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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 23 — Nov. 18, 2013
  • pp: 28877–28885
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High-coherence mid-infrared frequency comb

I. Galli, F. Cappelli, P. Cancio, G. Giusfredi, D. Mazzotti, S. Bartalini, and P. De Natale  »View Author Affiliations


Optics Express, Vol. 21, Issue 23, pp. 28877-28885 (2013)
http://dx.doi.org/10.1364/OE.21.028877


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Abstract

We report on the generation of a frequency comb around 4330 nm with an unprecedented coherence of the single teeth. Generating the comb within a Ti:sapphire laser cavity by a difference-frequency process and using a phase-lock scheme based on direct digital synthesis, we achieve a tooth linewidth of 2.0 kHz in a 1-s timescale (750 Hz in 20 ms). The generated per-tooth power of 1 μW ranks this comb among the best ever realized in the mid-infrared in terms of power spectral density.

© 2013 OSA

1. Introduction

The optical frequency comb synthesizer (OFCS) is nowadays a well established milestone for visible/near-infrared metrology [1

1. T. Udem, J. Reichert, R. Holzwarth, and T. W. Hänsch, “Absolute optical frequency measurement of the cesium D1line with a mode-locked laser,” Phys. Rev. Lett. 82, 3568–3571 (1999). [CrossRef]

4

4. P. Maddaloni, P. Cancio, and P. De Natale, “Optical comb generators for laser frequency measurement,” Meas. Sci. Technol. 20, 052001 (2009). [CrossRef]

]. Such a device has already found widespread application both as phase/frequency reference, thanks to its absolute traceability, and as source, thanks to its simultaneous broadband coverage and high coherence, providing unprecedented resolution, precision and sensitivity for spectroscopy [5

5. L. Consolino, G. Giusfredi, P. De Natale, M. Inguscio, and P. Cancio, “Optical frequency comb assisted laser system for multiplex precision spectroscopy,” Opt. Express 19, 3155–3162 (2011). [CrossRef] [PubMed]

7

7. S. Avino, A. Giorgini, M. Salza, M. Fabian, G. Gagliardi, and P. De Natale, “Evanescent-wave comb spectroscopy of liquids with strongly dispersive optical fiber cavities,” Appl. Phys. Lett. 102, 201116 (2013). [CrossRef]

].

For MIR-combs, values for total and per-tooth powers of 1.5 W and 30 μW respectively, have been achieved [15

15. F. Adler, K. C. Cossel, M. J. Thorpe, I. Hartl, M. E. Fermann, and J. Ye, “Phase-stabilized, 1.5 W frequency comb at 2.8–4.8 μm,” Opt. Lett. 34, 1330–1332 (2009). [CrossRef] [PubMed]

]. In terms of spectral purity, the best reported values for the teeth linewidth are 30–40 kHz in a 1–2-s timescale [15

15. F. Adler, K. C. Cossel, M. J. Thorpe, I. Hartl, M. E. Fermann, and J. Ye, “Phase-stabilized, 1.5 W frequency comb at 2.8–4.8 μm,” Opt. Lett. 34, 1330–1332 (2009). [CrossRef] [PubMed]

, 24

24. S. A. Meek, A. Poisson, G. Guelachvili, T. W. Hänsch, and N. Picqué, “Fourier transform spectroscopy around 3 μm with a broad difference frequency comb,” Appl. Phys. B (2013). [CrossRef]

]. However, frequency down-conversion of combs from the NIR to the MIR may sometimes severely degrade the coherence [26

26. T. W. Neely, T. A. Johnson, and S. A. Diddams, “High-power broadband laser source tunable from 3.0 μm to 4.4 μm based on a femtosecond Yb:fiber oscillator,” Opt. Lett. 36, 4020–4022 (2011). [CrossRef] [PubMed]

].

