Applied Physics B

, Volume 114, Issue 4, pp 573–578

Fourier transform spectroscopy around 3 μm with a broad difference frequency comb

Authors

  • Samuel A. Meek
    • Max Planck Institut für Quantenoptik
  • Antonin Poisson
    • Max Planck Institut für Quantenoptik
    • Institut des Sciences Moléculaires d’Orsay, CNRSUniversité Paris-Sud
  • Guy Guelachvili
    • Institut des Sciences Moléculaires d’Orsay, CNRSUniversité Paris-Sud
  • Theodor W. Hänsch
    • Max Planck Institut für Quantenoptik
    • Fakultät für PhysikLudwig-Maximilians-Universität München
    • Max Planck Institut für Quantenoptik
    • Institut des Sciences Moléculaires d’Orsay, CNRSUniversité Paris-Sud
    • Fakultät für PhysikLudwig-Maximilians-Universität München
Article

DOI: 10.1007/s00340-013-5562-7

Cite this article as:
Meek, S.A., Poisson, A., Guelachvili, G. et al. Appl. Phys. B (2014) 114: 573. doi:10.1007/s00340-013-5562-7

Abstract

We characterize a new mid-infrared frequency comb generator based on difference frequency generation around 3.1 μm. High power per comb mode (>10−7 W/mode) is obtained over a broad spectral span (>750 nm, >790 cm−1). The source is used for direct absorption spectroscopy with a Michelson-based Fourier transform interferometer.

1 Introduction

Laser frequency combs [1] are opening up new opportunities for broad-spectral-bandwidth direct absorption spectroscopy. In recent times, a variety of novel techniques have demonstrated improved capabilities in terms of sensitivity, acquisition times, resolution and/or accuracy. Such promising experiments have been initially developed [25] in the near-infrared region, where ultrashort-pulse lasers are conveniently available. For spectroscopic applications, the mid-infrared spectral region (2–20 μm; 500–5,000 cm−1) is more appealing because most molecules have strong and characteristic fundamental rovibrational transitions that provide a “fingerprint” of the molecule. As the technology in this region is technically more demanding, considerable efforts have been undertaken in recent years to develop new frequency comb generators based on the lasers directly emitting in the mid-infrared [6], nonlinear frequency conversion either by difference frequency generation [710] or by optical parametric oscillation [11, 12], or Kerr nonlinearity in high-quality factor whispering-gallery mode microresonators pumped by a continuous-wave laser [13]. Concurrently, a number of spectrometric techniques have been explored [1416] to efficiently analyze the comb light. Mid-infrared frequency comb sources and their applications have been discussed in a recent review [17].

2 Experimental setup

In this study, we report on the applicability of a new high-power mid-infrared frequency comb to the direct absorption frequency comb spectroscopy. A schematic of this source is shown in Fig. 1. An erbium-doped fiber oscillator with a repetition frequency of 100 MHz emits around 1.55 μm (6,450 cm−1) and seeds two erbium-doped fiber amplifiers. The output of one of those is launched into a highly nonlinear fiber that broadens the spectrum by self-phase modulation. The part of the broadened spectrum centered at 1.04 μm (9,615 cm−1) is filtered out, and the stretched pulses are amplified in two steps with an ytterbium preamplifier and an ytterbium power amplifier. The ytterbium preamplifier and amplifier are in a housing, which is temperature stabilized slightly above the room temperature (typically 25 °C). The pulses at the output of the power amplifier are compressed by means of a prism and a transmission grating. The outputs of the erbium (signal, 100 fs, 400 mW, autocorrelation trace and spectrum: Fig. 2b, d) and ytterbium (pump, 100 fs, 1.4 W autocorrelation trace and spectrum: Fig. 2c, d) amplifiers pass through a half-wave plate to get beams linearly polarized parallel to the crystal optical axis, as required by the quasi-phase-matching condition. They are then combined on a dichroic mirror, and the temporal superposition of the pulses is achieved by introducing extra optical delay into the path of the erbium amplifier beam. The beams are focused onto a 3-mm-long periodically poled MgO-doped lithium niobate crystal (MgO-PPLN) with poling period varying from 27 to 32 μm. The crystal temperature is stabilized to 92 °C. The generated mid-infrared idler beam is isolated by means of a wedged germanium filter. The mid-infrared beam has an average power of 150 mW and a peak spectral power density of about 0.4 mW/nm. Figure 2a, b, c shows the autocorrelation trace of the idler, signal and pump, respectively. The autocorrelation trace of the idler (Fig. 2a) has a full width at half-maximum of 150 fs, corresponding to a pulse duration of 106 fs for a Gaussian pulse shape. The shape of the autocorrelation trace, however, indicates some spectral chirp, consistent with the width of the associated spectrum (Fig. 2d). The spectrum (Fig. 2d) of the idler is centered at 3.1 μm (3,225 cm−1), with a spectral bandwidth from 2.7 to 3.45 μm (3,700 to 2,900 cm−1). The phase-matching bandwidth of MgO-PPLN is calculated to be about 200 cm−1, assuming a narrow single-frequency pump. The broad spectral width of the pump beam (about 97 nm–900 cm−1; Fig. 2d) enables generation of mid-infrared radiation over a much wider bandwidth. The lines present in the idler spectrum are due to atmospheric water vapor. Slight tunability from 2.3 to 3.6 μm (4,345–2,775 cm−1) is obtained by adjusting the optical delay between pump and signal.
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Fig. 1

