5050 OPTICS LETTERS / Vol. 39, No. 17 / September 1, 2014 Mid-infrared frequency comb for broadband high precision and sensitivity molecular spectroscopy I. Galli,1 S. Bartalini,1 P. Cancio,1 F. Cappelli,1,4 G. Giusfredi,1 D. Mazzotti,1,* N. Akikusa,2 M. Yamanishi,3 and P. De Natale1 1 Istituto Nazionale di Ottica (INO)—CNR and European Laboratory for Non-linear Spectroscopy (LENS), Via Carrara 1, 50019 Sesto Fiorentino FI, Italy 2 Development Bureau Laser Device R&D Group, Hamamatsu Photonics KK, Shizuoka 434-8601, Japan 3 4 Central Research Laboratories, Hamamatsu Photonics KK, Shizuoka 434-8601, Japan Current address: Institut für Quantenelektronik—ETH, Auguste-Piccard-Hof 1, 8093 Zürich, Switzerland *Corresponding author: [email protected] Received June 6, 2014; revised July 14, 2014; accepted July 17, 2014; posted July 17, 2014 (Doc. ID 213106); published August 20, 2014 We report on the experimental demonstration of the metrological and spectroscopic performances of a mid-infrared comb generated by a nonlinear downconversion process from a Ti:sapphire-based near-infrared comb. A quantum cascade laser at 4330 nm was phase-locked to a single tooth of this mid-infrared comb and its frequency-noise power spectral density was measured. The mid-infrared comb itself was also used as a multifrequency highly coherent source to perform ambient air direct comb spectroscopy with the Vernier technique, by demultiplexing it with a high-finesse Fabry–Perot cavity. © 2014 Optical Society of America OCIS codes: (140.4050) Mode-locked lasers; (140.5965) Semiconductor lasers, quantum cascade; (190.4223) Nonlinear wave mixing; (140.3590) Lasers, titanium; (120.2230) Fabry-Perot. http://dx.doi.org/10.1364/OL.39.005050 Since the advent of optical frequency combs (OFCs) , high-precision spectroscopy has been benefiting from a tool with unprecedented metrological features [2,3]. Nowadays, the development of OFCs in the visible and near-infrared (NIR) domains has reached such a maturity level as to be commercially available. As a consequence, the number of applications of OFCs is growing more and more, ranging from the calibration of astronomical spectrographs , to the development of optical atomic clocks . The frequency coverage of OFCs is rapidly improving , up to the extreme ultraviolet  and down to the terahertz  spectral regions. The extension of OFCs to the mid-infrared (MIR) is of particular interest , since many molecular species have strong ro-vibrational transitions in this spectral region. Therefore, OFC-assisted MIR precision spectroscopy is a powerful tool both for investigating the internal structure of molecules (and all related fundamental physical laws and/or constants) and for identifying and quantifying trace gases (even down to sub-ppt concentrations ) in several application fields (atmospheric monitoring, security, medical diagnostics, and so on). Different approaches have been pursued to generate MIR OFCs: with difference-frequency generation (DFG) processes either between a NIR OFC and a continuouswave (CW) laser [11–13] or between two spectral portions of the same NIR OFC [14–18]; with optical parametric oscillators pumped by NIR OFCs [19–22]; with χ 3 nonlinear processes occurring in high-Q monolithic microresonators [23,24]. Quantum cascade lasers (QCLs) parallel OFCs in their rapid development and represent a mature technology for providing powerful, tunable, and narrow-linewidth coherent sources for precise measurements in the mid/far-infrared spectral region . Phase/frequency 0146-9592/14/175050-04$15.00/0 stabilization of QCLs against OFCs was achieved with different schemes. MIR QCLs were phase-locked to NIR OFCs either by optical