Research On Interleaving Filters And Thermo-Optic Switches Based On Planar Lightwave Circuit

Optical fiber communication is transferring form point-to-point transmission to mesh network. Traditional optical-electronical-optical (OEO) transformation limits the data rate and adds the cost. With reconfigurable optical add/drop multiplexer (ROADM) for signal switching and routing, the OEO transformation can be bypassed, reduceing equipment size and cost. ROADM based on planar lightwave circuit (PLC) technology is characterized by small size, large scale and low cost. Silica waveguide on silicon wafer (SiO₂/Si) is believed to be the best waveguide technology for optical passive components integration. Single-chip integrated ROADM based on SiO₂/Si waveguide shows advantages in market competition and challenge in technical approach.This paper researches interleaving filters and thermo-optic switches based on SiO₂/Si waveguide, which are the bases for ROADM on chip. Our research work includesInterleaveing filter was designed with lattice structure lightwave circuit, which is composed of cascaded couplers and delay-lines. Directional couplers in the lattice circuit were extended to different complicated coupler structures and recursion equations for circuit parameters were derived. For half-band filter, and half-band filter with the last delay-line inversed due to layout requirement, design euqations for circuit parameters were also presented.For three interleaver types with directional couplers, stabilized couplers and GMMI-MZ-GMMI (GMMI, generized multimode interference) couplers, the influence of parameter error and process error on device crosstalk was simulated. The simulation results show that the last two interleaver types have better process tolerance. Simplified analytical analysis meets the simulation results well.Above three interleaver types were designed and fabricated. The devices show good performance. The insertion loss is less than 2dB, crosstalk is less than -17dB, and 0.5dB-bandwidth is more than 110GHz (for 200GHz channel spacing). It was first reported in this paper that GMMI-MZ-GMMI couplers were applicated in interleaver design, and show good performance and fabrication tolerance.Experimental results of the three interleaver types meet the above simulation results well. The last two types really show better crosstalk on the same process conditions. For interleaver with different coupler structures, with crosstalk and fabrication tolerance consideration, theoretical and experimental research was first carried out systematically in this paper.Two types of thermo-optic switch were designed and fabricated. The devices show good performance. The insertion loss is less than 0.85dB, extinction ratio is more than 21.73dB and response time is less than 3ms. Asymmetric extionction ratio for bar state and cross state was observed. Self-contained theoretical analysis was first given based on good uniformity of PLC process.For MMI coupler with tapered multimode waveguide, a more precise design formula was derived. MMI couplers with three tapering ratios were designed and fabricated. The excess loss is less than 0.18dB and uniformity is better than 0.17dB.