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.
Robert Sams and Alan Fried, "Potential Sources of Systematic Errors in Tunable-Diode-Laser Absorption Measurements," Appl. Spectrosc. 40, 24-29 (1986)