PhD Defense: "Integrated Photonics Technologies for Free Space Optical Communications"

Hongwei Zhao

June 6th (Thursday), 12:00pm
Engineering Science Building (ESB), Room 2001

Free space optical communications can support much higher data rates than radio frequency technologies and allow for greater flexibility in transmitter and receiver design and optimization. Commercial-off-the-shelf (COTS) components provide a ready solution to assemble free space optical systems. However, communications from small spacecraft, where both performance and reliability are crucial, require optical components with lower cost, size, weight and power (CSWaP), while demonstrating high output optical power and power-efficient modulation formats. Integrated photonic technologies enable complex single-chip implementations of complex functionality with low CSWaP, therefore, are desirable for free space links.

Indium phosphide (InP) is the most developed platform for photonic integrated circuits. Of interest, is the advancement of this platform for applications that demand high performance, especially high output power in free space communications. In this work, two transmitter types were fabricated; one based on an offset quantum wells (OQW) platform, and the other on a quantum well intermixing (QWI) platform. The OQW-based transmitter consists of a widely tunable laser, a high-speed SOA, a Mach-Zehnder modulator, and an output SOA. This transmitter demonstrates a 44-nm tuning range, >45 dB side mode suppression ratio, 14.5 dBm off-chip power, and a data rate of 7 Gbps. The second transmitter, based on QWI, utilized an alternate epitaxial structure to achieve a lower confinement factor for higher SOA output saturation power. This QWI transmitter consists of a distributed Bragg reflector laser, a high-speed SOA, an electro-absorption modulator, and an output SOA. The measured off-chip power was 19.5 dBm, and a data rate of 20 Gbps was demonstrated. Based on the improved performance with the new epitaxial structure, a novel platform for high-power InP PIC transmitters integrated with low-confinement and high-power SOAs is proposed. A low-confinement SOA on the novel platform demonstrated a maximum optical power of 23.8 dBm.

Reconfigurable metasurfaces show promise to achieve robust point-to-point laser beam steering in free space links. We have also done some preliminary work on the gate-tunable metasurfaces based on indium silicon oxide. Future efforts will be made towards beam steering by using reconfigurable metasurfaces along with high-power transmitters.

About Hongwei Zhao:

photo of hongwei zhaoHongwei Zhao received the B.S. degree from the Huazhong University of Science and Technology, Wuhan, China, in 2008, and the M.S. degree from the Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China, in 2011. She is currently working toward the Ph.D. degree in the Integrated Photonics Lab, Electrical and Computer Engineering Department, University of California Santa Barbara, Santa Barbara, CA, USA. Her research interests include silicon photonics with emerging materials (such as graphene, indium tin oxide, and indium silicon oxide), InP-based photonic integrated circuits for free space communications, and monolithic integration of III-V optical devices on Si substrate.