Gardiner, Tom

Reid, Derryck Telford

Zhang, Zhaowei

English

We present the first implementation of mid-infrared dual-comb spectroscopy with an optical parametric oscillator. Methane absorption spectroscopy was demonstrated with a resolution of 0.2 cm(-1) (5 GHz) at an acquisition time of similar to 10.4 ms over a spectral coverage at 2900-3050 cm(-1). The average power from each individual mid-infrared comb line was similar to 1 mu W, representing a power level much greater than typical difference-frequency-generation sources. Mid-infrared dual-comb spectroscopy opens up unique opportunities to perform broadband spectroscopic measurements with high resolution, high requisition rate, and high detection sensitivity. (C) 2013 Optical Society of America</p

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    Heriot-Watt University Research Gateway                                                          Heriot-Watt UniversityMid-infrared dual-comb spectroscopy with an optical parametric oscillatorZhang, Zhaowei; Gardiner, Tom; Reid, Derryck TelfordPublished in:Optics LettersDOI:10.1364/OL.38.003148Publication date:2013Link to publication in Heriot-Watt Research GatewayCitation for published version (APA):Zhang, Z., Gardiner, T., & Reid, D. T. (2013). Mid-infrared dual-comb spectroscopy with an optical parametricoscillator. Optics Letters, 38(16), 3148-3150. 10.1364/OL.38.003148General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Mid-infrared dual-comb spectroscopy with an opticalparametric oscillatorZhaowei Zhang,1,2,* Tom Gardiner,2 and Derryck T. Reid11Institute of Photonics and Quantum Sciences, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK2National Physical Laboratory, Hampton Road, Teddington, London TW11 0LW, UK*Corresponding author: z.zhang@hw.ac.ukReceived April 23, 2013; accepted July 12, 2013;posted July 26, 2013 (Doc. ID 189369); published August 14, 2013We present the first implementation of mid-infrared dual-comb spectroscopy with an optical parametric oscillator.Methane absorption spectroscopy was demonstrated with a resolution of 0.2 cm−1 (5 GHz) at an acquisition time of∼10.4 ms over a spectral coverage at 2900–3050 cm−1. The average power from each individual mid-infrared combline was ∼1 μW, representing a power level much greater than typical difference-frequency-generation sources.Mid-infrared dual-comb spectroscopy opens up unique opportunities to perform broadbandspectroscopic measure-ments with high resolution, high requisition rate, and high detection sensitivity. © 2013 Optical Society ofAmericaOCIS codes: (300.6300) Spectroscopy, Fourier transforms; (300.6390) Spectroscopy, molecular; (120.6200)Spectrometers and spectroscopic instrumentation; (190.4970) Parametric oscillators and amplifiers.http://dx.doi.org/10.1364/OL.38.003148The mid-infrared (mid-IR) region, defined here as from2.5 to 25 μm( 4000 − 400 cm−1), contains the fundamentalrovibrational absorption bands of many molecules and isoften referred to as the molecular fingerprint region.High-resolution spectroscopy in this region employsFourier-transform infrared (FTIR) spectrometers as thetools of choice for the identification of molecular species,quantitative analysis of complex mixtures, the investiga-tion of dynamic systems, biological and biomedical spec-troscopy, microspectroscopy, and hyperspectral imaging[1]. Conventionally, mid-IR FTIR spectroscopy is con-ducted with a thermal light source (e.g., a Globar) anda spectrometer based on a Michelson interferometer con-taining a scanning mechanical delay line. However, theuse of spatially incoherent thermal light sources limitsthe detection sensitivity of FTIR techniques and theirapplicability in free-space sensing or microspectroscopydue to their low spectral brightness and poor direction-ality. In addition, the acquisition rate and spectral reso-lution of traditional