Polarization-independent tunable optical filters based on liquid crystal polarization gratings

摘要

We introduce and demonstrate a novel tunable optical filter that is insensitive to input polarization. While the most obvious application of this novel filter is in compact spectroscopy, all technologies that are dependent on tunable passband fiters can benefit from it. Analogous to Lyot and Solc fiters, this filter is constructed of multiple liquid crystal polarization gratings (LCPGs) of different thicknesses. LCPGs are switchable, anisotropic, thin diffraction gratings which exhibit unique properties including diffraction at visible and infrared wavelengths that can be coupled between only the zero- and first-orders, with nearly 100% and 0% experimentally verified efficiencies. Most relevant to the filter concept introduced in this work, the transmittance of the LCPG zeroth order is independent of the incident polarization. When combined with an elemental spatial filter, polarization-independent bandpass tuning can be achieved with minimum loss. The unique filter design enables a high peak transmittance (～ 90%) that is difficult in competing polarizer-based technologies. In this work we derive the core principles of the tunable filter, present preliminary experimental data, and discuss the capabilities of the filter in terms of finesse, 3dB bandwidth (full-width at half-maximum), and free-spectral-range. We will also evaluate the most likely practical limitations imposed by material properties and fabrication.