In this Letter we present a scheme for generating a highly coherent MIR-comb through an intracavity DFG process [14

14. I. Galli, S. Bartalini, S. Borri, P. Cancio, G. Giusfredi, D. Mazzotti, and P. De Natale, “Ti:sapphire laser intracavity difference-frequency generation of 30 mW cw radiation around 4.5 μm,” Opt. Lett. 35, 3616–3618 (2010). [CrossRef] [PubMed]

]. The 1040-nm portion of the spectrum of a visible/NIR OFCS (NIR-comb) is amplified in an Yb-doped fiber and then mixed with the intracavity radiation of a Ti:sapphire (Ti:Sa) laser in a MgO:PPLN non-linear crystal. A MIR-comb centered around 4330 nm is thus generated.

We demonstrate that excess frequency noise of the NIR-comb can be efficiently removed in the downconverted one by properly using a direct digital synthesis (DDS) scheme. This leads to a 2.0 kHz tooth linewidth (in a 1-s timescale) of the generated MIR-comb. In addition, the high repetition rate (fr = 1 GHz) and the intracavity power-boosted DFG determine an average per-tooth power of 1 μW and thus a power spectral density at the μW/kHz level, comparable to the best results achieved with OPO-based MIR-combs.

2. Experimental setup

Fig. 1 Experimental setup: an Yb fiber amplifier (FA) is seeded by the NIR portion of the spectrum of the NIR-comb. The amplifier output is used as signal in a MgO:PPLN multi-period crystal to generate MIR radiation, where the pump is the Ti:Sa intracavity radiation. The MIR-comb beam is coupled into a high-finesse cavity and beaten with a room temperature distributed feedback quantum cascade laser (DFB-QCL) for characterization purposes. PLL: phase-locked loop, DDS: direct digital synthesis, APD: avalanche photodiode, BSp: beam splitter, PZT: piezoelectric transducer, DM: dichroic mirror, MgO:PPLN: periodically poled lithium niobate crystal doped with magnesium oxide, GM: gold mirror, OC: output coupler, Pol. det.: polarization detection and electronic control loop, BSt: beam stopper, M: mirror, SM: spherical mirror, L: lens, Ti:Sa: titanium sapphire crystal, PD: photovoltaic detector.
Fig. 2 NIR-comb spectrum before and after the Yb fiber amplifier. The 1.6-nm-wide gray region indicates the portion of the spectrum effectively involved in the MIR-comb generation, essentially limited by the phase-matching bandwidth of the DFG process.

The MIR-comb beam is coupled to a high-finesse cavity to study the frequency noise power spectral density (FNPSD) and beaten with a previously frequency calibrated QCL at 4.33 μm in order to characterize its frequency components. A sequence of beat-notes spaced by 1 GHz are measured as the QCL frequency is scanned. This confirms the value of the center wavelength emission of the generated MIR-comb and the value of fr as expected, otherwise difficult to measure due to the lack of fast photodetectors in the MIR region.

3. Characterization

3.1. The MIR-comb teeth

The 1-m-long high-finesse cavity (free spectral range FSR = 150 MHz) is made of two planoconcave ZnSe mirrors, with high-reflectivity coatings on the concave surfaces (6 m radius of curvature) [13

13. I. Galli, S. Bartalini, P. Cancio, G. Giusfredi, D. Mazzotti, and P. De Natale, “Ultra-stable, widely tunable and absolutely linked mid-IR coherent source,” Opt. Express 17, 9582–9587 (2009). [CrossRef] [PubMed]

]. At this wavelength (4.33 μm) the finesse is 8000. To maximize the transmitted signal we have matched fr with the following Vernier ratio [28

28. F. Adler, M. J. Thorpe, K. C. Cossel, and J. Ye, “Cavity-enhanced direct frequency comb spectroscopy: Technology and applications,” Annu. Rev. Anal. Chem. 3, 175–205 (2010). [CrossRef]

