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

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 F _LCC 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 F _LCC. 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 V _LCC 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 F _LCC 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.