injection locking  or by heterodyne beating  with OFC-referenced DFG radiation. Similarly, sum-frequency generation with NIR OFCs was used for upconverting MIR QCLs and heterodyne beating them with different spectral portions of the same OFCs . Recently, direct comb emission from a QCL was demonstrated [29,30]. Here we report on the metrological and spectroscopic performances of a MIR OFC generated by a nonlinear downconversion process from a NIR OFC. The MIR OFC served both as an accurate and highly stable phase/frequency reference for a CW quantum cascade laser at 4330 nm, thus enabling high-precision spectroscopy with a powerful and narrow linewidth MIR source, and as a multifrequency highly coherent MIR source to perform high-sensitivity broadband direct comb spectroscopy in ambient air with the Vernier technique. By using the experimental setup shown in Fig. 1 and described, with more details, in , we generated a MIR OFC and phase-locked a room-temperature CW distributed-feedback (DFB) QCL around 4330 nm to a single tooth of the comb. This was done by superimposing a fraction of the QCL beam (about 1 mW) with the MIR OFC beam and sending them to a liquid-N2 -cooled HgCdTe photodiode (200-MHz bandwidth, PD1 in Fig. 1). Figure 1 shows the detected beat-note between the QCL and the nearest comb tooth, as recorded by a RF spectrum analyzer. The beat-note frequency was phase-locked to a local oscillator provided by a RF signal generator, by using a home-made phase-detection electronics with hybrid analog/digital architecture. The phase-error signal was processed by a proportional-integral controller. The servo bumps confirm a phase-locking bandwidth of © 2014 Optical Society of America September 1, 2014 / Vol. 39, No. 17 / OPTICS LETTERS CO2 in air PD1 Beat-note detection Empty cavity PD2 MIR OFC Removable beam-splitter -40 Free running Locked RBW=4 kHz Mirror Beat-note (dBm) Room temp. DFB-QCL @4300 nm -50 -60 -70 -80 -600 -400 -200 0 200 400 600 Relative frequency (kHz) Fig. 1. Experimental setup. Inset: beat-note between the QCL and one MIR OFC tooth, in both free-running and phase-locked conditions. about 400 kHz, mainly limited by the QCL frequency modulation behavior. The phase-locking efficiency η 73% was estimated as the ratio between the areas of the resolution-bandwidth-limited carrier and the whole 2 beat-note spectrum. From the formula η e−ϕrms we calculated a residual rms phase-noise ϕrms 0.56 rad, as an estimate of the phase-locked loop performance. We remark that the possibility of phase locking the QCL to one arbitrary MIR OFC tooth within its working range (which for a standard DFB QCL spans about 300 GHz) makes the MIR OFC itself a sort of universal absolute frequency reference for this class of lasers. A free-running QCL linewidth of about 400 kHz is inferred from the free-running beat-note of Fig. 1, by considering the few kilohertz linewidth we measured for the MIR OFC tooth . Instead, a full characterization of the phase-locked QCL linewidth behavior at different time scales can be only retrieved by measuring its frequency-noise power spectral density (FNPSD). To that purpose, we used an empty 1-m-long high-finesse Fabry–Perot (F–P) cavity (finesse F ≈ 8000 at 4330 nm) as a frequency-to-amplitude converter. Figure 2 shows a comparison between FNPSDs of the phase-locked QCL measured in this work and of the MIR OFC measured in our previous work . Both spectra have been compensated for the F–P cavity cutoff, corresponding to a 8.5 μs ringdown time for resonant photons. The quite good agreement between the two recorded noise spectra confirms the validity of what we assumed in the previous work, i.e., that measuring the