The paper presents the results of manufacturing and characterization of a broad band in-line hybrid device using a nematic liquid crystal as an active cladding for biconical tapered optical fiber. Two different liquid crystal mixtures denoted as 1550* and E7 were used for electric and temperature control in a broad wavelength range. An optical fiber tapers with a waist of 10±0.5 μm and losses lower than 0.5 dB in a whole broad band spectrum range were applied. Such taper waist diameter makes the whole waist as core for light propagation, where the surrounding air becomes the cladding. Additionally, such diameter enables an effective control of molecules orientation. Performance of a tuned cladding was studied in an electric field in the range between 0 V and 160 V in the room temperature equal to 20 °C. Influence of induced liquid crystal molecules reorientation was measured at a broad wavelength range (500-1700 nm).
The research is based on properties of a tapered optical fiber, ability to influence external factors on the propagating light beam in the fiber and anisotropic properties of the liquid crystal (LC) mixture that surrounds a tapered region. The combination of these elements is possible by creating a liquid crystal cell consisting of a layer orientating a liquid crystal (LC) between conductive layers. In that type of a sandwich construction, we can change the orientation of LC molecules under influence of the applied electrical voltage. The second interesting feature of LC is change in a structure that results from the degree of molecules’ alignment. When temperature rises, structure of LC changes from smectic through nematic to an isotropic liquid. Both the molecules orientation as well as the LC structure changes modified a refractive index of structure, which directly affects the propagating light beam in a tapered optical fiber. The spectral characteristics in the 500 -1100 nm range for a LC cell filled with a mixture of 1550* oriented orthogonally to the cross section of the tapered optical fiber are presented. Changes on spectral characteristics depended on manipulation of such parameters as: electrical voltage (0-200 V), frequency (1-10 Hz), shape of electrical signal, and time course for different voltage in the range 20-180 V.