A Novel Electrically Tunable Long-Period Fiber Grating Using a Liquid Crystal Cladding Layer

Abstract

The main aim of the presented work is the integration of a long-period fiber grating and a liquid crystal into a hybrid structure, in order to develop an innovative fiber optic device controlled by an external electric field. The studied long-period fiber grating was fabricated using UV irradiation in a boron co-doped fiber (PS1250/ 1500, manufactured by Fibercore). As a liquid crystal we used a typical 5CB nematic liquid crystal. The sensing mechanism of the proposed loss filter relies on long-period fiber grating attenuation bands sensitivity to optical properties of the liquid crystal layer. The results obtained show that the long-period fiber grating with a nanosized liquid crystal layer exhibits one order of magnitude higher electrical sensitivity and a lower level of the voltage control than the long-period fiber grating with a micro-sized liquid crystal layer.PACS IntroductionIn order to achieve light coupling between two co--propagating modes, a grating formed in an optical fiber usually requires a pitch of several hundred micrometers. This type of grating is called a long-period fiber grating (LPFG). Compared to other optical devices, LPFGs have a number of unique advantages such as low-level back reflection, low insertion losses and compact construction (the grating is an intrinsic fiber device). The LPFGs have found a variety of applications in optical communications [1,2]. Tuning of the LPFGs is very attractive since it can offer a form of dynamic spectral control [3,4]. The central attenuation wavelength of the LPFGs is highly sensitive to temperature, strain, bend or surrounding refractive index (SRI) changes. For these reasons filters based on the LPFGs have generated a significant interest for applications in the sensing field as well.Liquid crystals (LCs) are attractive candidates as a coating material since their properties can be controlled through temperature, optical, electrical and magnetic fields [5][6][7]. There are three key points for a good design in order to combine a LPFG and a LC into one component: selection of the LC refractive indices, inherent SRI sensitivity of the LPFG and the LC layer thickness. Majority of LCs have refractive indices that are higher than the refractive index of the silica glass. Thus, the LPFG sensitive to SRI is limited to the range where SRI is higher than the refractive index of the fiber cladding. In this case, the core modes couple to leaky modes [8] and the dependence on the resonance wavelength on the SRI is not so straightforward. So far, most of the studies have focused on the analysis of the SRI sensitivity when the