Hybrid and Heterogeneous Photonic Integrated Circuits for Radar and Remote Sensing Applications

Microwave Photonics (MWP) is an interdisciplinary field that leverages photonic technologies to address complex functions in microwave systems, offering advantages such as low-loss signal distribution, wide bandwidth, and immunity to electromagnetic interference. MWP finds applications in areas like photonic microwave generation, optically controlled phased array antennas, and radio-over-fiber systems. Photonics-based radar systems, in particular, greatly benefit from these advantages, enabling low phase noise, ultra-wide bandwidth, and flexible signal generation. This dissertation explores the integration of photonic technologies to enhance the performance of radar and remote sensing systems. Due to the inherent complexity of MWP systems and the limitations of current monolithic platforms, this work investigates innovative multi-platform integration techniques to develop fully integrated systems with advanced functionalities. The first chapters holds the theory behind MWP systems and the state-of-the-art of photonics for radar and remote sensing systems and of the main Photonic Integrated Circuit (PIC) platforms and multi-platform integration techniques. Then, the research activities are organized into several chapters, each focusing on systems that use different integration methods. Chapter 2 focuses on monolithic platforms and details the development of an RF receiver with down-conversion capability for electronically scanned active array antenna systems using Silicon- on-Insulator (SOI) technology. Two packaged architectures, both relying on an external Mode- Locked Laser (MLL), are developed. The first architecture consists of a cascade of a Mach- Zehnder Modulator (MZM) and a Balanced Photodetector (BPD), achieving 48 dB of RF-to-IF conversion gain, in line with previous implementations in SOI, and an Spurious Free Dynamic Range (SFDR) of 94 dB · Hz2/3. The second architecture employs two very narrow-band optical filters to select the MLL lines, eliminating the need for electrical filtering after photodetection. Its very limited conversion gain emphasizes the need for active materials to provide on-chip amplification. The subsequent chapter delves into the butt-coupling approach for hybrid integration of distinct platforms. Indium Phosphide (InP) and Silicon Nitride (SiN) chips are die-attached to realize a multibeam beamformer for a Synthetic Aperture Radar (SAR) receiver and an RF scanner for Electronic Support Measurement (ESM) systems. The former leverages the Scan-on-Receive (SCORE) method to achieve precise and continuous beamforming-on-receive of wideband signals from a 12-element antenna array, synthesizing up to three simultaneous receiving beams. The Optical Beamforming Network (OBFN) implemented in low-loss SiN is a Blass matrix. Experimental results demonstrate successful frequency down-conversion from X- band at 9.65 GHz to a low intermediate frequency at 1.35 GHz with a measured conversion loss of 50 dB. Furthermore, multi-beam beamforming operations of the PIC receiver module are, for the first time, experimentally demonstrated through radiation diagrams showing beam steering at different pointing angles. Two distinct architectures implement a 6-channel RF scanner that work up to 40 GHz. In one, frequency scanning is achieved using an electrical oscillator up- converted in the optical domain, while the other relies on a tunable laser. For both architectures, system simulations and PIC designs are presented. Additionally, preliminary device-level testing, enabled by the hybrid approach, is reported for both the SiN and InP chips. The core of both architectures are optical filters implemented as SiN microring resonator-assisted Mach-Zehnder Interferometer (MZI), which show transmission bandwidth as low as 2 GHz, and record values of Free Spectral Range (FSR) and out-of-band rejection of 36 GHz and 57 dB, respectively. Next, the design and device-level characterization of an integrated frequency converter, enabled by butt-coupling of InP and Lithium Niobate-on-Insulator (LNOI) chips, is presented. The LNOI platform is attractive for its potential to implement low half-wave voltage (Vπ) and high bandwidth MZMs, which are crucial for MWP systems. Measurements conducted on the LNOI MZM show a modulation efficiency of 10 V/cm and a bandwidth of 23 GHz, though performance is expected to consistently improve with new fabrication processes. Chapter 4 explores the Micro-Transfer Printing (MTP) technique for heterogeneously integrating InP Semiconductor Optical Amplifiers (SOAs) onto an SOI platform, resulting in the design of a frequency converter for radar transceivers and a coherent Multiple-In-Multiple-Out (MIMO) radar constellation. Device-level characterization of the printed SOAs shows a maximum on-chip gain of 9 dB, which is relatively low compared to InP SOAs on native substrates. System-level testing of the fully packaged frequency converter revealed a limited conversion gain due to damage in the pig-tailed fiber array. However, considering the damage, a higher SOA gain, and the addition of a Trans-Impedance Amplifier (TIA) to electrically amplify the output photocurrent, the system could achieve a gain comparable to standard electronics. System simulation and design of PIC for coherent MIMO SAR constellation is then reported. For this application photonics allows for the flexible generation of multi-band signals and centralized generation in a primary satellite with coherent distribution to all the secondary satellites of the SAR signals over Free Space Optical (FSO) links. Moreover, the use of integrated technologies enables Size Weight and Power Consumption (SWaP) reduction. Numerical analysis shows the proposed system has a Noise Equivalent Sigma Zero (NESZ) < -29.6 dB, satisfying the SAR system requirements. MTP-SOAs on the consolidated SOI platform are intended to decrease the overall conversion gain. Polarization multiplexing is used to consistently reduce system complexity. Finally, measurements of heterogeneously integrated external cavity lasers, enabled by wafer bonding of III-V materials onto SiN and LNOI platforms, are presented. The combination of wafer bonding, which offers significantly low alignment tolerances and compatibility with high- volume production, along with the low-loss properties of the SiN platform and the strong electro- optic characteristics of the LNOI platform, makes these heterogeneous platforms extremely promising for future MWP systems. This research advances MWP systems for radar and remote sensing applications by utilizing innovative integration methods. My primary contributions involved system simulations, PIC design, and testing at both the device level (with bare PICs) and the system level (with fully packaged PICs) to demonstrate system functionalities. Although the photonic integrated platforms reported in this work exhibit specific limitations, multi-platform integration techniques, particularly heterogeneous ones, offer a promising direction for future developments capable of meeting the demanding requirements of MWP systems.