Fourier transform spectroscopy around 3 μm with a broad difference frequency comb
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- Meek, S.A., Poisson, A., Guelachvili, G. et al. Appl. Phys. B (2014) 114: 573. doi:10.1007/s00340-013-5562-7
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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.
Laser frequency combs  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 [2–5] 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 , nonlinear frequency conversion either by difference frequency generation [7–10] or by optical parametric oscillation [11, 12], or Kerr nonlinearity in high-quality factor whispering-gallery mode microresonators pumped by a continuous-wave laser . Concurrently, a number of spectrometric techniques have been explored [14–16] to efficiently analyze the comb light. Mid-infrared frequency comb sources and their applications have been discussed in a recent review .
2 Experimental setup
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
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  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 [22–24] 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.
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.