Compact On-chip Silicon Micro-nano Polarizers Based On Waveguide Gratings

Silicon-based photonic integration technology has been rapidly emerging in recent years.Due to the high polarization mode dispersion of silicon-based waveguides,on-chip polarization management is particularly important for silicon-based photonic integration circuits.The polarization filtering scheme based on a polarizer has received a lot of attention because of its simple implementation,low cost,and the advantage of improving on-chip integration.In this paper,we propose and design several on-chip polarizers with low insertion loss,high extinction ratio,and small size based on silicon-based waveguide gratings.The main research contents and innovations of this paper are as follows.1.Two types of transverse electric(TE)/transverse magnetic(TM)polarizers based on vertical/horizontal asymmetric directional couplers are designed using subwavelength gratings.A folded compact cascade scheme is constructed to improve the operating bandwidth of the polarizer and reduce the device size.The numerical analysis results show that the insertion loss of the TE polarizer is 0.13 d B at 1550 nm,the extinction ratio is 37.64 d B,and the 30 d B bandwidth can reach 220 nm with a size of only 22 ?m;the insertion loss of the TM polarizer is 0.4 d B at 1550 nm,the extinction ratio is 37.73 d B,and the 30 d B bandwidth can reach 62 nm with a size of 40 ?m.It can be seen that the two proposed polarizers have better bandwidth characteristics and extinction ratio,comparable device size and insertion loss.2.A TM pass polarizers based on a slot waveguide Bragg grating is proposed.By setting Bragg gratings in the slot area,the device achieves large extinction ratio and operating bandwidth in a small size.At the same time,a mode converter is designed at the input and output to reduce the insertion loss.Numerical simulations show that the insertion loss of this polarizers is 0.26 d B at 1550 nm,the extinction ratio is 36.2 d B and the 30 d B bandwidth can reach 113 nm,and the grating area size is only 13 ?m.It can be seen that this design achieves high polarization extinction ratio and small size while keeping the insertion loss low.In order to reduce the intensity of light reflected back to the input side of the Bragg grating polarizer,this paper also proposes a TM polarizer based on the inverse mode transformation Bragg grating.The reflection mode is reduced by transforming it into a higher-order mode and using the mode cutoff condition to change it into a radiation mode to leak into the cladding space.The analysis results show that the device has a very low insertion loss ~0.08 d B at the central wavelength of 1550 nm,an extinction ratio of 51.83 d B,and a 40 d B bandwidth of 48.34 nm.At the same time,the reflection loss at the central wavelength is reduced to-12.6 d B,which greatly reduces the impact of reflected light on other devices.To the best of our knowledge,this device design idea can provide a theoretical reference for the design of practical Bragg grating polarizers.