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Register a new userSpectroscopy of the Methane { u}3 Band with an Accurate Mid-Infrared Coherent Dual- Comb Spectrometer
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We demonstrate a high-accuracy dual-comb spectrometer centered at 3.4 mu m. The amplitude and phase spectra of the P, Q, and partial R-branch of the methane { u}3 band are measured at 25 MHz to 100 MHz point spacing with ~kHz resolution and a signal-to-noise ratio of up to 3500. A fit of the absorbance and phase spectra yield the center frequency of 132 rovibrational lines. The systematic uncertainty is estimated to be 300 kHz, which is 10-3 of the Doppler width and a tenfold improvement over Fourier transform spectroscopy. These data are the first high- accuracy molecular spectra obtained with a direct comb spectrometer.
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Dual-frequency comb spectroscopy has emerged as a disruptive technique for measuring wide-spanning spectra with high resolution, yielding a particularly powerful technique for sensitive multi-component gas analysis. We present a spectrometer system based on dual electro-optical combs with subsequent conversion to the mid-infrared via tunable difference frequency generation, operating in the range from 3 to 4.7 $mu$m. The simultaneously recorded bandwidth is up to 454(1) GHz and a signal-to-noise ratio of 7.3(2) x 10$^2$ Hz$^{-1/2}$ can be reached. The conversion preserves the coherence of the dual-comb within 3 s measurement time. Concentration measurements of 5 ppm methane at 3.3 $mu$m, 100 ppm nitrous oxide at 3.9 $mu$m and a mixture of 15 ppm carbon monoxide and 5 % carbon dioxide at 4.5 $mu$m are presented with a relative precision of 1.4 % in average after 2 s measurement time. The noise-equivalent absorbance is determined to be less than 4.6(2) x 10$^{-3}$ Hz$^{-1/2}$.
Dual-comb spectroscopy has been proven a powerful tool in molecular characterization, which remains challenging to implement in the mid-infrared (MIR) region due to difficulties in the realization of two mutually locked comb sources and efficient photodetection. An effective way to overcome those limitations is optical upconversion; however, previously reported configurations are either demanding or inefficient. Here we introduce and experimentally demonstrate a variant of dual-comb spectroscopy called cross-comb spectroscopy, in which a MIR comb is upconverted via sum-frequency generation (SFG) with a near-infrared (NIR) comb with a shifted repetition rate and then interfered with a spectral extension of the NIR comb. We experimentally demonstrate a proof-of-concept measurement of atmospheric CO2 around 4.25 micrometer, with a 350-nm instantaneous bandwidth and 25000 resolved comb lines. Cross-comb spectroscopy can be realized using up- or down-conversion and offers an adaptable and efficient alternative to dual-comb spectroscopy outside the well-developed near-IR region, where having two mutually coherent sources and efficient photodetection is challenging. Moreover, the nonlinear gating in cross-comb spectroscopy promises a superior dynamic range compared to dual-comb spectroscopy.
Two semiconductor optical frequency combs consuming less than 1 W of electrical power are used to demonstrate high-sensitivity mid-infrared dual-comb spectroscopy in the important 3-4 $mu$m spectral region. The devices are 4 millimeters long by 4 microns wide, and each emits 8 mW of average optical power. The spectroscopic sensing performance is demonstrated by measurements of methane and hydrogen chloride with a spectral coverage of 33 cm$^{-1}$ (1 THz), 0.32 cm$^{-1}$ (9.7 GHz) frequency sampling interval, and peak signal-to-noise ratio of ~100 at 100 $mu$s integration time. The monolithic design, low drive power, and direct generation of mid-infrared radiation are highly attractive for portable broadband spectroscopic instrumentation in future terrestrial and space applications.
Four-wave-mixing-based quantum cascade laser frequency combs (QCL-FC) are a powerful photonic tool, driving a recent revolution in major molecular fingerprint regions, i.e. mid- and far-infrared domains. Their compact and frequency-agile design, together with their high optical power and spectral purity, promise to deliver an all-in-one source for the most challenging spectroscopic applications. Here, we demonstrate a metrological-grade hybrid dual comb spectrometer, combining the advantages of a THz QCL-FC with the accuracy and absolute frequency referencing provided by a free-standing, optically-rectified THz frequency comb. A proof-of-principle application to methanol molecular transitions is presented. The multi-heterodyne molecular spectra retrieved provide state-of-the-art results in line-center determination, achieving the same precision as currently available molecular databases. The devised setup provides a solid platform for a new generation of THz spectrometers, paving the way to more refined and sophisticated systems exploiting full phase control of QCL-FCs, or Doppler-free spectroscopic schemes.
Infrared spectroscopy is a powerful tool for basic and applied science. The molecular spectral fingerprints in the 3 um to 20 um region provide a means to uniquely identify molecular structure for fundamental spectroscopy, atmospheric chemistry, trace and hazardous gas detection, and biological microscopy. Driven by such applications, the development of low-noise, coherent laser sources with broad, tunable coverage is a topic of great interest. Laser frequency combs possess a unique combination of precisely defined spectral lines and broad bandwidth that can enable the above-mentioned applications. Here, we leverage robust fabrication and geometrical dispersion engineering of silicon nanophotonic waveguides for coherent frequency comb generation spanning 70 THz in the mid-infrared (2.5 um to 6.2 um). Precise waveguide fabrication provides significant spectral broadening and engineered spectra targeted at specific mid-infrared bands. We use this coherent light source for dual-comb spectroscopy at 5 um.
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