## High-coherence mid-infrared frequency comb |

Optics Express, Vol. 21, Issue 23, pp. 28877-28885 (2013)

http://dx.doi.org/10.1364/OE.21.028877

Acrobat PDF (1117 KB)

### 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

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

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]

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]

*μ*W 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]

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. 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]

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]

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]

*f*= 1 GHz) and the intracavity power-boosted DFG determine an average per-tooth power of 1

_{r}*μ*W and thus a power spectral density at the

*μ*W/kHz level, comparable to the best results achieved with OPO-based MIR-combs.

## 2. Experimental setup

*N*×

_{s}*f*, with

_{r}*N*order number of one of the down-converted NIR teeth) must be replicated by the

_{s}*pump*frequency, to be canceled by difference. An intermediate stable oscillator operating around

*signal*frequencies (a Nd:YAG laser) and a DDS electronic scheme similar to that described in [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]

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]

*ν*and

_{p}*ν*respectively) are beaten with the NIR-comb nearest teeth and the beat note frequencies are where

_{Y}*f*and

_{p}*f*are the beat note frequencies of the ECDL and the Nd:YAG respectively,

_{Y}*f*and

_{r}*f*are the repetition rate and the carrier envelope offset of the NIR-comb respectively, and

_{o}*N*is the integer identifying the tooth used for the beat note.

_{x}*f*is canceled from

_{o}*f*using standard RF mixing, then the DDS multiplies

_{Y}*f*+

_{Y}*f*by a factor (

_{o}*N*−

_{p}*N*)/

_{s}*N*. A phase-locked loop (PPL2) is used to control the

_{Y}*pump*beat-note

*f*against the DDS output with a large bandwidth (2 MHz). In these conditions

_{p}*f*= (

_{p}*f*+

_{Y}*f*)(

_{o}*N*−

_{p}*N*)/

_{s}*N*and using eq. (1) and (2) the

_{Y}*pump*frequency can be written as and the frequency of the MIR-comb tooth (

*idler*) obtained as difference between the

*pump*laser and the

*signal*NIR-comb tooth results It is worth noting that an absolute frequency traceability of the generated

*ν*is obtained by controlling the frequency of the Nd:YAG laser against the nearest tooth of the NIR-comb: a PLL (PLL1 in Fig. 1) with a bandwidth of 10 Hz corrects for Nd:YAG frequency drifts, without perturbing its linewidth. Moreover, from eq. (4) we note that the frequency fluctuations of

_{i}*ν*depends only on

_{i}*ν*and are independent from NIR-comb parameters (

_{Y}*f*and

_{r}*f*), giving narrow MIR-comb teeth. The factor (

_{o}*N*−

_{p}*N*)/

_{s}*N*is about 1/4 in our case and, considering a Nd:YAG laser linewidth of about 5 kHz in a 1-s timescale, a linewidth of about 1 kHz of the MIR-comb

_{Y}*N*teeth is expected. It is important to observe that a perfect cancellation of

_{s}*f*can only be obtained for the tooth

_{r}*N*, whereas, for the tooth

_{s}*N*+

_{s}*m*(

*m*is the integer that enumerates the MIR-comb teeth), the additional fluctuations amount to

*m*×

*δf*. The fluctuations for a generic MIR-comb tooth can be expressed as In our case the two terms are of the same order of magnitude, thus contributing to the noise at the same level (see the next section).

_{r}*μ*m 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

*f*as expected, otherwise difficult to measure due to the lack of fast photodetectors in the MIR region.

_{r}## 3. Characterization

### 3.1. The MIR-comb teeth

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]

*μ*m) the finesse is 8000. To maximize the transmitted signal we have matched

*f*with the following Vernier ratio [28

_{r}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]

*L*

_{0}. 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

*f*constant, if the cavity length

_{r}*L*is changed, other resonances can be observed between the comb and the cavity, for

*L*

_{1}=

*L*

_{0}+

*λ*/6 (second peak),

*L*

_{2}=

*L*

_{0}+

*λ*/3 (third peak) and

*L*

_{3}=

*L*

_{0}+

*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

*L*only the central comb tooth

_{n}*m*

_{0}

*of the corresponding subset is resonant with a cavity mode. Neglecting the cavity dispersion, the frequency mismatch of the teeth*

_{n}*m*(again one every three) with the nearest cavity resonance is given by [

*f*− (20/3)(

_{r}*c*/2

*L*)](

_{n}*m*−

*m*

_{0}

*). Therefore the width of the peak at*

_{n}*L*=

*L*is where

_{n}*M*

_{tot}is the total number of the MIR-comb teeth. Eq. (7) allows to estimate

*M*

_{tot}. As an example, the measured width of the third peak is

*W*

_{pk}= 630 kHz FWHM. Since in this case a total teeth number

*M*

_{tot}= 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

*μ*W. 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.

