DUAL COMB FOURIER TRANSFORM SPECTROSCOPY

Author Institution: Max Planck Institut fur Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany; $^2$ Ludwig Maximilians-Universitat Munchen, Fakultat fur Physik, Schellingstrasse 4/III, 80799 Munchen, Germany; Max Planck Institut fur Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany; $^2$ Ludwig Maximilians-Universitat Munchen, Fakultat fur Physik, Schellingstrasse 4/III, 80799 Munchen, Germany; $^3$ Institut des Sciences Moleculaires d'Orsay, CNRS, Universite Paris-Sud, Batiment 350, 91405 Orsay, France. Email: [email protected] advent of laser frequency combs a decade ago has already revolutionized optical frequency metrology and precision spectroscopy. Extensions of laser combs from the THz region to the extreme ultraviolet and soft x-ray frequencies are now under exploration. Such laser combs have become enabling tools for a growing tree of applications, from optical atomic clocks to attosecond science.\\ Recently, the millions of precisely controlled laser comb lines that can be produced with a train of ultrashort laser pulses have been harnessed for highly multiplexed molecular spectroscopy. Fourier multi-heterodyne spectroscopy, dual comb spectroscopy, or asynchronous optical sampling spectroscopy with frequency combs are emerging as powerful new spectroscopic tools. Even the first proof-of-principle experiments have demonstrated a very exciting potential for ultra-rapid and ultra-sensitive recording of complex molecular spectra. Compared to conventional Fourier transform spectroscopy, recording times could be shortened from seconds to microseconds, with intriguing prospects for spectroscopy of short lived transient species. Longer recording times allow high resolution spectroscopy of molecules with extreme precision, since the absolute frequency of each laser comb line can be known with the accuracy of an atomic clock.\\ The spectral structure of sharp lines of a laser comb can be very useful even in the recording of broadband spectra without sharp features, as they are e.g. encountered for molecular gases or in the liquid phase. A second frequency comb of different line spacing permits the generation of a comb of radio frequency beat notes, which effectively map the optical spectrum into the radio frequency regime, so that it can be recorded with a single fast photodetector, followed by digital signal analysis. In the time domain, a pulse train of a mode-locked femtosecond laser excites some molecular medium at regular time intervals. A second pulse train of different repetition frequency interferometrically samples the transient response or ??free induction decay ?? of the medium, akin to an optical sampling oscilloscope

FOURIER TRANSFORM SPECTROSCOPY WITHOUT MICHELSON INTERFEROMETERS

Author Institution: Laboratoire de Photophysique Moleculaire, CNRS, Batiment 350, Universite Paris-Sud, 91405 Orsay cedex, FranceMichelson interferometers have been for decades the main component of Fourier transform spectrometers. With the advent of femtosecond frequency combs, this obviousness is called into question.\\ In this talk, a Fourier transform spectrometer based on two frequency combs (2C-FTS) will be presented. One of the frequency combs serves as interferometer.\\ 2C-FTS bears on the same principle as traditional FTS. The key is to make a down conversion of the optical frequencies characterizing the absorption spectrum of interest, so to allow practical measurements. A single detector may be used to record, generally as a function of time, the data called interferogram. The analyzed spectrum is recovered by a Fourier transform operation. In 2C-FTS, the optical frequency down conversion is obtained from the interference between two similar frequency combs. Two different frequency combs with slightly different repetition rates beat with each other. Acquisition time is then of the order of a few milliseconds. This spectrometer has no moving part. It is compact and able to provide detailed and sensitive spectra of single events

DUAL COMB FOURIER TRANSFORM SPECTROSCOPY IN THE GREEN REGION

Author Institution: Laser and Optics Research Center, U.S. Air Force Academy, Suite 2A31, 2354 Fairchild Drive, Colorado Springs, Colorado 80840, U.S.A; Max Planck Institut fur Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany; Max Planck Institut fur Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany; Max Planck Institut fur Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany; Institut des Sciences Moleculaires d'Orsay, CNRS, Universite Paris-Sud, Batiment 350, 91405 Orsay Cedex, France; Ludwig Maximilians-Universitat Munchen, Fakultat fur Physik, Schellingstrasse 4/III, 80799 Munchen, Germany; Max Planck Institut fur Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany; Ludwig Maximilians-Universitat Munchen, Fakultat fur Physik, Schellingstrasse 4/III, 80799 Munchen, GermanyLaser combs in combination with other advancing tools of laser science, nonlinear optics, photonics, and electronic signal processing have the potential to vastly enhance the range and capabilities of molecular laser spectroscopy.\\ The high versatility of frequency comb sources can indeed harness new techniques for ultra-rapid and ultra-sensitive recording of complex molecular spectra. The recent proof-of-principle demonstrations of dual comb Fourier transform spectroscopy have mostly been carried out in the near-infrared region, around 1.0 and 1.5 $\mu$m. The mode-locked ytterbium- or erbium-doped fiber femtosecond laser systems emitting in this range indeed require few adjustment thanks to their guided light and permit reliable unattended operation. With expanded wavelength coverage and continued improvements in speed and sensitivity, dual comb spectroscopy should find use as a novel, time-domain spectroscopic analytical tool. As far as molecular spectroscopy is concerned, the mid-infrared and visible-ultraviolet wavelength regions show both the potential for specificity and sensitivity for tracing molecules. In particular, the visible-ultraviolet region complements the mid-infrared molecular fingerprint range, as it provides access to many electronic transitions, in particular belonging to reactive species.\\ In this contribution, we report on our progress in the implementation of dual comb spectroscopy in the 520 nm green region. We present preliminary results on a powerful new sensitive ultra-rapid tool for linear rovibronic absorption spectroscopy, based on frequency-doubled ytterbium-doped fiber lasers and we discuss its intriguing prospects for spectroscopy of short lived transient species

