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Gas spectroscopy with a free-running dual-comb semiconductor disk laser

Dual-comb spectroscopy [1] combines extremely high measurement precision with very fast data acquisition. However, the expensive and complex laser systems which usually comprise two modelocked lasers that need to be tightly stabilized to achieve good mutual coherence are limiting the practicality of this method. We present a very compact, simple and cost-efficient source in form of a dual-comb based on a modelocked integrated external-cavity surface-emitting laser (MIXSEL) [2]. The dual-comb MIXSEL generates simultaneously two gigahertz pulse trains with a difference in pulse repetition rate of 4 MHz from a single laser cavity by utilizing an intracavity birefringent crystal [3] (Fig. 1a). The semiconductor technology of the MIXSEL allows to tailor the wavelength of the laser to the region of interest via bandgap engineering potentially from the UV to the mid-IR [4, 5]. This wavelength flexibility can be used to optimize the optical spectrum of the source to the spectroscopic region of interest. Here, we perform a proof of concept dual-comb spectroscopy demonstration on water vapor at a wavelength of 968 nm. Both beams are brought to the same polarization and then half of the power is sent through a gas cell of 40 m length with water vapor (60% relative humidity) while the other half is detected directly and serves as a reference (Fig. 1a). The Fourier transformation of the two time signals reveals two corresponding microwave combs (Fig. 1b and c) which are the result of the interference of the optical laser combs with different repetition rates. From the two microwave combs the water absorption can be extracted and is in very good agreement with the HITRAN database with a standard deviation of the residual difference of 0.017 (Fig. 1d). This measurement is achieved with a totally free-running laser system, which is enabled by the high initial coherence of the two laser beams that share the same cavity. For applications requiring longer measurement times to enhance the signal-to-noise ratio and to avoid long-term drifts, we will present a very simple stabilization scheme that only requires two feedback loops using directly the microwave spectrum without the need for more complex f-to-2f interferometry [6] (Fig. 1a). In conclusion, the dual-comb MIXSEL drastically reduces the complexity of the system and can open up dual-comb spectroscopy to a wide range of commercial applications.