]:
fr=203FSR=203c2L0
(6)
which corresponds to a given cavity length L0. In this condition, the cavity selects a subset of comb teeth (one every three), that gives rise to the peak shown in Fig. 3(a) (the first peak of Fig. 3(b)). While keeping fr constant, if the cavity length L is changed, other resonances can be observed between the comb and the cavity, for L1 = L0 + λ/6 (second peak), L2 = L0 + λ/3 (third peak) and L3 = L0 + λ/2 (fourth peak), where λ is the mean wavelength of the radiation [Fig. 3(b)]. However, the condition expressed in eq. (6) is valid only for the first subset of teeth. For the other cavity lengths Ln only the central comb tooth m0n of the corresponding subset is resonant with a cavity mode. Neglecting the cavity dispersion, the frequency mismatch of the teeth m (again one every three) with the nearest cavity resonance is given by [ fr − (20/3)(c/2Ln)](mm0n). Therefore the width of the peak at L = Ln is
Wpk=(fr203c2Ln)Mtot
(7)
where Mtot is the total number of the MIR-comb teeth. Eq. (7) allows to estimate Mtot. As an example, the measured width of the third peak is Wpk = 630 kHz FWHM. Since in this case
fr203c2(L0+λ/3)=1.44kHz
(8)
a total teeth number Mtot = 440 is obtained. Calculations on the other peaks give consistent results that, recalling the total comb power, allow to estimate an average per-tooth power of 1 μW. This is also in agreement with the expected spectral coverage of the MIR-comb retrieved by the phase-matching bandwidth of the DFG process at this wavelength.

Fig. 3 Transmission peaks of the high-finesse cavity recorded for different cavity detunings. a) The peak corresponding to eq. (6), recorded in two different ECDL phase lock schemes: with DDS implemented (black line, 30 kHz FWHM), and with simple phase lock to the nearest NIR-comb tooh (red line, 400 kHz FWHM). b) 1-FSR-wide cavity scan (inset) with zooms on consecutive longitudinal resonances spaced by FSR/3 = 50 MHz.

3.2. MIR-comb coherence

In order to estimate the coherence of the MIR-comb we have used the high-finesse cavity as frequency-to-amplitude converter to retrieve the FNPSD of the radiation in the condition established by eq. (6). The factor used to convert amplitude fluctuations to frequency fluctuations is the slope of the first peak of Fig. 3 at half maximum. Due to the photons average lifetime in the cavity, the latter acts as a second order low-pass filter with a 9.4 kHz cutoff frequency. The spectrum reported in Fig. 4 is compensated for this cutoff. Using Elliott’s formula to calculate the linewidth of the comb teeth [29

29. D. S. Elliott, R. Roy, and S. J. Smith, “Extracavity laser band-shape and bandwidth modification,” Phys. Rev. A 26, 12–18 (1982). [CrossRef]

], we obtain a value of 2.0 kHz FWHM in a 1-s timescale and 750 Hz in a 20-ms timescale. This is to our knowledge the narrowest measured linewidth of a MIR-comb. Taking into account the MIR-comb power, we calculate a per-tooth power spectral density of 0.5 μW/kHz (in a 1-s timescale), which is comparable with the best values obtained with OPO-based MIR-combs [15

15. F. Adler, K. C. Cossel, M. J. Thorpe, I. Hartl, M. E. Fermann, and J. Ye, “Phase-stabilized, 1.5 W frequency comb at 2.8–4.8 μm,” Opt. Lett. 34, 1330–1332 (2009). [CrossRef] [PubMed]

, 16

16. K. L. Vodopyanov, E. Sorokin, I. T. Sorokina, and P. G. Schunemann, “Mid-IR frequency comb source spanning 4.4–5.4 μm based on subharmonic GaAs optical parametric oscillator,” Opt. Lett. 36, 2275–2277 (2011). [CrossRef] [PubMed]

]. It is worth noting that this power level is in a range suitable for direct comb spectroscopy in this spectral region.

Fig. 4 FNPSD related to the MIR-comb radiation retrieved by using the high-finesse cavity as frequency-to-amplitude converter. The spectrum analyzer was set in max-hold acquisition mode to be sure to collect the maximum amplitude for each frequency interval. The spectrum is compensated for the 9.4 kHz cavity cutoff.
Inset: profiles of the MIR-comb power spectrum in different timescales, calculated using Elliott’s formula [29].

