Photonic Integrated Receivers for Broadband Optical Wireless Communication

Optical wireless communication (OWC) offers many benefits over existing wireless technologies that rely on radio frequencies, making it a promising candidate for future wireless networks. When using narrow optical beams, OWC brings a number of advantages, including a very wide unlicensed spectrum, huge bandwidth, low latency, high security, high privacy, and high energy efficiency. The focus of this thesis is on an optical receiver for indoor OWC. In the indoor OWC scenario, the transmission and reception angles can be quite large, which requires wide-angle beam steering and wide-angle reception at the OWC transmitters and receivers. The OWC receivers require a large aperture to capture as much light as possible, a wide field-of-view to capture light from various directions, and a high bandwidth for high-capacity data transmission. However, one cannot improve one parameter without interfering with the other. Firstly, the bandwidth of top-illuminated photodiodes which are commonly used in OWC is inversely proportional to their active area, i.e. their aperture. This is because with an increase in the area of a photodiode, its capacitance increases, negatively affecting the bandwidth. The second compromise is between the aperture and the field of view (FoV), which is caused by the law of etendue. The larger the aperture, the narrower the FoV becomes. Photonic integrated circuits (PICs) offer the possibility to separate light collection and light detection. In this way, both can be optimized separately. Light can be coupled into a waveguide on the chip by means of a surface grating coupler (SGC) array and detected with high-speed uni-traveling carrier photodiode (UTCPD) photodiodes. Additionally, it is possible to include extra processing, since the light is already in a waveguide on the chip. For example, an amplifier can be placed in front of the photodiode to increase the sensitivity of a receiver. An arrayed waveguide grating router (AWGR) can be used to separate wavelength-division multiplexing (WDM) channels, which enables scaling up the transmission capacity proportionally to the number of channels, or a coherent receiver with 90°- hybrid and balanced photodiodes, which improves sensitivity and enables receiving higher-order modulation formats to increase spectral efficiency. Furthermore, PICs are scalable; it is straightforward to expand functionality by combining features, e.g. AWGR and a coherent receiver for receiving coherent dense WDM channels. However, each additional feature increases the complexity of the design and fabrication. This thesis presents the research efforts to develop a novel wide-FoV, wide-bandwidth, and large-aperture PIC-based receiver for optical wireless communication. Three PIC fabrication platforms are considered for light collection: silicon-on-insulator (SOI), silicon nitride (SiN), and indium-phosphide membrane on silicon (IMOS); SOI and SiN for low losses, high integration density, and complementary metal oxide semiconductor (CMOS) compatibility, and IMOS for its active devices and potential of monolithic integration of active photonic chips with CMOS electronics. An array of large SGCs targets the aperture and the field-of-view requirements of OWC receivers. A −7-dB 360° horizontal FoV, a vertical −3-dB FoV of > 6° , and aperture of 240 μm diameter are achieved with an array of four silicon photonics focusing grating couplers, where each grating coupler faces a different cardinal direction, and their outputs are combined with 1×2 multi-mode interferometer (MMI) couplers into a single waveguide. Error-free wireless transmission of 8 Gbps non-return-to-zero on-off keying was achieved by using the silicon photonics SGC array for light collection. In addition, an array of six fiber-to-chip focusing SGCs with 360° FoV fabricated on the IMOS platform is presented. 100 Gbps orthogonal frequency-division multiplexing (OFDM) is achieved in a wireless communication system over 3 meters using a SiN grating coupler for light collection. High-capacity transmission is achieved with wavelength division multiplexing – error-free 100 Gbps (4x channels of 25 Gbps nonreturn-to-zero (NRZ) on-off keying (OOK)), and coherent transmission – 270 Gbps (45 GBaud, 64-QAM) – with SGCs used for light collection from free space. Due to the fact that the grating couplers are polarization-sensitive devices, transmission of the same data on two polarization states is validated, so that the PIC receiver can be arbitrarily oriented with respect to the transmitter. Lastly, a monolithic transceiver is proposed, where local oscillator laser is shared between coherent receiver and transmitter, with an array of eight grating couplers for light collection and an optical antenna for uplink transmission. Building upon these results, future work could explore how to increase the vertical field of view of grating couplers by means of varying pitch, duty cycle, and etching depth, or with microlenses, enlarging the aperture with more elements in array, how to create an angle diversity receiver, and how to package a PIC OWC receiver. The work presented in this thesis provides a path towards advanced photonic integrated receivers for optical wireless communication.