### 3.2. MIR-comb coherence

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]

*μ*W/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. 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]

## 4. Conclusion

*μ*m, 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

*μ*W for each single tooth was achieved, which means a per-tooth power spectral density of 0.5

*μ*W/kHz (in a 1-s timescale). The generated spectrum spans 27 nm, with a center wavelength tunable from 4.2 to 5.0

*μ*m.

30. E. Baumann, F. R. Giorgetta, W. C. Swann, A. M. Zolot, I. Coddington, and N. R. Newbury, “Spectroscopy of the methane ν_{3}band 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]

## Acknowledgments

## References and links

1. | T. Udem, J. Reichert, R. Holzwarth, and T. W. Hänsch, “Absolute optical frequency measurement of the cesium D |

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. |

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. |

4. | P. Maddaloni, P. Cancio, and P. De Natale, “Optical comb generators for laser frequency measurement,” Meas. Sci. Technol. |

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 |

6. | A. Marian, M. C. Stowe, J. R. Lawall, D. Felinto, and J. Ye, “United time-frequency spectroscopy for dynamics and global structure,” Science |

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. |

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. |

9. | A. Schliesser, N. Picqué, and T. W. Hänsch, “Mid-infrared frequency combs,” Nat. Photon. |

10. | A. Hugi, G. Villares, S. Blaser, H. C. Liu, and J. Faist, “Mid-infrared frequency comb based on a quantum cascade laser,” Nature |

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. |

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. |

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 |

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. |

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. |

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. |

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 |

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. |

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. |

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. |

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. |

22. | P. Maddaloni, P. Malara, G. Gagliardi, and P. De Natale, “Mid-infrared fibre-based optical comb,” New J. Phys. |

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. |

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. |

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. |

27. | T. W. Hänsch and B. Couillaud, “Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity,” Opt. Commun. |

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. |

29. | D. S. Elliott, R. Roy, and S. J. Smith, “Extracavity laser band-shape and bandwidth modification,” Phys. Rev. A |

30. | E. Baumann, F. R. Giorgetta, W. C. Swann, A. M. Zolot, I. Coddington, and N. R. Newbury, “Spectroscopy of the methane ν |

31. | L. D. Carr, D. DeMille, R. V. Krems, and J. Ye, “Cold and ultracold molecules: science, technology and applications,” New J. Phys. |

**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

- 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]
- 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]
- 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]
- P. Maddaloni, P. Cancio, and P. De Natale, “Optical comb generators for laser frequency measurement,” Meas. Sci. Technol.20, 052001 (2009). [CrossRef]
- 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]
- 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]
- 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]
- 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]
- A. Schliesser, N. Picqué, and T. W. Hänsch, “Mid-infrared frequency combs,” Nat. Photon.6, 440–449 (2012). [CrossRef]
- 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]
- 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]
- 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]
- 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]
- 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]
- 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]
- 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]
- 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]
- 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]
- 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]
- 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]
- 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]
- P. Maddaloni, P. Malara, G. Gagliardi, and P. De Natale, “Mid-infrared fibre-based optical comb,” New J. Phys.8, 1–8 (2006). [CrossRef]
- 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]
- 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]
- 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]
- 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]
- T. W. Hänsch and B. Couillaud, “Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity,” Opt. Commun.35, 441–444 (1980). [CrossRef]
- 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]
- D. S. Elliott, R. Roy, and S. J. Smith, “Extracavity laser band-shape and bandwidth modification,” Phys. Rev. A26, 12–18 (1982). [CrossRef]
- 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. A84, 062513 (2011). [CrossRef]
- 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]

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