4. Conclusion

In conclusion we have demonstrated a highly coherent MIR-comb centered around 4.33 μm, with a tooth linewidth of 2.0 kHz and 750 Hz in a timescale of 1 s and 20 ms, respectively. An average power of 1 μW for each single tooth was achieved, which means a per-tooth power spectral density of 0.5 μW/kHz (in a 1-s timescale). The generated spectrum spans 27 nm, with a center wavelength tunable from 4.2 to 5.0 μm.

The combination of such unprecedented spectral features enables particularly demanding applications, including its use as broadband MIR source for direct comb spectroscopy [30

30. E. Baumann, F. R. Giorgetta, W. C. Swann, A. M. Zolot, I. Coddington, and N. R. Newbury, “Spectroscopy of the methane ν3band with an accurate midinfrared coherent dual-comb spectrometer,” Phys. Rev. A 84, 062513 (2011). [CrossRef]

] or for manipulation of cold molecular samples, which exhibit transition linewidths below 1 kHz [31

31. L. D. Carr, D. DeMille, R. V. Krems, and J. Ye, “Cold and ultracold molecules: science, technology and applications,” New J. Phys. 11, 055049 (2009). [CrossRef]

].

Acknowledgments

This work was financially supported by Ente Cassa di Risparmio di Firenze, by the Laserlab-Europe Consortium within the ALADIN project framework, by the Extreme Light Infrastructure (ELI) European project and by the Progetto Operativo Nazionale (PON) PON01_01525 Monitoraggio Innovativo per le Coste e l’Ambiente Marino (MONICA) funded by the Italian Ministry of Education, University and Research (MIUR).

References and links

1.

T. Udem, J. Reichert, R. Holzwarth, and T. W. Hänsch, “Absolute optical frequency measurement of the cesium D1line with a mode-locked laser,” Phys. Rev. Lett. 82, 3568–3571 (1999). [CrossRef]

2.

T. Udem, J. Reichert, R. Holzwarth, and T. W. Hänsch, “Accurate measurement of large optical frequency differences with a mode-locked laser,” Opt. Lett. 24, 881–883 (1999). [CrossRef]

3.

S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, T. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84, 5102–5105 (2000). [CrossRef] [PubMed]

4.

P. Maddaloni, P. Cancio, and P. De Natale, “Optical comb generators for laser frequency measurement,” Meas. Sci. Technol. 20, 052001 (2009). [CrossRef]

5.

L. Consolino, G. Giusfredi, P. De Natale, M. Inguscio, and P. Cancio, “Optical frequency comb assisted laser system for multiplex precision spectroscopy,” Opt. Express 19, 3155–3162 (2011). [CrossRef] [PubMed]

6.

A. Marian, M. C. Stowe, J. R. Lawall, D. Felinto, and J. Ye, “United time-frequency spectroscopy for dynamics and global structure,” Science 306, 2063–2068 (2004). [CrossRef] [PubMed]

7.

S. Avino, A. Giorgini, M. Salza, M. Fabian, G. Gagliardi, and P. De Natale, “Evanescent-wave comb spectroscopy of liquids with strongly dispersive optical fiber cavities,” Appl. Phys. Lett. 102, 201116 (2013). [CrossRef]

8.

I. Galli, S. Bartalini, S. Borri, P. Cancio, D. Mazzotti, P. De Natale, and G. Giusfredi, “Molecular gas sensing below parts per trillion: Radiocarbon-dioxide optical detection,” Phys. Rev. Lett. 107, 270802 (2011). [CrossRef]

9.

A. Schliesser, N. Picqué, and T. W. Hänsch, “Mid-infrared frequency combs,” Nat. Photon. 6, 440–449 (2012). [CrossRef]

10.

A. Hugi, G. Villares, S. Blaser, H. C. Liu, and J. Faist, “Mid-infrared frequency comb based on a quantum cascade laser,” Nature 492, 229–233 (2012). [CrossRef] [PubMed]

11.

A. A. Savchenkov, D. Eliyahu, W. Liang, V. S. Ilchenko, J. Byrd, A. B. Matsko, D. Seidel, and L. Maleki, “Stabilization of a Kerr frequency comb oscillator,” Opt. Lett. 38, 2636–2639 (2013). [CrossRef] [PubMed]

12.

