Research On Spectral Characteristics Of An Optic U-shaped Fiber Cavity Integrated By Liquid Crystal

Recently,integration of an optic fiber structure with functional material is appealing thanks to its ability to improve performance of the fiber device.The optic U-shaped fiber cavity have an opening space that allows a direct interaction between light and the external material,generally leading to a high refractive index sensitivity compactness,high integration,and so on.Among those functional materials,liquid crystal(LC)has good electro-optic and thermo-optic effect and has been widely used in optoelectronic devices and display fields.Therefore,by combining LC with an optic U-shaped fiber cavity,we may have an optical device with high integration,good performance and miniaturization in size.In this thesis,we have reported an LC-integrated U-shaped fiber cavity.The spectral characteristics,and the tuning effects of electric field and temperature have been investigated and discussed in detail.Such a device shows good potential in polarization-related optics,sensing,thermal characterization of LC material,tunable photonic device,and so on.The main content of this thesis includes:(1)A photonic device is formed by integrating LC with an optic U-shaped fiber cavity.It is found that the spectral characteristics can be tuned by electric field.We have adopted a PI film to orientate the molecule of LC and let the long axis of the LC molecule to be parallel to the optical fiber.The applied electric field can re-orientate the LC molecule so that the polarization effect occur for light propagating along the fiber axis.The tuning effects of electric field to the two orthogonal polarizations are measured and analyzed.As an increase of electric field,the spectrum corresponding to the ordinary light keeps almost unvaried,while the spectrum corresponding to the extraordinary light becomes large gradually.The measured sensitivity is around 4.42 dB/kV/cm within an electric-field range of 0.48 kV/cm-2.08 kV/cm.This device has advantages of high sensitivity,compact size,good stability,fast response and so on.(2)The thermal tuning characteristics for an LC-integrated U-shaped fiber cavity are studied in detail.The interference spectra corresponding to the ordinary and extraordinary waves can have different responses to the temperature variation.With an increase of temperature,the interference spectra of the ordinary light general redshifts,while the spectrum of extraordinary light blueshifts.As temperature approaches the clearing point of LC,the spectra shift more and more quickly.But when the temperature is above the clearing point,the polarization effect vanishes in this case,the interference spectrum shifts towards the short wavelength as temperature increases.Moreover,the temperature dependences of the LC refractive index in near-infrared region have also been analyzed.Our results should be valuable for future applications of the devices in the fields of sensing and tunable photonic devices.