Ground-based prototype quantum cascade laser heterodyne radiometer for atmospheric studies

Abstract

The advent of quantum cascade lasers has provided matured continuously tunable solid state laser sources emitting from mid-infrared to terahertz wavelengths. Such sources, used as local oscillators, offer the practical prospect of aircraft, high altitude platform, and satellite deployment of compact and shot noise limited heterodyne radiometers for Earth observation and astronomy. A ground-based prototype of a quantum cascade laser heterodyne radiometer operating in the mid-infrared has been developed and is presented. The instrument design and concepts are described, together with evaluation of the instrument in the laboratory and during field measurements of atmospheric ozone. In this study the best performance achieved by the prototype quantum cascade laser heterodyne radiometer was a signal-to-noise ratio of three times the theoretical shot-noise limit. The prototype has allowed the main sources of excess noise to be identified as residual optical feedback in the local oscillator optical path and a lack of mechanical and thermal stability in the local oscillator collimation system. Instrument improvements are currently being implemented and enhanced performance is expected in the near future.

The Methane Isotopologues by Solar Occultation (MISO) Nanosatellite Mission: Spectral Channel Optimization and Early Performance Analysis

“…HIROS exploits the heritage from several ground-based LHR prototypes that have been demonstrated in solar occultation mode for several atmospheric constituents, including CH 4 [26][27][28]. Hoffmann et al [28] provide a wealth of details on the TIR LHR approach and performance, which are directly relevant to HIROS.…”

Thermal infrared laser heterodyne spectroradiometry for solar occultation atmospheric CO<sub>2</sub> measurements

“…The forward model also includes an LHR instrument model (Weidmann et al, 2007a), primarily to work out the SNR delivered by the instrument as function of instrumental parameters such as integration time, spectral resolution, and sampling resolution. With adequate detectors, laser heterodyne radiometry is ideally LO shot-noise-limited (Weidmann et al, 2007a).…”

“…With the objective to provide complementary instrumentation addressing these shortfalls, an alternative approach for generating high spectral resolution spectra of atmospheric transmittance is considered here. This paper reports on the technical development of a laband ground-based thermal infrared (TIR) laser heterodyne spectroradiometer (LHR) aiming at accurately characterizing total column and vertical profiles of CO 2 in solar occultation (solar absorption-atmospheric transmission) mode, building on past instruments successfully used for the passive remote sounding of a variety of trace gases in the atmosphere (Tsai et al, 2012;Weidmann et al, 2007aWeidmann et al, , b, 2011a. The prototype instrument has been installed at the Rutherford Appleton Laboratory near Chilton (Didcot), UK, and operated in fair weather conditions since May 2015.…”

“…LHR combines the unique advantages of very high spectral resolution (resolving power ∼ 10 6 ) and high sensitivity (ideally shot-noise-limited) with a narrow field of view (FOV). The use of single-mode semiconductor lasers as LO, in this case a quantum cascade laser (QCL), restricts the spectral coverage of the LHR and narrow spectral window optimization is required to specifically targets one or a few trace gases (Weidmann et al, 2007a). High spectral resolution, combined with a high signal-to-noise ratio (SNR), is useful to accurately measure an atmospheric line shape, and thus to derive total column trace gas amounts to a high degree of precision or, alternatively, to deconvolve altitudinal (height-resolved) information, both immediately useful in the context of carbon cycle and anthropogenic emission studies.…”

Compact Setup of a Tunable Heterodyne Spectrometer for Infrared Observations of Atmospheric Trace-Gases

“…It provides ultra high spectral resolution of with further improvements, this leads to a substantial decrease in the size and weight of the receiver's framework compared to existing instruments [1,20].…”