Post on 20-Jun-2020
Fiber comb lasers for spectroscopy
M. E. Fermann, K. F. Lee, A. Mills, C. Mohr, J. Jiang
IMRA America Inc.
1044 Woodridge Av., MI
Acknowledgements
DESY Ingmar Hartl
Stanford/CREOL Nick Leindecker, Konstantin Vodopyanov
BAE Systems Peter G. Schunemann
Univ. Colorado Thomas Schibli, Jun Ye
Univ. Milano Marco Marangoni, Davide Gatti
Univ. Naples Livio Gianfrani
Overview
• Introduction
• Mid IR comb lasers
Tm fiber combs
OPOs
• Spectroscopy applications
comb referencing of QCLs
QCL spectroscopy
• Summary
Frequency combs & standard molecular spectroscopy techniques
Frequency combs for molecular spectroscopy:
cw: QCLs
Photo-acoustic spectroscopy FTIR
Mass specSingle Line Multi-Line
Spectroscopic Techniques Mass Techniques
Requires ionization
Fragmentation
Measures M/Z ratio
only competes with MS in terms of cost; to compete with MS:
• need broadband mid IR measurements
• need high spectral resolution (≈ 10 – 100 MHz for unambiguous gas analysis)
• need high sensitivity (=sufficient spectral brightness)
For scientific applications:
• narrow linewidth allows proper evaluation of lineshapes,
allowing collision effects to be accounted for.
.
Broadband: gas mixtures
Low power per resolution
bandwidth
Narrow linewidth,
High power,
Slow signal acquisition
1 Line at a time…
Very Sensitive
Wide dynamic range
� � �
How much IR power do we need?
Glowbar spectral power density = 0.01 mW/nm(adapted for 1 GHz resolution FTIR, inversely proportional to FTIR resolution)
Ref. S.P Davis et al., Fourier Transform Spectrometry, Academic Press(2001)
Coherent IR laser source should have:
spectral power density > 1 mW/nm
Incoherent fiber SC spectral power density = 1 mW/nm(at 3000 nm with a fluoride fiber)
Comb technology can give us around 100 times better
sensitivity than incoherent sources with same spectral power
Comb technology should produce 100 times better
sensitivity to make the additional expense of combs tolerable
OPGaAs
Spectral power density of current
coherent mid IR sources
PPLN
GaSechalcogenide
fiber
PPLN
black line = glow bar with 1 GHz res. FTIR
Wavelength (µm)
Red line = incoherent fiber supercontinuum sources
Why mid-IR?
Comparison of molecular absorption linestrength:
Source: HITRAN database
Mid-IR combs promise higher sensitivity for molecular sensing.
courtesy:
S. Diddams
F. Adler
Tm fiber based comb sources
Tm-doped Fiber Properties
efficient cladding pumping at 790 nm
�excellent power scaling capability
(1 kW cw-power demonstrated)
Moulton et al., JSTQE 15, 85 (2009)
Emission cross-section of silica Tm fiber
300 nm
bandwidth
Jackson and King, JLT 17, 948 (1999)
large bandwidth & gain
� ideal for fs-pulse amplification
400 MHz Tm soliton comb oscillator
WDM
TSF DCF
PBS
SA
Graphene
Modulator
SMF
QWP
Low RIN 1564 nm 2 W pump
1.0
0.8
0.6
0.4
0.2
0.0
Inte
nsity
[a.u
.]
-400 0 400Delay [fs]
-1.0
-0.5
0.0
0.5
1.0
Phase [rad]
FWHM 58fs
FROG Intensity Retrieved Phase
1.0
0.8
0.6
0.4
0.2
0.0
In
ten
sity (
a.u
.)
22002100200019001800
Wavelength (nm)
-1.0
-0.5
0.0
0.5
1.0
Ph
ase
(rad
)
retrieved spectrum retrieved phase measured spectrum
Time bandwidth product
0.31, soliton pulse
Intracavity
dispersion
-17,000 fs2
400 MHz Tm soliton amplifier
WDM
TSF DCF
PBS
SA
Graphene
Modulator
SMF
QWP
Low RIN 1564 nm 2 W pump DCF
1.0
0.8
0.6
0.4
0.2
0.0
Inte
nsi
ty (
a.
u.)
205020001950190018501800Wavelength (nm)
Tm Amplifier 8
6
4
2
0
Inte
nsity
(a. u
.)
