Optimal design of a high-speed polymer Mach-Zehnder interferometer electro-optic switch over 260 GHz

Abstract

By using the proposed 3-D mode propagation analysis method and point-matching method, a polymer multimode interference (MMI) Mach-Zehnder interferometer (MZI) electro-optic (EO) switch is designed and optimized for enhancing the EO modulating efficiency and matching the impedance and the velocity. The designed switch possesses low driving voltages of ±1.375 V with a short EO active region length of 5 mm under 1550 nm wavelength, and the estimated cutoff switching frequency is up to 263 GHz due to the less mismatch between the lightwave velocity and microwave velocity.The 3-dB lightwave bandwidth is 60 nm, and within the wavelength range of 1520-1580 nm, the insertion loss and crosstalk are less than 6.71 and -30 dB, respectively.Polymer assisted EO switches using the linear EO effect should be fabricated with a short device length, low switching voltage, low crosstalk, low insertion loss, fast switching speed and relaxed manufacture tolerance [1,2] . During the design of such devices, firstly, the high EO modulation efficiency is desirable for decreasing the switching voltage and minimizing the device size, secondly, the match of impedance is required for improving the switching features especially under high switching frequency by avoiding the reflection of microwave signal, and thirdly, the match between microwave refractive index and lighwave refractive index is essential for increasing the switching speed and thus expanding the microwave band. To achieve these features, we previously reported the four-section micro-strip push-pull directional coupler (DC) switch [3] and the shielded push-pull DC switch [4] .The interest of the application for multimode interference (MMI) coupler in integrated optoelectronic devices has been increased dramatically [5][6][7] , due to the advantages of its compactness, relaxed fabrication tolerance, large optical bandwidth, polarization insensitivity and suitability for device integration. Therefore, the MMI-based Mach-Zehnder interferometer (MZI) EO switches, in some degree, show superior characteristics and relaxed manufacturing tolerance to the DC switches. In view of the above considerations, an impedance-matched polymer MZI EO switch comprising two 3-dB MMI couplers is designed and optimized for enhancing the EO modulation efficiency and matching the impedance and the velocity. Fig.1 shows the schematic diagram, the cross-section in the EO active region, and the schematic diagram of the MMI waveguide with length L MMI and width W MMI for the designed polymer MZI EO switch. The device consists of two symmetric 3 dB MMI couplers as the power splitter and power combiner, respectively, an MZI EO region with length L EO , and a push-pull dual-driving electrode structure comprising two upper operation electrodes with width W, gap G and thickness b 3 and a grounded under electrode. The refractive index and amplitude bulk attenuation coefficient are n 1 = 1.643 and 1 =2.0 dB/cm for the cross-linked core material AJ309 [8,9] , n 2 = 1.461 and 2 =0.25 dB/cm for the upper/under buffer...