There are various potential sources of systematic error in tunable-diode-laser absorption measurements. In this paper, we discuss two such sources that are associated with multimode lasing. The first is caused by secondary modes which are passed by a monochromator and are absorbed by lines not intentionally being studied. In the second source of error, different spatial modes may appear in different arms of a double-beam system. Both types of errors can give rise to rather dramatic systematic errors in frequency and concentration calibrations.

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