Electrically controllable IR Lyot filter has high transmission

Lyot optical filters take advantage of birefringence to form bandpass filters; if the birefringent material is liquid crystal (LC), the filters can be made tunable. Researchers at Ryukoku University (Seta, Japan) have created an infrared (IR) Lyot filter that contains a polarization beamsplitter, two LC retarders, and a LC layer between two silicon pentaprisms, set up so that light is incident on the LC layer at Brewster’s angle. The LC transmits p-polarized light while reflecting s-polarized light via total internal reflection.

Four electrodes with three voltage drops (V1, Vp, and V2) are attached to two mirrors and both pentaprisms; the LC retarders are sandwiched between the mirrors and the silicon (both of which are electrically conductive). As the voltages were varied, the broadband transmission spectrum, which ranges from 2 to 6 μm, changed shape, shifting the peak transmission from 2.1 to 3.8 μm. The pentaprism-based device was compared to a conventional tunable LC Lyot filter consisting of a polarizer, LC cell, and an analyzer. While the conventional Lyot filter showed broader tunability from 2.5 to about 5 μm, the pentaprism device had a much higher transmission of about 40% compared to less than 10% for the conventional filter. The researchers are now working on a similar pentaprism-based Lyot filter that has only one, rather than three, LC layers, making the filter easier to control electrically. Contact Mitsunori Saito at [email protected].