Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Phase-locking of a 2.7-THz quantum cascade laser to a mode-locked erbium-doped fibre laser


Mode-locked femtosecond lasers have revolutionized the field of optical metrology by allowing the realization of ultra-stable phase-coherent links between the optical-frequency domain and the radiofrequency range1,2,3,4. In this work we have used the electro-optic effect in ZnTe (ref. 5) to demonstrate that the frequency and the phase of a 2.7 THz quantum cascade laser6 can be actively stabilized to the nth harmonic of the 90 MHz repetition rate (frep) of a commercial, mode-locked erbium-doped fibre laser7. The beating between the stabilized quantum cascade laser frequency and the harmonic of frep yield a signal-to-noise ratio of 80 dB in a bandwidth of 1 Hz. The technique is inherently broadband, that is, it is applicable to any quantum cascade laser source provided that its frequency falls within the spectral bandwidth of the femtosecond laser (5 THz)8,9. Furthermore, it is an ideal tool with which to control the phase of different quantum cascade lasers using light and compact fibre technology rather than superconducting bolometer mixers10,11.

This is a preview of subscription content, access via your institution

Access options

Rent or buy this article

Prices vary by article type



Prices may be subject to local taxes which are calculated during checkout

Figure 1: Schematic of the electro-optic detection set-up.
Figure 2: Principle of the detection technique.
Figure 3: QCL electrical and optical characteristics.
Figure 4: Experimental set-up.
Figure 5: RF spectra.


  1. Udem, Th., Holzwarth, R. & Hansch, T. W. Optical frequency metrology. Nature 416, 233–237 (2002).

    ADS  Google Scholar 

  2. Jones, D. J. et al. Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis. Science 288, 635–699 (2000).

    Article  ADS  Google Scholar 

  3. Reichert, J. et al. Phase coherent vacuum-ultraviolet to radio frequency comparison with a mode-locked laser. Phys. Rev. Lett. 84, 3232–3235 (2000).

    Article  ADS  Google Scholar 

  4. Maddaloni, P., Cancio, P. & De Natale, P. Optical comb generators for laser frequency measurement. Meas. Sci. Technol. 20, 1–19 (2009).

    Article  Google Scholar 

  5. Sakai, K. (Ed.) Terahertz Optoelectronics (Springer, 2005).

    Book  Google Scholar 

  6. Barbieri, S. et al. 2.9 THz quantum cascade lasers operating up to 70 K in continuous wave. Appl. Phys. Lett. 85, 1674–1676 (2004).

    Article  ADS  Google Scholar 

  7. Loffler, T. et al. Continuous wave THz imaging with a hybrid system. Appl. Phys. Lett. 90, 091111 (2007).

    Article  ADS  Google Scholar 

  8. Amy-Klein, A. et al. Absolute frequency measurement in the 28-THz spectral region with a femtosecond laser comb and a long-distance optical link to a primary standard. Appl. Phys. B 78, 25–27 (2004).

    Article  ADS  Google Scholar 

  9. Amy-Klein, A. et al. Absolute frequency measurement of a SF6 two-photon line by use of a femtosecond optical comb and sum-frequency generation. Opt. Lett. 30, 3320–3322 (2005).

    Article  ADS  Google Scholar 

  10. Rabanus, D. et al. Phase locking of a 1.5 THz quantum cascade laser and use as a local oscillator in a heterodyne HEB receiver. Opt. Express 17, 1159–1168 (2009).

    Article  ADS  Google Scholar 

  11. Khosropanah, P. et al. Phase locking of a 2.7 THz quantum cascade laser to a microwave reference. Opt. Lett. 34, 2958–2960 (2009).

    Article  ADS  Google Scholar 

  12. Mittleman, D. M. Sensing with THz Radiation (Springer, 2003).

    Book  Google Scholar 

  13. Reix, J.-M. et al. The Hershel/Planck programme, technical challenges for two science missions, successfully launched. Acta Astronomica 34, 130–148 (2009).

    Google Scholar 

  14. Tonouchi, M. Cutting edge terahertz technology. Nature Photon. 1, 97–105 (2007).

    Article  ADS  Google Scholar 

  15. Capasso, F. et al. Quantum cascade lasers: ultrahigh-speed operation, optical wireless communication, narrow linewidth and far-infrared emission. IEEE J. Quantum Electron. 38, 511–532 (2002).

