Porch swing bearing for the linear motion of the mirror in Michelson interferometer for mid-infrared low resolution Fourier transform spectrometer was studied experimentally using the modulation depth of the collimated laser beam. The mirror tilting was measured to be lower than 5 μrad over 3？mm mirror travel using two different bearings assemblies. Additionally, the manufacturing tolerances of the bearing type were proved to be loose enough not to limit the interferometer application. These demonstrate that the porch swing without any adjustment mechanisms provides the sufficient motion linearity. 1. Introduction Inexpensive and robust portable Fourier transform infrared (FTIR) spectrometers for gas analysers are still needed, because the existing solutions have some weaknesses like expenses, bulky size, or sensitivity to the temperature variations. The most sensitive component in FTIR spectrometer is the interferometer. We have aimed to design such an interferometer with 25？mm beam diameter and with 2？cm？1 resolution in the mid-infrared region. Numerous interferometer designs can be found, for example, from the papers by Jackson [1] and Kauppinen et al. [2] and from the book by Griffiths and de Haseth [3]. We have selected Michelson interferometer with plane mirrors, because it has only a few, even inexpensive, optical components. Using Michelson interferometer, it is possible to minimize the size and cost of the interferometer structure while keeping the beam diameter and the throughput constant. Additionally, we will not use any dynamic alignment of the optics which are commonly applied in most solutions. One of the key parameters in interferometer design is the modulation depth of the interferogram. In Michelson interferometer built using plane mirrors, the modulation depth is practically mainly affected by the tilting of optics which causes an angle between the output beams from interferometer. The shearing is the second major phenomenon determining the modulation depth. It is caused by the lateral shift of the output beams. With the plane mirror setup, the shearing is usually negligible because the plane mirrors do not shift the beams with each other. If the cube corners would be used as the end mirrors, the output beams would not tilt but they may be shifted with each other which would decrease the modulation depth especially when the size of the radiation source is finite. Maximizing the modulation depth sets the requirements for the movement of the mirror. The movable mirror has to remain parallel to the image of the second mirror formed by
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