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Completed TRL 1 (started at 1, targeting 3)
OSAs are a common tool in the laboratory to measure laser wavelength. They are not usually used for remote sensing because they are less flexible than spectrometer systems and the calibration needed to make accurate measurements is not known. We will examine the performance of two different OSA’s, one heterodyne laser based and one Fourier transform based, in the laboratory using calibrated reference gases. The calibration technique is unique in that it takes advantage of our cavity enhanced absorption spectroscopy (CEAS) cells that were developed in previous IRAD’s. These cells provide long optical paths (>1 km) in small physical dimensions (<30 cm). As a result, the reference gas (CO2) can be held at atmospheric mixing ratios and pressures, which is essential to measure pressure and molecular broadening effects accurately. In contrast, a standard 1-m cell would need an atmosphere of pure CO2 to simulate the atmospheric absorbance. A better alternative is to use CEAS.
We plan to leverage our past work developing CEAS for OH, NO2, and O3 to then use this technique to create long path lengths of CO2 concentrations in a small (short) cell. CEAS turns a physically small cells (0.3 meters or less) into an effective length (path length) of 100s or 1000s of meters depending on the mirrors used.
By combining the compact calibration process that can be achieved using a CEAS cell with a COTS OSA, we hope to overcome the common systematic errors that have previously limited these instruments (OSAs) potential. Specially, for the FT based OSA, the calibration cell will help to address the instrument spectral broadening due to lower resolution. Likewise, for the heterodyne based OSA, the calibration cell will serve as an absolute metric for any wavelength drift.
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