Optical properties of actively controlled reflection and transmission gratings.

摘要

Reflection and transmission gratings have found a wide variety of applications as optical filters and beam steering elements. In this work we have studied the optical properties of reflection and transmission gratings whose diffraction properties could be actively controlled. Two different material systems were utilized for the study. Reflection gratings in optical fibers were used and reflection and transmission gratings were fabricated holographically in a polymer dispersed liquid crystal (PDLC) material.; The optical properties of refractive index-shifted gratings were studied using the fiber Bragg gratings. It was found that narrow, high transmission spikes developed inside a high reflectivity stopgap when the refractive index of a section of the grating is shifted. The refractive index-shift was achieved using the thermo-optic effect. Experimental as well as theoretical results are presented and discussed.; The optical properties of electrically switchable reflection and transmission gratings fabricated in polymer dispersed liquid crystal materials were also studied. The PDLC material is electro-optic and therefore by applying an external electric field to the gratings the diffraction properties are modified. Gratings were fabricated holographically. From the study of the transmission properties of the reflection gratings we found that the reflection of the structures can be switched off by applying an external electric field and that the reflectivity is polarization insensitive for normal incidence. We also studied the diffraction properties of PDLC transmission gratings. In our analysis of the diffraction properties of these electrically-switchable liquid crystal gratings we found that it was necessary to use a generalized two-wave coupled mode theory that includes the effects of the optical anisotropy of the liquid crystal. We found that the morphology of the PDLC gratings depends on the specific PDLC mixture used to fabricate the grating.