FTIR spectrometers are limited,respectively, by the velocity and displacement of thescanning mechanical delay line used in these systems.Mode-locked lasers are broadband optical sourcespossessing exceptional spatial coherence, and when con-figured in the mid-IR region they have been shown to bepromising sources for FTIR spectroscopy [2]. Such lasersources inherently support high detection sensitivity, arecompatible with the use of multipass cells for trace gassensing, and enable long-range open-path sensing andFTIR microspectroscopy. In the frequency domain, adja-cent longitudinal cavity modes of a mode-locked laserare separated by a precise interval in both frequencyand phase, forming a frequency comb. In a dual-combspectrometer, two laser frequency combs with slightlydifferent comb-line spacings (or pulse repetition rates,f rep) and stabilized carrier-envelope-offset (CEO)frequencies (f CEO) are heterodyned after passing througha sample, yielding a down-converted radio-frequency(RF) comb encoding information about the absorptionexperienced by the combs [3–11]. This RF comb mani-fests itself as an interferogram, analogous to thatobtained using a mechanically scanning Michelson FTIRspectrometer, but the absence of moving parts in a dual-comb spectrometer overcomes the speed and resolutionlimitations of its conventional counterpart. The resolu-tion is only limited by the comb-line interval (the pulserepetition frequency), and the recording time is theinverse of the difference in the pulse repetitionfrequencies.Until now, only a limited number of dual-comb spec-troscopy experiments were conducted in the mid-IRregion, and all such systems were implemented withlow-efficiency and low-power difference frequency gen-eration frequency combs [3,4,8]. Recently, we introduceda new dual-comb source comprising a single synchro-nously pumped optical parametric oscillator (OPO)driven by two independent lasers [12,13] and capable ofproducing two asynchronous—but otherwise identical—mid-IR pulse sequences. Here, we describe the firstapplication of this system to mid-IR dual-comb spectros-copy, from which high-resolution absorption spectra ofmethane were obtained in the spectral range of2900–3050 cm−1 with an acquisition rate of 96 Hz.The asynchronous mid-IR comb sources were basedon the scheme detailed in [13]. Two independent pumpsources of identical design and performance, but slightlydifferent repetition rates, were combined on a 50∶50 di-electric beam splitter. Each source was composed of afiber amplifier seeded by a mode-locked Yb:KYW laser.The average power, center wavelength, 3 dB spectralbandwidth, and pulse width of each pump source were2.5 W, 1058 nm, 18 nm (4.82 THz), and ∼3 ps, respec-tively. The output from the beam splitter was used topump an OPO incorporating a 20 mm long magnesiumoxide-doped periodically poled lithium niobate (MgO:PPLN) crystal, generating two idler channels of broad-band mid-IR pulses. The high group-delay dispersion ofthe OPO cavity, mainly contributed by the MgO:PPLNcrystal, made it possible for two optical pulse sequencesof nearly identical wavelengths but with different repeti-tion rates to be resonant simultaneously [13]. These asyn-chronous mid-IR pulse sequences left the OPO through a3148 OPTICS LETTERS / Vol. 38, No. 16 / August 15, 20130146-9592/13/163148-03$15.00/0 © 2013 Optical Society of Americacurved resonator mirror and possessed excellent spatialoverlap. The average power, center wavelength, −3 dBspectral bandwidth and pulse width of each mid-IR idlerchannel were 100 mW, 3313 nm, 172 nm (4.70 THz), and∼3 ps respectively. The pulses from both idler channelswere strongly chirped, with a duration-bandwidth prod-uct of 14.The layout of the mid-IR dual-comb spectrometer isshown in Fig. 1. The repetition frequency of one channelwas stabilized to a 100 MHz RF reference source, whilethe repetition-frequency