A FM-TDLAS-instrument for simultaneous measurement of two atmospheric trace gases has been constructed. The instrument is designed to enable time or frequency multiplex operation after minor setup changes. Noise properties for the frequency modulated setup were investigated. It was found that the noise from both lasers combines as expected by square root addition. Additional noise from strong absorption lines has been observed. Consequences of these findings on the design of multi-species. FM-TDLAS-instruments are discussed.
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