Liquid crystal based spectral control for applications in optical communication and metrology

Abstract

The molecules of nematic liquid crystals (LCs), aligned in one average direction, constitute a birefringent medium. The molecules in nematic LCs of positive anisotropy (ne>no) get aligned in the direction of applied electric field due to the induced dipoles, which results in modification in the birefringence of this medium by applying the voltage. Being an electro-optical medium, the broadband spectra passing through the nematic LCs can be manipulated by the application of voltage. We demonstrate both theoretically and experimentally a tunable optical frequency ruler and a spectral switch using voltage application across the LCs. The tunable optical frequency ruler finds application in identifying an unknown spectral line in the given spectra suitable for optical metrology applications. The LC based spectral switch, which can be controlled several orders of magnitude faster then its mechanical counterpart, is used in a proposed scheme for efficient spectral shift based data encoding and decoding in free-space based optical communication. We also demonstrate theoretically and experimentally, the tunability in spectral width selection by rotating the LC cell placed between the two cross-polarizers. It may be useful in the situations where a tunable spectral bandwidth source is of prime importance.