1. Controlling QCLs for Frequency Metrology from the Mid-IR to the THz range
Paolo De Natale
IQCLSW 2018

2. The Playground
Relatore:
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Evento:
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3. Why mid-Infrared Molecular fingerprint region
Fundamental ro-vibrational transitions of simple molecules
Scaling linestrengths with respect to the visible and near-IR spectral regions
Mid-IR frequency combs, narrow-linewidth laser sources

4. The mid-IR Toys Mid infrared coherent sources QCLs (and ICLs)
High-power
~ 1 MHz linewidth
limited tunability
Nonlinear sources (DFG, OPO)
widely tunable
low power (DFG), high power (OPO)
Intrinsically metrological (DFG)
Key light sources for mid infrared sensing and spectroscopy come from Material Science Research: nonlinear sources on one hand, semiconductor devices like Interband and Quantum Cascade Lasers on the other hand.

5. The Frequency Comb Synthesizer

6. Back to the beginnings of Frequency Combs: the role of phase
Bellini&Hänsch Opt. Lett. 25, 1049 (2000)

7. Infrared frequency combs
Classical visible/near-infrared frequency combs:
Alternative:
Ti:Sa bulky OFC
Fiber OFC
microresonator
Relatore:
Dimensions: cm
C2-cascaded very promising:
PRA 91, (2015)
PRL 121, (2018)
Nome relatore
Methods for generating IR-combs:
- nonlinear frequency conversion
- quantum cascade lasers
Evento:
QCL-comb
OPO
DFG

8. Difference Frequency Generation - DFG

9. The Source: DFG in PP-Crystals

10. Referencing a 3-μm source to a MIR comb
Maddaloni et al., New J. Phys. 8, 262 (2006)
Malara et al. Opt. Express 16, 8242 (2008)
Alternative approach using dual-branch Er-doped pulsed fiber laser: Gambetta et al. Opt.Lett. 33, 2671 (2008)

11. Intracavity DFG mid-IR Comb as direct spectroscopic source
Our approach: intracavity DFG
NIR-comb:
mode-locked Ti:Sa with fr ≈ 1 GHz
Bandwidth = 27 nm
Tunability = 4.2 – 5.0 μm
Power = 0.5 mW
Galli et al., Opt. Express 21, (2013).

12. A “simple” Comb-Assisted DFG Setup

13. Cavity-Enhanced Saturation Spectroscopy
CO2 (0001−0000) R(56)
saturated-absorption
line at cm−1
D. Mazzotti, et al., Opt. Lett. 30 , 997 (2005)
uncertainty of 800 Hz (10-11) in the absolute frequency
Main limitation comes from transit time

14. Pushing up Sensitivity by Cavity Enhancement
4.5 mm
FSR = 150 MHz

15. Coupling IR radiation into an enhancement cavity
Cavity width is 9 kHz (measured with cavity ring-down technique)‏
Cavity drift is about 1 kHz/s
measured FWHM:
16 kHz 5 ms)‏
28 kHz 2.4 s)‏
34 kHz 24 s)‏
270 kHz 240 s)‏
Appl. Phys.B 102, (2011)

16. A smart DFG link to Comb
I. Galli et al., Optics Express 17, 9582 (2009)
‏ original scheme proposed by Telle et al., Appl. Phys. B (2002))

17. Narrowing the IR linewidth
Intrinsic IR linewidth is ~ 10 Hz
Integrated IR linewidth is ~ 1 kHz 1 ms)
Ultra-stable, widely tunable and absolutely linked mid-IR coherent source
I. Galli et al., Optics Express 17, 9582 (2009)‏

18. Comb-assisted QCL set-up
Spectroscopy
Sum frequency generation:
Nd:YAG power on PPLN Crystal: 1.2 W
QC Laser power on PPLN Crystal: 2.0 mW
Generated 858 nm: 7 mW
Beat-note detection:
858 nm: 7 mW
Laser Diode power: 100 uW
Beat-note amplitude: -35 dBm; SNR = 45 dB

19. Absolute frequency measurements on CO2
Doppler Spectroscopy
Sub-Doppler Spectroscopy
13CO2 ( ) P(30)
12CO2 ( ) P(30)
kHz-level precision on dip-center
frequency determination
nc = ± 2 MHz
S. Bartalini et al., Opt. Lett. 32, 988 (2007)
S. Borri et al., Opt. Express 16, (2008)

20. Measuring the QCL’s frequency-noise: principle
amplitude fluctuations
Frequency-noise power spectral density
molecular absorption
frequency fluctuations
The discriminator has its own
frequency response.