Experimental setup used to generate femtosecond mid-infrared pulses and perform absorption spectroscopy. A femtosecond erbium-doped oscillator is used to generate the 1.04-μm pump (through spectral shift in a nonlinear fiber and amplification) and the 1.55-μm signal beams. The two beams are mixed in a periodically poled lithium niobate crystal. The generated 3.1-μm mid-infrared beam is filtered out. It interrogates a gas cell, and it is analyzed by a Fourier transform spectrometer. HWP: half-wave plate

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Fig. 2

Interferometric autocorrelation traces of the a idler (red), b signal (green) and c pump (blue) pulses of the difference frequency generation source. d Spectra of the idler (red), signal (green) and pump (blue) radiation, measured with a Fourier transform spectrometer at 0.2-nm (0.2 cm−1) resolution (idler) or grating spectrometer at 1.6-nm (16 cm−1) resolution (pump and signal)

Spectral broadening of a fiber laser output in a nonlinear fiber can introduce noise, causing a loss of coherence between the pulses. We thus characterize the coherence of the pump by measuring the heterodyne beat between the pump and a continuous-wave ytterbium fiber laser (specified line width <10 kHz at 120 μs). Figure 3a shows the result of the measurement of the free-running beat note at the wavelength of 1.04 μm, with a 10-kHz resolution bandwidth and 1.25-s sweep time. The contrast in the beat note is higher than 25 dB, and its full width at 3-dB is about 30 kHz. The mid-infrared spectrum is also investigated by measuring the heterodyne beat between the idler and a continuous-wave optical parametric oscillator (OPO) tunable between 2.4 and 3.2 μm (specified line width <1 MHz at 1 s) with a InGaAsSb photodetector that has a 100-MHz bandwidth. The result suggests good pulse-to-pulse coherence. Figure 3b shows the result of the measurement of the free-running beat note at 3.0 μm with an integration bandwidth of 300 kHz, chosen to limit the spread in beat-note frequency due to variation in the unlocked OPO source.
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Fig. 3

a Free-running beat note between a line of the pump and a continuous-wave ytterbium fiber-doped laser. b Free-running beat note between the mid-infrared idler frequency comb and a continuous-wave OPO. The 100-MHz line is the repetition frequency of the comb, and the other lines are the beat notes of comb lines with the OPO. c Measurement of the total power of the idler radiation over 25 h. d Relative intensity noise of the idler, signal and pump from 50 Hz to 500 kHz

The total idler output power (Fig. 3c) is measured with a thermal detector. It varies by only about 5 % over the course of 24 h. Although the idler output power and spectrum depend strongly on the optical delay, it was found unnecessary to actively stabilize this degree of freedom. The main source for long-term instabilities in the temporal overlap between the pump and signal pulses was identified during the first tests of the comb as arising from the ytterbium preamplifier and amplifier. A simple temperature stabilization of the housing of the ytterbium systems maintaining them at a few degrees above room temperature was found to satisfactorily solve this issue and lead to the results of Fig. 3c.

To quantify the magnitude of noise sources, relative intensity noise (RIN) measurements at each stage of the system are performed by monitoring (Fig. 3d) each intermediate output with a fast photodiode and using a signal analyzer to obtain a power spectral density of the RIN. The pump beam is the main source of RIN in the system, and we infer that the main source of RIN originates in the nonlinear fiber. The RIN is almost flat from 1 to 30 MHz with the values of −149 and −124 dBc/Hz for the signal and the pump, respectively. For the idler, the RIN depends strongly on the temporal overlap of the pulses; while the minimum value is around −119 dBc/Hz in this frequency range, the intensity noise can be as much as 30 dB higher. Therefore, we optimize the alignment of the system by simultaneously monitoring the spectrum and the intensity noise of the idler radiation. Interferometers, like Michelson or dual-comb spectrometers, however, allow for efficient amplitude noise cancellation when the two outputs of the interferometer are subtracted. This seems an important prerequisite for efficient spectroscopic measurement with this light source.

