Silicon photonic devices consisting of nanowire waveguides are a promising technology for on-chip integration in future optical telecommunication and interconnection systems based on silicon-large scale integration fabrication. However, the accommodation of variable optical components on a chip remains challenging due to the small size of microchips. In this study, we investigated the characteristics of a microelectromechanical silicon nanowire waveguide switch with a gap-variable coupler. Due to its capacitive operation, the proposed waveguide switch consumed negligible power relative to switches that use a thermo-optical effect and carrier injection. The proposed switch was characterized using analyses based on coupled-mode theory for rectangular waveguides, as well as a simulation using the finite difference time domain method. A 232 single switch with an improved configuration and a 236 multiple switch composed of the 232 switches was designed and fabricated by a combination of electron beam lithography, fast-atom beam etching and hydrofluoric acid vapor sacrificial etching. The properties of the switches were measured and evaluated at a wavelength of 1.55 mm. 1 The monolithic fabrication of silicon waveguides and silicon electronics is useful for future integration in opto-electronic systems. Due to the high refractive index of silicon (,3.5 at a wavelength of 1.5 mm), silicon waveguide circuits can be miniaturized to be several orders of magnitude smaller than silica waveguide circuits. Several types of circuits that employ submicron-scale silicon waveguides, such as waveguide splitters/couplers and micro-rings, 2-4 have been reported. In addition, a waveguide switch that uses a thermo-optical effect 5 and an ultra-fast silicon waveguide light modulator based on changes in the refractive index of silicon by carrier injection 6 have been reported. Recently, submicron-scale waveguide switches for optical path changes using the nano-mechanical motions of electrostatic actuators have been reported. 7 Due to the very low power consumption associated with capacitive operation, this technology suits the large-scale integration and reduced energy consumption requirements of telecommunication systems. [8][9][10][11][12][13][14][15][16] Among these waveguide circuits, coupler switches have attracted interest due to the use of non-contact low-loss mechanisms.13,15 A coupler switch using InP waveguides was operated by applying electrostatic forces between the freestanding waveguides. 13 In the case of a parallel-electrode actuator, a coupler gap smaller than two-thirds of the initial gap is not consistently controllable due to force instability. Using an in-plane comb-drive actuator, stable low-voltage operation has been realized for silicon waveguides.

Submicron-wide silicon waveguide coupler with gap variable mechanism is proposed for a compact optical waveguide switch. Two freestanding silicon waveguides are placed parallel with a submicron gap. The gap is changed by electrostatic comb-drive micro-actuators to control the coupling coefficient of the coupler. The fabricated device consisted of the silicon waveguides of 400 nm in width and 260 nm in thickness. The total size of the switch was 100 μm wide and 150 μm long. Decreasing the gap between the waveguides to 110 nm, the output intensity at drop port became a maximum while the output intensity at through port became a minimum. The extension ratio of the switch output was 17 dB for the waveguide displacement of 300 nm.

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