21. Observing QCL Intrinsic Linewidth
Williams et al., Opt. Lett. 24, 1844 (1999)
S. Bartalini et al., Phys. Rev. Lett. 104 , (2010)
“The intrinsic limits of Quantum-Cascade Lasers”
Physics Today , p.20, (May 2010)
Myers et al., Opt. Lett. 27, 170 (2002)

22. …but linewidth likes it hot!
1/f noise 200 times lower
Intrinsic linewidth: 260 Hz (four times lower)
Current noise from driver more important
Collaboration with Hamamatsu Photonics
For state-of the–art, see Bartalini presentation, this afternoon
Borri et al., IEEE J. Quantum Electron. 47, 984 (2011)

23. Linewidth Results for THz QCLs
THz QCLs are suitable for metrological grade experiments!
f<10 kHZ:
Excess noise not arising
from the current driver
10 kHZ< f<10 MHz
The current noise from the driver is dominant
f>10MHz
White noise: intrinsic LW
Dn = p Nw ~ 90 Hz
Vitiello, M. S. et al. Quantum-limited frequency fluctuations in a terahertz laser. Nat. Photon. 6, 525–528 (2012).

24. How to calculate the Free-running Linewidth
100-ms linewidth: ~ 1.3 MHz
1-ms linewidth: ~ 70 kHz
- Elliot et al., “Extracavity band-shape and bandwidth modification”, Phys. Rev. A 26, 12 (1982)
- Di Domenico et al., “Simple approach to the relation between laser frequency noise and laser line shape”,
Appl. Opt. 49, 4801 (2010)

25. The importance of a low-noise current supply
How large can be the current-noise contribution to the laser linewidth?
250 MHz
1 MHz
25 kHz
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26. Ultra-narrow laser linewidth
by frequency-locking to a stable reference (molecular sub-Doppler feature)...
Doppler-free polarization spectroscopy with a quantum cascade laser at 4.3 mm
S. Bartalini et al.
Optics Express 17, 7440 (2009)‏
… or by phase-locking to a near-IR frequency comb.

27. Injection Locking: the setup

28. Injection Locking: the FNPSD
Power spectra calculated from the FNPSD
FWHM
DFG
21 kHz
Injection-locked QCL
49 kHz
Free-running QCL
2.3 MHz

29. A CaF2 WGMR for mid-IR applications
CaF2 toroidal WGMR
3.6 mm diameter --> FSR= mm
Coupling: Al2O3 (sapphire) prisms
Q-factor ~2 x 1.5 mm (F=250k)
~4 x 107 @ 4.3 mm (F=11k)
~100 ns, L=20 m
1.55 mm laser (Koheras, 4 kHz linewidth)
80kHz mode width

30. QCL frequency locking Optical self-injection locking
Electronic locking
DFB 4.3 mm (Hamamatsu)
700 mA threshold, 10 mW
low-noise home-made current driver
M. Siciliani, S. Borri et al., Laser and Photonics Reviews 10, 153 (2016)

31. Measuring the laser frequency noise
Free-running QCL:
700 kHz FWHM (1 s timescale)
Locked QCL:
15 kHz FWHM (1 s timescale)
10 kHz (1 ms)
M. Siciliani, S. Borri et al., Laser and Photonics Reviews 10, 153 (2016)

32. QCLs: noise suppression
Three techniques:
Bartalini et al., PRL 104, (2010)
Opt. Lett. 37, 1011, (2012)
Opt. Expr. 37, 4811 (2012)
Optical injection locking
Phase lock
Frequency lock
Reference
Comb-referenced DFG source
Molecular absorption line
Linewidth
48 kHz
500 Hz
760 Hz
excess noise
Appl. Phys. Lett. 102, (2013)
Intrinsic linewidth:
inter-subband transition limit
CO2 high resolution spectroscopy
The results
noise
reduction
Molecular Physics, 111, (2013)

33. Accuracy improvement of absolute frequency measurements
From
Doppler-limited direct-absorption spectroscopy
with a free running QCL To
Sub-Doppler saturated-absorption spectroscopy
with a comb-assisted QCL
accuracy improved by
more than 3 orders of magnitude

34. QEPAS signal is proportional to the optical power
Intracavity QEPAS
QEPAS signal is proportional to the optical power
merging QEPAS with Cavity-Enhanced Absorption Spectroscopy
QTF
4.3 μm
Mode-matching Lens
See V. Spagnolo talk on Thursday

35. Intracavity QEPAS-Results
background due to CO2 absorption in air path, removed by lorentzian fit
sensitivity improved by a factor 250 with respect to standard QEPAS
good linearity with concentration
300 ppt detection limit on CO2 (linestrength ~ cm/mol) with 10 seconds averaging time
S.Borri et al., Intracavity quartz-enhanced photoacoustic sensor, Appl. Phys. Lett. 104, (2014)

