Validation of open-path dual-comb spectroscopy against an O background: erratum.

This erratum corrects errors that appear in Opt. Express31, 5042 (2023).10.

Removing biases in dual frequency comb spectroscopy due to digitizer nonlinearity.

Operation of any dual-comb spectrometer requires digitization of the interference signal before further processing. Nonlinearities in the analog-to-digital conversion can alter the apparent gas concentration by multiple percent, limiting both precision and accuracy of this technique. This work describes both the measurement of digitizer nonlinearity and the development of a model that quantitatively describes observed concentration bias over a range of conditions.

Open-path dual-comb spectroscopy of methane and VOC emissions from an unconventional oil well development in Northern Colorado.

We present results from a field study monitoring methane and volatile organic compound emissions near an unconventional oil well development in Northern Colorado from September 2019 to May 2020 using a mid-infrared dual-comb spectrometer. This instrument allowed quantification of methane, ethane, and propane in a single measurement with high time resolution and integrated path sampling. Using ethane and propane as tracer gases for methane from oil and gas activity, we observed emissions during the drilling, hydraulic fracturing, millout, and flowback phases of well development.

Validation of open-path dual-comb spectroscopy against an O background.

Dual-comb spectroscopy measures greenhouse gas concentrations over kilometers of open air with high precision. However, the accuracy of these outdoor spectra is challenging to disentangle from the absorption model and the fluctuating, heterogenous concentrations over these paths. Relative to greenhouse gases, O concentrations are well-known and evenly mixed throughout the atmosphere.

Precise multispecies agricultural gas flux determined using broadband open-path dual-comb spectroscopy.

Advances in spectroscopy have the potential to improve our understanding of agricultural processes and associated trace gas emissions. We implement field-deployed, open-path dual-comb spectroscopy (DCS) for precise multispecies emissions estimation from livestock. With broad atmospheric dual-comb spectra, we interrogate upwind and downwind paths from pens containing approximately 300 head of cattle, providing time-resolved concentration enhancements and fluxes of CH, NH, CO, and HO.

Dual-comb photoacoustic spectroscopy.

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.

Compact mid-infrared dual-comb spectrometer for outdoor spectroscopy.

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.

Estimating vehicle carbon dioxide emissions from Boulder, Colorado, using horizontal path-integrated column measurements.

We performed 7.5 weeks of path-integrated concentration measurements of CO, CH, HO, and HDO over the city of Boulder, Colorado. An open-path dual-comb spectrometer simultaneously measured time-resolved data across a reference path, located near the mountains to the west of the city, and across an over-city path that intersected two-thirds of the city, including two major commuter arteries.

Dual-comb spectroscopy with tailored spectral broadening in SiN nanophotonics.

SiN waveguides, pumped at 1550 nm, can provide spectrally smooth, broadband light for gas spectroscopy in the important 2 μm to 2.5 μm atmospheric water window, which is only partially accessible with silica-fiber based systems. By combining Er fiber frequency combs and supercontinuum generation in tailored SiN waveguides, high signal-to-noise dual-comb spectroscopy spanning 2 μm to 2.

Speed-dependent Voigt lineshape parameter database from dual frequency comb measurements at temperatures up to 1305 K. Part II: Argon-broadened HO absorption, 6801-7188 cm.

We report argon-broadened water vapor transition parameters and their temperature dependence based on measured spectra spanning 6801-7188 cm from a broad-bandwidth, high-resolution dual frequency comb spectrometer. The 25 collected spectra of 2% water vapor in argon ranged from 296 K to 1305 K with total pressure spanning 100 Torr to 600 Torr. A multispectrum fitting routine was used in conjunction with a quadratic speed-dependent Voigt profile to extract broadening and shift parameters, and a power-law temperature-dependence exponent for both.

Open-path dual comb spectroscopy to an airborne retroreflector.

We demonstrate a new technique for spatial mapping of multiple atmospheric gas species. This system is based on high-precision dual-comb spectroscopy to a retroreflector mounted on a flying multi-copter. We measure the atmospheric absorption over long open-air paths to the multi-copter with comb-tooth resolution over 1.

Speed-dependent Voigt lineshape parameter database from dual frequency comb measurements up to 1305 K. Part I: Pure HO absorption, 6801-7188 cm.

We measure speed-dependent Voigt lineshape parameters with temperature-dependence exponents for several hundred spectroscopic features of pure water spanning 6801-7188 cm. The parameters are extracted from broad bandwidth, high-resolution dual frequency comb absorption spectra with multispectrum fitting techniques. The data encompass 25 spectra ranging from 296 K to 1305 K and 1 to 17 Torr of pure water vapor.

Intercomparison of Open-Path Trace Gas Measurements with Two Dual Frequency Comb Spectrometers.

We present the first quantitative intercomparison between two open-path dual comb spectroscopy (DCS) instruments which were operated across adjacent 2-km open-air paths over a two-week period. We used DCS to measure the atmospheric absorption spectrum in the near infrared from 6021 to 6388 cm (1565 to 1661 nm), corresponding to a 367 cm bandwidth, at 0.0067 cm sample spacing.

The Role of Sulfur Dioxide in Stratospheric Aerosol Formation Evaluated Using In-Situ Measurements in the Tropical Lower Stratosphere.

