Error analysis and fabrication of low-stepped mirrors

In this study, a static and light Fourier transform infrared spectrometer based on stepped mirrors and a grid beam splitter was proposed. By introducing two stepped mirrors into the interference system, the optical path difference is discretized and the 2-dimensional sampling of the interferogram is obtained. Furthermore, by introducing the grid beam splitter into the interference system, the volume and weight are decreased. Stepped mirrors as the core optical devices of such a spectrometer, its step height consistency, face flatness and the structure's precision determine the spectral sampling interval, resolution and noise of the system. We propose a method based on MOEMS technology involving multiple depositions accompanied by a 50% reduction in thickness at every iteration to fabricate a low-stepped mirror with 32 steps and 0.625 μm in step height. The test results show that the root-mean-square of roughness is 1.72 nm and that the average height of the real steps is 626.9 nm. The effect of the height error on the recovered spectrum is analyzed. In order to reduce the influence of this error, two methods are proposed:one is through using tooling factor to reduce the monitoring error of the film thickness, thus reducing the height error; the other is through using the least-squares approximation cosine polynomial algorithm to correct the recovered spectrum. The spectrum-constructing error(SCE) is reduced to 2.34%, which meets the requirements of spectral restoration. Finally, the experiment was carried out using low stepped mirrors and the interferograms were obtained before and after the addition of the sample. The absorption spectrum of the sample acetonitrile can be obtained using a Fourier transform.