| Tunable optical filter are one of the key components in modern optical applications and have been widely used in various optical devices and systems, especially in optical communications and spectrometers. Various types of optical filters have been developed, such as Fabry-Perot cavities, thin-film interference filters, Bragg gratings, solid spaced filters, diffraction gratings, Mach-Zehnder interferometer filters, and waveguide filters. Most of the current optical filter can either tune the central wavelength or the bandwidth. The development of the telecommunication and the hyper-spectral/multi-spectral imaging system is requiring the optical filters with the ability of tuning both the central wavelength and the bandwidth. So far, however, only a few bandwidth-variable tunable filters have been reported.Liquid crystal (LC) is a highly birefringent material and its effective refractive index is varied with the applied voltage. Therefore, LC is a good candidate for tunable filter and has been widely used in tunable filters. However, all the current liquid crystal based filters can either tune the central wavelength or the bandwidth.By doping some chiral dopants into a liquid crystal host, cholesteric liquid crystal (CLC) can be obtained, which exhibits periodic helical structure. CLCs have many unique properties. Among them, the most interesting feature is that its reflection band can be easily modified by the external factors including electric, magnetic, acoustic fields, temperature, and light irradiation, which provides the possibility of assembling tunable devices, such as tunable filter and tunable lasers. Tunable filters based on CLCs have been studied and demonstrated. However, the bandwidth variable tunable optical filter based on CLC has not been reported yet.In this thesis, we focused our work on developing bandwidth variable tunable optical filters by studying the optical properties of CLC. The content mainly includes the following three parts.1) The effect of the incident angle on CLC’s reflection band has been investigated. It is observed that the CLCs’ reflection band is blue shifted with the increase of the incident angle. Based on this property, an optical tunable filter with variable bandwidth has been developed and demonstrated by configuring two CLCs in a reflection mode, in which the incident light is first reflected by one CLC and then by the other one. Both the wavelength and bandwidth of the filter can be tuned by changing the incident angle of the two CLCs. The wavelength can be tuned from573.8nm to513.4nm and the bandwidth can be varied from80nm to10nm.2) The effect of the chiral concentration on the reflection band of CLCs was studied and it was observed that the central wavelength of the reflection band is highly relied on the chiral concentration. Based on this property, we fabricated two spatially distributed CLCs by injecting four CLC mixtures with different chiral concentration one by one. By configuring the two spatially distributed CLCs in a reflection mode, we developed and demonstrated a tunable optical filter with variable bandwidth. By changing the spatial position of the two spatially distributed CLCs, the wavelength and the FWHM can be tuned. The wavelength can be widely tuned from480.9nm to618.7nm and the FWHM can be varied from59.6nm to27.5nm3) The thermal effect on the reflection band of the CLCs has been studied. It was observed that the reflection band will be blue shifted with the increase of the temperature. Based on this property, a bandwidth-variable tunable optical filter has been developed and demonstrated by configuring two CLCs in a reflection mode. By changing the temperature of the two CLCs, the filter’s wavelength and the bandwidth can be tuned. The wavelength can be widely tuned from598nm to534nm and the bandwidth can be varied from85.8nm to26.2nm. |
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