Stratospheric aerosols (SAs) are a variable component of the Earth's albedo that may be intentionally enhanced in the future to offset greenhouse gases (geoengineering). The role of tropospheric-sourced sulfur dioxide (SO) in maintaining background SAs has been debated for decades without in-situ measurements of SO at the tropical tropopause to inform this issue. Here we clarify the role of SO in maintaining SAs by using new in-situ SO measurements to evaluate climate models and satellite retrievals.

The use of intrapartum ultrasound to diagnose malpositions and cephalic malpresentations.

Fetal malpositions and cephalic malpresentations are well-recognized causes of failure to progress in labor. They frequently require operative delivery, and are associated with an increased probability of fetal and maternal complications. Traditional obstetrics emphasizes the role of digital examinations, but recent studies demonstrated that this approach is inaccurate and intrapartum ultrasound is far more precise.

Accurate frequency referencing for fieldable dual-comb spectroscopy.

We describe a dual-comb spectrometer that can operate independently of laboratory-based rf and optical frequency references but is nevertheless capable of ultra-high spectral resolution, high SNR, and frequency-accurate spectral measurements. The instrument is based on a "bootstrapped" frequency referencing scheme in which short-term optical phase coherence between combs is attained by referencing each to a free-running diode laser, whilst high frequency resolution and long-term accuracy is derived from a stable quartz oscillator. The sensitivity, stability and accuracy of this spectrometer were characterized using a multipass cell.

Synchronization of Clocks Through 12 km of Strongly Turbulent Air Over a City.

We demonstrate real-time, femtosecond-level clock synchronization across a low-lying, strongly turbulent, 12-km horizontal air path by optical two-way time transfer. For this long horizontal free-space path, the integrated turbulence extends well into the strong turbulence regime corresponding to multiple scattering with a Rytov variance up to 7 and with the number of signal interruptions exceeding 100 per second. Nevertheless, optical two-way time transfer is used to synchronize a remote clock to a master clock with femtosecond-level agreement and with a relative time deviation dropping as low as a few hundred attoseconds.

Cardiotocographic findings in the second stage of labor among fetuses delivered with acidemia: a comparison of two classification systems.

Background: The RCOG classification system of CTG trace is widely used for the analysis of the fetal heart rate during the first and second stage of labor. Other authors proposed specific classification systems for the second stage traces.

Objective: To evaluate the accuracy of RCOG and Piquard cardiotocographic patterns classification systems in predicting fetal acidemia in the second stage of labor.

Tight real-time synchronization of a microwave clock to an optical clock across a turbulent air path.

The ability to distribute the precise time and frequency from an optical clock to remote platforms could enable future precise navigation and sensing systems. Here we demonstrate tight, real-time synchronization of a remote microwave clock to a master optical clock over a turbulent 4-km open air path via optical two-way time-frequency transfer. Once synchronized, the 10-GHz frequency signals generated at each site agree to 10 at one second and below 10 at 1000 seconds.

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.

Broadband Phase Spectroscopy over Turbulent Air Paths.

Broadband atmospheric phase spectra are acquired with a phase-sensitive dual-frequency-comb spectrometer by implementing adaptive compensation for the strong decoherence from atmospheric turbulence. The compensation is possible due to the pistonlike behavior of turbulence across a single spatial-mode path combined with the intrinsic frequency stability and high sampling speed associated with dual-comb spectroscopy. The atmospheric phase spectrum is measured across 2 km of air at each of the 70,000 comb teeth spanning 233  cm(-1) across hundreds of near-infrared rovibrational resonances of CO(2), CH(4), and H(2)O with submilliradian uncertainty, corresponding to a 10(-13) refractive index sensitivity.

Comb-calibrated laser ranging for three-dimensional surface profiling with micrometer-level precision at a distance.

Non-contact surface mapping at a distance is interesting in diverse applications including industrial metrology, manufacturing, forensics, and artifact documentation and preservation. Frequency modulated continuous wave (FMCW) laser detection and ranging (LADAR) is a promising approach since it offers shot-noise limited precision/accuracy, high resolution and high sensitivity. We demonstrate a scanning imaging system based on a frequency-comb calibrated FMCW LADAR and real-time digital signal processing.

Speckle phase noise in coherent laser ranging: fundamental precision limitations.

Frequency-modulated continuous-wave laser detection and ranging (FMCW LADAR) measures the range to a surface through coherent detection of the backscattered light from a frequency-swept laser source. The ultimate limit to the range precision of FMCW LADAR, or any coherent LADAR, to a diffusely scattering surface will be determined by the unavoidable speckle phase noise. Here, we demonstrate the two main manifestations of this limit.

Comb-calibrated frequency-modulated continuous-wave ladar for absolute distance measurements.

We demonstrate a comb-calibrated frequency-modulated continuous-wave laser detection and ranging (FMCW ladar) system for absolute distance measurements. The FMCW ladar uses a compact external cavity laser that is swept quasi-sinusoidally over 1 THz at a 1 kHz rate. The system simultaneously records the heterodyne FMCW ladar signal and the instantaneous laser frequency at sweep rates up to 3400 THz/s, as measured against a free-running frequency comb (femtosecond fiber laser).