Research Of Polarization-independent Grating For C Band

Wavelength division multiplexer(WDM)is one of the key components in optical communication system.Its performance plays an important role in improving the capacity and transmission quality of the system.As an important wavelength division multiplexing device,the diffraction grating has the advantages of parallel demultiplexing,low insertion loss,narrow channel spacing and small device size.The polarization will change when light propagates in an optical fiber.The diffraction efficiency of a conventional grating is closely related to the polarization.If the non-polarized light is split directly by dispersion,the energy loss is large.It is often necessary to add a conversion device to convert the polarization of the incident light first.In this paper,the grating can achieve high diffraction efficiency in any polarization,avoiding the use of polarization conversion devices.High-efficiency polarization-independent gratings are of great importance for improving the energy utilization of optical communications.In this paper,two kinds of C-band polarization-independent gratings-metal/multilayer-dielectric grating(MMDG)and multi-layer dielectric grating(MLDG)are studied,and corresponding design schemes are given,and theoretical analysis and calculation are carried out.The main work of contents are as follows:(1)Based on the rigorous coupled wave analysis,the grating is analyzed under the incident of TE and TM polarized light,and the calculation formula of diffraction efficiency is obtained.The calculation and optimization procedure of grating diffraction efficiency are written.(2)The 12001ines/mm metal/multilayer-dielectric gratings were studied.Ag and Au were used as the high-reflection materials on substrate,and the grating layer was a two-layer structure.The gratings with different refractive index materials were optimized for structure,and the corresponding angular and spectral properties were studied.The Si/SiO2 composite structure based on the Au substrate is preferably selected,and the diffraction efficiency of TE and TM waves in the C-band is greater than 95%,and the difference between the efficiency of the two is less than 3.1%.Based on the optimized structure,the effects of duty cycle,groove depth and matching layer thickness on the diffraction efficiency of the-1 order reflection are analyzed.The variation of the diffraction efficiency of the trapezoidal grating teeth with the bottom angle and the bottom duty ratio is considered.(3)The Si and SiO2 double-layer structure was used as the grating tooth,and the multilayer dielectric film was used as the reflective layer to study the multi-layer dielectric grating of 1200 lines/mm.The evaluation function including process tolerance is constructed.The genetic algorithm is used to optimize multiple structural parameters.The theoretical result of the average diffraction efficiency is greater than 98.9%in C-band and the diffraction efficiency difference between TE and TM polarization is less than 1%.Tolerance analysis shows that the grating can achieve diffraction efficiency of center wavelength higher than 90%in the range of 230-330nm Si etching and 0.48-0.61 duty cycle.The SiO2 tooth depth is controlled at 330-520nm,the average diffraction efficiency is greater than 90%.Compared to the MMDG grating,the requirement for sidewall straightness is reduced.(4)Preparation of Si/SiO2 double layer MLDG.A film structure of the grating is deposited on the glass substrate,and then a photoresist mask is formed using holographic lithography,and finally the trench shape is transferred to the underlying SiO2 and Si layers by ICP etching to complete grating preparation experiment.