Rapid, broadband, precision, absorption spectroscopy of carbon dioxide near 5000 cm-1 was performed using femtosecond laser frequency combs. Home-built, polarization-maintaining, 200 MHz fiber laser sources based on supercontinuum broadening from an Er oscillator were developed. The optical spectrum spanned from 4500 cm-1 to 10000 cm-1 (1000 nm to 2200 nm) with an average power of 250 mW, and contained discrete, equally-spaced elements every 200 MHz throughout the supercontinuum. After generation, the laser light was passed through laboratory gas cells or open air where the absorption features from the sample gas were imprinted onto the laser light. Two detection schemes were implemented for spectroscopic measurements. The first effort involved a virtually-imaged phased array- (VIPA-) based spectrometer, which allowed for spatial separation of the discrete optical elements, and subsequent imaging on an array detector. The bandwidth of an individual measured spectrum was 250 cm-1, limited by the size of the detector array. The spectral resolution was roughly 0.05 cm-1. Time resolution of the spectrometer was on the order of hundreds of microseconds over the full detected bandwidth, and the detectable absorption changes were on the level of 0.1%. A second spectroscopic method currently being investigated is multiheterodyne detection of two individual laser sources with repetition rates differing by roughly 10 kHz. The two individual lasers are passed through a carbon dioxide sample and beat together on a fast photodiode. The recorded interferogram in the RF domain is Fourier-transformed into an absorption spectrum in the optical domain. In this presentation, performance of the VIPA-based spectrometer and the multiheterodyne detection will be compared. Additionally, the difference between laboratory and open-air measurement sensitivities will be discussed as well as progress toward a fieldable instrument.

Rapid, broadband, precision spectroscopy of carbon dioxide near 5000 cm-1 using optical frequency combs

249th National Meeting (2015)

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