D. Mazzotti, P. Cancio, G. Giusfredi, P. De Natale, and M. Prevedelli, “Frequency-comb-based absolute frequency measurements in the mid-infrared with a difference-frequency spectrometer,” Opt. Lett. 30, 997–999 (2005). [CrossRef] [PubMed]

13.

I. Galli, S. Bartalini, P. Cancio, G. Giusfredi, D. Mazzotti, and P. De Natale, “Ultra-stable, widely tunable and absolutely linked mid-IR coherent source,” Opt. Express 17, 9582–9587 (2009). [CrossRef] [PubMed]

14.

I. Galli, S. Bartalini, S. Borri, P. Cancio, G. Giusfredi, D. Mazzotti, and P. De Natale, “Ti:sapphire laser intracavity difference-frequency generation of 30 mW cw radiation around 4.5 μm,” Opt. Lett. 35, 3616–3618 (2010). [CrossRef] [PubMed]

15.

F. Adler, K. C. Cossel, M. J. Thorpe, I. Hartl, M. E. Fermann, and J. Ye, “Phase-stabilized, 1.5 W frequency comb at 2.8–4.8 μm,” Opt. Lett. 34, 1330–1332 (2009). [CrossRef] [PubMed]

16.

K. L. Vodopyanov, E. Sorokin, I. T. Sorokina, and P. G. Schunemann, “Mid-IR frequency comb source spanning 4.4–5.4 μm based on subharmonic GaAs optical parametric oscillator,” Opt. Lett. 36, 2275–2277 (2011). [CrossRef] [PubMed]

17.

I. Ricciardi, E. De Tommasi, P. Maddaloni, S. Mosca, A. Rocco, J.-J. Zondy, M. De Rosa, and P. De Natale, “Frequency-comb-referenced singly-resonant OPO for sub-doppler spectroscopy,” Opt. Express 20, 9178–9186 (2012). [CrossRef] [PubMed]

18.

S. Borri, I. Galli, F. Cappelli, A. Bismuto, S. Bartalini, P. Cancio, G. Giusfredi, D. Mazzotti, J. Faist, and P. De Natale, “Direct link of a mid-infrared QCL to a frequency comb by optical injection,” Opt. Lett. 37, 1011–1013 (2012). [CrossRef] [PubMed]

19.

I. Galli, M. S. de Cumis, F. Cappelli, S. Bartalini, D. Mazzotti, S. Borri, A. Montori, N. Akikusa, M. Yamanishi, G. Giusfredi, P. Cancio, and P. De Natale, “Comb-assisted subkilohertz linewidth quantum cascade laser for high-precision mid-infrared spectroscopy,” Appl. Phys. Lett. 102, 121117 (2013). [CrossRef]

20.

S. Bartalini, P. Cancio, G. Giusfredi, D. Mazzotti, P. De Natale, S. Borri, I. Galli, T. Leveque, and L. Gianfrani, “Frequency-comb-referenced quantum-cascade laser at 4.4 μm,” Opt. Lett. 32, 988–990 (2007). [CrossRef] [PubMed]

21.

A. Mills, D. Gatti, J. Jiang, C. Mohr, W. Mefford, L. Gianfrani, M. Fermann, I. Hartl, and M. Marangoni, “Coherent phase lock of a 9 μm quantum cascade laser to a 2 μm thulium optical frequency comb,” Opt. Lett. 37, 4083–4085 (2012). [CrossRef] [PubMed]

22.

P. Maddaloni, P. Malara, G. Gagliardi, and P. De Natale, “Mid-infrared fibre-based optical comb,” New J. Phys. 8, 1–8 (2006). [CrossRef]

23.

A. Ruehl, A. Gambetta, I. Hartl, M. E. Fermann, K. S. E. Eikema, and M. Marangoni, “Widely-tunable mid-infrared frequency comb source based on difference frequency generation,” Opt. Lett. 37, 2232–2234 (2012). [CrossRef] [PubMed]

24.

S. A. Meek, A. Poisson, G. Guelachvili, T. W. Hänsch, and N. Picqué, “Fourier transform spectroscopy around 3 μm with a broad difference frequency comb,” Appl. Phys. B (2013). [CrossRef]

25.

