Pulse interaction induced systematic errors in dual comb spectroscopy.

Systematic errors are observed in dual comb spectroscopy when pulses from the two sources travel in a common fiber before interrogating the sample of interest. When sounding a molecular gas, these errors distort both the line shapes and retrieved concentrations. Simulations of dual comb interferograms based on a generalized nonlinear Schrodinger equation highlight two processes for these systematic errors. Self-phase modulation changes the spectral content of the field interrogating the molecular response but affects the recorded spectral baseline and absorption features differently, leading to line intensity errors. Cross-phase modulation modifies the relative inter-pulse delay, thus introducing interferogram sampling errors and creating a characteristic asymmetric distortion on spectral lines. Simulations capture the shape and amplitude of experimental errors which are around 0.1% on spectral transmittance residuals for 10 mW of total average power in 10 meters of common fiber, scaling up to above 0.6% for 20 mW and 60 m.

Precision Doppler shift measurements with a frequency comb calibrated laser heterodyne radiometer.

We report precision atmospheric spectroscopy of CO2 using a laser heterodyne radiometer (LHR) calibrated with an optical frequency comb. Using the comb calibrated LHR, we record spectra of atmospheric CO2 near 1572.33 nm with a spectral resolution of 200 MHz, using sunlight as a light source. The measured CO2 spectra exhibit frequency shifts by approximately 11 MHz over the course of the 5-h measurement, and we show that these shifts are caused by Doppler effects due to wind along the spectrometer line of sight. The measured frequency shifts are in excellent agreement with an atmospheric model, and we show that our measurements track the wind-induced Doppler shifts with a relative frequency precision of 2 MHz (3 m·s-1) for a single 10 s measurement, improving to 100 kHz (15 cm·s-1) after averaging (equivalent to a fractional precision of a few parts in 1010). These results demonstrate that frequency comb calibrated LHR enables precision velocimetry that can be of use in applications ranging from climate science to astronomy.

Cepstral analysis for baseline-insensitive absorption spectroscopy using light sources with pronounced intensity variations.

This paper presents a data-processing technique that improves the accuracy and precision of absorption-spectroscopy measurements by isolating the molecular absorbance signal from errors in the baseline light intensity (Io) using cepstral analysis. Recently, cepstral analysis has been used with traditional absorption spectrometers to create a modified form of the time-domain molecular free-induction decay (m-FID) signal, which can be analyzed independently from Io. However, independent analysis of the molecular signature is not possible when the baseline intensity and molecular response do not separate well in the time domain, which is typical when using injection-current-tuned lasers [e.g., tunable diode and quantum cascade lasers (QCLs)] and other light sources with pronounced intensity tuning. In contrast, the method presented here is applicable to virtually all light sources since it determines gas properties by least-squares fitting a simulated m-FID signal (comprising an estimated Io and simulated absorbance spectrum) to the measured m-FID signal in the time domain. This method is insensitive to errors in the estimated Io, which vary slowly with optical frequency and, therefore, decay rapidly in the time domain. The benefits provided by this method are demonstrated via scanned-wavelength direct-absorption-spectroscopy measurements acquired with a distributed-feedback (DFB) QCL. The wavelength of a DFB QCL was scanned across the CO P(0,20) and P(1,14) absorption transitions at 1 kHz to measure the gas temperature and concentration of CO. Measurements were acquired in a gas cell and in a laminar ethylene-air diffusion flame at 1 atm. The measured spectra were processed using the new m-FID-based method and two traditional methods, which rely on inferring (instead of rejecting) the baseline error within the spectral-fitting routine. The m-FID-based method demonstrated superior accuracy in all cases and a measurement precision that was ≈1.5 to 10 times smaller than that provided using traditional methods.

Baseline-free quantitative absorption spectroscopy based on cepstral analysis.

The accuracy of quantitative absorption spectroscopy depends on correctly distinguishing molecular absorption signatures in a measured transmission spectrum from the varying intensity or 'baseline' of the light source. Baseline correction becomes particularly difficult when the measurement involves complex, broadly absorbing molecules or non-ideal transmission effects such as etalons. We demonstrate a technique that eliminates the need to account for the laser intensity in absorption spectroscopy by converting the measured transmission spectrum of a gas sample to a modified form of the time-domain molecular free induction decay (m-FID) using a cepstral analysis approach developed for audio signal processing. Much of the m-FID signal is temporally separated from and independent of the source intensity, and this portion can be fit directly with a model to determine sample gas properties without correcting for the light source intensity. We validate the new approach in several complex absorption spectroscopy scenarios and discuss its limitations. The technique is applicable to spectra obtained with any absorption spectrometer and provides a fast and accurate approach for analyzing complex spectra.

Broadband dual-frequency comb spectroscopy in a rapid compression machine.

We demonstrate fiber mode-locked dual-frequency comb spectroscopy for broadband, high-resolution measurements in a rapid compression machine (RCM). We apply an apodization technique to improve the short-term signal-to-noise-ratio (SNR), which enables broadband spectroscopy at combustion-relevant timescales. We measure the absorption on 24345 individual wavelength elements (comb teeth) between 5967 and 6133 cm-1 at 704 µs time resolution during a 12 ms compression of a CH4-N2 mixture. We discuss the effect of the apodization technique on the absorption spectra, and apply an identical effect to the spectral model during fitting to recover the mixture temperature. The fitted temperature is compared against an adiabatic model, and found to be in good agreement with expected trends. This work demonstrates the potential of DCS to be used as an in situ diagnostic tool for broadband, high-resolution measurements in engine-like environments.