Adaptive real-time dual-comb spectroscopy

ARTICLE Received 24 Sep 2013 | Accepted 4 Feb 2014 | Published 27 Feb 2014 DOI: 10.1038/ncomms4375 OPEN Adaptive real-time dual-comb spectroscopy Takuro Ideguchi1,*, Antonin Poisson2,*, Guy Guelachvili2, Nathalie Picqué1,2,3 & Theodor W. Hänsch1,3 The spectrum of a laser frequency comb consists of several hundred thousand equally spaced lines over a broad spectral bandwidth. Such frequency combs have revolutionized optical frequency metrology and they now hold much promise for significant advances in a growing number of applications including molecular spectroscopy. Despite an intriguing potential for the measurement of molecular spectra spanning tens of nanometres within tens of microseconds at Doppler-limited resolution, the development of dual-comb spectroscopy is hindered by the demanding stability requirements of the laser combs. Here we overcome this difficulty and experimentally demonstrate a concept of real-time dual-comb spectroscopy, which compensates for laser instabilities by electronic signal processing. It only uses free-running mode-locked lasers without any phase-lock electronics. We record spectra spanning the full bandwidth of near-infrared fibre lasers with Doppler-limited line profiles highly suitable for measurements of concentrations or line intensities. Our new technique of adaptive dual-comb spectroscopy offers a powerful transdisciplinary instrument for analytical sciences. 1 Max Planck Institut für Quantenoptik, Laser Spectroscopy Division, Hans-Kopfermann-Strasse 1, 85748 Garching, Germany. 2 Institut des Sciences Moléculaires d’Orsay, CNRS, Bâtiment 350, Université Paris-Sud, Orsay 91405, France. 3 Ludwig-Maximilians-Universität München, Fakultät für Physik, Schellingstrasse 4/III, 80799 München, Germany. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to N.P. (email: ). NATURE COMMUNICATIONS | 5:3375 | DOI: 10.1038/ncomms4375 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4375 ourier transform spectroscopy1,2 is the mature solution to a variety of problems3–8 from the research laboratory to the manufacturing floor. The Michelson-based Fourier transform interferometer is known for more than 40 years as a benchtop instrument of great value in analytical sciences because it measures a broad range of optical frequencies simultaneously with accuracy and sensitivity. In recent years, novel techniques9–31 have been developed in which a laser frequency comb32,33 directly interrogates a vast number of transitions of a molecular sample. In particular, the precisely spaced spectral comb lines may be harnessed for dual-comb Fourier transform spectroscopy14–31. Dual-comb spectroscopy holds much promise for outperforming absorption measurements performed by Michelson-based Fourier transform spectroscopy: the recording time and resolution may be improved a millionfold, with potential broad spectral span in any spectral region. The technique may thus open up new opportunities for laboratory spectroscopy, as well as for field-based sensors. Furthermore, recent proof-of-principle experiments29–31 demonstrate an intriguing potential for nonlinear multiplex spectroscopy, with possible applications ranging from microscopy30 to sub-Doppler precision measurements31. However, the technique of dual-comb spectroscopy has not realized its full potential yet, mostly because of the difficulty of synchronizing the pulse trains of two comb lasers within interferometric precision. In this letter, we present a technique of real-time dual-comb spectroscopy that only requires free-running femtosecond oscillators and electronic signal processing, using commercially available components. Long measurement times, complex locking systems and computer algorithms are not needed. We demonstrate the suitability of our technique for the measurement of Doppler-broadened line profiles. We first explain the principle and challenges of dual-comb spectroscopy. We then describe a technique, which makes it possible to record real-time highquality molecular spectra with free-running femtosecond lasers. We show its experimental performance with commercial fibre lasers emitting around 1.55 mm. We finally discuss its limitations as well as the new opportunities opened up by adaptive dualcomb spectroscopy. F Results Stability requirements in dual-comb spectroscopy. Dual-comb spectroscopy can be considered with the metaphor of a sampling oscilloscope. Pulses from comb 1 excite molecular free induction decay and pulses from comb 2 sample the waveform of this decay interferometrically. For simplicity, we at first ignore carrier–envelope phase shifts. At best, interferometric samples are taken at intervals 1/f (for example, 10  8 s) where f is the repetition frequency of comb 2 (for example, 100 MHz). If Df is the difference of comb repetition frequencies (for example, 100 Hz), consecutive interferometric samples result from pulse pairs showing a time separation increased by an amount Dt ¼ Df/f 2 (for example, 10  14 s). The ‘sampling oscilloscope’ thus effectively stretches the waveform of the free induction decay signal by a factor s ¼ f/Df (for example, 106). Signal frequencies in this waveform are transformed down from the optical to the radio frequency region by the same factor s. In the experiment, the signal of the photodetector is electronically low-pass filtered to suppress the pulse repetition frequency f. The time-stretched waveform appears thus as a continuous electronic signal. This signal is digitized at a constant clock rate determined by the data acquisition board of the computer. For ideally stable frequency combs, the waveform thus sampled can be Fourier transformed to reveal the signal 2 spectrum. In the real world, major difficulties arise from the residual instabilities of the frequency combs, even when these benefit from state-of-the-art stabilization. The time intervals between excitation and sampling pulses are indeed subject to some variations dt, which appear in the detector signal stretched in time by the factor s. Let us now consider the slippage of the carrier phase relative to the pulse envelope owing to laser dispersion. If the carrier– envelope slippage frequencies of the two combs differ by fce, the relative phases of pump and probe pulse will change by an additional 2p fce/f between two interferometric samples, and all frequencies in the Fourier spectrum of the detector signal will be translated by fce. As long as fce is constant, the spectral translation can be accounted for if fce is measured or if the absolute frequency of some spectral feature is known. Below, we describe an electronic signal processing technique that compensates for both timing fluctuations and phase fluctuations. In the frequency domain, dual-comb spectroscopy can be understood by considering the two frequency combs of (...truncated)