difference (Δf) between the twochannels was free-running at 96 Hz (unlocked). The CEOfrequencies of both channels were also free-running.The asynchronous mid-IR pulse sequences passedthrough a 20 cm cell, containing a 1 atm mixture of nitro-gen with a low concentration (∼0.7%) of methane. Thesignal was detected by a thermoelectrically cooled MCZT(HgCdZnTe) photovoltaic detector with a bandwidth of100 MHz. In the time domain, one mid-IR pulse sequenceis sampled by the other with a slightly different pulse rep-etition rate, at a sampling interval of Δf∕f 2rep, forming atime-domain interferogram. In the frequency domain, thelaser frequency lines from the two asynchronous combsheterodyne with each other, generating an RF comb witha comb spacing of Δf. After Nyquist filtering by a 50 MHzlow-pass filter, the time-domain signal was sampled by ahigh-resolution 14 bit data acquisition card with a 50 MHzbandwidth at a sampling rate of 100 MSamples∕s.The detected time-domain interferogram is shown inFig. 2. The intense spikes are the interferograms corre-sponding to the temporal overlap of two pulses, andthese repeat with a period of 10.4 ms (1∕Δf). One ofthe mode-locked Yb:KYW seed lasers was running in adouble-pulse mode, and this behavior was transferredto the mid-IR pulse sequence in the OPO. Thus, the inter-ferogram shows two bursts in each period, with twoseries of bursts with a period of 10.4 ms and separatedfrom each other by 5.8 ms. A Fourier transform wasperformed over a single interferogram to reveal theRF comb (the lower axis in Fig. 3), which was scaledto the optical domain with a scaling factor off rep∕Δf  1.04 × 106,asshownby theupper axisinFig. 3.We implemented a 0.2 ms time window over the inter-ferogram, so the transformed spectrum shown in Fig. 2had a resolution of 5 kHz for the RF spectrum and 5 GHz(0.2 cm−1) for the optical spectrum. Spectra with muchhigher resolution but lower signal-to-noise ratio (SNR)could be achieved from the same interferogram by useof a broader time window.Shown in Fig. 4 are normalized optical spectra, ob-tained from four single measurements, and a spectrumobtained by averaging over eight measurements. It isclear that useful spectroscopic data are accessible fromjust a single interferogram, but also that the SNR can beimproved significantly by averaging. It is noteworthy thatthe acquisition rate was 96 Hz; an individual measure-ment was completed within a time duration ofMLL 1 & AmplifierMLL 2 & Amplifier BSPPLN CH4 cell Mid-IR fast detector Optical parametric oscillator Mid-IRdual-comb Data acquisition & PCGe filter Fig. 1. Layout of the mid-IR dual-comb spectrometer. MLL,mode-locked laser; BS, beam splitter; PPLN, periodically poledlithium niobate.Fig. 2. Time-domain interferogram (Δf  96 Hz). One of themid-IR pulse sequences operated in a double-pulse mode.Fig. 3. Fourier-transform spectrum expressed in the RF(lower axis) and optical (upper axis) domains.Fig. 4. Normalized methane absorption spectra for singlemeasurements (top 4 traces, black) and an averaged spectrum(bottom trace, blue).August 15, 2013 / Vol. 38, No. 16 / OPTICS LETTERS 314910.4 ms, and within 1 min, 5760 measurements could beconducted, implying a potential enhancement of 75 timesin the spectral SNR over a single measurement [1]. In thisexperiment, since the CEO frequencies of the dual mid-IRcombs were neither stabilized or monitored in real time,information about the absolute optical frequency was notdirectly available from the Fourier transform. Rather, weobtained the absolute optical frequency by aligning thefrequencies in the Q branch of the methane absorptionspectrum to the known value at 3317 cm−1. Another op-tion to determine the absolute optical frequency is by us-ing passive optical filters with known central frequenciesas frequency references [14].Shown in Fig. 5 is a comparison between the