FNPSD of many comb teeth together (simultaneously tuned on the same F–P cavity fringe side) yields the same result as measuring it for a single comb tooth. It is worth noting that this further confirmation of the MIR OFC high coherence degree was enabled by the phase-locked QCL, used as a tracking oscillator “amplifying” a single comb tooth, otherwise too weak to be measured. By properly integrating the FNPSD spectrum of the phase-locked QCL , we can retrieve a linewidth of about 4.6 kHz over 1 s, with a narrowing factor of about 100. This value is a bit larger than what was measured for the MIR OFC (2 kHz), mainly due to residual 1∕f frequency noise of the QCL. Such a single-frequency narrow-linewidth, absolutely referenced MIR coherent source with tens of milliwatts of power can serve as a unique tool to perform high-precision/ sensitivity molecular spectroscopy, also exploiting recently developed sub-Doppler techniques . Alternatively, the multifrequency MIR OFC itself can be used for broadband direct comb spectroscopy. A convenient technique to go this second route is the Vernier technique , which proved to be able to resolve all individual teeth of the MIR OFC, as described below. We would like to point out that this approach was enabled by the high average power (about 1 μW) and narrow linewidth (about 5 kHz) of each tooth. Since our F–P cavity has a free spectral range FSR ≈ 150 MHz and the signal NIR OFC has a repetition rate f r ≈ 1 GHz, the simplest Vernier ratio to make the MIR OFC teeth simultaneously resonant with the F–P cavity (neglecting the mirrors dispersion) is f r0 Fig. 2. Comparison between FNPSDs measured for the MIR OFC (see ) and for the phase-locked QCL, retrieved by using the F–P cavity as a frequency-to-amplitude converter. The MIR comb trace is noisier than the QCL one, due to lower available power. 5051 20 20 c FSR ; 3 3 2L0 (1) where L0 ≈ 1 m is the fixed resonance length of the F–P cavity. In order to resolve the single teeth, we chose a repetition rate f r of the NIR OFC that was deliberately slightly detuned from the value f r0 that perfectly satisfies the Vernier condition in Eq. (1). We scanned the cavity length by applying a voltage ramp to three PZT actuators moving one of the two mirrors. We plot in Fig. 3 the measured spectrum of the MIR OFC, as transmitted by the empty F–P cavity and detected by a liquid-N2 -cooled InSb photodiode (1-MHz bandwidth, PD2 in Fig. 1). The MIR OFC spectrum exhibits a FWHM of about 200 GHz, due to the spectral bandwidth of quasi-phase-matching (QPM) conditions, with strong amplitude inhomogeneities, mainly due to absorption from CO2 in ambient air over 2 m path length. The absolute frequency scale was retrieved in two steps. First, a relative, linear frequency scale was calculated by fitting the length-to-voltage response of the PZT actuators to a 5th-order polynomial curve, by using the comb teeth as frequency markers with spacing equal to 3f r . Second, the molecular 5052 OPTICS LETTERS / Vol. 39, No. 17 / September 1, 2014 Fig. 3. MIR OFC spectrum measured with the Vernier technique: only one of the three subcombs is shown, with no amplitude normalization. absorption features in ambient air were used (as a “natural” wavemeter) to retrieve the integer order numbers of the teeth. The spectrum plotted in Fig. 3 was measured by averaging 80 scans of the F–P cavity length and the measured full width at half-maximum (FWHM) of each single tooth corresponds to 55 kHz, which is wider than the cavity resonance linewidth (about 19 kHz), mainly due to cavity drift during the averaging time (4.75 s). Given the 20∕3 Vernier ratio, when scanning the cavity length by a complete FSR, three