Subwavelength Silicon-Wire Photonics

Abstract

A new technology using hydrogen annealing and thermal oxidation is introduced to make subwavelength photonic wires as well as to achieve three-dimensional photonic integration on a silicon-on-insulator (SOI) substrate. Due to the hydrogen-annealing-induced profile transformation, the silicon wire with very smooth sidewall is accomplished, showing the propagation loss as low as 1.26 dB/cm. Moreover, a tapered beam spot-size converter is developed and monolithically integrated with subwavelength silicon wires. Owing to the low on-chip optical loss, nonlinear optical processes such as four-wave mixing, Raman emission and amplification, and anti-Stokes Raman conversion are observed at the pump power around just tens of milliwatts. To achieve large-scale integration of photonic circuits, waveguide couplers are essential to device integration and functioning. Thus a very compact two-dimensional multimode interference coupler with dual-layer silicon photonic wires is demonstrated for the first time using the similar fabrication technology. Two devices, a 1×1 cross power coupler and a 1×4 power splitter, are presented and characterized. The experimental result shows a 1.1-nm 3-dB transmission bandwidth, and on-chip coupling losses of 8.6 dB and 2.84 dB with respect to the 1×1 cross coupler and 1×4 splitter. This work shows the possibility to realize photonic integrated circuits in three dimensions on a silicon-on-insulator (SOI) substrate. For realizing active functions such as optical switch, signal modulation and attenuation on photonic circuits via the interference-type devices, it is important to control optical phase without amplitude dependency. However, it is challenging to be accomplished by using the conventional p-i-n or pn phase modulators based on the plasma dispersion effect due to the strong free-carrier absorption. A deformable silicon wire actuated by micro-electro-mechanical-systems (MEMS) offers an efficient way to control optical phase without causing much amplitude variation. By mechanically stretching the waveguide physical length, a 0.1-pi phase shift could be achieved via actuation of a single actuator at 240 V. The amplitude variation during the mechanical actuation is measured to be within 0.063 dB for both TE- and TM-polarized waves. To the authors’ best knowledge, this is the first time that a waveguide phase modulator decoupling from amplitude variation has been realized based on MEMS and silicon photonic technologies.