PhD defence of Jinsong Zhang – Advanced Integrated Silicon Photonic Devices and Circuits for Optical Interconnects

Abstract

The ever-increasing throughput of global networks has been driving the evolution of optical communication in the past decades. Optical transceivers are the key component in optical communication to transmit and receive data through optical fiber. Silicon photonics leverages its compatibility with the mature process of complementary metal-oxide-semiconductor (CMOS) fabrication to enable low-cost and high-yield mass production, making it a promising platform for optical transceivers.

In this thesis, we cover the design and characterization of advanced silicon photonic devices and circuits for optical interconnects. The thesis can be separated to two parts. In the first part, we demonstrate three designs of passive optical devices that utilize subwavelength gratings (SWG) to improve the devices in various aspects. In the first design, by inserting a SWG slot in the middle of a multimode interference (MMI) coupler to achieve 1310/1550 nm multiplexing, we managed to reduce the length of the device to 37 μm while achieving calculated 1-dB-IL bandwidths of 192 nm and 123 nm at the two target bands. The second design is a broadband all-silicon multi-band transverse-magnetic-pass (TM-pass) polarizer where we carefully utilize different regimes of Bragg gratings for both TE and TM modes. The device achieves 343-nm bandwidth with IL < 0.4 dB and polarization extinction ratio (PER) > 20 dB in simulation. In the third design, a broadband silicon photonic waveguide crossing enabled by SWG lateral cladding is proposed. The device achieves a calculated maximum IL of 0.229 dB and a maximum crosstalk of -35.6 dB over a 415-nm wavelength range from 1260 nm to 1675 nm, which covers the whole band for optical communication. All three designs are tested experimentally and the results are shown to well match the simulation.

In the second part, we introduce two works about the co-design of silicon photonic circuits and optical communication systems. The first work is the development of two low-complexity digital signal processing (DSP) algorithms to improve the performance of the optical digital subcarrier multiplexing (DSCM) transmission system on a silicon photonic transmitter. In the experiment with 64 GBd 4-bit/s/Hz DSCM signal containing 8 subcarriers through 43.2 km of standard single-mode fiber (SSMF), the two algorithms combined bring a power budget increase of 4.159 dB at the HD-FEC threshold. For the second work, we propose a novel silicon photonic receiver with phase-retrieving capability based on the spectrally efficient silicon asymmetric self-coherent detection (ASCD). In 40-km transmission experiments, a record net electrical spectral efficiency (ESE) of 7.10 bit/s/Hz per wavelength and per polarization is achieved, where a net 208-Gb/s 32QAM transmission is demonstrated using 29.3-GHz electrical bandwidth.