Xiaoguang Tu

Review of Silicon Photonics Foundry Efforts

Silicon photonics have progressed to a point where the next step for commercialization depends on the accessibility of manufacturing foundries. The implementation of a fabless foundry model using standardized process technology platforms is crucial for that to occur. Research and development (R&D) foundries are beginning to play bigger roles in transforming silicon photonics into a mature technology for mass production. R&D foundry services such as multi-project wafer (MPW) shuttles, customized process developmental runs and small volume manufacturing are discussed. The development of commercial foundries for low cost, high volume production is also shown to be underway, and key results from an on-going effort to set-up a manufacturing silicon photonics foundry line are presented.

Low power 50 Gb/s silicon traveling wave Mach-Zehnder modulator near 1300 nm

A silicon traveling-wave Mach-Zehnder modulator near 1300 nm is demonstrated to operate at 50 Gb/s with a differential 2 Vpp signal at 0 V reverse bias, achieving a 800 fJ/bit power consumption.

Thermal independent Silicon-Nitride slot waveguide biosensor with high sensitivity

As the sensitivity and detection limit of photonic refractive index (RI) biosensor increases, the temperature dependence becomes a major challenge. In this paper, we present a Mach-Zehnder Interferometer (MZI) biosensor based on silicon nitride slot waveguides. The biosensor is designed for minimal temperature dependence without compromising the performance in terms of sensitivity and detection limit. With air cladding, the measured surface sensitivity and detection limit of MZI biosensor reach 7.16 nm/(ng mm(-2)) and 1.30 (pg mm(-2)), while achieving a low temperature dependence is 5.0 pm/° C. With water cladding, the measured bulk sensitivity and detection limit reach 1730(2π)/RIU and 1.29 × 10(-5) RIU respectively. By utilizing Vernier effect through cascaded MZI structures, the measured sensitivity enhancement factor is 8.38, which results in a surface detection limit of 0.155 (pg mm(-2)).

50-Gb/s silicon optical modulator with traveling-wave electrodes

We demonstrate silicon Mach-Zehnder Interferometer (MZI) optical modulator with 50.1-Gb/s data rate and 5.56 dB dynamic extinction ratios. The phase shifter is composed by a 4 mm-long reverse-biased p-n junction with a modulation efficiency (V(π) · L(π)) of ~26.7 V · mm and phase shifter loss of ~1.04 dB/mm at V(bias) = -6 V. The measured electro-optic bandwidth reaches 25.6 GHz at V(bias) = -5 V. Compensation doping method and low loss traveling-wave electrodes are utilized to improve the modulator performance. Measurement result demonstrates that reasonable choosing of working point and doping profile of the silicon optical modulator is critical in order to match the performance requirement of the real application.

Fabrication of low loss and high speed silicon optical modulator using doping compensation method

Compared with an optical modulator based on lithium niobate, the total loss of the current high speed silicon modulator is still too high for commercial use. Reduction of the total loss always comes along with the degradation of the other two characteristics including modulation efficiency or switching speed. In this paper, we reduce the phase shifter loss through optimizing the doping level out of the depletion region while keeping the modulation efficiency and switching speed at a high level. Compensated doping method is utilized to optimize the doping level on the cross section of the phase shift. With doping compensation, the Loss·Efficiency figure-of-merit (FOM) of 4 mm phase shifter is reduced from 25.8 dB·V to 19.4 dB·V while still keeping the small signal 3 dB-bandwidth at about 10 GHz. After doping profile optimizing, the measured bandwidth of the phase shifter with doping compensation can even reaches 17 GHz with a Loss·Efficiency FOM of about 25.4 dB·V.

Low-loss silicon slot waveguides and couplers fabricated with optical lithography and atomic layer deposition

We demonstrate low-loss silicon slot waveguides patterned with 248 nm deep-UV lithography and filled with atomic layer deposited aluminum oxide. Propagation losses less than 5 dB/cm are achieved with the waveguides. The devices are fabricated using low-temperature CMOS compatible processes. We also demonstrate simple, compact and efficient strip-to-slot waveguide couplers. With a coupler as short as 10 µm, coupling loss is less than 0.15 dB. The low-index and low-nonlinearity filling material allows nonlinearities nearly two orders of magnitude smaller than in silicon waveguides. Therefore, these waveguides are a good candidate for linear photonic devices on the silicon platform, and for distortion-free signal transmission channels between different parts of a silicon all-optical chip. The low-nonlinearity slot waveguides and robust couplers also facilitate a 50-fold local change of the waveguide nonlinearity within the chip by a simple mask design.

Silicon Optical Interconnect Device Technologies for 40 Gb/s and Beyond

Important active technologies, modulators, photodetectors, and thermooptics for low-energy silicon optical interconnects are discussed. High-speed performance up to 40 Gb/s is reported for the silicon modulators and germanium photodetectors, and approaches for further improvement in speed and efficiency are presented. Low-voltage avalanche multiplication is demonstrated, giving a gain-bandwidth product of 75 GHz, while the combined effects of multiplication gain and the Franz-Keldysh effect enable a 5-μm-long germanium photodetector to achieve responsivity in the L-band that is comparable to that in the C-band. With trench-based thermal isolation, a low switching power of 0.4 mW is achieved for a thermooptic switch.

DQPSK/QPSK modulation at 40–60 Gb/s using low-loss nested silicon Mach-Zehnder modulator

44.6-Gb/s DQPSK and 50-to-64 Gb/s QPSK modulation are demonstrated using low-loss nested Silicon MZ modulator with fiber-to-fiber loss of 10dB. Dispersion tolerance of +/-80 ps/nm is observed in the DQPSK transmission.

Electrical tracing-assisted dual-microring label‑free optical bio/chemical sensors

We propose and demonstrate a novel electrical tracing-assisted dual-microring resonator-based optical sensor system in silicon-on-insulator substrate. The system comprises one microring resonator-based sensing element and another microring resonator-based tracing element integrated with electrical controller. The resonance wavelength shift of sensing microring induced by the refractive index change is traced and determined by direct voltage supply of the electrical tunable tracing microring. Such optical sensing system eliminates the traditional wavelength-scanning method thus provide a cost effective sensing scheme. Proof-of-principle demonstration by testing polyelectrolyte multilayer shows the sensitivity of ~4.0 mW/ng∙mm-2 and the detection limit of ~5.35 pg/mm2.

Efficient silicon nitride grating coupler with distributed Bragg reflectors

In this paper we have designed, fabricated and characterized a high efficiency Silicon nitride grating coupler at 1490 nm. Distributed Bragg reflectors as bottom mirrors are employed to improve the coupling efficiency by reflecting the downward traveling light. The peak coupling efficiency obtained is about -2.5 dB and the 1-dB bandwidth is 53 nm. The fabrication process is CMOS-compatible and is ready to be integrated with photonic circuits.