High-Speed Group IV Photonics 

     Si photonics or Group IV photonics have been extensively applied in high-speed data transmission systems. Two key components of high-speed Si photonics are Si/Ge modulators and photodetectors. Unlike III-V based counterparts, Si/Ge modulators and photodetectors are easily integrated with other passive components and can be implemented by advanced CMOS technologies. It is advantageous to realize a multi-channel transceivers almost without extra cost. In our lab, we focus on the design and modeling of high-speed Si/Ge optoelectronic components and develop a new device structure with better performance.  

Si Traveling-Wave Mach-Zehnder Modulator

A low-voltage dual-drive push-pull Si MZM operated at 1550 nm with optical 25 Gbaud PAM-4 transmission and 6 dB extinction ratio at a driving voltage of 2.5 VPP is presented, which is compatible with the supply voltage of common CMOS electronics. It potentially can be applied for 50 Gb/s data transmission for optical communication. We also demonstrate the integrated modulator with CMOS driver and successfully modulate 25Gb/s OOK signal.

Fringe-Field Carrier-Depletion Modulator

A high-speed carrier-depletion silicon modulator based on a fringe field pn junction design is presented. Due to the strong fringe field, the size of heavily doped regions can be reduced and away from the waveguide core, whereas large modulation efficiency is still accomplishable. The VπL is 1.8 V-cm and the phase shifter loss is 1.3 dB/mm. The figure of merit (FOM), defined by the product of VπL and phase shifter loss, is estimated to be 23.4 dB-V. Meanwhile, the phase shift is linear to the driving voltage.

Integrated all-Si Traveling-Wave Optical MZM Modulators and Photodetectors Operating beyond 40 GHz

We present high-speed, traveling-wave Si Mach-Zehnder modulators and Si defect-mediated sub-bandgap photodetectors monolithically integrated on a Si-only photonics platform without incorporation of Ge epitaxial growth process. Through constructing a detailed equivalent circuit model on the components, we design the device structure and traveling-wave electrodes for operating the device with bandwidth beyond 40 GHz. The experimental results show the 3-dB bandwidths of the Si modulator and photodetectors are 35 GHz and 44 GHz, respectively, generally agreeing well with our design.

A High-Speed and Low-Breakdown-Voltage Silicon Avalanche Photodetector

A silicon avalanche photodetector with a low break- down voltage of −6.78 V is demonstrated by narrowing down the intrinsic layer width of interdigitated p-i-n junctions to ∼150 nm. It reaches the physical limit of avalanche breakdown in which the performance degradation caused by the Zener tunneling process is negligible. Dark current <350 pA at −4 V is measured, and a responsivity (gain) exceeding 1.424 A/W (110) at −6.75 V is obtained with an 850-nm laser illumination. The intrinsic bandwidth is determined to be 10 GHz, suggesting our device is applicable for a 10-Gb/s high-speed optical receiver application and beyond.

We incorporate an inductor design at the metal trace of the Si MRM to introduce an additional inductance in the circuit. This inductance can yield an inductive peaking effect to enhance signal transmission at the peaking frequency. Meanwhile, adjusting the operating wavelength or resonant wavelength of the MRM can introduce an optical peaking effect, depending on the optical frequency detuning. Via engineering both the inductance and the optical frequency detuning, the modulation band of the Si MRM can be tailored with more degrees of freedom. We present two types of modulation schemes; one is with a high EO conversion gain at a certain frequency band for microwave photonics, and the other is with a broad, flat EO conversion spectrum for high-speed data transmission. A very high-speed Si MRM with a flat modulation band over 95 GHz (corresponding to a maximum NRZ bitrate of 126 Gbit/s) is expected to be accomplished.