F. Zhu, H. Hundertmark, A. A. Kolomenskii, J. Strohaber, R. Holzwarth, and H. A. Schuessler, “High-power mid-infrared frequency comb source based on a femtosecond Er:fiber oscillator,” Opt. Lett. 38, 2360–2362 (2013). [CrossRef] [PubMed]

26.

T. W. Neely, T. A. Johnson, and S. A. Diddams, “High-power broadband laser source tunable from 3.0 μm to 4.4 μm based on a femtosecond Yb:fiber oscillator,” Opt. Lett. 36, 4020–4022 (2011). [CrossRef] [PubMed]

27.

T. W. Hänsch and B. Couillaud, “Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity,” Opt. Commun. 35, 441–444 (1980). [CrossRef]

28.

F. Adler, M. J. Thorpe, K. C. Cossel, and J. Ye, “Cavity-enhanced direct frequency comb spectroscopy: Technology and applications,” Annu. Rev. Anal. Chem. 3, 175–205 (2010). [CrossRef]

29.

D. S. Elliott, R. Roy, and S. J. Smith, “Extracavity laser band-shape and bandwidth modification,” Phys. Rev. A 26, 12–18 (1982). [CrossRef]

30.

E. Baumann, F. R. Giorgetta, W. C. Swann, A. M. Zolot, I. Coddington, and N. R. Newbury, “Spectroscopy of the methane ν3band with an accurate midinfrared coherent dual-comb spectrometer,” Phys. Rev. A 84, 062513 (2011). [CrossRef]

31.

L. D. Carr, D. DeMille, R. V. Krems, and J. Ye, “Cold and ultracold molecules: science, technology and applications,” New J. Phys. 11, 055049 (2009). [CrossRef]

OCIS Codes
(140.3070) Lasers and laser optics : Infrared and far-infrared lasers
(140.4050) Lasers and laser optics : Mode-locked lasers
(190.4223) Nonlinear optics : Nonlinear wave mixing

ToC Category:
Nonlinear Optics

History
Original Manuscript: September 16, 2013
Revised Manuscript: October 18, 2013
Manuscript Accepted: October 21, 2013
Published: November 15, 2013

Virtual Issues
Nonlinear Optics (2013) Optics Express

Citation
I. Galli, F. Cappelli, P. Cancio, G. Giusfredi, D. Mazzotti, S. Bartalini, and P. De Natale, "High-coherence mid-infrared frequency comb," Opt. Express 21, 28877-28885 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-23-28877


