Tuning of liquid-crystal birefringence using a square ac variable frequency voltage

International audience; We demonstrate that the birefringence of the liquid-crystal cell (LCC) can be varied by applying different frequency values of a single applied ac square voltage. For the experimental evaluation of the birefringence, associated with a certain wavelength λ, as a function of the frequency FLCC of the electrical signal applied to the LCC, we use, for the first time to our knowledge, what we call here a frequency-dependent transmission technique. It consists in measuring the transmission responses between crossed and parallel polarizers as a function of the frequency FLCC. Experimental tests were carried out using a 7 μm-thick E63 nematic LCC and a laser source emitting at λ=1.55 μm with a launch power of −3 dBm. The tuning voltage VLCC applied to the LCC is an alternative square wave electrical signal whose frequency ranges from 0.5 to 15 kHz. The peak to peak amplitude of the electrical signal is 5 V. The curve of the measured variations of the optical path difference of the LCC versus the frequency FLCC has a positive slope. Application to the tuning of the center wavelength of the transmission response of a one stage hybrid birefringent filter is shown as a proof-of-principle test