Polarization-Independent Tunable Interleaver Based On Liquid Crystals

A tunable optical filter is an optical device that can select a narrow wavelength from a broadband light source. It is one of the key parts for wavelength division multiplexing and all-optical communication. For a conventional tunable filter, the band edge is not sharp and the top is not flat, which can not meet the requirement of the optical communication. The development of the technology and the rapid increase of information need a tunable optical filter with better performance. Especially in the optical communication system, in order to reduce errors, a tunable optical filter with rectangle shape is preferred. A rectangle wave shape has been obtained by inserting a Gires-Tournois cavity into one or two arms of a Michelson Interferometer. The reason is that the surfaces of the Gires-Tournois exhibit some reflection. A portion of light will be reflected back and forward inside the cavity and therefore form nonlinear phase variation relating to the wavelength. With suitable parameters such as reflectivity and thickness, a nearly rectangle wave shape can be obtained.So far, the methods for tuning the central wavelength of a Michelson Gires-Tournois Interferometer (MGTI) include mechanic tuning, piezoelectric ceramics, acousto-optic modulation, and liquid crystal, etc. Each one has its drawback. For example, the mechanic tuning method is not stable and can be easily affected by the mechanic vibration. Piezoelectric ceramics’power consumption is high. Acousto-optic modulation is bulky. Liquid crystal is attractive because of its lower power consumption, portability and good stability. However, a liquid crystal device is usually sensitive to the light polarization, which is not desired for many applications, especially in optical communications because the polarization state is unexpected when the light propagates in a fiber. In this thesis, we developed several configurations based on liquid crystal to overcome its sensitivity to the light polarization. Below is the main content.1) Two nematic liquid crystals with orthogonal rubbing direction are insensitive to the incident polarization. Based on this property, we applied two orthogonal nematic liquid crystals as the phase regulator in MGTI. By designing the MGTI with suitable configuration and parameters, we obtained nearly rectangle shape output. The incident light with s, p and unpolarized polarization has been simulated, respectively. The results show that the MGTI with two orthogonally rubbed nematic liquid crystals is indeed polarization independent.2) The electro-optic properties of a90°twisted nematic liquid crystal (TNLC) has been studied, which shows that it is polarization independent when the applied voltage is above3V. By replacing the phase regulator in the MGTI with a90°twisted nematic liquid crystal, we simulated the performance of the MGTI. By setting suitable parameters, we obtained the output spectra with nearly rectangle shape. The incident light with s, p and unpolarized polarization has been investigated, respectively and found that the output wave shape is the same at the same applied voltage when the applied voltage is above3V, which indicates that the MGTI with90°TNLC is polarization independent as the applied voltage is above3V. In the meantime, the tolerance of the thickness, reflection of the90°TNLC based GT was analyzed.3) Blue phase liquid crystal is insensitive to the light polarization due to its orthogonal helical structure. Therefore, it is very attractive for many applications. Here, we used the blue phase liquid crystal as the phase regulator of a MGTI and simulated its performance. By setting suitable parameters and configurations, a nearly rectangle output wave shape has been obtained. The incident light with s, p and unpolarized polarization has been investigated, respectively and found that the output wave shape is the same at the same applied voltage, which indicates that the MGTI with blue phase liquid crystal is polarization independent. In the meantime, the tolerance of the thickness, reflection of the blue phase liquid crystal based GT was analyzed.