subcombs were measured, each one with teeth spacing 3f r and shifted with respect to the others by f r . After properly interlacing the three subcombs, we obtained an overall MIR OFC with teeth spacing f r , whose central portion is shown in Fig. 4. The transmission spectrum through 2 m path length in ambient air, as simulated by the HITRAN Fig. 4. Central portion of the MIR OFC air transmission spectrum measured with the Vernier technique. Circles, squares, and triangles identify the three interlaced subcombs. The HITRAN simulation is plotted below and the CO2 ro-vibrational transitions of two bands are also labeled; ν3 → 000 1 − 000 0, ν3 → 011 1 − 011 0. 2012 database [34,35], is also plotted in the same figure. The agreement between the experimental MIR OFC transmission spectrum in air and the HITRAN simulation is very good. Actually, the spectrum of the generated MIR OFC depends both on the parent NIR OFC and on QPM conditions. Therefore, to make the visual comparison above easier, the experimental data plotted in Fig. 4 were normalized to make the amplitude of the MIR OFC teeth as flat as possible. This was done by dividing the original spectrum of each subcomb by an envelope Gaussian curve with the same center and width. In conclusion, we have shown that a low-frequencynoise MIR OFC can be exploited to phase lock a QCL to one of its spectral teeth, thus absolutely referencing the QCL frequency and narrowing its linewidth by about 2 orders of magnitude. Furthermore, the comb nature of this absolute frequency reference also makes it a unique tool either for simultaneously referencing several MIR lasers with Cs-clock traceability, or for phase locking one MIR laser to another, acting as a transfer oscillator with hundreds of gigahertz bandwidth. Finally, we have shown that this MIR OFC is suitable as a direct source for broadband high-precision molecular spectroscopy at atmospheric pressure. When detecting a sample gas with lower concentration than ambient CO2 (∼400 ppm) and/or weaker lines, the achieved sensitivity can be pushed even further by using the F–P cavity itself as a spectroscopy cell with ∼5 km effective absorption length. For all these purposes the wide tunability of this setup (4.2–5.0 μm), limited by both the Ti:sapphire laser and the spectral bandwidth of the high-finesse F–P cavity, is one of its selling points. We gratefully acknowledge funding from LaserlabEurope, grant agreement no. 284464; EU 7th Framework Program; Programma Nazionale di Ricerche in Antartide (PNRA), 2013/AC4.01 research project; Italian Ministry of Foreign Affairs; “Broadcon” Italy-Israel bilateral project; ESFRI Roadmap; Extreme Light Infrastructure (ELI) project. References 1. T. Udem, R. Holzwarth, and T. W. Hänsch, Nature 416, 233 (2002). 2. S. A. Diddams, J. Opt. Soc. Am. B 27, B51 (2010). 3. P. Maddaloni, M. Bellini, and P. De Natale, Laser-Based Measurements for Time and Frequency Domain Applications: A Handbook, Series in Optics and Optoelectronics (CRC Press, 2013). 4. T. Steinmetz, T. Wilken, C. Araujo-Hauck, R. Holzwarth, T. W. Hänsch, L. Pasquini, A. Manescau, S. D’Odorico, M. T. Murphy, T. Kentischer, E. Schmidt, and T. Udem, Science 321, 1335 (2008). 5. D. Akamatsu, M. Yasuda, H. Inaba, K. Hosaka, T. Tanabe, A. Onae, and F.-L. Hong, Opt. Express 22, 7898 (2014). 6. P. Maddaloni, P. Cancio, and P. De Natale, Meas. Sci. Technol. 20, 052001 (2009). 7. A. Cingöz, D. C. Yost, T. K. Allison, A. Ruehl, M. E. Fermann, I. Hartl, and J. Ye, Nature 482, 68 (2012). 8. L. Consolino, A. Taschin, P. Bartolini, S. Bartalini, P. Cancio, A. Tredicucci, H. E. Beere, D. A. Ritchie, R. Torre, M. S. Vitiello, and P. De Natale, Nat. Commun. 3, 1040 (2012). 