    Article  ADS  Google Scholar 

  16. Köhler, R. et al. Terahertz semiconductor-heterostructure laser. Nature 417, 156–159 (2002).

    Article  ADS  Google Scholar 

  17. Williams, B. S. Terahertz quantum cascade lasers. Nature Photon. 1, 517–525 (2007).

    Article  ADS  Google Scholar 

  18. Yasui, T. et al. Real-time monitoring of continuous-wave terahertz radiation using a fiber-based, terahertz-comb-referenced spectrum analyzer. Opt. Express 17, 17034–17043 (2009).

    Article  ADS  Google Scholar 

  19. Duvillaret, L., Raillant, S. & Coutaz, J.-L. Electro-optic sensors for electric field measurements. I. Theoretical comparison among different modulation techniques. J. Opt. Soc. Am. B 19, 2692–2702 (2002).

    Article  ADS  Google Scholar 

  20. Nahata, A., Weling, A. S. & Heinz, T. F. A wideband coherent terahertz spectroscopy system using optical rectification and electro-optic sampling. Appl. Phys. Lett. 69, 2321–2323 (1996).

    Article  ADS  Google Scholar 

  21. Barbieri, S. et al. Heterodyne mixing of two far-infrared quantum cascade lasers by use of a point-contact Schottky diode. Opt. Lett. 29, 1632–1634 (2004).

    Article  ADS  Google Scholar 

  22. Barkan, A. et al. Linewidth and tuning characteristics of terahertz quantum cascade lasers. Opt. Lett. 29, 575–577 (2004).

    Article  ADS  Google Scholar 

  23. Hensley, J. M. et al. Spectral behaviour of a terahertz quantum cascade laser. Opt. Express 17, 20476–20483 (2009).

    Article  ADS  Google Scholar 

  24. Demichel, O. et al. Surface plasmon photonic structures in terahertz quantum cascade lasers. Opt. Express 14, 5335–5345 (2006).

    Article  ADS  Google Scholar 

  25. Santarelli, G. et al. Heterodyne optical phase-locking of extended-cavity semiconductor lasers at 9 GHz. Opt. Commun. 104, 339–344 (1994).

    Article  ADS  Google Scholar 

  26. Blanchard, A. Phase-Locked Loops, Ch. 12 (Wiley, 1976).

    Google Scholar 

  27. Suizu, K., Miyamoto, K., Yamashita, T. & Ito, H. High-power terahertz-wave generation using DAST crystal and detection using mid-infrared powermeter. Opt. Lett. 32, 2885–2887 (2007).

    Article  ADS  Google Scholar 

  28. Dhillon, S. et al. Terahertz transfer onto a telecom optical carrier. Nature Photon. 1, 411–415 (2007).

    Article  ADS  Google Scholar 

  29. Yokoyama, S., Nakamura, R., Nose, M., Araki, T. & Yasui, T. Terahertz spectrum analyzer based on a terahertz frequency comb. Opt. Express 16, 13052–13061 (2008).

    Article  ADS  Google Scholar 

Download references


The authors thank M. Amato for technical assistance and acknowledge partial financial support from the Délégation Générale pour l'Armement (contract no. 06.34.020) and the initiative C-Nano Ile-de-France (contract TeraCascade). Device fabrication was carried out at the CTU-IEF-Minerve, which was partially funded by the Conseil Général de l'Essonne.

Author information

Authors and Affiliations



S.B. conceived and performed the experiment, analysed the data and wrote the paper. P.G. performed the experiment and fabricated the QCL. G.S. conceived and performed the experiment, analysed the data and contributed to the manuscript preparation. L.D. and W.M. contributed to the experimental set-up. C.S. gave conceptual advice, and contributed to data analysis and to the manuscript preparation. R.C. contributed to fabrication of the QCL and to manuscript preparation. H.E.B. and D.A.R. carried out growth of the QCL.

Corresponding author

Correspondence to Stefano Barbieri.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Rights and permissions

Reprints and permissions

About this article

Cite this article

Barbieri, S., Gellie, P., Santarelli, G. et al. Phase-locking of a 2.7-THz quantum cascade laser to a mode-locked erbium-doped fibre laser. Nature Photon 4, 636–640 (2010).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

This article is cited by


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing