Polarization gratings, which have the important properties of being highly efficient for first order diffraction and having high polarization sensitivity, can be applied to beam splitters, displays, spectro-polarimeters, and so on. Usually, polarization gratings are fabricated utilizing the periodic anisotropy of liquid crystal molecules. However, high tolerance to light, heat, and humidity is required, in particular, for optical communication applications. Therefore, a polarization grating based on an inorganic material is more suitable than one based on an organic one. We propose a silicon polarization grating with form birefringence induced by an anisotropic surface microstructure with features shorter than the wavelength of light (sub-wavelength anisotropic structure), allowing the birefringence to be controlled through selection of the dimensions of the periodic structure. In this paper, we describe the design of sub-wavelength structures of half wave plates using a thin film of silicon. By optimizing the line width, the line spacing, and the thickness of the film, a transmittance of more than 99% at an incident wavelength of 1550 nm was obtained. A polarization grating based on this half wave plate was designed. The orientation of the half wave plate structure was rotated in a particular direction. Furthermore, we evaluated the wavelength dependence and incident angle dependence of the diffraction efficiency using a finite-difference time-domain method.
A wavelength cross-connect switch (WXC) is proposed and demonstrated. The cross-connect optics have orthogonal imaging systems that operate differently in the switching and spectral planes. The switching plane has 2f Fourier optics with a Rayleigh length. On the other hand, the spectral plane has 4f imaging optics. Two types of switching engines, microelectromechanical system (MEMS) mirrors and liquid crystal on silicon (LCOS), are applied for the same cross-connect optics. The 5×5 WXC with MEMS mirrors has a 100 GHz channel spacing, which is compatible with the International Telecommunication Union (ITU) grid. On the other hand, the WXC with LCOS has a variable channel spacing. The characteristics of two types of WXC are compared. In addition, the port count, which is one of the important parameters, is discussed.