We recently demonstrated a field-deployed dual frequency comb laser spectrometer capable of locating and sizing methane sources down to 1.6 grams/minute (which is equivalent to approximately one quarter of the human breathing rate) from a distance of 1 km. The system couples open-path methane concentration measurements over long distances together with wind information in a Bayesian inversion framework to locate sources within the monitoring region. We are now applying the technology for leak detection at operational oil and gas facilities. We will discuss the evolution of the project from laboratory proof-of-concept to controlled field testing to initial implementation in an industrial setting. We will also discuss the challenges of field deployment in real environments, which include remotely operating stabilized mode-locked frequency combs and maintaining a sensing network through rain, snow, and fog.

Dual-comb spectroscopy holds the promise as real-time, high-resolution spectroscopy tools. It had been applied to measure the spectral features of a wide variety of samples. With the help of nonlinear optical spectral broadening schemes, gases with absorption in different spectral windows can be monitored using the dual-comb methodK. Cossel, et al., Optica, 4, 724 (2017). Among them, methane is one that attracts much attention due to its important role in the greenhouse effect. Dual-comb spectroscopy based on a single mode-locked fiber laser has attracted more attention due to its simplification compared to the dual lasers systems. High-resolution optical spectroscopy had been demonstrated around the Erbium (Er) gain window by measuring the absorption of acetylene gas cell and other devicesX. Zhao, et al. Opt. Express 24, 21833 (2016). It had been suggested that certain degree of mutual coherence between two combs without stabilization, which is important for dual-comb applications, could exist due to some common-mode noise cancellation mechanisms. In this work, dual-comb spectroscopy based on the nonlinear spectral broadening of a single laser dual comb source at the 1550 nm Er window is further investigated in ultralength (UL) wavelength band where the absorption of methane can be observed. The absorption line of methane around 1648 nm, which is  100 nm away from the lasing wavelengths of our seed laser, can be clearly obtained. The spectral signal to noise ratio improves significantly with the increase of the number of averaged interferograms. Despite the extra noise introduced by the nonlinear spectral broadening process, the absorption line of methane can still be resolved with good quality. K. Cossel, et al., Optica, 4, 724 (2017).. X. Zhao, et al. Opt. Express 24, 21833 (2016)..

The development of accurate absorption models for high pressure, high temperature environments is complicated by the increased relevance of higher order collisional phenomena on the absorption lineshape (e.g. line mixing, collision-induced absorption, finite duration of collisions). Accurate reference spectroscopy at these conditions is important for the study of combustion systems and remote sensing of dense planetary atmospheres. We present a new high pressure, high temperature absorption spectroscopy facility at the University of Colorado Boulder. This facility is coupled with a dual frequency comb absorption spectrometer to record broadband (~1500cm⁻¹), high resolution (~0.0066cm⁻¹) spectra in a controlled environment at high pressures and temperatures. Measurements of the NIR spectrum of carbon dioxide will be compared to modeled spectra extrapolated from the HITRAN 2016 database as well as other published models that include line mixing corrections. This comparison gives insight into the effectiveness of existing absorption models in the high pressure, high temperature limit as well as the improvements required to accurately model absorption spectra in harsh systems.

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 emissionsE. 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 leaksK. 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.

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Footnotes:

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). K. C. Cossel, et al. “Open-path dual-comb spectroscopy to airborne retroreflector.” Optica 4, 724–728 (2017).

We develop a spectroscopic method combining the broadband spectral coverage and frequency resolution of a frequency comb with the optical path enhancement of a resonant optical cavity. The method requires only one frequency comb laser. We present measurements of the methane absorption spectrum performed in ambient air in the mid-IR from 3μm to 4μm using a comb derived from difference frequency generation. The resonant cavity provides the dual purpose of optical path enhancement along with Vernier filtering of the comb modes, allowing individual comb modes to be resolved with a grating. As the resonant cavity length is scanned, the transmitted comb lines in the whole spectral range of the cavity reflectivity are continuously recorded with a mid-IR camera, yielding an absorption spectrum in seconds. We show how this method can be realized with fewer moving parts and with broader spectral coverage than similar techniques reported in the literature, along with showing how the technique can be modified to suit different mid-IR detectors. This work was supported by Robert A. Welch Foundation, grant No. A1546, the Qatar Foundation, grant NPRP 8-735-1-154.

