Dual-comb spectroscopy of water vapor with a free-running semiconductor disk laser

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Science  16 Jun 2017:
Vol. 356, Issue 6343, pp. 1164-1168
DOI: 10.1126/science.aam7424
  • Fig. 1 Current state-of-the-art performance in frequency comb characteristics.

    (A) A commercial OFC (e.g., Menlo FC1500-250-WG MVIS) can cover an octave of optical spectrum. The large bandwidth, however, strongly reduces the average power per comb line to about 50 nW. Optically pumped semiconductor disk lasers (SDLs) can provide more than 1 mW of average power per comb line. With pulse durations as short as 100 fs, their optical spectrum spans up to 17 nm at a center wavelength around 1 μm (41). The center wavelength of ultrafast SDLs can be shifted with semiconductor band-gap engineering. (B) Large spectral coverage of optically pumped SDLs. Overview of continuous-wave and passively mode-locked SDL results in terms of center operation wavelength and average output power on a logarithmic scale. The plotted results are summarized in recent review papers (1419).

  • Fig. 2 Dual-comb source and operating principle.

    (A) Dual-comb MIXSEL. The diode-pumped dual-comb MIXSEL is a specialized SDL for which both the gain and the saturable absorber are integrated into the same semiconductor wafer. The semiconductor MIXSEL chip forms one cavity-end mirror, and the output coupler forms the other. In addition, two intracavity elements are used: first, an etalon to adjust the center wavelength, and second, a birefringent crystal for polarization splitting and dual-comb generation. The output coupler can be mounted on a piezo actuator to adjust the cavity length (and then the repetition rate). The dual-comb MIXSEL generates two collinear perpendicularly polarized (polarization indicated by circles and arrows) mode-locked pulse trains with a small difference in the pulse repetition frequency due to the difference in optical path length in the birefringent crystal. Thus, we obtain two collinear OFCs). A polarizing beam splitter (PBS), with an optical axis at an angle of 45° to the polarization axes of the OFCs, combines both OFCs into the same polarization, which then optically interfere on a photodetector (PD). A microwave spectrum analyzer (MSA) directly measures the microwave frequency comb, which manifests as a comb structure (comb1) between dc and the pulse repetition rates frep,1 and frep,2. The pink rectangle indicates the microwave comb. dBc, decibels relative to the carrier. (B) The microwave frequency comb results from the optical interference on the PD between all the optical frequencies of the two collinear and combined OFCs with their offset in pulse repetition rate Δfrep, which can be adjusted with the optical path length in the birefringent crystal. With this dual-comb principle, we obtain a direct link between the optical and the microwave frequencies.

  • Fig. 3 Dual-comb spectroscopy results.

    (A) Schematic of the experimental setup with two combined dual-comb OFCs. A reference microwave spectrum is measured with PD1, and the microwave spectrum after the multipass gas cell is simultaneously measured with PD2, making the setup insensitive to phase errors. (B) Microwave frequency combs for the reference measurement (blue) and the measurement with the gas cell (red), which is modulated because of the absorption of the water vapor. (C) Absorption measurement with the free-running system on water vapor and comparison with the HITRAN database, with the residual error shown below in purple. (D) Same measurements as in (C) with the stabilized microwave comb.

  • Fig. 4 Stabilization of the microwave comb and optical mode stability.

    (A) Schematic of the stabilization setup. By using a beam splitter (BS), both OFCs (red and blue lines) are superimposed in cross-polarization on PD4 to prevent optical interference and to simply measure and stabilize the difference in pulse repetition rate Δfrep. With a PBS, both combined OFCs are superimposed in the same polarization on PD3, where the microwave comb is measured and one of the comb lines is stabilized. Electr. ref., electronic reference. (B) Microwave spectrum of the free-running microwave comb1 (Fig. 2A) with a resolution bandwidth (RBW) of 10 kHz. (C) Magnification of a single microwave comb line. The blue line shows the fluctuations of a free-running comb line. The maximum of the trace is recorded for 10 s with a RBW of 10 kHz. The red line shows a stabilized comb line with a RBW of 300 Hz. The green line demonstrates that all comb lines are simultaneously stabilized if the red line, and additionally Δfrep, are stabilized (RBW 300 Hz) (36). (D) Measurement of the absolute stability of the optical modes. The microwave spectrum shows the beat signal between the single-frequency laser and one optical comb line with a RBW of 100 kHz. The fluctuations of the beat frequency and the consequent fluctuations of the optical comb line are measured with a frequency counter over time in case of the free-running system (blue line) and with the microwave comb stabilization active (red line). The fluctuations of the single-frequency laser are measured with a wavelength meter (gray line). Stabilizing the microwave comb drastically improves the long-term stability of the optical comb lines.

Supplementary Materials

  • Dual-comb spectroscopy of water vapor with a free-running semiconductor disk laser

    S. M. Link, D. J. H. C. Maas, D. Waldburger, U. Keller

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    • Materials and Methods
    • Figs. S1 to S3
    • Caption for Movie S1
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    Movie S1
    Dual-comb spectroscopy demonstration of water vapor with the free-running dualcomb MIXSEL in the laboratory. The laser cavity is shown during operation.

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