It is well known that optical interference often is a major problem in laser-based gas spectroscopy. In conventional laser-based gas spectrometers, the optical interference originates from e.g. etalon-type interference effects in (or between) optical components and multi-pass gas cells. Throughout the years, great efforts have been devoted to handle this problem. Proper optical design is important to minimize interference from the beginning (e.g. by tilting optical components, avoiding transmissive optics and using anti-reflection coating), but interference patterns can not be completely avoided and are often difficult to separate from gas absorption. Since gas spectroscopy often involves measurement of small absorption fractions (down to 10), appropriate handling of interference is crucial. Utilised countermeasures include customized optical design, tailored laser modulation, mechanical dithering, signal post-processing, sample modulation, and baseline recording and interference subtraction.
In the case of GASMAS, optical interference is particularly cumbersome. This is related to the severe speckle-type interference that originates from the interaction between laser light and highly scattering solid materials. Since this highly non-uniform interference is generated in same place as the utility signal, it cannot be removed by design. The optical properties of the porous material under study determines the interference pattern, and the level of interference is not seldom much stronger than actual gas absorption signals. Random mechanical dithering (e.g. laser beam dithering and/or sample rotation ) has been found effective in GASMAS. However, this approach converts stable interference into a random noise that must be averaged away, thus requiring longer acquisition times. Baseline recording and interference subtraction may be applicable in some GASMAS applications, as may other of the methods described above.