3
3 – Results : carbon dioxide absorption Wavelength tunability
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1 12C16O2, 31112-01101 band R(16) line
2 12C16O2, 30012-00001 band R(30) line
3 12C16O2, 31112-01101 band R(15) line
4 12C16O2, 31112-01101 band R(14) line
5 12C16O2, 30012-00001 band R(28) line
The frequency agility of the laser diode allows us to readily probe other spectral regions or other molecules
Frequency agility is mainly limited by the bandwidth of the amplifiers
Use of optical amplifiers at other telecom bands access to a wide spectral range
3
4- Conclusions and perspectives
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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 mid- infrared).
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 AUSSOIS – FRISNO13 2015
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and for financial supports :
• IXCORE Research Foundation
• PARI PHOTCOM Regional Council of Burgundy
• Labex ACTION
and THANKS :
to Julien Fatome, Bertrand Kibler, Christophe Finot, Gil Fanjoux, Vincent Tissot, Philippe Morin
THANKS FOR YOUR ATTENTION
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
2 – Experimental breakthrough : generation of two mutually-coherent combs with a single continuous laser
2 9
Laser Diode 1570 nm / 4 mW
Electrical Clock - 300 MHz
Electrical Clock - 300,1 MHz
Electrical Pulse Generato ( 50 ps)
Electrical Pulse Generator (50 ps)
Intensity Modulator
Electrical Comb 300 MHz
Electrical Comb 300,1 MHz
Optical Comb 300 MHz
Optical Comb 300,1 MHz
EDFA
OSICS TLS-50 YENISTA 1568,77 nm - 1607,47 nm 4mW - 10 mW Linewidth ∼1 MHz typical
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
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
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
MANLIGHT L Band - 20 dBm
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
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
3
2 – Experiment : need for optical spectral filtering
2 11
λ0
Frequency (THz)
… …
Frequency (kHz) 100 kHz
200 kHz
N x 100 kHz
Low frequency detection ( RF domain )
300 kHz
…
Solving the problem of spectral overlapping with an optical filter
Optical filter
Filter YENISTA : Spectral range : 1480 nm to 1620 nm Spectral bandwidth : 32 pm to 650 pm Filtering slope : 800 dB/nm
∼ 4 to 80 GHz @ 1570 nm that to say
∼ 1 to 26 MHz for the down converted spectrum
Down converted spectrum
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
2 – Experiment : frequency comb characterization
2 14
Direct measurement of the temporal profile of an optical pulse with the ultrafast oscilloscope (33 GHz)
RF comb with frep= 300 MHz Zoom on the peak at 300 MHz
∼ 2 Hz
Characterization of the electrical combs with the RF spectrum analyzer
Characterization with the optical spectrum analyzer (OSA)
Characterization of the optical pulses generated at the output of the intensity modulators
∼ 50 ps
50 100 150 200 250
Time (ps)
Inte
nsity
(arb
. uni
ts)
1
2
3
4
5
6
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
∼ 0.2 nm @ -10dB
3
2 – Experiment : comb characterization
2 13
Electrical Spectrum Analyzer
Optical Spectrum Analyzer
OSA
YOKOGAWA AQ6370 Resolution : 20 pm -> 2 nm Dynamical range : 45 dB -> 57 dB
AGILENT N9010A Range : 9kHz – 26,5 GHz Resolution : 2 Hz
Ultrafast Oscilloscope
DSO
AGILENT Infiniium DSO-X 93304Q Bandwidth : 33 GHz 80 GSa/s
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
2 – Experiment : spectral broadening of the two frequency combs by self-phase modulation
2 15
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
Spectral broadening versus power at the HNLF input
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
Intensity Modulator
EDFA PC High Nonlinear Fiber
Frequency Comb
∼ 15 nm @ -10dB
3
2 – Experiment : spectral broadening of the two frequency combs by wave-breaking
2 13
Spectro-temporal representation of a pulse at different propagation distances Fig.2 from : C. Finot et al., JOSAB 25, p. 1938 (2008).
Flat-top spectrum High degree of coherence
ξ=z/LD τ=t/To
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
2 – Experiment : spectral characterization examples
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After the DCF After the optical filter After absoption in CO2
Optical spectra directly measured with the OSA
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
2 – Experiment : jitters of the two combs
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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 spectral broadening
162 ps
After
100 ps
100 ps
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
2 – Experiment : jitters of the two combs
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Histogram obtained with the 33 GHz oscilloscope
8417 optical pulses – Trigger by the electrical pulse
100 ps
After spectral broadening
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
2 – Experiment : comb coherence measurement
2 17
Comb coherence after spectral broadening - Characterization with the RF spectrum analyzer
Interference line between the two combs at 3 MHz at the DCF
input 2.5 Hz output 2.5 Hz
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
Comb at the DCF input ( peak at 300 MHz ) Comb at the DCF output ( peak at 300 MHz )
1 Hz 1 Hz
The nonlinear fiber does not induce any additional jitter on each comb
The counter-propagation of the two combs in the same nonlinear fiber is highly suitable for generating a coherent dual-comb
Coherence of each Comb
Mutual coherence
between the two Combs
3
3 – Results : carbon dioxide absorption Different pressure and different C12/C13 ratios
2 27
Spectrum 100 mBar / natural CO2 (1 % C13)
Carrier wavelength : 1569 nm
frep = 300 MHz ; ∆frep = 103 kHz
Temporal window = 525 µs
Average over 100 spectra
C12 : R36 1569,494 nm
C12 : R20 1569,544 nm
Reference
C12 : R38 1569,250 nm
λo = 1569,00 nm
Recording time ∼ 50 ms
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
3 – Results : carbon dioxide absorption Wavelength tunability
2 29
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
C12 : R14 1572,66 nm
λo = 1572,45 nm
Reference
Recording time ∼ 50 ms
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
3 – Results : carbon dioxide absorption Wavelength tunability
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C12 : R6 1574,034 nm
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
λo = 1573,90 nm
Reference
Recording time ∼ 50 ms
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
3 – Results : carbon dioxide absorption Wavelength tunability
2 31
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
C12 : 1569,25 nm
C12 :R40 1569,012 nm
C12 : 1569,19 nm
C12 :R38 1569,228 nm
C13 : 1569,075 nm
λo = 1568,80 nm
Reference
Recording time ∼ 50 ms
DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
3
3 – Results : carbon dioxide absorption Wavelength tunability
2 31 DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015
frep1
frep2
Absorption line
Elec
tric
fiel
d
Optical frequency (THz)
∆frep Radio frequency (MHz)
3
3 – Biomedical application : diagnosis of air breathing
2 31 DUAL COMB SPECTROSCOPY AUSSOIS – FRISNO13 2015