Design of polarization-insensitive transmission gratings for optical communication

Abstract

With the development of optical communication, especially the wide applications of dense wavelength division multiplexing (DWDM) technology, all-optical communication is attracting more and more attention. Wavelength selective switch (WSS) is a new technology, which has the advantage of free switching of individual wavelength. The wavelength division devices play an important role in above mentioned optical communication system. Holographic gratings is one kind of DWDM device, which are characterized by dispersion, high diffraction efficiency and polarization-insensitive. Nowadays, holographic gratings have been widely used in optical communication systems. However, the polarization-insensitive transmission gratings have high aspect ratio structure, which are very hard to transfer the pattern from photoresist mask to substrate (fused silicon). In this paper, the aspect ratio is effectively reduced by adopting LaK9 as substrate with a high index of refraction relatively. The structure parameters of polarization-insensitive gratings should be designed and optimized, such as period, duty cycle and depth. The diffraction efficiency under TE polarization mode and TM polarization mode was discussed respectively in detail in this paper. At the same time, the bandwidth of the designed gratings is discussed. As a result, a proper gratings were designed with high diffraction efficiency (>90%) under TE polarization mode and TM polarization mode, which could be applied in optical communication. The ion etching difficulty is eased by reducing the aspect ratio.