Investigation of the field-of-view properties in Lyot liquid crystal tunable filter

The field of view (FOV) is a critical parameter for the applications of liquid crystal tunable filters (LCTFs). This work systematically investigates the impact of FOV on the filtering characteristics of the Lyot-type LCTF and establishes a quantitative evaluation method. First, phase retardation model of FOV was constructed to derive formulas for both spectral transmission and wavelength shift. Then, the variation characteristic of filter wavelength with the azimuth angle was analyzed for different FOVs, and the quantified evaluation method of the FOV effect was proposed based on the maximum wavelength shift. In succession, the change rule of maximum wavelength shift with the FOV and filtering wavelength are simulated. Furthermore, the influence of liquid crystal (LC) birefringence on the maximum wavelength shift was analyzed and the results indicate that larger Δn results in larger effective FOV. The proposed model and evaluation method were validated experimentally. For a single-stage filter, the experimental results show excellent agreement with the simulations. However, for a cascaded LCTF, the measured transmittance curve deviates from that of the simulated significantly, which is caused by the LC cell thickness mismatch in the experiment. It illustrates that the LC cell thickness not only satisfies the demanded phase retardation of all the stages, but also the recursive relation d_i = 2^i-1d₁, which is often overlooked in the LCTF design. All the results confirm that the proposed model and evaluation method can effectively characterize the FOV properties of LCTFs. This work will provide a foundation for designing the LCTFs with large FOV.