NONLINEAR DUAL-COMB SPECTROSCOPY WITH TWO-PHOTON EXCITATION

Author Institution: Max-Planck-Institut fur Quantenoptik, Hans-Kopfermann-Stra\ss{}e 1, D-85748 Garching, GermanyDual frequency comb spectroscopy has proven to be a powerful method for acquiring broadband, high resolution spectra with measurement times that are much shorter than in traditional moving-mirror Fourier transform spectroscopy. Because the measurements are carried out with femtosecond lasers, this technique has great potential for decreasing the measurement times and improving the signal-to-noise ratio of nonlinear spectroscopic measurements, such as two-photon excitation or Raman processes. In the case of two-photon excitation, an entire spectrum can be obtained at a given signal level using dual-comb spectroscopy in the same time that a measurement of a single transition frequency would be obtained with a continuous laser of the same average power. In this presentation, I will show the latest results in extending the dual-comb technique to two-photon excitation spectroscopy, with measurements on gas-phase rubidium and liquid-phase dye samples. In our realization of dual-comb spectroscopy, two frequency combs with slightly different repetition rates are combined on a beam splitter and directed into a sample, and we measure the intensity of the resulting fluorescence as a function of time. Because of the different repetition rates, the time delay between a pulse from the first comb and the next pulse from the second comb changes linearly with time, simulating the action of the moving mirror in a traditional Michelson interferometer. The Fourier transform of the measured time-domain interferogram produces a radio-frequency spectrum that can be directly converted to a broadband optical spectrum through a linear scaling of the frequency. To achieve the highest possible resolution, it is necessary to compensate the residual relative fluctuations of the repetition rate and the carrier-envelope offset frequency of the frequency combs. Measuring RF beatnotes of each comb with two CW lasers provides two error signals that can be used to correct the recorded interferograms. This correction is applied differently for one-photon and two-photon spectra, providing a method of distinguishing the two

Sensitive and simple frequency comb fourier transform spectrometer with a multipass cell

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VERSATILE AND SENSITIVE DUAL COMB FOURIER TRANSFORM SPECTROSCOPY

Author Institution: Institut des Sciences Moleculaires d'Orsay, CNRS, Universite Paris-Sud, Batiment 350, 91405 Orsay Cedex, France; Max Planck Insitut fur Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany; Ludwig Maximilians-Universitat Munchen, Fakultat fur Physik, Schellingstrasse 4/III, 80799 Munchen, Germany; Institut des Sciences Moleculaires d'Orsay, CNRS, Universite Paris-Sud, Batiment 350, 91405 Orsay Cedex, France; Max Planck Insitut fur Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany; Ludwig Maximilians-Universitat Munchen, Fakultat fur Physik, Schellingstrasse 4/III, 80799 Munchen, GermanyFourier transform spectroscopy based on time-domain interferences between two slightly detuned frequency comb sources holds much promise for the real-time diagnostic of gaseous, liquid or solid-state samples. In one very recent example, cavity-enhanced absorption spectroscopy with two infrared frequency combs has demonstrated a dramatically enhanced sensitivity, compared to conventional Fourier spectroscopy, with intriguing implications for instantaneous trace gas analysis. It however remains challenging to match continuously the comb and cavity modes across a broad spectral bandwidth during the time of a measurement.\\ An obvious alternative for reaching a long interaction path is a conventional multipass cell. Additionally, differential detection schemes may be devised to increase the dynamic range of the interferometric measurements, thus providing enhanced signal to noise ratio. Experimental demonstrations will be given in the 1.5 $\mu$m region with a dual comb set-up based on two Er-doped fiber femtosecond lasers. The versatility and performances of these solutions will be compared to the cavity-enhanced dual comb technique and other state-of-the-art alternatives. \end{document