4002000-200-400
Delay (fs)
102.4 fs
Ref.: J. Jiang, “500 MHz, 58fshighly coherent Tm fiber soliton laser”
CLEO 2012 PDL, paper CTh5C.4.
2.5 – 5.0 W
SC: 1.1 – 2.3 µmHNLF
TDF
TDF
C.-C. Lee et al., Optics Express 20, 5264 (2012)C.-C. Lee et al., CLEO (2012), paper JTu1M.3.
Graphene Modulator design
Nobel Prize in Physics for 2010 to
Andre Geim & Konstantin Novoselov
Graphene loss modulator for fast fceo control
Small effect at DC dueto gain response
Large effect , sincegain cannot follow
locking performance (500MHz comb)
-80
-60
-40
-20
Inte
nsity
[dB
]
2.42.22.01.81.61.41.21.0Wavelength [µm]
FT-IR measurement OSA measurement
f2f
-80
-60
-40
-20
0
RF
Pow
er [d
Bm
], tr
aces
offs
et b
y 20
dB
-2 -1 0 1 2Frequency Offset [MHz]
fceo lock
-80
-60
-40
-20
0
RF
Pow
er [d
Bm
], tr
aces
offs
et b
y 20
dB
-2 -1 0 1 2Frequency Offset [MHz]
fceo lock
Tm comb lock to SF diode laser
-80
-60
-40
-20
0
RF
Pow
er [d
Bm
], tr
aces
offs
et b
y 20
dB
-2 -1 0 1 2Frequency Offset [MHz]
fceo lock
Tm comb lock to SF diode laser
out of loop beat with SF Yb fiber laser
Coherent SC generated with Tm fiber sysem
0 2 4 6 8 10 12 14 16 180
20
40
60
80
100
G aA sLiN bO3
W avelength (µm )
Tra
nsm
issi
on (
%)
LiNbO3
Advantages of QPM GaAs
Large nonlinear coefficient (94 pm/V)
Non-critical quasi-phase-matching & long interaction lengths for reduced cw threshold (up to 70 mm)
High transparency out to 15 µm
High thermal conductivity (55 W/mK)
Low absorption losses (<0.005 cm-1)
ZnGeP2
Re.: L. A. Eyres et al., All-epitaxial fabrication of thick, orientation-
patterned GaAs films for nonlinear optical frequency conversion,
Appl. Phys. Lett., 79, 904 (2001)
The Future of Mid-IR Frequency Conversion
Quasi-Phase Matched (QPM) GaAs:
mid-IR generation with OP-GaAs OPO Collaboration with:
Nick Leindecker & Konstantin L. Vodopyanov
(CREOL) &Peter G. Schunemann (BAE Systems)
Leindecker et al., Opt. Expr., 30, 7046 (2012)
2.6 – 6.1 µm output
Threshold pump energy ≈ 100 pJ,
Compatible with multi-GHz rep. rates
OPGaAs
crystal
Tm fiber
pump
output
How to lock OPO?
signalidler
• Use spurious doubled signal output from OPO
for OPO cavity length control
• Rep. rate and fceo of OPO automatically locked
OPO lock performance
• cavity length changes move
the cw laser + OPO beat
• feedback beat to OPO mirror
position (5 kHz bandwidth)
• phase lock to pump
• linewidth below 100 mHz
• >80% power in carrier
• <500 mrad cumulative phase
noise
lock
Frequency combs assisted QCL
molecular spectroscopy
• QCL line narrowing
• Precision frequency scanning with QCLs
• Application to molecular spectroscopy
12
10
8
6
4
2
Inte
nsity
x106 x10
6
Frequency
12
10
8
6
4
2
Inte
nsity
x106 x10
6
Frequency
fCEO independent non-linear mixing schemes
① Comb down-conversion (DFG scheme)
② CW-laser up-conversion comb (SFG scheme)
QCLf
DFG n mf f f= − m CEO REPf f mf= + m CEO REPf f nf= +
SFG m QCLf f f= +
120.6604
x106
Frequency1.95
µm1.6
µm9
µm
Gatti et al. Optics Express, 2011, 19, 17520
Comb assisted spectroscopy: Tm fiber comb & 9 µm QCL
1.95 µm
LO
Servo
QCL
Current
Driver
Quantified Line-Narrowing
• Observed line narrowing from 520 kHz to 25 kHz in 1 ms.
