Advances in Silicon Photonics: Integrated Laser Stabilization, Isolation and Applications

Date of Award

Spring 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Applied Physics

First Advisor

Rakich, Peter

Abstract

Silicon photonics has revolutionized integrated optical systems, offering compact, scalable, and high-performance solutions for applications in telecommunications, metrology, and quantum technologies. However, key challenges remain in achieving stable, high-performance on-chip lasers due to excess phase noise, frequency instability, and sensitivity to unwanted optical feedback. The absence of CMOS-compatible optical isolators and ultra-stable frequency references further limits the robustness of integrated photonic systems. This dissertation presents novel approaches to address these challenges by developing integrated laser stabilization techniques and broadband optical isolation solutions, leveraging micro-Fabry-Perot (uFP) reference cavities and acousto-optic interactions. A crucial requirement for high-performance integrated photonics is a stable and narrow-linewidth laser source. This work develops an approach for co-packaging ultra-high-quality factor, low-noise \textmu FP cavities with photonic integrated circuits to enable advanced laser stabilization techniques such as self-injection locking (SIL) and Pound-Drever-Hall locking directly on-chip. The uFP cavities used in this study are fabricated via wafer-scale processing, resulting in vacuum-sealed resonators with sub-milliliter volumes. These cavities provide exceptional frequency stability with minimal thermal noise, making them ideal for integrated photonic applications. To efficiently interface uFP cavities with on-chip lasers, this work introduces a novel reflection transformation circuit (RTC) that enables stable optical coupling. A significant challenge in using Fabry-Perot cavities for laser stabilization is their high-reflectivity response, which can destabilize laser operation. The RTC overcomes this issue by reshaping the reflection spectrum, allowing for stable SIL and PDH locking without optical circulators. Using this approach, we demonstrate a SIL laser with a record-low phase noise (-97 dBc/Hz at 10 kHz offset) and fractional frequency stability of 5*10^(-13) at 10 ms in an integrated silicon photonic system. These results establish a path toward ultra-stable, narrow-linewidth semiconductor lasers for applications such as high-precision spectroscopy, optical frequency metrology, and atomic clocks. Beyond laser stabilization, this dissertation addresses the critical challenge of on-chip optical isolation. Traditional isolators rely on magneto-optic materials that are incompatible with CMOS processing, limiting their integration with photonic circuits. The lack of a practical on-chip isolator reduces the scalability of integrated photonic systems, as unwanted optical feedback can degrade laser coherence and introduce noise. To overcome this limitation, we develop a terahertz-bandwidth nonmagnetic optical isolator based on acousto-optic interactions in silicon photonic waveguides. By leveraging electrically driven phonon scattering, this approach enables broadband optical isolation without requiring magneto-optic materials. Through multimode waveguide dispersion engineering, we demonstrate an isolation bandwidth exceeding 2THz with insertion losses as low as -2dB. This level of performance surpasses existing integrated optical isolation techniques, offering a practical solution for protecting on-chip lasers from destabilizing back-reflections. These innovations in laser stabilization and optical isolation represent a significant advancement in silicon photonics, addressing fundamental limitations in integrated photonic systems. The co-packaging of uFP reference cavities with photonic circuits provides a new level of stability and coherence for integrated laser sources, enabling ultra-low-noise microwave photonics, coherent optical communications, and high-precision sensing. Meanwhile, the development of a CMOS-compatible, broadband optical isolator enhances the robustness of integrated lasers and amplifiers, making fully integrated photonic systems more scalable and reliable. Beyond telecommunications and metrology, the technologies demonstrated in this work have broader implications for emerging fields such as quantum photonics, where coherence and stability are essential for quantum computing and sensing. The ability to integrate ultra-stable laser sources and nonreciprocal optical components on a single chip paves the way for miniaturized, portable precision photonic systems with applications in navigation, remote sensing, and imaging.

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