3 Spectroscopic measurements

To demonstrate the potential for spectroscopy, we insert (Fig. 1) a 5-cm-long single-pass cell filled with 67 hPa acetylene in natural abundance in the idler’s beam path and we analyze the light transmitted by the cell with a Michelson-based Fourier transform spectrometer. The beam is attenuated before being focused onto the J-stop of the interferometer. Figure 4 shows a spectrum at 0.2 nm (0.2 cm−1) resolution over the full spectral span, while the inset zooms onto some of the acetylene lines and gives their assignment [18]. The crowded spectrum is mainly composed of the lines of the ν3 and ν2 + ν4 + ν5 cold bands of 12C2H2, with rovibrational levels interacting in a Fermi-type resonance. The resolution is currently limited by the Fourier transform spectrometer, but the repetition frequency of the comb, 100 MHz, allows for Doppler-limited measurements. The frequency comb generator used as a light source for absorption Fourier transform spectroscopy features the same advantages as those already pointed out in other publications [6]: Compared to an incoherent blackbody source, the spectral brightness is a factor of 105 higher, enabling direct absorption measurements with higher signal to noise. A significant difference with respect to the previous reports is that the source spans a broad range in the region of the CH, OH and NH stretches in molecules.
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Fig. 4

Spectrum of C2H2 in the 3.0-μm region illustrating the spectral bandwidth capabilities of the spectrometric technique. The ν3 and ν2 + ν4 + ν5 bands are observed. The left graph shows the entire spectral domain covered in a single recording, while the right inset shows a portion of the spectrum expanded in wavelength and intensity scales

4 Discussion

Figure 5 summarizes the main characteristics of the new frequency comb generator and compares them to a nonexhaustive selection of the state-of-the-art recent reports, chosen for the specific characteristics they demonstrate, in spectral span or central wavelength or power per comb mode. The recent review article [17] discusses in a more detailed manner various implementations and achievements that have been reported so far. When compared to other frequency comb sources based on difference frequency generation, the system achieves high power per comb line (>10−7 W/mode) over a 750-nm (750 cm−1) span. Thus, the difference frequency comb generator reaches a domain that has traditionally been occupied by OPOs. Although noisier than an OPO, a fiber-based difference frequency generation source is easier to operate, as it is mostly alignment free. Interestingly, the generation of difference frequencies between the different teeth of a single-comb oscillator leads to comb lines with a frequency that is an integer multiple of the repetition frequency. The carrier-envelope offset frequency is indeed fixed to zero. We note that a similar system with 250 MHz repetition frequency and 120 mW average power has been described [19] simultaneously to our present report.
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Fig. 5

Typical spectral regions and power per comb mode reached with mid-infrared femtosecond lasers (blue), difference frequency generation (DFG) (yellow), optical parametric oscillators (OPOs) (green) and microresonator-based Kerr combs (violet). Performances of a few selected frequency comb sources are represented with black and white lines, with citations shown in brackets. The width of each bar represents the spectral span (regardless of their tunability) over which the respective comb has at least the power per mode indicated by its position on the ordinate. The characteristics of the source investigated in the present work are displayed in red. This figure is adapted from a figure displayed in [17]

This frequency comb generator has been developed with the goal of dual-comb spectroscopy. The coherence, the repetition frequency and the spectral span make this generator well suited for dual-comb spectroscopy of gas-phase samples in the 3-μm (3,300 cm−1) region. Excellent results have been reported [15] on dual-comb spectroscopy of methane in the 3.4 μm (2,950 cm−1) range, but the difference frequency generation sources had a spectrum spanning only 40 nm (35 cm−1) with a tunability of 170 nm (150 cm−1). We thus envision our frequency comb generator to expand the capabilities of such highly multiplex spectroscopy due to its 750 nm (790 cm−1) simultaneous spectral coverage. Cancellation of the carrier-envelope offset frequency is expected to significantly simplify experimental implementation, either with stabilized [3, 15] or with free-running [20, 21] setups.

Furthermore, on the basis of the recent demonstrations of nonlinear dual-comb spectroscopy [2224] in the near-infrared region, the high power per comb mode makes it possible to envision mid-infrared nonlinear dual-comb spectroscopy, e.g., sum-frequency generation at surfaces or two-photon excitation of rovibrational molecular transitions.

Acknowledgments

The frequency comb source has been developed in collaboration with Menlo Systems GmbH within a Eurostars project. Research was conducted in the scope of the European Laboratory for Frequency Comb Spectroscopy. Support by the Max Planck Foundation and the Munich Center for Advanced Photonics is also acknowledged.

Copyright information

© Springer-Verlag Berlin Heidelberg 2013