Large-scale reconfigurable silicon photonic integrated circuits for RF photonics and optical signal processing

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University of Delaware
The demand for flexible wideband receivers that can be software-reconfigured for various bands and communication standards is paramount. However, implementing frequency-agile radio receivers faces significant challenges due to the limited frequency selectivity and tuning range of existing RF frontend solutions. Surface acoustic wave (SAW) filters, commonly used for off-chip filtering, are bulky, expensive, and inflexible. Researchers have explored alternative off-chip approaches such as MEMS switches and dual conversion receiver architectures, but these solutions often result in large size, weight, power and cost (SWaP-C), which is undesirable for both military and commercial wireless devices. Additionally, other solutions like RF MEMS or LTCC-based filters suffer from drawbacks such as large size, high voltage requirements, limited rejection for strong blockers, and limited tuning range. Active CMOS filters face limitations in tuning range and linearity, while mixer-first architectures using reciprocal mixing switches in nano-scale CMOS are constrained by losses and limited operating frequency. ☐ RF photonics, on the other hand, has emerged as a promising solution to overcome these challenges. It offers the advantages of large bandwidth and high tunability inherent to light. Traditional RF photonic systems have relied on discrete photonic components made of LiNbO3, which are costly, inefficient, and power-hungry. In contrast, silicon-based RF photonics leverages the mature CMOS processes developed in the semiconductor industry, enabling the integration of electronics with Photonic Integrated Circuits (PICs) on a large scale. Silicon photonics provides the potential for revolutionizing flexible wideband RF receivers by offering reliability, low manufacturing costs, and reduced energy consumption, all within a smaller form factor. ☐ However, achieving successful integration of Silicon Photonic Integrated Circuits (PIC) with RF electronics necessitates a cohesive design and verification platform capable of co-simulating both photonic and electronic circuits. The design tools for photonic integrated circuits (PICs) have evolved independently from electronic circuit simulation tools, resulting in a disconnection between these two domains. Bridging this gap is essential to enable efficient co-simulation of photonic circuits with interfacing electronic circuits. To address this, current work introduces a complex frequency chirp-based method for rapid frequency-domain simulation of PICs. The study investigates the trade-offs in selecting simulation parameters for achieving desired frequency response accuracy and simulation time, taking into account factors such as windowing and frequency chirp profile. The presented method can result in over 1000x improvement in simulation time of frequency sweeps of higher-order optical resonant circuits. ☐ In any flexible RF photonic system, the Electro-Optic (EO) modulator plays a crucial role as a key component, requiring both reconfigurability and high linearity. While traditional lithium niobate (LiNbO3) Mach-Zehnder Modulators (MZMs) have been widely used due to their superior linearity, silicon-based EO modulators have not achieved the same level of performance. To address this, the present work focuses on the experimental demonstration of a Ring Assisted Mach Zehnder Modulator (RAMZM) fabricated using a silicon photonic foundry process. This RAMZM modulator allows for linearization in the optical domain, and can be reconfigured to linearize around a user-specified center frequency and bias conditions, even in the presence of process, voltage, and temperature variations. The developed automatic reconfiguration algorithm, enabled by DACs, ADCs, TIAs, and digital configuration engine, linearizes the RAMZM modulator to achieve a spurious-free dynamic range (SFDR) exceeding 113 dB.Hz^2/3, assuming a shot-noise limited link. Moreover, a novel biasing scheme is introduced for RAMZMs, which significantly enhances the modulation slope efficiency, resulting in a tone gain of more than 13 dB compared to its standard operation. This reconfigurable electro-optic modulator can be seamlessly integrated into integrated RF photonic System-on-Chips (SoCs), offering the advantages of integration and cost-effectiveness. ☐ Furthermore, within a flexible RF photonic frontend, it is crucial for optical filters to have high tunability across a broad range of center frequencies and wide bandwidths. Additionally, these filters are expected to be frequency agile, meaning that their tunability and reconfigurability should be rapid and automatic. Therefore, this work demonstrates a software-configurable integrated optical filter capable of on-the-fly reconfiguration based on user specifications (filter topology, center frequency, bandwidth, and rejection). The digital configuration engine automatically reconfigures the filter with high fidelity even when process and temperature variations are present. The design is fabricated using AIM Photonics' Active SiP process, and I validated the reconfiguration algorithm for a second-order filter with 3dB bandwidth of 3 GHz, 2.2 dB insertion loss and >30 dB out-of-band rejection using only two reference laser wavelength settings. ☐ Moreover, in high-performance RF photonic systems, the incorporation of additional functionalities like optical mixing, true-time delay lines, frequency generation, and beamforming necessitates the use of dedicated silicon photonic IPs. Designing these Application-Specific Photonic Integrated Circuits (ASPICs) involves complex and time-consuming design cycles such as simulation, layout, fabrication, packaging, and testing, which require significant engineering effort and incur high costs. To address this challenge, the introduction of a general-purpose reconfigurable PIC holds the potential to revolutionize the field of integrated photonics, similar to the impact of electronic field-programmable gate arrays (FPGAs) in the electronics industry. Such a reconfigurable PIC would enable rapid design exploration and could cut the design cycle time from months to just a few hours. Therefore, in this work, general-purpose silicon photonics-enabled optical mesh structures have been designed and fabricated. These structures are large-scale mesh architecture fabricated using a CMOS-compatible SiP foundry process interfaced with on-chip monitors and an electronic hardware backend. A wide range of optical circuits is synthesized automatically within these meshes with the help of the electronic hardware backend.
Filter, Modulator, Photonic integrated circuit, Programmable photonics, RF Photonics, Silicon photonics