measuredmethane absorption spectrum (averaged over eight inter-ferograms) and the simulated spectrum, based on theHITRAN database and a methane concentration of0.65% at 1 atm with a resolution of 0.2 cm−1. The agree-ment between the experimental and simulated spectra isexcellent for the P branch, while we believe the discrep-ancy in the Q branch is due to experimental uncertaintiesin the normalization procedure we used.Dual-comb spectroscopy can be understood as a formof multiheterodyne spectroscopy in which the beat be-tween laser-frequency comb 1 (f 1  f CEO;1  n1nrep;1)and laser-frequency comb 2 (f 2  f CEO;2  n2f rep;2) gen-erates a downshifted RF comb f  f 1–f 2  δ  NΔf,where δ  f CEO;1 − f CEO;2 represents the difference be-tween the CEO frequencies of the two channels. To fullyexploit the potential of mid-IR dual-comb spectroscopy, δmust be stabilized [5,8] or monitored [7,10,11], thus en-abling precise determination of the absolute optical fre-quency, ensuring high spectral resolution for resolvingeach individual comb tooth and facilitating higher SNRthrough the application of intensive coherent averaging.In this work, δ was free-running; however, the accuratealignment between the P-branch spectrum and simulateddata implies that δ was passively stable during the 0.2 msinterferogram window. Further work to implement adual-comb spectroscopy measurement with a fully stabi-lized system [15] is in progress.In summary, we have reported the first dual-combasynchronous spectroscopy with an OPO operating inthe important mid-IR molecular fingerprinting region.The average power of the individual mid-IR comb linewas approximately 1 μW, representing a huge improve-ment over previously reported mid-IR dual-comb sys-tems. These initial results demonstrate the benefitsand practicality of mid-IR dual-comb OPO spectroscopyin terms of scanning speed, resolution, SNR, and spectralcoverage.The authors gratefully acknowledge financial supportfrom the UK Engineering and Physical Sciences Re-search Council under grants EP/H018190/1 and EP/H000011/1 and from the UK National Physical Laboratoryunder its Strategic Research Programme.References1. P. R. Griffiths and J. A. De Haseth, Fourier TransformInfrared Spectrometry (Wiley, 2007).2. K. A. Tillman, R. R. J. Maier, D. T. Reid, and E. D.McNaghten, Appl. Phys. Lett. 85, 3366 (2004).3. F. Keilmann, C. Gohle, and R. Holzwarth, Opt. Lett. 29, 1542(2004).4. A.Schliesser, M. Brehm, F. Keilmann,and D. vander Weide,Opt. Express 13, 9029 (2005).5. I. Coddington, W. C. Swann, and N. R. Newbury, Phys. Rev.Lett. 100, 013902 (2008).6. B. Bernhardt, E. Sorokin, P. Jacquet, R. Thon, T. Becker, I.T. Sorokina, N. Picqué, and T. W. Hänsch, Appl. Phys. B100, 3 (2010).7. J.-D. Deschenes, P. Giaccarri, and J. Genest, Opt. Express18, 23358 (2010).8. E. Baumann, F. R. Giorgetta, W. C. Swann, A. M. Zolot, I.Coddington, and N. R. Newbury, Phys. Rev. A 84, 062513(2011).9. A. Schliesser, N. Picqué, and T. W. Hänsch, Nat. Photonics6, 440 (2012).10. T. Ideguchi, A. Poisson, G. Guelachvili, N. Picqué, and T. W.Hänsch, arXiv:1201.4177 (2012) (to be published).11. T. Ideguchi, A. Poisson, G. Guelachvili, T. W. Hänsch, andN. Picqué, Opt. Lett. 37, 4847 (2012).12. Z. Zhang, J. Sun, T. Gardiner, and D. T. Reid, Opt. Express19, 17127 (2011).13. Z.Zhang,C. Gu,J. Sun,C. Wang,T. Gardiner,and D. T.Reid,Opt. Lett. 37, 187 (2012).14. P. Giaccari, J.-D. Deschênes, P. Saucier, J. Genest, and P.Tremblay, Opt. Express 16, 4347 (2008).15. Z. Zhang, X. Fang, T. Gardiner, and D. T. Reid, Opt. Lett. 38,2077 (2013).Fig. 5. Comparison between the measured and simulated ab-sorption spectrum (fitting parameters: 0.65% methane at 1 atm;20 cm cell; resolution  0.2 cm−1).3150 OPTICS LETTERS / Vol. 38, No. 16 / August 15, 2013

Mid-infrared dual-comb spectroscopy with an optical parametric oscillator

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