High Speed Silicon-On-Insulator Optical Modulators Based On The Free Carrier Plasma Dispersion Effect.

Silicon-based optoelectronic integrated circuits for future communications and interconnect applications require high speed silicon (Si) optical modulators with GHz and tens of GHz bandwidth. The design, fabrication, and performance of three Si waveguide modulators meeting this speed criterion are presented in this thesis. They are based on the free-carrier plasma dispersion effect, where a change in free carrier density results in the needed refractive index change for optical phase modulation. An MZI structure is then used to convert this phase modulation into the desired intensity modulation. The first device, with a cross-section of 2.5 μm x 2.3 μm, is based on charge accumulation of a metal-oxide-semiconductor (MOS) capacitor embedded inside a silicon-on-insulator (SOI) waveguide. It has a 12 nm silicon dioxide gate that is sandwiched between a p-type poly-Si layer and an n-type crystalline Si layer. It has a bandwidth of 2.5 GHz and is the first Si modulator to break the GHz barrier. Using custom-designed drive circuitry, it transmitted data at 4 Gb/s with 1.3 dB extinction ratio (ER) and 4 dB of on-chip optical loss. The second modulator is an improvement upon the first where the p-type poly-Si is replaced by a lower loss crystalline Si layer. Its device cross-section is also reduced to 1.6 μm x 1.6 μm to improve mode-charge overlap and therefore phase efficiency. Furthermore, its doping levels are increased for higher speed operation. These improvements enabled this modulator to become the first Si device to experimentally demonstrate 10 GHz bandwidth. Using the same custom-designed drive circuitry, it transmitted data at 10 Gb/s with 3 8 dB ER. The third modulator, with 0.6 μm x 0.6 μm cross-section, is based on carrier depletion of a p-n diode embedded inside an SOI waveguide. To achieve high-speed performance, a travelling-wave design is used to allow co-propagation of the electrical and optical signals along the length of the device. The resulting modulator has a bandwidth of >30 GHz and demonstrated 40 Gb/s data transmission with 1.1 dB ER. This p-n diode based modulator remains the fastest Si modulator to date