• Possible to line narrow 2 MHz bandwidth mid-IR sources to kHz
level using GPS-referenced combs!
• Narrower line widths possible with optically referenced combs.
Spectral noise density
1.0
0.8
0.6
0.4
0.2
0.0
Nor
mal
ized
Inte
nsity
10005000-500-1000Frequency (kHz)
1 msQCL: Free RunningComb: RF LockedComb: Free Running
Reconstructed lineshape
Optics Letters. Mills, Gatti, Fermann, et al. 37 (2012) 4083.
1st Application:
Ammonia spectroscopy
Ammonia Spectroscopy
ν2 band of Ammonia
500x10-21
400
300
200
100
0
Inte
nsity
cm
/mol
ecul
e
120011001000900800
Frequency (cm-1
)
Current Hitran line-center accuracy ~ 3 MHz.
Rapid, comb-assisted spectroscopy enables highly reproducible scans over multiple lines.
QCL tuning range
sR(6)
k=1…6160x10-21
140
120
100
80
60
40
20
0
Inte
nsity
(cm
/mol
ecul
e)
111011081106110411021100
Frequency (cm-1
)
150
100
50
0
x10-2
1
1103.481103.461103.44
Tm oscillator can tune 5.5 GHz or 0.2 cm-1 continuously.
Doppler width can be measured to give the thermodynamic
temperature if molecular mass (M) and Boltzmann constant (kB)
is known.
Precise knowledge νline, lineshape, and stability of T, gives kB.
Precision Spectroscopy and Thermometry
2ln 2lineD B
Tk
c M
νν∆ =
Precision Thermometry History using NH3 :
• Previously done with single lines of CO2 lasers
• with huge datasets to reduce the statistic uncertainty
Multi-line fitting
• Remove uncertainty of fit
• Three closely spaced lines
• Same pressure and temperature
Thermometry
Measured temperature dependent on true line shape.
Quantify accuracy of line shape by looking at Td/Tmeasured.
• be independent of pressure
• “= 1”
Linear scaling law (νD~ν0)
• Reduces the statistical uncertainty on Tgas
• Voigt Line shape: TD/Tmeasure has negative slope across a pressure range.
• Demonstrates comb-assisted spectroscopy’s accuracy in observation of collisional effects.
• Voigt profile is insufficient to measure temperature.
• Speed dependent Voigt profile
has 0 slope across pressure range.
Thermometry: Line shape
How about phase-locking of tunable external
cavity QCLs?
Line narrow QCLs ~120 cm-1 tuning range.
-2
-1
0
1
2
Vo
ltag
e
20x10-6151050
Time (s)
Lock Not Engaged Frequency "Lock"Time trace of error signal. Unlocked one rails.
-50
-40
-30
-20
-10
0
10
dB
120100806040200Frequency (MHz)
Spectrum Analyzer: Red: UnlockedBlue: Locked (Div 16=Div 4)
-50
-40
-30
-20
-10
0
10
dB
120100806040200Frequency (MHz)
Spectrum Analyzer: Unlocked
Frequency-locking of EC QCL
EC QCL linewidth = 25 MHz
Error signal of QCL:
Most noise arises from
High bandwidth temperature
control feedback loop
Blue = QCL locked
Linewidth = 3 MHz
CO2 sub-Doppler spectroscopy
4.46 µm CO2
Sub- Doppler spectroscopy with EC QCL
Frequency accurary:
10 - 100 kHz
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
Vo
ltag
e (
V)
2239.02238.52238.02237.52237.0
Frequency (cm-1
)
10-25
10-24
10-23
10-22
10-21
10-20
10-19
10-18
Stre
ng
th (cm
/mo
lecu
le)
Scan of CO2 absorption features
• Low noise OPO combs pumped by
high power Tm fiber combs
• OPOs are ready for prime-time in molecular
spectroscopy
• Ultimate aim is to be better than mass spectrometry
• QCLs locked to fiber combs offer useful tools for
precise line-shape analysis
• Collisional effects in multiple gas systems can now be
studied
Summary
IMRA Prototypes
• Tm, Ho, Yb fiber amplifiers
• Mid-IR laser technology
• frequency conversion
dual Tm comb system
• cw fiber pump lasers
• Control & feedback
High energy Tm fs source