Abstract

Liquid crystals have attractive properties since they exhibit a large birefringence change at low applied voltages and a good optical transparency in the IR. A parallel alignment LC cell compared with twisted or perpendicular alignment in the IR is employed for three reasons: the dark state is insensitive to wavelength; highly stable LC mixtures can be formulated with large positive ε_o large Δn and low viscosity; and a relatively thinner LC layer that does not seriously weaken the modulation efficiency is usable. The cell made with a pair of BaF AR-coated windows (coating In₁O₁ infrared conductive film) is in the polarizer-LC cell-analyzer configuration. An extended beam of a 10.6-µm Co₂ laser with a few tens of µW cm² transmitted a 10-µm LC layer of E₁ driven by 2.4-3.2 V (1 kHz). An optical response time of 75 ms is achieved from this modulator utilizing the controlled birefringence effect. This time is smaller than that calculated from τ = r₁d²/k₁₁π², because of the difference between the reorientation time and optical response time, varying by a factor of 2–4. Using the same cell with E₁₀, the 65-ms optical response time is achieved. The smaller time is due to its better parameter of r₁/k₁₁ although Δn is a little large. The choice of LC is discussed by calculating the two important parameters of response time (τ = r₁d²/k₁₁π²) and dynamic range (δ = 2 π dΔn/λ) with r₁k₁₁ and Δn.

© 1991 Optical Society of America