Abstract
The paper describes the concept of a compact, lightweight heterodyne NIR spectro-radiometer suitable for atmospheric sounding with solar occultations, and the first measurement of CO2 and CH4 absorption near 1.65 μm with spectral resolution λ/δλ~10(8). A highly stabilized DFB laser was used as local oscillator, while single model silica fiber Y-coupler served as a diplexer. Radiation mixed in the single mode fiber was detected by a balanced couple of InGaAs p-i-n diodes within the bandpass of ~3 MHz. Wavelength coverage of spectral measurement was provided by sweeping local oscillator frequency in the range of 1.1 cm(-1). With the exposure time of 10 min, the absorption spectrum of the atmosphere over Moscow has been recorded with S/N ~120, limited by shot noise. The inversion algorithm applied to this spectrum resulted in methane vertical profile with a maximum mixing ratio of 2148 ± 10 ppbv near the surface and column density 4.59 ± 0.02·10(22) cm(-2).
Highlights
High resolution spectroscopy is widely used in a variety of applications in space research, astrophysics, environmental science and technology as a powerful analytical tool allowing for accurate measurements of species abundance, isotopic ratios, velocity fields and other parameters of target objects
The experiment described in this paper has proven feasibility of heterodyne detection in the near infrared range using commercial tunable diode laser as local oscillator and single mode fiber optics for beam combining
Achieved spectral resolution is determined by local oscillator (LO) line width and stability and constitutes about 2.5 MHz, which corresponds to resolving power of λ/δλ~108
Summary
High resolution spectroscopy is widely used in a variety of applications in space research, astrophysics, environmental science and technology as a powerful analytical tool allowing for accurate measurements of species abundance, isotopic ratios, velocity fields and other parameters of target objects. High resolution spectroscopy (λ/δλ~107-108) allows for Doppler measurements of wind fields in the atmospheres of the Earth and other planets, implemented in the infrared spectral range in only a few instruments to date and resulted in seminal results on the dynamics of planetary atmospheres [3,4] and remote sensing of the Earth atmosphere [5,6]. Few attempts of heterodyne spectro-radiometry have been made to date in the SWIR and NIR ranges [10,11], despite the availability of high precision lasers, detectors and fiber optics. In this paper we present the technique of heterodyne detection of solar radiation passed through the atmosphere in the range 1.1-2.1 μm with spectral resolution up to λ/δλ~108, first proposed in [12]. Further development of the proposed technique may result in efficient remote sensing instruments for precise measurements of atmospheric composition, structure and dynamics
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