Mid-infrared dual-comb spectroscopy has the potential to supplant conventional high-resolution Fourier transform spectroscopy in applications that require high resolution, accuracy, signal-to-noise ratio, and speed. Until now, dual-comb spectroscopy in the mid-infrared has been limited to narrow optical bandwidths or to low signal-to-noise ratios. Using a combination of digital signal processing and broadband frequency conversion in waveguides, we demonstrate a midinfrared dual-comb spectrometer that can measure comb-tooth resolved spectra across an octave of bandwidth in the mid-infrared from 2.6-5.2 µm with sub-MHz frequency precision and accuracy and with a spectral signal-to-noise ratio as high as 6500. As a demonstration, we measure the highly structured, broadband cross-section of propane (C3H8) in the 2860-3020 cm -1 region, the complex phase/amplitude spectrum of carbonyl sulfide (COS) in the 2000 to 2100 cm -1 region, and the complex spectra of methane, acetylene, and ethane in the 2860-3400 cm -1 region.Mid-infrared spectroscopy is a powerful technique for the multispecies detection of trace gases with applications ranging from the detection of hazardous materials, to environmental monitoring and industrial monitoring. Compared to the near-infrared, where laser sources are more plentiful, the techniques for measuring mid-infrared spectra are more limited. Mid-infrared spectra are most commonly acquired by Fourier transform spectroscopy (FTS), which provides accurate and high resolution spectra but requires a scanning delay arm and blackbody source leading to large instruments and long acquisition times. Dual-comb spectroscopy (DCS) is a high-performance alternative to conventional FTS providing high resolution, absolute frequency accuracy, fast acquisition times, long interaction lengths, broad bandwidth coverage, and high signal-to-noise ratio [1,2]. The advantages of speed and long path length are of particular relevance to non-laboratory applications, for example in open-path atmospheric monitoring or industrial process monitoring [3][4][5][6]. However, up until now, DCS has only been demonstrated with its full panoply of advantages in the near-infrared, from ~ 1 to 2 µm [7-10]. The near-infrared has much more limited applications compared to the mid-infrared since molecular crosssections are typically 1000 times weaker, if they exist at all. DCS in the mid-infrared has indeed been actively pursued [11][12][13][14][15][16][17][18][19][20][21][22][23][24], yet it is not competitive with high-resolution conventional FTS, limited by the coherence and/or the bandwidth of mid-infrared comb sources.Quantitative broadband mid-infrared DCS requires addressing strong overlapping requirements on the underlying mid-infrared frequency combs: they must produce broad and relatively flat optical spectra while maintaining mutual coherence over the measurement time. Without coherence, adjacent comb teeth blend together, sacrificing orders of magnitude in spectral resolution and obscuring both the frequency and amplit...

Spectrally resolved photoacoustic imaging is promising for label-free imaging in optically scattering materials. However, this technique often requires acquisition of a separate image at each wavelength of interest. This reduces imaging speeds and causes errors if the sample changes in time between images acquired at different wavelengths. We demonstrate a solution to this problem by using dual-comb spectroscopy for photoacoustic measurements. This approach enables a photoacoustic measurement at thousands of wavelengths simultaneously. In this technique, two optical-frequency combs are interfered on a sample and the resulting pressure wave is measured with an ultrasound transducer. This acoustic signal is processed in the frequency-domain to obtain an optical absorption spectrum. For a proof-ofconcept demonstration, we measure photoacoustic signals from polymer films. The absorption spectra obtained from these measurements agree with those measured using a spectrophotometer. Improving the signal-to-noise ratio of the dual-comb photoacoustic spectrometer could enable high-speed spectrally resolved photoacoustic imaging.

The microscale integration of mid-and longwave-infrared photonics could enable the development of fieldable, robust chemical sensors, as well as highly efficient infrared frequency converters. However, such technology would be defined by the choice of material platform, which immediately determines the strength and types of optical nonlinearities available, the optical transparency window, modal confinement, and physical robustness. In this work, we demonstrate a new platform, suspended AlGaAs waveguides integrated on silicon, providing excellent performance in all of these metrics. We demonstrate low propagation losses within a span of nearly two octaves (1.26 to 4.6 µm) with exemplary performance of 0.45 dB/cm at λ = 2.4 µm. We exploit the high nonlinearity of this platform to demonstrate 1560 nm-pumped second-harmonic generation and octave-spanning supercontinuum reaching out to 2.3 µm with 3.4 pJ pump pulse energy. With mid-IR pumping, we generate supercontinuum spanning from 2.3 to 6.5 µm. Finally, we demonstrate the versatility of the platform with mid-infrared passive devices such as low-loss 10 µm-radius bends, compact power splitters with 96 ± 1% efficiency and edge couplers with 3.0 ± 0.1 dB loss. This platform has strong potential for multi-functional integrated photonic systems in the mid-IR. arXiv:1905.01380v1 [physics.app-ph] 3 May 2019 have suitable optical transparency [23], and strong optical nonlinearities are also required for the generation or broadening of frequency combs in the mid-IR [15]. While significant Kerr nonlinearity is present in silicon, germanium and chalcogenide materials, they lack intrinsic second-order optical nonlinearities for highly efficient frequency conversion [6,7,24,25] and electro-optic modulation [26].Alternatively, group III-V materials possess many desirable properties for multi-functional integrated photonic systems including a high refractive index, strong second-and third-order optical nonlinearities, and wide optical transparency windows into the LWIR. A practical advantage of these materials is the ability to grow a chemically selective etch stop underneath a high-quality epitaxial device (donor) film, enabling wafer or chip-bonding film transfer techniques for heterogeneous integration [27,28]. This has enabled high-index-contrast III-V waveguides on other substrates such as oxidized silicon and sapphire [29][30][31][32][33]. However, to take full advantage of the broad transparency window supported by III-V semiconductors, it is necessary to pursue alternative geometries such as air-clad suspended waveguides. But even this approach requires a degree of caution, as most materials readily form surface oxide layers that also introduce absorption. Undercut etching has been used to suspend GaAs waveguides engineered for mid-IR difference frequency generation [34]. While this represents a promising step in the development of nonlinear mid-IR photonics with III-V materials, many issues remain, such as the propagation loss in the mid-IR region, atmospheric stabilit...

This manuscript describes the design of a robust, mid-infrared dual-comb spectrometer operating in the 3.1-µm to 4-µm spectral window for future field applications. The design represents an improvement in system size, power consumption, and robustness relative to previous work while also providing a high spectral signal-to-noise ratio. We demonstrate a system quality factor of 2×106 and 30 hours of continuous operation over a 120-meter outdoor air path.