A small size tunable diode laser (TDL) spectrometer with dimensions of only 280 x 190 x 130 mm (11 x 7.5 x 5 inch) was built for automated field use. The water-protected package also contains batteries and a data storage device for an independent operation of 15 hours. Mass of the complete instrument is 5 kg. Smallest absorptions recorded in fully auto-mated operation are in the 10-5 range. Data is stored as direct absorptions and as derivative as well. The instrument can be equipped with near infrared (NIR) laser diodes depending on target species (e.g. water vapor or methane). A high dyna-mic range is achieved by periodically tuning the laser over various molecular absorption lines of different line strength using temperature scan. The absorption path is set up using a retroreflector, the pathlength can be between 0.5 meters and 100 meters depending on requirements. The instrument is targeted at reliable and sensitive field measurements in fully automated and unattended mode, including e.g. aircraft and environmental measurements.
A 1530nm multimode diode laser with an external resonator containing a grating achieved a single-mode tuning range of 100nm. Etalon effects and modehops during the frequency tuning of the laser were minimized by special design features. Absorptions of water vapor were recorded intracavity and extracavity. The intracavity experiments were impeded by modehops induced by the absorption itself. This effect could be reduced by increasing the selectivity of the resonator. Modeling of the optical properties of the resonator allows the quantitative specification of the optical parameters for a singlemode intracavity tunable diode laser (TDL) operation. This enables a resonator design to be realized which achieves an optimum stability and selectivity for a singlemode TDL for both extracavity applications.
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