Direct frequency comb spectroscopy provides high-resolution spectra over a broad bandwidth. Its high sensitivity has also enabled real time detection for gas sensing and chemical reaction kineticsT. Q. Bui, B. J. Bjork, P. B. Changala, T. L. Nguyen, J. F. Stanton, M. Okumura, J. Ye, Direct measurements of DOCO isomers in the kinetics of OD+CO, Science Advances, 4, eaao4777 (2018) Previous work has focused in the near-infrared or mid-infrared (1 - 5 μm), but there are stronger absorption lines in the > 5 μm wavelength region. In addition, at longer wavelengths spectral congestion is significantly reduced owing to the decreasing strength of intramolecular vibrational energy redistribution. We have developed a new frequency comb spectrometer within 8.5 – 9.5 μm. The light source is a synchronously pumped optical parametric oscillator (OPO)-based frequency comb using a 2 μm Tm fiber comb as the pump wave. In direct frequency comb spectroscopy, several options exist to read out the spectrum, such as FTIR or highly dispersive optics like a virtually-imaged phased array (VIPA)L. Nugent-Glandorf, T. Neely, F. Adler, A. J. Fleisher, K. C. Cossel, B. Bjork, T. Dinneen, J. Ye, S. A. Diddams, Mid-infrared virtually imaged phased array spectrometer for rapid and broadband trace gas detection, Opt. Lett. 37, 3285 (2012) In this work, an immersion grating and a reflective grating are used as cross dispersers and each comb mode is mapped to a 2D image in the same way as a VIPA spectrometer. Immersion gratings have been applied in astronomy and have resolving power (λ/∆λ) exceeding 10⁵, which is suitable for high-resolution real-time comb spectroscopy. We report work done with this new spectrometer.

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Footnotes:

T. Q. Bui, B. J. Bjork, P. B. Changala, T. L. Nguyen, J. F. Stanton, M. Okumura, J. Ye, Direct measurements of DOCO isomers in the kinetics of OD+CO, Science Advances, 4, eaao4777 (2018). L. Nugent-Glandorf, T. Neely, F. Adler, A. J. Fleisher, K. C. Cossel, B. Bjork, T. Dinneen, J. Ye, S. A. Diddams, Mid-infrared virtually imaged phased array spectrometer for rapid and broadband trace gas detection, Opt. Lett. 37, 3285 (2012).

Here we present a new mid-infrared frequency comb system for rapid spectral acquisition using a virtually imaged phased array (VIPA) spectrometer.L. Nugent-Glandorf et al., Opt. Lett. 37, 3285 (2012) difference-frequency generation comb, tuneable from 4.4 μm to 4.7 μm, was used to interrogate a single-pass absorption cell containing either N₂O or CO dilute in either N₂ or air. Precision molecular spectroscopy capabilities at timescales of less than 1 ms will be presented, and progress toward cavity-enhanced and time-resolved comb spectroscopiesA.J. Fleisher et. al., J. Phys. Chem. Lett. 5, 2241 (2014)ill be discussed.

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Footnotes:

L. Nugent-Glandorf et al., Opt. Lett. 37, 3285 (2012)A A.J. Fleisher et. al., J. Phys. Chem. Lett. 5, 2241 (2014)w

r0pt Figure By exploiting scanning Fabry-Pérot micro-cavity resonator (SMART) we developed a simple and compact spectrometer capable of resolving the mode structure of an optical frequency comb, with a frequency resolution limited only by the comb tooth linewidth. The SMART approach can be adopted in any spectral window from UV to THz and represents an easy and completely “locking-free” approach to direct-comb spectroscopy, drastically reducing the system complexity when compared to the more conventional methods like the VIPA, dual- comb and Vernier. Furthermore, high-speed/high-sensitivity detection and straightforward absolute calibration of the optical-frequency axis are still granted by the SMART spectrometer, together with an ultimate resolution limited only by the optical linewidth of the frequency comb source adopted for the measurement. We present an application to broadband and high-precision spectroscopy of acetylene at 1.54 μm. Also, by means of an auxiliary 2500-finesse cavity exploited as a sample with narrow- transmission features, we show the ability of the SMART approach to resolve the 400 kHz resonances of the auxiliary cavity, demonstrating a final resolution well below than the 20 MHz linewidth of the transmission-modes of the SMART micro-resonator employed for optical- detection.