Thermo-optical effects and fiber optic sensing device based on polymer dispersed liquid crystals.

In this paper an experimental study of thermo-optical properties of polymer dispersed liquid crystals (PDLC), prepared by PIPS (polymerisation-induced phase separation) in bulk and in confined cylindrical geometry, is presented. The transmittivity of PDLC in bulk as a function of temperature proves the existence of temperature optical switching. We have also demonstrated the existence of optical bistability, which could be interesting in develop of logical optical devices as optic memory elements. A bistable all fiber optic sensors based on PDLC is also presented. In this device PDLC permits at the same time the optomechanical interconnection of two fibers and the modulation of the light crossing the device. As the modulation can be controlled by external temperature, the device has been proved to be suitable for the realisation of a heat flow sensor. Without any optimisation of the device we have obtained an ON-OFF contrast of 8 dB and a response time comparable with other conventional device using nematic LC's. This sensor is compact, rugged and is cheap, because it does not require a complex fabrication and alignment technology. It presents the typical advantages of both the fiber optic sensors and the liquid crystal technology. We note that its main advantage is a small thermal capacity, which is comparable with electronic device as thernistors, and it represents a significant improvement for the sensor based on liquid crystals. Further theoretical studies are necessary in order to understand in depth it thermo-optical characteristics.