Coherent, multi-heterodyne spectroscopy using stabilized optical frequency combs

The broadband, coherent nature of narrow-linewidth fiber frequency combs is exploited to measure the full complex spectrum of a molecular gas through multi-heterodyne spectroscopy. We measure the absorption and phase shift experienced by each of 155,000 individual frequency comb lines, spaced by 100 MHz and spanning from 1495 nm to 1620 nm, after passing through a hydrogen cyanide gas. The measured phase spectrum agrees with Kramers-Kronig transformation of the absorption spectrum. This technique can provide a full complex spectrum rapidly, over wide bandwidths, and with hertz-level accuracy.Comment: 4 pages, 3 figure

DUAL-COMB SPECTROSCOPY IN THE OPEN AIR

Dual-comb spectroscopy is arguably the natural successor to FTIR. Based on the interference between two frequency combs, this technique can record broadband spectra with a resolution better than 0.0003 wn. Like FTIR, dual-comb spectroscopy measures an entire spectrum simultaneously, allowing for suppression of systematic errors related to temporal dynamics of the sample. Unlike FTIR it records the entire spectrum with virtually no instrument lineshape or error in the frequency axis. The lack of moving parts in dual-comb spectroscopy means that spectra can be recorded in milliseconds to microseconds with the desired signal-to-noise being the only real constrain on the minimum recording time. Finally the high spacial beam quality of the frequency combs allows for increased sensitivity through long interaction paths either in free-space, multi-pass cells or enhancement cavities. This talk will explore the recent use of dual-comb spectroscopy in the near-infrared to measure atmospheric carbon dioxide, methane and water concentrations over a 2-km outdoor open-air path. Due to many of the strengths just mentioned, precisions of $<$1 ppm for CO$_2$ and $<$3 ppb for CH$_4$ in 5 min are achieved making this system very attractive for carbon monitoring at length scales relevant to carbon transport models. Additionally this presentation will address recent work on robust, compact, and portable dual-comb spectrometers as well as dual-comb spectroscopy further into the IR

DYNAMIC REGIONAL AND CITY SCALE SENSING OF GHG’S USING A DUAL-COMB SPECTROMETER

The output of a laser frequency comb is composed of 100,000+ perfectly spaced, discrete wavelength elements or comb teeth, which act as a massively parallel set of single frequency (CW) lasers with highly stable, well-known frequencies. In dual-comb spectroscopy (DCS), two such frequency combs are interfered on a single detector yielding absorption information for each individual comb tooth. This approach combines the strengths of both CW laser spectroscopy and broadband spectroscopy providing high spectral resolution and broad optical bandwidths, all with a single-mode, high-brightness laser beam and a simple, single photodetector, detection scheme. Inter comparisons of DCS instruments in the 1.55-1.7um region have shown that atmoshpheric CO2 and CH4 concentrations can be retrieved with precisions of 0.14\% and 0.35\% respectively making this an attractive source for quantifying greenhouse gas emissions\footnote{E. M. Waxman, et al. “Intercomparison of open-path trace gas measurements with two dual-frequency-comb spectrometers.” Atmos Meas Tech 1,3295–3311 (2017)}. Here we show that DCS can be employed for dynamic regional monitoring using an unmanned aerial systems (UAS) to identify and quantify methane leaks\footnote{K. C. Cossel, et al. “Open-path dual-comb spectroscopy to airborne retroreflector.” Optica 4, 724–728 (2017)}. Additionally, we will show that much larger scale (multi-kilometer) fixed path measurements can be used for continuous monitoring of city scale CO2 emissions. A preliminary demonstration of this technique in Boulder Colorado shows reasonable agreement with the city’s own bottom up emission projections

Mid-Infrared Optical Frequency Combs based on Difference Frequency Generation for Molecular Spectroscopy

Mid-infrared femtosecond optical frequency combs were produced by difference frequency generation of the spectral components of a near-infrared comb in a 3-mm-long MgO:PPLN crystal. We observe strong pump depletion and 9.3 dB parametric gain in the 1.5 \mu m signal, which yields powers above 500 mW (3 \mu W/mode) in the idler with spectra covering 2.8 \mu m to 3.5 \mu m. Potential for broadband, high-resolution molecular spectroscopy is demonstrated by absorption spectra and interferograms obtained by heterodyning two combs.Comment: 11 pages, 8 figure

Broadband dual-comb hyperspectral imaging and adaptable spectroscopy with programmable frequency combs

We explore the advantages of a free-form dual-comb spectroscopy (DCS) platform based on time-programmable frequency combs for real-time, penalty-free apodized scanning. In traditional DCS, the fundamental spectral resolution, which equals the comb repetition rate, can be excessively fine for many applications. While the fine resolution is not itself problematic, it comes with the penalty of excess acquisition time. Post-processing apodization (windowing) can be applied to tailor the resolution to the sample, but only with a deadtime penalty proportional to the degree of apodization. The excess acquisition time remains. With free-form DCS, this deadtime is avoided by programming a real-time apodization pattern that dynamically reverses the pulse periods between the dual frequency combs. In this way, one can tailor the spectrometer's resolution and update rate to different applications without penalty. We show operation of a free-form DCS system where the spectral resolution is varied from the intrinsic fine resolution of 160 MHz up to 822 GHz by applying tailored real-time apodization. Because there is no deadtime penalty, the spectral signal-to-noise ratio increases linearly with resolution by 5000x over this range, as opposed to the square root increase observed for postprocessing apodization in traditional DCS. We explore the flexibility to change resolution and update rate to perform hyperspectral imaging at slow camera frame rates, where the penalty-free apodization allows for optimal use of each frame. We obtain dual-comb hyperspectral movies at a 20 Hz spectrum update rate with broad optical spectral coverage of over 10 THz