Ultra-Compact Rectangle Filter Based on Silicon Substrate
Objective Optical signal processing is widely used in modern communication systems due to its large bandwidth,low loss,and capability for easy parallel transmission.Rectangle filters,as key components in optical signal processing systems,have become a prominent research focus because they meet the requirements of wavelength division multiplexing systems and are well-suited for broadband access applications.Integrated rectangular filters typically use structures such as waveguide gratings(WGs),Mach-Zehnder interferometers(MZIs),and microring resonators(MRRs).Although these designs achieve good performance,they often suffer from drawbacks like large size and complex manufacturing processes.In this paper,we propose an ultra-compact,silicon-based rectangular filter with a high shape factor,which significantly advances the miniaturization of wavelength division multiplexing systems and enhances the performance of elastic optical networks,on-chip optical interconnects,and broadband access systems.Methods Our design utilizes silicon-on-insulator(SOI)technology,where the top silicon layer has a thickness of 220 nm and the buried oxide layer is 2 μm thick.The waveguide is etched to a depth of 220 nm to form a strip waveguide,with an additional 2 μm of SiO2 deposited on top(Fig.3).The filter consists of a single MRR with a slot fabricated in the resonator,also etched to a depth of 220 nm.By fine-tuning the slot's length(ls)and width(ws),and the gaps(g1 and g2)between the ring and the straight waveguide,we create an ultra-small rectangle filter.To mitigate the effects of fabrication errors,we incorporate two micro-heaters in the coupling regions of the MRR.The filtering performance is significantly enhanced by adjusting the voltage applied to these heaters.Results and Discussions The gaps between the ring resonator and the input and output waveguides,denoted as g1 and g2 respectively,and the slot dimensions(length ls and width ws)within the ring resonator,are optimized using the finite difference time domain(FDTD)method.Starting with g1=0.120 μm,g2=0.110 μm,and ls=0.1 μm,the initial output spectrum at the add port demonstrates excellent rectangularity,minimal loss,and zero ripple.Further optimization of ws,by varying it from 0.115 μm to 0.175 μm,yields an optimal value of 0.155 μm.Subsequent refinements to g1 and g2 establish their optimal values at 0.120 and 0.115 μm,respectively.The resulting optimized rectangular filter(Fig.6)has a shape factor of approximately 3.13,a sidelobe ratio(SLR)of approximately 37.92 dB,and an insertion loss(IL)of approximately 6.23 dB.Given that the filter's minimum feature size exceeds 100 nm,fabrication via electron beam lithography is feasible,ensuring high precision.However,due to manufacturing challenges,slight deviations in the dimensions of the MRR coupling region and slot can significantly influence performance,necessitating an analysis of fabrication errors.We examine the filter's performance with a±30 nm deviation in g1 and g2(case 1).The worse performance is observed when g1 and g2 deviate by-30 nm,resulting in a shape factor of 3.95.The filter's performance with a±30 nm deviation in ls and ws(case 2)shows the poorest results when both ls and ws have deviations of-30 nm,with a shape factor of 3.94.To compensate for fabrication errors,we introduce two heaters in the MRR's coupling regions,adjusting their temperatures(T1 and T2)to optimize filtering performance.With T1=T2=46.34℃,the shape factor improves to 3.44,the SLR is 33.63 dB,and the IL is 6.29 dB for case 1.For case 2,with T1=49.90℃and T2=45.59℃,the shape factor reaches 3.57,the SLR reaches 40.66 dB,and the IL reaches 6.57 dB.To further reduce loss,the add and drop ports are merged,and a thermoelectrode is added to the upper branch,yielding an optimized performance:a shape factor of 5.42,an IL of 3.73 dB,an SLR of 24.00 dB,and a ripple coefficient of 1.15 dB,with a core filter size of 2580 μm2.Conclusions In this paper,we propose a rectangular filter based on a single MRR with a slot,achieving rectangular filtering by adjusting the spacing within the MRR and the slot length.The filter exhibits a 3 dB bandwidth of approximately 0.51 nm,a shape factor of approximately 3.13,an SLR of approximately 37.92 dB,an IL of approximately 6.23 dB,and a compact size of only 1200 μm2.Compared to prior designs,this filter's ultra-compact structure uses a single MRR,reducing size by a factor of 50 and enhancing shape factor performance by approximately 9%.The analysis of output spectra under fabrication errors further underscores the device's robustness.Lastly,to minimize losses,merging the add and the drop ports and incorporating a thermal electrode in the upper branch yield improved performance.
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