The Research On Low-Power Optical Switch Based On Silicon Photonic Crystal Nanobeam Cavities

The constantly developing optical communication systems are driving demand for larger-scale optical matrix switches with low power consumptions and high capacity transmissions.Optical switching is currently perceived as a promising technique in optical communication systems including optical add–drop multiplexing(OADM)systems and optical cross-connects(OXCs).The cost-effective and power-efficient optical switching networks require optical switches with low power consumption and high capacity.A 2 × 2 optical switch,capable of routing an optical signal from an input port to an unoccupied output port,is a key component for these optical switching networks.Such 2 × 2 units,when cascaded and interconnected,are the “building blocks” that enable highradix N × N switch fabric offering large numbers of routing channels.Silicon-based optical switches can offer competitive advantages including compact device footprint,complementary metal oxide semiconductor(CMOS)compatible fabrication,and low power consumption.In previous reports,various schemes have been demonstrated to implement EO switches based on the high-speed free-carrier plasma dispersion(FCD)effect in silicon.Among them,microring resonators(MRRs)and Mach-Zehnder interferometers(MZIs)are two common optical configurations.However,MRRs and MZIs have physical lengths of several tens or even hundreds of micrometers,leading to relatively large device footprints and high power dissipations caused by free carrier absorption(FCA)in these EO switches.In recent years,photonic crystal nanobeam(PCN)cavities have attracted considerable attention for implementing many new optoelectronic devices,including optical filters,EO modulators and so on.In a PCN cavity,photons are strongly confined within a small area and light-matter interaction is highly enhanced,thus the mode volume shrinks to the fundamental limit of V =(?/2n)3.Since the mode volume and light-matter interaction have effect on the switching energy,an optical switch based on PCN cavities could significantly reduce the device footprint and the power consumption.My thesis focuses on a scheme to implement a 2×2 optical switch based on two PCN cavities,which have been demonstrated as follows:1)Structural optimalization of the silicon PCN cavity with ultra-small mode volume: The proposed 2×2 silicon optical switch is a compact four-port system consisting of two cascaded PCN cavities.In each PCN cavity,the central nanobeam waveguide is evanescently coupled to two identical bus waveguides with equal coupling strengths.The central nanobeam waveguide etched with an array of air-holes forms a Fabry-Perot(F-P)cavity,which consists of a central-taper section and two side-reflector sections.The central-taper section is optimized to reduce the scattering loss and provide high phase matching between the photonic crystal Bloch mode and the waveguide mode.The side-reflector sections are designed as two symmetrical mirrors to reflect light to the central-taper section.Such PCN cavities have ultra-small mode volumes,leading to compact device footprints and low switching powers in the optical switches based on PCN cavities.2)2×2 optical couplers based on PCN cavities: based on the coupled-mode analysis,a PCN cavity is evanescently coupled to two identical bus waveguides with equal coupling strengths,which implements a stand-mode 2×2 optical coupler.The power distributes equally at the four ports of the single naobeam cavity based optical device at the resonant wavelength,i.e.,the transmission efficiency at each port reaches a maximum of 25% in the 2×2 optical coupler based on the single PCN cavity.In order to realize a high drop-port transmission,two identical PCN nanobeam cavities are cascaded with properly chosen phase difference between the two connecting bus waveguides.Based on the coupled mode theory,most light power goes to the drop port if the phase difference between the two arms |?1 – ?2| is equal to ? radians,thus the 2×2 optical coupler based on dual PCN cavity is achieved.3)A 2 × 2 thermo-optic(TO)switch implemented by dual PCN cavities: We propose and experimentally demonstrate a 2 × 2 thermo-optic(TO)crossbar switch implemented by dual PCN cavities within a silicon-on-insulator(SOI)platform.By thermally tuning the refractive index of silicon,the resonance wavelength of the PCN cavities can be red-shifted.With the help of the ultra-small mode volumes of the PCN cavities,submilliwatt power is needed to change the switching state.Based on the experimental measurements,we achieved a 2 × 2 TO nanobeam switch with an ultrasmall switching power and high thermal tuning efficiency.4)Research on a compact 2×2 electro-optic(EO)switch based on dual PCN cavities: We propose a compact 2×2 electro-optic(EO)switch with ultralow switching energy,which is implemented by dual silicon photonic crystal nanobeam(PCN)cavities.The PCN cavities embedded with PN junctions are optimized to reduce the optical mode volume and increase the carrier-photon overlap,whereby the drive voltage and power consumption of the EO switch can be minimized.The device is designed on a silicon-on-insulator platform with a small device footprint.Based on the finite-difference-time-domain(FDTD)simulation results,we demonstrated the feasibility of the 2 × 2 EO nanobeam switch.