Development and test of a broadband absolute frequency reference with sub-MHz precision based on a GHz mode locked laser source
Résumé
Space lidar instruments for missions like AEOLUS or MERLIN require advanced high-power laser systems with according technical and financial effort. In order to increase the impact of such missions, it is advantageous to expand the versatility of their instruments. In the case of trace gas sensing, the ability to detect multiple trace gas species with the same instrument greatly enhances their value. Multi-species trace gas differential absorption lidar (DIAL) systems require absolute frequency referencing across large spectral bandwidths. While absorption cell based references need individual lasers at online and offline wavelengths for each species, a broadband mode locked laseroffering a frequency combcan provide the required frequency accuracy over the complete spectral range of the lidar instrument. In the frame of the LEMON project, we developed a combined design for an absolute frequency reference based on a wavemeter for coarse frequency determination (<500 MHz accuracy) and a broadband mode locked laser for precise frequency detection by means of heterodyne beat generation. It features a large spacing of 1 GHz and is optimized for spectroscopic lidar applications covering the spectral range from 980 nm to 1100 nm and 1500 nm to 2300 nm. The achieved accuracy of <100 kHz of the optical frequency, satisfies the requirements needed to atmospheric gas analysis from space. The broadband approach offers a cost-effective solution to address multiple gas species simultaneously. The system can also be adapted to different spectral ranges of interest for gas spectroscopy and other applications.
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