36. Molecular sensing at parts-per-quadrillion (10-15)
Saturated Absorption Cavity Ringdown-SCAR set-up

37. Linear Cavity Ringdown/Serial measurements vs….
absorption cell
Fabry-Perot Cavity
Fast Photodiode
Laser
AOM
ADC ca
Measurement of the Cavity Ring Down time variations
P Cavity output Power
t off-abs. decay time
t on-abs decay time
gc cavity decay rate
gg gas abs. decay rate
L cavity length
R mirror reflectivity
A other abs. losses
a absorption coefficient

38. Nonlinear Cavity Ringdown/Simultaneous measurements: Saturated absorption Cavity Ringdown-SCaR
I<IS gC
I>IS
empty cavity +
gas absorption
gC + gg
like
empty cavity
Slope increase from region A to B is proportional to the absorption coefficient and hence to the gas concentration
G. Giusfredi et al. Phys. Rev. Lett. 104, (2010)

39. SCAR: the resolution Phys. Rev. Lett. 104, 110801 (2010)
transition of 17OCO
100 mW Power
250 kHz
Phys. Rev. Lett. 104, (2010)

40. Measuring Radiocarbon dioxide at parts-per-quadrillion (10-15)
modern sample
T = 170 K
P = 12 mbar
3 scans, 11 minutes
14N216O
( ) Q(12)e
14C16O2
( ) P(20)
Optica, (2016)
A CNR-Italy spin-off company
now commercializes compact
trace-gas sensors based on this technology:

41. Repeatability CO2 P= 13 mbar T= 170 K
Repeatability over 4 different days
Theory: JOSAB 32, 2223 (2015)
Experiment: Optica 3, 385 (2016)
Single measurement time = 12 min Precision ~3% on natural abundance
Total averaging time = 16 h Precision ~ 0.35% on natural abundance (4 ppq)

42. Moving deeper into the (Far) Infrared

43. Outlook: two parallel approaches to THz metrology
Optical Frequency Comb
Near-IR Optical Frequency COMB
Near-IR Optical Frequency COMB
THz Frequency COMB
Characterization and stabilization
of THz cw sources:
Down
Conversion
THz QCLs
THz QCL combs
THz spectroscopy
Stabilization and referencing
of near-IR cw sources
Down
Conversion
Comb-based Metrological-grade
cw THz source for
THz spectroscopy
THz light
Pump Source(s)
Down Conversion
DFG generation in
a surface linear waveguide
Diapositiva introduttiva con animazioni. Ricordarsi di integrare i contenuti (puntini di sospensione)!

44. A THz Frequency Comb Nome relatore Total power = 1 mW
Usually employed for THz TDS, but…
About 70-80% of the QCL power
effectively phase-locked
S/N = 50 dBm (1 Hz – RBW)
Electronic Bandwidth = 200 kHz
P = 250 mW
≈ 13 ns = 77.5 MHz
Pump laser:
<1 ps
Nome relatore
THz QCL
Total power = 1 mW
Power per tooth = 30 pW
Evento:
zero-CEO frequency
6-octaves-spanning COMB
(more than teeth)
Nome evento
L. Consolino et al., Nat. Comm. 3, 1040 (2012).

45. THz comb-referenced spectroscopy
THz-comb-assisted Spectroscopy
n0 = (10) THz
Precise determination
of the line center
Comparison with state-of-the-art alternative techniques:
S/N > 100
expected
error on nc:
~ 10 kHz
Transition linewidth ~ MHz (FWHM)
Doppler limited spectroscopy
S. Bartalini et al, Phys. Rev. X 4, (2014).

46. QCLs linked to the primary frequency standard
Mid-IR optical reference  Towards a full stabilization of the WGMR
Insero et al., Optics Letters 41, (2016)

47. Measuring CO* frequencies at 11 digits accuracy
Insero et al., Scientific Reports 7, (2017)

48. Thank you for your attention!
The Team
FIRENZE
Paolo De Natale
Saverio Bartalini
Simone Borri
Giulio Campo
Francesco Cappelli
Luigi Consolino
Michele De Regis
Iacopo Galli
Giovanni Giusfredi
Davide Mazzotti
Pablo Cancio Pastor
Thank you for
your attention!
The CNR-INO startup company ( that provided the Ultralow-noise current drivers for powering the QCLs
NAPOLI
Saverio Avino
Maurizio De Rosa
Gianluca Gagliardi
Pasquale Maddaloni
Pietro Malara
Simona Mosca
Maria Parisi
Iolanda Ricciardi