Development,Justification,and Applications of a Mid-Infrared Polarization-Division Interferometer

We report the development of a polarization-division interferometer (PDI) for the mid-infrared region. This interferometer uses a self-designed beamsplitter constructed in-house from a BaF₂ polarizer and a matching substrate. In conjunction with a linear polarizer in front of the source and two roof-top mirrors, one in each arm of the interferometer, the PDI divides the input beam into two orthogonal linear polarization components, recombines them for interference at the beamsplitter, and directs the output beam at 90° to the direction of the input beam. Light exiting the interferometer is manipulated entirely with lenses, to avoid polarization distortions that are inherent to the reflecting surfaces of the mirrors. Details of the instrumental design for this mid-infrared PDI are presented. The performance of the PDI is evaluated by measuring the circular dichroism of α-pinene and camphor and the linear dichroism of oriented polypropylene and polystyrene. These measurements establish the utility of the PDI to measure transmission, circular dichroism, and linear dichroism spectra simultaneously without need for any additional components. The dichroic multiplex advantage (ability to measure dichroism in the entire mid-infrared region from a single measurement) and throughput advantage are demonstrated.