9. A. Schliesser, N. Picque, and T. W. Hänsch, Nat. Photonics 6, 440 (2012). September 1, 2014 / Vol. 39, No. 17 / OPTICS LETTERS 10. I. Galli, S. Bartalini, S. Borri, P. Cancio, D. Mazzotti, P. De Natale, and G. Giusfredi, Phys. Rev. Lett. 107, 270802 (2011). 11. P. Maddaloni, P. Malara, G. Gagliardi, and P. De Natale, New J. Phys. 8, 262 (2006). 12. E. Baumann, F. R. Giorgetta, W. C. Swann, A. M. Zolot, I. Coddington, and N. R. Newbury, Phys. Rev. A 84, 062513 (2011). 13. I. Galli, F. Cappelli, P. Cancio, G. Giusfredi, D. Mazzotti, S. Bartalini, and P. De Natale, Opt. Express 21, 28877 (2013). 14. A. Gambetta, R. Ramponi, and M. Marangoni, Opt. Lett. 33, 2671 (2008). 15. T. W. Neely, T. A. Johnson, and S. A. Diddams, Opt. Lett. 36, 4020 (2011). 16. A. Ruehl, A. Gambetta, I. Hartl, M. E. Fermann, K. S. E. Eikema, and M. Marangoni, Opt. Lett. 37, 2232 (2012). 17. F. Zhu, H. Hundertmark, A. A. Kolomenskii, J. Strohaber, R. Holzwarth, and H. A. Schuessler, Opt. Lett. 38, 2360 (2013). 18. S. A. Meek, A. Poisson, G. Guelachvili, T. W. Hänsch, and N. Picqué, Appl. Phys. B 114, 573 (2014). 19. N. Leindecker, A. Marandi, R. L. Byer, and K. L. Vodopyanov, Opt. Express 19, 6296 (2011). 20. N. Coluccelli, H. Fonnum, M. Haakestad, A. Gambetta, D. Gatti, M. Marangoni, P. Laporta, and G. Galzerano, Opt. Express 20, 22042 (2012). 21. Z. Zhang, X. Fang, T. Gardiner, and D. T. Reid, Opt. Lett. 38, 2077 (2013). 22. K. A. Ingold, A. Marandi, C. W. Rudy, K. L. Vodopyanov, and R. L. Byer, Opt. Lett. 39, 900 (2014). 23. P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, and T. J. Kippenberg, Nature 450, 1214 (2007). 24. C. Y. Wang, T. Herr, P. Del’Haye, A. Schliesser, J. Hofer, R. Holzwarth, T. W. Hänsch, N. Picqué, and T. J. Kippenberg, Nat. Commun. 4, 1345 (2013). 25. S. Bartalini, M. S. Vitiello, and P. De Natale, Meas. Sci. Technol. 25, 012001 (2014). 5053 26. S. Borri, I. Galli, F. Cappelli, A. Bismuto, S. Bartalini, P. Cancio, G. Giusfredi, D. Mazzotti, J. Faist, and P. De Natale, Opt. Lett. 37, 1011 (2012). 27. I. Galli, M. Siciliani de Cumis, F. Cappelli, S. Bartalini, D. Mazzotti, S. Borri, A. Montori, N. Akikusa, M. Yamanishi, G. Giusfredi, P. Cancio, and P. De Natale, Appl. Phys. Lett. 102, 121117 (2013). 28. A. A. Mills, D. Gatti, J. Jiang, C. Mohr, W. Mefford, L. Gianfrani, M. Fermann, I. Hartl, and M. Marangoni, Opt. Lett. 37, 4083 (2012). 29. A. Hugi, G. Villares, S. Blaser, H. C. Liu, and J. Faist, Nature 492, 229 (2012). 30. J. B. Khurgin, Y. Dikmelik, A. Hugi, and J. Faist, Appl. Phys. Lett. 104, 081118 (2014). 31. D. S. Elliott, R. Roy, and S. J. Smith, Phys. Rev. A 26, 12 (1982). 32. G. Giusfredi, S. Bartalini, S. Borri, P. Cancio, I. Galli, D. Mazzotti, and P. De Natale, Phys. Rev. Lett. 104, 110801 (2010). 33. C. Gohle, B. Stein, A. Schliesser, T. Udem, and T. W. Hänsch, Phys. Rev. Lett. 99, 263902 (2007). 34. L. S. Rothman, I. E. Gordon, Y. Babikov, A. Barbe, D. C. Benner, P. F. Bernath, M. Birk, L. Bizzocchi, V. Boudon, L. R. Brown, A. Campargue, K. Chance, E. A. Cohen, L. H. Coudert, V. M. Devi, B. J. Drouin, A. Fayt, J.-M. Flaud, R. R. Gamache, J. J. Harrison, J.-M. Hartmann, C. Hill, J. T. Hodges, D. Jacquemart, A. Jolly, J. Lamouroux, R. J. Le Roy, G. Li, D. A. Long, O. M. Lyulin, C. J. Mackie, S. T. Massie, S. Mikhailenko, H. S. P. Müller, O. V. Naumenko, A. V. Nikitin, J. Orphal, V. Perevalov, A. Perrin, E. R. Polovtseva, C. Richard, M. A. H. Smith, E. Starikova, K. Sung, S. Tashkun, J. Tennyson, G. C. Toon, V. G. Tyuterev, and G. Wagner, J. Quant. Spectrosc. Radiat. Transfer 130, 4 (2013). 35. Harvard–Smithsonian Center for Astrophysics (CfA), V. E. Zuev Insitute of Atmosperic Optics (IAO), “HITRAN on the Web,” (2014).