All-mirror wavefront division interferometer for mid-infrared spectrometry

In this contribution, we address the core of any Fourier transform (FT) spectrometer$\unicode{x2013}$the interferometer$\unicode{x2013}$in perspective of the recent emergence of spatially coherent broadband infrared (IR) sources. As a result, we report on the design of a wavefront-division interferometer for spectroscopic applications in the mid-IR and beyond. The theoretical framework of the proposed wavefront division interferometer is discussed, and an analytical solution to determine the far-field interference pattern is derived. The solution is verified by both optical propagation simulations and experimentally. In view of the practical significance, we apply the wavefront division interferometer for FTIR spectroscopy. It features a simple architecture, ultra-broad achromaticity (limited only by the spectral profiles of the mirrors), high optical throughput, variable arms split ratio, and a two-fold increase in scan length and spectral resolution (demonstrated up to 0.2 $cm^{-1}$), respectively. Further, the employed design inherently enables the measurement of the complex refractive index. Experimental verification of the mentioned properties is provided by coupling the spectrometer with a mid-IR supercontinuum source for various applied spectroscopic studies: high-resolution transmission measurements of polymers (polypropylene) and gas (methane), as well as reflectance measurements of dried pharmaceuticals (insulin products on a metal surface).