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References

  1. T. Udem, J. Reichert, R. Holzwarth, and T. W. Hänsch, “Absolute optical frequency measurement of the cesium D1line with a mode-locked laser,” Phys. Rev. Lett.82, 3568–3571 (1999). [CrossRef]
  2. T. Udem, J. Reichert, R. Holzwarth, and T. W. Hänsch, “Accurate measurement of large optical frequency differences with a mode-locked laser,” Opt. Lett.24, 881–883 (1999). [CrossRef]
  3. S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, T. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett.84, 5102–5105 (2000). [CrossRef] [PubMed]
  4. P. Maddaloni, P. Cancio, and P. De Natale, “Optical comb generators for laser frequency measurement,” Meas. Sci. Technol.20, 052001 (2009). [CrossRef]
  5. L. Consolino, G. Giusfredi, P. De Natale, M. Inguscio, and P. Cancio, “Optical frequency comb assisted laser system for multiplex precision spectroscopy,” Opt. Express19, 3155–3162 (2011). [CrossRef] [PubMed]
  6. A. Marian, M. C. Stowe, J. R. Lawall, D. Felinto, and J. Ye, “United time-frequency spectroscopy for dynamics and global structure,” Science306, 2063–2068 (2004). [CrossRef] [PubMed]
  7. S. Avino, A. Giorgini, M. Salza, M. Fabian, G. Gagliardi, and P. De Natale, “Evanescent-wave comb spectroscopy of liquids with strongly dispersive optical fiber cavities,” Appl. Phys. Lett.102, 201116 (2013). [CrossRef]
  8. I. Galli, S. Bartalini, S. Borri, P. Cancio, D. Mazzotti, P. De Natale, and G. Giusfredi, “Molecular gas sensing below parts per trillion: Radiocarbon-dioxide optical detection,” Phys. Rev. Lett.107, 270802 (2011). [CrossRef]
  9. A. Schliesser, N. Picqué, and T. W. Hänsch, “Mid-infrared frequency combs,” Nat. Photon.6, 440–449 (2012). [CrossRef]
  10. A. Hugi, G. Villares, S. Blaser, H. C. Liu, and J. Faist, “Mid-infrared frequency comb based on a quantum cascade laser,” Nature492, 229–233 (2012). [CrossRef] [PubMed]
  11. A. A. Savchenkov, D. Eliyahu, W. Liang, V. S. Ilchenko, J. Byrd, A. B. Matsko, D. Seidel, and L. Maleki, “Stabilization of a Kerr frequency comb oscillator,” Opt. Lett.38, 2636–2639 (2013). [CrossRef] [PubMed]
  12. D. Mazzotti, P. Cancio, G. Giusfredi, P. De Natale, and M. Prevedelli, “Frequency-comb-based absolute frequency measurements in the mid-infrared with a difference-frequency spectrometer,” Opt. Lett.30, 997–999 (2005). [CrossRef] [PubMed]
  13. I. Galli, S. Bartalini, P. Cancio, G. Giusfredi, D. Mazzotti, and P. De Natale, “Ultra-stable, widely tunable and absolutely linked mid-IR coherent source,” Opt. Express17, 9582–9587 (2009). [CrossRef] [PubMed]
  14. I. Galli, S. Bartalini, S. Borri, P. Cancio, G. Giusfredi, D. Mazzotti, and P. De Natale, “Ti:sapphire laser intracavity difference-frequency generation of 30 mW cw radiation around 4.5 μm,” Opt. Lett.35, 3616–3618 (2010). [CrossRef] [PubMed]
  15. F. Adler, K. C. Cossel, M. J. Thorpe, I. Hartl, M. E. Fermann, and J. Ye, “Phase-stabilized, 1.5 W frequency comb at 2.8–4.8 μm,” Opt. Lett.34, 1330–1332 (2009). [CrossRef] [PubMed]
  16. K. L. Vodopyanov, E. Sorokin, I. T. Sorokina, and P. G. Schunemann, “Mid-IR frequency comb source spanning 4.4–5.4 μm based on subharmonic GaAs optical parametric oscillator,” Opt. Lett.36, 2275–2277 (2011). [CrossRef] [PubMed]
  17. I. Ricciardi, E. De Tommasi, P. Maddaloni, S. Mosca, A. Rocco, J.-J. Zondy, M. De Rosa, and P. De Natale, “Frequency-comb-referenced singly-resonant OPO for sub-doppler spectroscopy,” Opt. Express20, 9178–9186 (2012). [CrossRef] [PubMed]
  18. S. Borri, I. Galli, F. Cappelli, A. Bismuto, S. Bartalini, P. Cancio, G. Giusfredi, D. Mazzotti, J. Faist, and P. De Natale, “Direct link of a mid-infrared QCL to a frequency comb by optical injection,” Opt. Lett.37, 1011–1013 (2012). [CrossRef] [PubMed]
  19. I. Galli, M. S. de Cumis, F. Cappelli, S. Bartalini, D. Mazzotti, S. Borri, A. Montori, N. Akikusa, M. Yamanishi, G. Giusfredi, P. Cancio, and P. De Natale, “Comb-assisted subkilohertz linewidth quantum cascade laser for high-precision mid-infrared spectroscopy,” Appl. Phys. Lett.102, 121117 (2013). [CrossRef]
  20. S. Bartalini, P. Cancio, G. Giusfredi, D. Mazzotti, P. De Natale, S. Borri, I. Galli, T. Leveque, and L. Gianfrani, “Frequency-comb-referenced quantum-cascade laser at 4.4 μm,” Opt. Lett.32, 988–990 (2007). [CrossRef] [PubMed]
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